3.1.1.

Magnification

Clinicians from different fields have recognized the usefulness of magnification.^{10,25,62,72,82,103,114,121}

The total magnification ($M_{\text{total}}$) of a surgical microscope is determined by all the four optical components in the microscope, namely the focal length of the objective lens ($f_{\mathrm{OBJ}}$), zoom value ($M_{\mathrm{ZOOM}}$), the focal length of binocular ($f_{\mathrm{TUBE}}$), and the magnifying power of eyepieces ($M_{\mathrm{EP}}$),¹¹⁵ as Eq. (1)

Magnification of modern surgical microscopes varies from $4\times$ to $40\times$ ^10,73,122 and is usually selected through a manual or motorized magnification changer. The zoom value is usually 6:1 but can be as high as 8:1.¹²³ For some microscopes, an additional magnification multiplier is applicable, which provides 40% more magnification.¹²⁴ Resolution measures the acuity improved by magnification. It is the ability of an optical system to distinguish two separate entities.⁷⁴ Human eyes have an inherent resolution of 0.2 mm¹²⁵ but with $20\times$ magnification, it can be increased to 0.01 mm.¹²⁶ This can add more confidence to surgeons, enhance the advanced surgical skills, and enable the use of many fine surgical instrumentations when they operate on fine anatomical structures.¹²¹

3.1.2.

Optics

The design of optics is vital to the image quality of a surgical microscope. Aberration is an inherent property of optical systems, and it causes the blur or distortion of images, which is adverse to the desire for a clear view. Monochromatic aberrations such as spherical aberration, coma, and astigmatism can be corrected but usually only for one color.¹²⁷ Chromatic aberration is a failure of a lens to focus all colors to the same point, because of which images show color fringes and lose sharpness. Chromatic aberration correction is necessary for optics in a surgical microscope not only because of the wide-band light source used but also due to the image enhancement in cameras, such as sharpening and edge enhancement, which enhances the image edge as well as the color fringes.¹²⁸ Achromatic lens, which is a combination of converging and diverging lens elements, was employed in early surgical microscopes to correct the primary spectrum, leaving the secondary spectrum being the main factor limiting the image quality.^16,19 The apochromatic lens is the answer to that problem. It not only corrects for two wavelengths (red and blue) to reduce spherical aberration but also utilizes the exceptional quality optical materials that have unusual and desirable characteristics to reduce chromatic aberration for three wavelengths (red, green, and blue).^19,129

3.1.3.

Focusing

Focusing is essential for a clear view. Surgeons would want the surgical site to be in focus throughout the surgery. However, the shape of organs or the deep cavities makes it impossible for the whole surgical site to be perfectly on the focal plane. Depth of focus, in other words, depth of field (DOF), is a term that indicates the area in front of and behind the point of perfect focus where the sharp focus is maintained. It depends on many factors, including but not limited to the quality of optical design, the size of objective lens aperture relative to the focal length of the objective lens, and the magnification of the object, and it is reciprocal of the resolution.¹¹⁵ A good surgical microscope should have an adequate depth of focus without sacrificing too much resolution to keep the scene sharp. Another important term is parfocal, which means an optical system can stay in focus even with magnification changes.¹³⁰ Due to the need of switching magnification during surgery, a surgical microscope being parfocal saves surgeons from repeated refocusing.

Microscopes need to be well focused before the operation, and when the position of the microscope is adjusted during surgery, refocus is needed. A fast focusing capability can save setup time for surgery. Various methods have been proposed for the automatic focusing of the surgical microscope. Nohda¹³¹ proposed an automatic focusing device for a stereoscopic microscope, which detects the position of the reflected image of infrared LED (IR-LED) using a focusing screen. The positions of the IR-LED and the focusing screen are conjugate with the in-focus position of the sample; hence, the reflected image of the infrared diode is at the center of the focusing screen when the sample is in focus. Jorgens and Faltermeier¹³² proposed using the interaction of an active light-projecting system and a passive video system to focus on both covered and uncovered objects illuminated by the transmitted and reflected lights. Vry et al.¹³³ proposed a high-precision optical arrangement for stereomicroscope autofocusing, where a cylinder optic is employed to project a bar-shaped mark onto the object. Many current microscopes are equipped with fast autofocusing optics, which uses two laser beams acting as a focusing reference to find a focus point rapidly. The focus point works for not only the main viewing position but also the assistant position and camera. Furthermore, methods have been developed to maintain the surgical microscope in focus at different viewing points. For example, Heller¹³⁴ proposed a mechanical control unit for a surgical microscope support stand, and the unit constrains the microscope to move along a spherical surface so the focusing status can be maintained.

3.2.1.

Light source

Except for the traditional incandescent bulbs used in old surgical microscopes, there are mainly three types of light sources, i.e., xenon lamp, halogen lamp, and the LED. LED can provide illumination in the visible wavelength range with good brightness, good stability, longer life, less power consumption, and extremely low heat; therefore, it is preferred in many ophthalmic and ENT microscopes.¹³⁹ However, LED as a surgical light source also has disadvantages: the higher color temperature and narrower wavelength range make the light not as close to sunlight; its spectrum is insufficient for fluorescence-guided applications especially ICG imaging, where an excitation light in the NIR range is needed; moreover, it is not easy to replace.

Xenon lamp and halogen lamp are two options to address these needs. Xenon lamp emits light with a broad spectrum from ultraviolet (UV) (185 nm) to infrared (2000 nm). The spectrum is relatively smooth in the visible range, but it has some spikes in the near-infrared (NIR) range. Xenon light has a color temperature of 4000 to 6000 K, which is similar to sunlight. Therefore, the bright-white light is able to offer a naturally colored view of the anatomy. Halogen lamp also covers a wide and continuous spectrum including visible and NIR light, but it has a slightly lower color temperature (3200 to 5000 K), which means the light does not look as “white” as xenon light. Both xenon and halogen lamps can provide a stable illumination with DC regulation power employed. Nevertheless, the surgical microscopes do not use all the wavelength range of xenon lamp or halogen lamp. Actually, UV light and infrared light above $\sim 1100\mathrm{nm}$ are filtered out for surgical microscopes to avoid various possible damages to the patient’s skin or eye caused by exposure in this wavelength range.^140,141 Xenon and halogen light sources are commonly used in neurosurgical microscopes because of the need for intraoperative fluorescence imaging. They are also utilized in some ophthalmic and plastic microscopes. For example, some ophthalmic microscopes may employ a dual-illumination system combining LED and halogen for Red Reflex and normal illumination. Both halogen lamps and xenon lamps emit much heat. Therefore, in a surgical microscope, the light source is installed away from optics, and a fiber guide is used to transmit light from the light source to optics without carrying the heat.

3.2.2.

Illumination arrangement

The tissue surface being viewed under a surgical microscope during operation is usually wet and highly reflective. The light that comes from an angle can be easily reflected away and cause a dark view, as Fig. 6(a) shows. Coaxial illumination is the solution to this situation. Different from lateral illumination where light comes from the side, coaxial illumination matches the optical axes of illumination and visualization (lens).^142,143 Illumination from the light source that locates on the side is diverted and projected almost parallel to the axis of the lens, as shown in Fig. 6(b). Therefore, light vertically illuminates the tissue surface and is reflected directly to the lens, not having much loss. Coaxial illumination reduces the diameter of the illuminated area,¹⁴⁴ moreover, it can be directed into narrow and deep cavities, which is helpful for neurosurgery, ENT surgery, and endodontics.^72,144

Fig. 6

Illustration of coaxial illumination and comparison with side illumination: (a) side illumination and (b) coaxial illumination.

The light path for coaxial illumination in nonophthalmic surgical microscopes, such as neuro or ENT microscopes, usually forms a small angle with the observation axis in the range of 6°.^144–146 In some contemporary surgical microscopes, it is called small angle illumination (SAI),¹²⁴ which provides a concentrated and evenly distributed light beam, a bright view, and an improved depth perception, as Fig. 7 shows. With SAI, the shadow that appears at the edge of the viewing field is significantly reduced. Illumination with an even smaller angle is important when it comes to certain ophthalmic interventions especially cataract operations, where the vertically impinging light gets diffusely reflected by the fundus and the pupil under operation shines reddish, which is called red reflex.^144,145,147 The production of red reflex requires a small angle between the illumination beam path and observation beam path, in the range of 0 deg to 2 deg.^144,146 Although the red reflex helps under certain circumstances, it does not help as much in revealing good plasticity without the shadows on the structures in the interior eye.¹⁴⁵ Therefore, both types of illumination, namely the 6 deg and the 0 deg illumination, are usually equipped in ophthalmic surgical microscopes.

Fig. 7

Comparison of illumination effect with and without SAI.¹²⁴

3.2.3.

Light management

A desirable illumination for a surgical microscope should provide a stable brightness for the viewing area regardless of the change of working distance or magnification. In fact, irradiance (irradiation of a surface, $\mathrm{W}/{\mathrm{m}}^2$) of a microscope light source increases with decreasing spot size and decreasing working distance.¹³⁶ With an unchanged illumination setting, the increased working distance can cause insufficient irradiance, while the decreased working distance excessive irradiance. Insufficient irradiance makes the view unclear, while excessive irradiance may cause soft tissue burns.¹⁴⁸ Similarly, decreased magnification, which enlarges the FOV, may lead to the dimness, while increased magnification may burn tissue outside of the FOV. To address this issue, many contemporary surgical microscopes are equipped with smart light management, which adjusts light intensity automatically with the change of working distance [Fig. 8(a)] or magnification [Fig. 8(b)].

Fig. 8

Light management related to working distance and spot size: (a) automatic adaption of light intensity with decreased working distance and (b) automatic adjustment of the illuminating area with increased magnification.

3.3.1.

Balancing and positioning

As is known, surgical microscopes should be quick and effortless to move and remain stationary once the position is established.¹³⁰ Balancing of the forces and moments from all directions should be achieved, otherwise, brakes or bracing devices are needed to hold the microscope in its position.¹⁵⁴ Many suspension structures and balancing apparatus have been developed for a fast and reliable balancing of microscope.^154–160 Modern surgical microscopes have made it an easy and time-saving process to balance. All six axes can get fully balanced with two pushes of a button, and intraoperative rebalance can be quickly and accurately accomplished with a single push of button on handgrip.

In recent years, a robotic autopositioning feature has been added to state-of-art surgical microscopes.^161–163 The robotic ability enables the microscope to orient the angle or change its focal length so surgeons can target a specific point of interest, which is probably identified in a preoperative imaging study. Oppenlander et al.¹⁶² developed the automatic positioning movement control with three options. The first option is “auto lock current point,” which makes surgeons lock onto a target by changing the angle and focal length of the microscope to keep it in focus at one point while being manually moved. The second option is “align parallel to plan,” which positions the microscope to a preset angle and focus without any need to adjust the microscope. The last one is “point to plan target,” which automatically adjusts the focus on a predefined target. In a newly developed robotic visualization system, two robotic positioning features, namely “point lock” and “position memory,” have brought many advantages in time, functionality, and ergonomics.⁶⁰ With “point lock,” the microscope head stays in focus when being manually or automatically moved during surgery, so the surgeon can visualize different angles of the same structure. “Position memory” makes the system able to “bookmark” positions and transit quickly and smoothly back to these positions with no need to rediscover. In some circumstances where the scope needs to be moved around to observe different structures or be temporarily removed to get an x-ray, “position memory” can save plenty of time getting the scope back to the same position. Previously, it was reported that around 40% of surgical duration was spent on adjusting the microscope. But with all these robotic positioning features, the surgical duration can be greatly reduced.⁶⁰

3.3.2.

Draping

Microscope drape is a very thin, transparent, and heat-resistant plastic film that houses the whole surgical microscope, and it includes a transparent optical lens enclosing objective lens and ocular-housing extensions.^164–166 The drape is seamed for sterile packaging to assure the microscope sterility during surgery.¹⁶⁷ To save the setup time of a surgical microscope, instant readiness is required. Meanwhile, the drape must have adequate ocular pockets, not reduce the working distance, not interfere with surgeons’ operation or obstruct the view. Bala¹⁶⁸ invented a microscope drape assembly, which has four ocular pockets for different needs and does not affect the working distance or visualization by locating the objective lens window support within the objective lens barrel.

Glare is one problem that comes with draping and illumination. As light passes through the objective lens and illuminates the surgical field, some of the light would be reflected by the lens cover on the drape, which can cause chromatic and spherical aberrations.¹⁴⁹ Removing the cover, however, can cause the contamination of surgical instruments. A dome-shaped objective lens cover¹⁶⁹ can not only reduce the reflection but also compromises the magnifying performance. Surgical microscope manufacturer has brought up a solution by replacing the sterile lens cover with a slanted one, and another attempt solution is to include the slanted lens cover in the sterile microscope drape. Both methods can eliminate glare, with the price of increased costs or system complexity. Langley¹⁴⁹ has invented a glare elimination device for surgical microscopes. The device includes three parts: the first part is to be attached to the surgical microscope, the second part is for the sterile drape, and the third part is a body to connect the other two and provide an angular offset. The device can be semipermanently attached to a surgical microscope with more convenience. Weaver et al.¹⁷⁰ proposed an apparatus that provides a secondary holder for a cover to be applied to the objective lens barrel. The additional cover can be rocked and rotated easily to a position where the view is not affected by glare.

3.3.3.

Control

Surgical microscopes can be controlled in various ways to facilitate easy use of the microscope and to free surgeons’ hands during surgery. Contemporary surgical microscopes are often equipped with footswitch devices¹⁷¹ for generating control commands, touch-screen^58,172,173 for operation mode selection or switching images intraoperatively, or joystick control¹⁷⁴ for highly precise micropositioning. Mouth switch^175–177 is a commonly employed controlling method. Surgeons can use the mouth switch to change signals simply by holding the levers with a mouth, in which way operation errors can be reduced, even with a large number of functions to control.¹⁷⁵ Eye controlling is another trend for surgical microscopes. Charlier et al.¹⁷⁸ proposed an eye-controlled surgical microscope, which used an IR-LED to illuminate the surgeon’s eye and a charge-coupled device (CCD) sensor to detect the reflected infrared light from the surgeon’s eye for movement tracking. Similarly, Roduit et al.¹⁵² proposed an eye-guided controlling technology, which used a CCD camera mounted on the right ocular of a microscope and continuously monitored the surgeon’s eye. With the eye-guided control function, surgeons can use their eyes to perform multiple tasks including access to built-in data display, laser aiming, and control of autofocusing. Voice control is a sterile remote control that facilitates operator in either sterile or nonsterile region and does not require the operator to take action.¹⁵¹ Furthermore, Pitskhelauri et al.¹⁵⁰ developed a device named Mari, which allows hands-free utilization of surgical microscopes. The device was attached to the eyepieces of a surgical microscope, and operators can use the joystick and electric switch to do multifunction control of the microscope.

3.3.4.

Ergonomics and maneuverability

Except premium optics, good illumination, and various image-guided surgical functions, one nonnegligible benefit of surgical microscope compared with traditional loupes is the ergonomics,^25,73,130 which guarantees a comfortable and flexible working position for surgeons and reduces the risk of back and neck musculoskeletal injuries.²⁵ Meanwhile, maneuverability is valued for the simplification of microscope operations.¹³⁰ Therefore, the microscopes of the time are equipped with a full range of movement and tilt of the optics carrier, as well as a selection of binoculars with full 360-deg rotation for different heights and positioning needs. Some microscopes have large HD monitors so that surgeons can all work with an upright position. Eye-to-object distance¹¹⁵ indicates the distance from the observer’s eye to the focus point of the microscope. Surgeons are likely to be more comfortable with a longer eye-to-object distance. In addition, new designs of surgical microscopes are trying to provide longer working distances up to 600 mm¹⁷⁹ to offer better ergonomics, easy maneuvering, and more working space that allows long instruments. For example, Horizontal Optics Technology, which is employed in the state-of-art surgical microscope, enables a compact optics carrier and further ergonomics.¹⁸⁰ Figure 9 shows how a surgical microscope can improve ergonomics for an endodontic surgeon as an example.

Fig. 9

Illustration of improved ergonomics with surgical microscope.¹⁸¹

3.4.1.

Stereopsis

Stereopsis is a key feature of binocular surgical microscopes. While the monocular depth cues lie in perspective projection, occlusion, size, shading, and motion parallax, the stereoscopic depth is based on the slight disparities between two images presented to two eyes.¹⁸⁷ Stereo microscopes use two afocal relay zoom lens systems for the two channels of a binocular tube, and their axes are parallel to and offset from the axis of the objective lens.¹⁸⁸ The light coming out of the objective lens is divided into two parts and forms two slightly different images into two channels. In surgery, especially when working with magnification, perspective, and size cues may be lost; therefore, the stereopsis brought by binocular is essential to provide a 3D impression of the surgical field. The depth information can aid the detection of diagnostically relevant shapes, orientations, and positions of anatomical features, especially when monocular cues are absent or unreliable.¹⁸⁷ For example, it is vital for dentists to construct 3D structures in patients’ mouth⁷⁴ and for neurosurgeons to understand complex volumetric relationships of neuroanatomical structures.⁶⁰

An optical design that enhances stereo visualization for surgical microscopes is FushionOptics technology,¹⁸⁹ which sets two separate beam paths in the optical head, providing the DOF and high resolution, respectively. The two paths are then merged in the observer’s brain into a single, optical spatial image. Because of this combination of depth and resolution, the interruptions for refocusing can be avoided.

3.4.2.

Share of view

There is usually more than one observer during surgery, which makes the “share of view” an important and necessary feature for surgical microscopes. In some procedures, meaningful assistance has to be given by a cosurgeon sharing the same view with surgical microscope.^25,117,190 It aids not only assistance but also teaching and participation of trainees.²⁵ The simplest way to share the identical surgical view is to use an optical splitter to split the light into two eyepieces.¹⁴² Nowadays, surgical microscopes can have multiple optical ports for the main observer, assistant observer, and external cameras. Some models have integrated HD cameras and monitors so the whole team can share the view on the screen.^{19,25,117,191}

3.4.3.

High-definition display and 3D visualization

Many new surgical microscope models, especially neurosurgical microscopes and ophthalmic microscopes, are equipped with HD video cameras and large HD monitors, so subtle details can be viewed more clearly and shared by the whole team.^56,58 In addition, 3D screens, which employ passive linear polarization technology, have been brought to the operating room to deliver depth perception.^22,192–194 Observers need to wear goggles to have a real-time 3D view, which gives a realistic appraisal of certain features. It was reported that screens possibly offer better contrast of the visual field than eyepieces and image injection in some cases.⁵⁸ Moreover, utilizing screens enables the heads-up display, which is beneficial for surgeons’ spinal health during long procedures.

A screen can show not only the white-light image of the surgical site but also other images, such as intraoperative OCT images, for surgical guidance. The images can be shown separately, overlaid on the white-light image,⁵⁸ or even in picture-in-picture endoscopic assistance view⁶⁰ for endoscopic microinspection tools, as shown in Fig. 10.

Fig. 10

Screens for visualization during surgery: (a) intraoperative OCT images shown separately on 6.5-in screen, with white-light image simultaneously on 21.5-in screen, and injected in oculars,⁵⁸ (b) surgeons using the 3D display in a seated position with goggles,²² and (c) picture-in-picture 3D visualization of endoscopic assistance.⁶⁰

3.5.1.

Augmented surgical microscope

The microscope-based AR systems have been found particularly useful in neurosurgery, which is the earliest adopter of AR.^210,212 The majority of applications for recent neurosurgical AR is tumor resection, followed by neurovascular surgery and spinal procedures.⁷ For tumor resection, AR with the segmented CT or MRI image helps with the margin definition during surgery, AR overlay of volumetric CT/MRI data with no additional surgical time or complications reduces both intensive care unit and hospital length of stay.²²⁹ AR for vascular neurosurgery has focused on the augmentation of stereomicroscopes,²¹² with either fluorescence from intraoperative ICG angiography or segmented preoperative CTA/MRA/digital subtraction angiography (DSA).^{57,209,230,231} The overlay of the target vasculature optimizes craniotomy placement, dural opening, and skin incision.^{57,212,230,231} Moreover, it is significant that the microscope-based AR system does not require the bayonet pointer typical for common neuronavigational systems.²¹⁰

The first augmented monoscopic surgical microscope,¹⁰⁶ which was used for cranial surgery, was proposed in 1985. The segmented 2D preoperative CT images were displayed in monocular and were registered to the operating table using an acoustic localizer system, but this system was unable to track tools in real time. The first augmented stereoscopic operating microscope was proposed in 1995.⁶³ It achieved multicolor displaying of segmented 3D cross-sectional MRI/CT data into both microscope oculars via a beam-splitter, either as solid or wire-mesh overlays. Fiducials on the skin surface were used for patient registration. The overlay accuracy was 2 to 3 mm, and the interactive update rate was 2 Hz. The system used an LED-based 3D localizer for calibration, microscope pose tracking, and patient tracking. Therefore, the microscope was capable of free movement while maintaining the overlay accuracy. Later in 2000,²³² several improvements were achieved to the system, including automated calibration, the bone-implanted fiducial added for registration, and the locking acrylic dental stent for patient tracking. The clinical overlay errors were 0.5 to 1.0 mm on bone fiducials and 0.5 to 4.0 mm on target structures. In 1996,²³³ the S.M.N. system developed by Carl Zeiss (Germany) as a neuronavigational system was first installed at Rennes Pontchaillou Hospital. The system was integrated with an OPMI ES surgical microscope, and it utilized a 3D optical localizer for tracking, which was comprised of infrared emitters and three linear 1D cameras. Fiducial markers such as bone or skin markers were used for registration. The virtual images were either MRI/CT slices or 3D rendered images, and they were both injected into the microscope ocular and displayed on the monitor. Inside of the microscope ocular, surgeons had access to menus, so they could interact with the user interface.

3.5.2.

Technical implement of augmented surgical microscopes

The procedures of an augmented surgical microscope include calibration of the optical system, tracking, registration, and display.²³² The AR accuracy mostly depends on the accuracy of the tracking technology, the registration procedure, the camera calibration, and the image scanning device (e.g., CT or MRI scanner).¹⁹⁸ Registration is the process of relating two or more data sets to each other to match their content.²³⁴ It is essential for augmented surgical microscopes because the system is very sensitive to misalignments of the virtual image and the real environment (the patient tissue). Calibration of binocular optics and registration limit application accuracy mostly.²³² The term tracking for AR refers to the pose estimation of objects in real time.²³⁴ The display of an augmented surgical microscope mostly refers to the image injection into microscope oculars but also the monitors that are equipped in some new-generation surgical microscopes.

Early image overlay in a microscope-based neuronavigational system injects 2D contours into one eyepiece.²⁰² Therefore, surgeons needed to either scroll through different image planes and merge them in mind or look away from the microscope for a 3D impression.²⁰⁵ The conceptual description of the AR idea of 3D stereoscopic overlay of the operating field in a surgical microscope appeared in 2003 proposed by Aschke et al.²⁰⁵ The process is divided into two phases, namely the preoperative phase and the intraoperative phase. In the preoperative phase, calibrations under different magnifications are accomplished to obtain lens error values. Patient image data, e.g., from MRI, fMRI, or ultrasound image are segmented manually and a reference model is generated. Then, the image data and the reference model are matched to patient’s anatomy by the registration process. In the intraoperative phase, new intraoperative image data such as intraoperative MRI are registered to the reference model, and the coordinates of intraoperative image data are utilized to segment edges.

The aim of calibration is to produce a projection matrix that will give the pixel position in the injected image of any 3D point relative to the frame of reference of the microscope.⁶³ The process determines all camera parameters including the correction of optical errors generated by nonperfect lenses.²⁰⁵ Usually, several calibrations need to be done under different magnifications because the lens error values change with zooms,^199,205,235 and the preoperative calibration process takes about 10 to 20 min.⁵⁷ Edwards et al.⁶³ used a calibration object that consisted of a number of localizer LEDs within the working region to enable the calculation of room coordinates. Mun et al.²²¹ proposed a calibration algorithm that uses image intensity rather than fiducials and is adaptable for different lens distortion models. In some recent papers, the checkerboard patterns were used to calibrate the stereo camera integrated into the microscope head, as Fig. 11 shows.²¹⁸,²³⁶

Fig. 11

Illustration of the calibration method using checkerboard pattern: (a) system setup and (b) various transforms involved with the calibration method.²³⁶

There are several tracking methods based on different devices. Friets et al.²⁰² used an ultrasound range finding system, while Doyle²³⁷ used a magnetic field digitizer instead. However, optical tracking is the most popular way^{63,199,206,210,232,233,235,238} and has been in use in modern operating rooms for intraoperative navigation. It is facilitated by the widely available cameras, and it does not require any wire connection between the system and the tracked object. LEDs, IR-LEDs, or other optical trackers can be attached to both the microscope and the patient, as shown in Fig. 12(a), and a 3D optical localizer is utilized to track these light points.^63,206,235 An acrylic dental stent with imaging and physical locators was developed and attached firmly to the patient’s upper teeth so the tracker was brought close to the volume of interest, and it allowed free movement of the patient’s head within the line-of-sight.²³² Garcia Giraldez et al.¹⁹⁹ integrated a 3D tracking camera onto the housing of a surgical microscope tube, and the camera tracked the movements of surgical tools and the patient, as shown in Fig. 12(b). Gard et al.²¹⁸ used an image-based method to track a green-colored instrument tip under a surgical microscope for trajectory creation in tympanoplasty.

Registration is most commonly accomplished with fiducial markers,^57,207,235 skin surface,⁶³ and manual procedures²³⁸ as well as skull-implanted or dental-fixed fiducials.^239,240 It is reported that fiducial markers or skin surface registration are the easiest, fastest, and most accurate ways with respect to manual registration,²¹⁰ and they are less invasive and laborious than skull-implanted or dental-fixed fiducials.²¹⁰ Overall, the registration errors for clinical AR range from 0.3 to 4.2 mm, mostly 2 to 3 mm.^{199,206,207,212,218,232,235,241}

When combined with surgical microscopes, the display method of AR is usually image injection^57,208 into either one microscope ocular or both. Displaying the bright-field image and the augmented image in two separate oculars may cause strain and fatigue since there is no cue for spatial coregistration of two types of images. Two embodiments of an augmented stereoscopic microscope using ICG fluorescence images were proposed by Romanowski et al.²⁴² In the single-channel embodiment, the objective lens receives both NIR and visible bright-field images of the examined object simultaneously. The augmentation module (beam splitter between the objective lens and ocular lenses) separates the NIR image from the visible image, and then the NIR image is processed to generate a synthetic image. The synthetic image is directed into an eyepiece, while the visible image is shown in two eyepieces with different light intensity proportions. In the dual-channel embodiment, two cameras capture the NIR image and the visible bright-field image, respectively. After image processing, the ICG fluorescence image is overlaid onto the optical image of the same FOV, and the combined image can be directed into both eyepieces. Figure 13 shows the schematic of a single-channel design of an augmented surgical microscope for NIR fluorescence imaging. Furthermore, the three ways for 3D data representation²³⁴ are slice rendering (one slice of the volume data), surface rendering that shows the transition between structures, or volume rendering. Figure 14 shows the overlay of injected 2D and 3D models during bypass surgery.

Fig. 12

Tracking methods: (a) surgical microscope with optical tracker (yellow arrow)²³⁵ and (b) surgical microscope with integrated tracking camera.¹⁹⁹

Fig. 13

Schematic of the optical pathway in an augmented microscope, augmentation provided in the right ocular.²⁰⁸

Fig. 14

Image injection of the occipital artery allows detailed appreciation of its sinuous course. (a) This image overlay indicates where the incision should be performed and guides the dissection of the artery. (b) The accurate superposition of the real and virtual arteries.²³⁰

3.5.3.

Challenges

Although AR has been a fast-developed technology, challenges still exist for it in clinical use, such as the registration error from compounded sources, the visual and tactile temporal asynchrony caused by system latency, the inattentional obstructed view by virtual component, and the time consumption for registration and verification.²¹⁰ Injection of 3D images into microscope oculars alleviates the visual fatigue caused by the accommodation-divergence discrepancy, but the focal plane of the virtual image remains incongruent with that of the target.²¹² The impaired depth perception has been a problem for all AR systems.^243,244 Binocular cues, partially offered by 3D stereoscopic displays, are not always sufficient for inferring the spatial relationships between objects in a 3D scene.²¹⁰ Another challenge is the organ and tissue shift, such as the brain shift when the arachnoid is opened, which compromises the reliability of the virtual model of the AR system and navigation. Furthermore, in some surgeries such as arteriovenous malformation (AVM) resection, the role of AR is still limited because arterial feeders in most AVMs are not resolvable with current systems.²¹² The future direction of clinical AR involves the improvement of the AR system, the refinement of virtual models, and the adaptation of new imaging techniques such as magnetoencephalography and transcranial magnetic stimulation.²¹⁰

3.6.1.

Fluorescence dyes

Although endogenous fluorescence (autofluorescence) of human tissue has attracted much attention in the biomedical imaging field, it requires UV-visible or NIR light source and probably needs to be combined with other techniques such as single-photon detection.^249,250 On the other hand, various fluorescent dyes have been invented for different surgical applications and put into use. Most invented dyes generate visible fluorescence, so that with the help of visible exogenous fluorescence, surgeons are provided with a high contrast of visualization, and they are able to separate different tissues with human eyes. Besides the visible exogenous fluorescence, some dyes that emit fluorescence in the NIR wavelength range, such as the ICG, are also helpful during surgery.

5-aminolevulinic acid (5-ALA) is a polar molecule to which the blood–brain barrier is virtually impermeable.^251,252 It induces accumulation of protoporphyrin IX (PpIX), which is a kind of porphyrin, in the malignant tissues.²⁴⁷ The accumulated PpIX has a strong absorption band in the violet spectral range (380 to 420 nm) and emits red fluorescence (620 to 710 nm) with peaks at 635 and 704 nm in tissue.^31,253–255 At the same time, there exists a broad band of endogenous fluorescence peaking at 520 nm. The excited fluorescence and endogenous fluorescence are superimposed, composing a spectrum, as Fig. 15 shows. Although the use of 5-ALA might cause minor side effects such as skin sensitization,^{109,256–259} it has been proved to be helpful for brain tumor resection. ^31,260

Fig. 15

Fluorescence spectrum of malignant glioma superimposed with protoporphyrin IX spectrum.²⁴⁷

Fluorescein sodium is an older compound that has been receiving new attention. It is a kind of fluorescent dye that accumulates in tumor tissues via disrupted blood–brain barrier.^27,248 The fluorescence is based on passive permeability,²⁴⁸ and it is usually used in brain surgeries to distinguish the bright yellow-stained tumor and pinkish brain tissue. Although fluorescein can cause some adverse reactions, the use of sodium fluorescein (Na-Fl) in fluorescence-guided brain surgery has been reported as safe and effective.^27,109,261

ICG is a cyanine dye that absorbs light ranging from 600 to 900 nm with an absorption peak above 800 nm.^262,263 The fluorescence is also based on passive permeability,²⁴⁸ and it has a very wide spectrum in the NIR range. Because of the longer wavelength of fluorescence, ICG angiography is capable of achieving a deeper imaging depth than fluorescein angiography. Furthermore, it can be eliminated from the body by the liver within several minutes, thus it is safe to use. Nowadays, ICG is commonly used in cardiac, circulatory, hepatic, and ophthalmic conditions.^54,264–269

3.6.2.

Integration of fluorescence imaging and microscope

The integration of the intraoperative fluorescence system and surgical microscope frees surgeons’ hands and achieves an uninterrupted operating flow. The fluorescence system usually consists of an excitation light source, one short-pass filter for excitation light, one long-pass filter for fluorescence signal, and a camera to capture the fluorescence (for image-injection observation or recording), as Fig. 16 shows. The excitation filter is positioned in front of the light source, and the barrier filter is in the observation optical path. The fluorescence signal that passes the barrier filter can either go to the eyepieces and be observed by surgeons or be captured by a CCD camera mounted on the surgical microscope. One basic requirement is that surgeons can switch from white to excitation light (i.e., violet-blue) illumination directly under the microscope without interrupting the operation.²⁴⁷ Moreover, a certain level of excitation light irradiance is needed to ensure the adequate intensity of the fluorescence signal. It is said that irradiance of $40\mathrm{mW}/{\mathrm{cm}}^2$ is sufficient with an individual light source that is close to the human tissue.²⁴⁷ However, when the light source is integrated into the microscope, it has a distance of $\sim 250\mathrm{mm}$ to the surgical field, which reduces the excitation light irradiance that falls in the surgical field; the fluorescence observed by the main surgeon can also be weakened due to the share of view.

Fig. 16

Schematic of a surgical microscope integrated with an intraoperative fluorescence imaging device.²⁷⁰

Kabuto et al.⁶ evaluated the effectiveness of fluorescence imaging by a surgical microscope with sodium fluorescein. A 380- to 500-nm band-pass filter and a 520-nm long-pass filter were applied for excitation and emission, respectively. The surgical microscope could be easily modified for fluorescent microscopy simply by inserting the blue filter [arrow in Fig. 17(b)] in the pathway of the light from a xenon or halogen lamp and inserting the yellow filter [double arrows in Fig. 17(b)] in the pathway of the reflected and excited lights. The optical filters and their installation are shown in Fig. 17.

Fig. 17

Two optical filters and their installation in the surgical microscope: (a) One (right) is a 380- to 500-nm wavelength band-pass filter (blue) for the excitation of fluorescein, and the other (left) is a 520-nm wavelength long-pass filter (yellow) for emission. (b) The surgical microscope can be easily modified for fluorescent microscopy simply by inserting the blue filter (arrow) in the pathway of the light from a xenon or halogen lamp and inserting the yellow filter (double arrows) in the pathway of the reflected and excited light.⁶

Kuroiwa et al.^271,272 demonstrated a surgical microscope system, in which the microscope (Zeiss OPMI CS-NC or Zeiss OPMI MD) was equipped with a glass interference filter as excitation filter and a Kodak Wratten No. 12 filter as a barrier (observation) filter. The switch between white light and fluorescence modes was realized via a switching apparatus in which the filters can be inserted. The system, as Fig. 18 shows, was proved to be useful during malignant glioma surgeries.

Fig. 18

Photograph and diagram of fluorescein surgical microscope system.²⁷¹

Similarly, Stummer et al.²⁴⁷ demonstrated a Zeiss OPMI CS-NC surgical microscope with a modified varioscope for fluorescence observation. The violet-blue illumination irradiance was $40\mathrm{mW}/{\mathrm{cm}}^2$ at a working distance of 250 mm. An electromagnetic filter switcher was employed to introduce the long-pass observation filters into the two observer light paths of the stereo surgical microscope. Because beam splitters were used in both light paths for a second observer or camera recording, the main surgeon could obtain 50% of the fluorescence signal.

Elliott et al.²⁷³ customized an illumination and imaging module enabling picomolar-sensitive NIR fluorescence imaging on a commercial Zeiss OPMI Pentero surgical microscope, as Fig. 19 shows. A continuous-wave 760-nm laser diode was employed as the excitation light source, and a dichroic mirror is used in combination with a long-pass filter to allow only light above 780 nm to enter the fluorescence-channel camera sensor. The customized system was tested with a new fluorescent agent named ABY-029 in live craniotomies in orthotropic tumor-bearing rats. Furthermore, Richter et al.²⁷⁴ combined fluorescence spectroscopy-based hand-held probe (HHF-probe) with fluorescence-guided resection surgical microscope (FGR-microscope) for tumor identification and resection. The surgical microscope adopted in this system was Leica M720 OH5 with the FL400 fluorescence module. The HHF-probe system included an excitation laser of 405 nm and a spectrometer operating in the range of 240 to 850 nm. The HHF-probe and FGR-microscope were used for the detection of 5-ALA-induced PpIX fluorescence in brain tissue. During surgery, the probe could be held very close to tissue. Clinical results show a better sensitivity of the probe and a better specificity of the microscope-integrated fluorescence module, and the combination of HHF-probe and FGR-microscope was proved to be beneficial especially in the tumor marginal zone.

Fig. 19

The detection module (blue) and the illumination model (yellow) are attached to the Zeiss Pentero OPMI head and can be used without affecting the standard operation of the microscope.²⁷³

Fig. 20

Microscope integration method of an iOCT system, the MIOCT beam is coupled into the microscope prior to the object lens but after the zoom.²³

3.7.1.

Intraoperative OCT and its applications

The principle of OCT is basically low-coherence interferometry. It typically uses NIR low-coherence (in other words, broad-band) light source, which can be superluminescent diodes, ultrashort pulsed lasers, or supercontinuum lasers.^277,301 The light in the OCT system is split into two arms, namely the reference arm and the sample arm. After the light being reflected from tissue, the light beams from two arms form an interference pattern only if the difference of distances they have traveled is less than a coherence length. To take advantage of this interference, scanning is needed to obtain the location of the microstructure of the sample. According to the domain in which scanning is fulfilled, OCT is classified as time-domain OCT (TD-OCT), and frequency-domain OCT (FD-OCT) also known as spectral-domain OCT (SD-OCT). In TD-OCT, the path length of the reference arm changes with time, while in FD-OCT (SD-OCT) the broadband interference is acquired with a spectral variation using either a spectrally scanning light source or a dispersive detector. The reflective profile from a single axial depth scan is called A-scan. To achieve a 2D visualization of the FOV, a lateral combination of a series of A-scans is required for a cross-sectional tomography (B-scan).

Intraoperative OCT has three types of devices: handheld OCT (HHOCT), needle-based probes, and microscope-integrated OCT (MIOCT).^275,286,302 When using HHOCT devices, surgeons have to remove the surgical microscope to place the HHOCT device over the patient’s eye. Needle-based probes overcome the above problem; however, it requires an assistant to hold.²⁷⁵ Microscope integration is a key advancement of OCT. Since this technology is now available integrated to ophthalmic surgical microscopes with an HD display to view intraoperative OCT images, it is possible for surgeons to give real-time feedback during the surgery. The integration achieves increased stability that handheld devices do not possess. Moreover, the X-Y-Z-foot-pedal control enabled by the microscope foot pedal facilitates rapid imaging with enhanced image reproducibility.³⁰³ Considering the fast scanning time of MIOCT, the minimal surgical pause is needed to switch between the surgical microscope and other OCT devices during the procedure, which greatly facilitates a smooth workflow. Furthermore, MIOCT would benefit procedures (1) with a reduced anterior chamber view, (2) where transparent or very thin structures are not readily visible with a surgical microscope, (3) that need to be monitored, and (4) where the intraoperative projection of preoperative imaging data is preferred.³⁰⁰

The early reported MIOCT systems have the limitation of an A-scan rate of $<2\mathrm{kHz}$,^{296–298,304,305} then it was improved to about 20 kHz.^306–308 Although they achieved axial resolution as good as $7\mu \mathrm{m}$²⁹⁷ and imaging depth of 2 mm,²⁹⁸ an acquisition time of 1 to 4 s was still required for a densely sampled volume. Moreover, they were incapable of real-time rendering of the volumetric data. Using a continuous high-resolution B-scan system, which consists of either two orthogonal B-scans or one single B-scan, live images could be generated of surgical procedures and maneuvers.^{289,298,306,308–311} With the outstanding computational speed of graphics processing units (GPU), live 3D OCT has been enabled for real-time volumetric data rendering.^312–314 Four-dimensional MIOCT (4D MIOCT) system with dual-channel head-up display was reported since 2015 to be capable of live volumetric imaging through time at the speed of up to $10\text{-}\mu \mathrm{m}$-scale volumes per second,^78,315,316 which is promising for ophthalmic surgery improvement.

3.7.2.

Integration of OCT with surgical microscope optics

The essentials of a microscope-integrated iOCT system are the microscope integration, a HUDs, and the scan control of OCT. The integration of the OCT module with a microscope is a challenging job, as it compromises the performance of either the OCT or the microscope to some extent.

In the first demonstrated MIOCT system, the optics of iOCT were separate from those of the microscope, and the MIOCT system was coupled to the surgical microscope with a dichroic mirror after the microscope objective lens, which resulted in a reduction of working distance.^275,317 In the following designs, the MIOCT system (or part of the system) was coupled into the microscope using either a dichroic mirror or a beam splitter cube. Based on the position of the dichroic mirror (or beam splitter) in the microscope and the optics shared by MIOCT and microscope, the integration method of MIOCT can be divided into two types.

The first design couples the MIOCT system using a dichroic mirror located prior to the microscope zoom module.^296,318,319 The optical beam of MIOCT is folded by the dichroic mirror into the path of one of the oculars, and then the merged beam goes through the zoom system and the main objective lens. Specifically, a real-time projection unit can be applied to project the generated OCT image to the other eyepiece.³¹⁹ The MIOCT beam can be coupled from a camera port of the microscope. Therefore, the MIOCT system does not change the height and the working distance of the microscope. This design can achieve a lateral resolution of 23 to $47\mu \mathrm{m}$ and an FOV (lateral scan size) of 4 to 28 mm, with the microscope working distance in the range of 232 to 290 mm. The resolution and FOV are mainly limited by the diffraction, the shared magnification, and the working distance of the microscope.²⁹⁶

In the second design, the MIOCT beam is coupled into the microscope using a dichroic mirror prior to the objective lens but after the zoom module, as Fig. 20 shows.^{23,108,188,309,310,318,320–327} In this way, the MIOCT beam is decoupled from the microscope ocular path, and the objective lens is the only optic that is shared by the microscope and the MIOCT. In some designs, the OCT is integrated with the illumination module, so the OCT beam is coupled into the illuminating beam path.^{108,320–322} The other form of MIOCT is an independent unit that is assembled onto the microscope.^{188,323,324,327} Since the MIOCT does not utilize the microscope magnification anymore, independent control of beam waist diameter and beam waist position is usually added.¹⁸⁸ Moreover, a telescope made of a reduction lens and a magnifier lens is applied after the microscope objective lens to magnify and provide wide-field image.^{23,188,309,310,325} Furthermore, using reflective elements and a tunable focus lens can help improve the transmission and the integration of MIOCT to the microscope.^23,310 However, this approach increases the height of the microscope and the distance between the surgical field and microscope ocular, both of which could have an adverse effect on the microscope ergonomics.²⁷⁵

4.1.1.

Neuro-oncology

Surgical microscopes can aid the visualization of tumor margin during resection surgery and help achieve total resection of the tumor.^27,109,389 It has been widely used for the surgical treatment of primary and metastatic brain tumors, as well as spinal tumors.

Brain tumor resection

Surgical microscope is essential for the resection of brain tumors, such as glioma,^52,260,272 meningioma,³¹ intracranial aneurysm (IA),^57,391,392 supratentorial tumor, pineal tumor, infratentorial tumor,²⁶¹ metastasis tumor,³⁹³ and pituitary tumor.³⁹⁴ Compared with gross-total resection, microscopic total resection has better local control of tumor, and potentially decrease the risk of local recurrence.³⁹⁵ Because of the macroscopically unclear tumor margin, imaging techniques are usually required to locate the tumor, such as intraoperative 5-ALA fluorescence or AR with MRI images. A neuronavigation system combined with a surgical microscope can also offer great help. A surgical microscope in brain tumor resection not only offers clear and bright visualization as a routinely used instrument but also works as a framework that facilitates imaging-guided resections.

Kabuto et al.⁶ produced two filters for the excitation and emission of fluorescein and fitted them with a Zeiss surgical microscope for the detection of glioma during resection. In the in vivo study on rats, the tumors were stained with clear yellowish-green color, the adjacent brain was weakly stained later than the tumor, and the normal brain was not stained. In the clinical study of patients, the glioblastomas that were enhanced in MR images were stained with a yellowish-green color, the peripheral area where invaded glioma cells existed was stained later and more weakly, and the normal brain was not stained at all. Kuroiwa et al.²⁷¹ developed a fluorescein surgical microscope with a BP450-490 filter and a Kodak Wratten No. 12 filter for malignant glioma resection. They tested the system on 10 malignant glioma patients. The results show that the system could enable adequate removal of the tumor within a short time. Later in 2016, they performed surgery on 30 cases of malignant glioma resection with their fluorescein surgical microscope,²⁷² and the operative finding coincided with historical ones, which proved the fluorescein surgical microscope a useful aid in glioma surgery. Similarly, Stummer et al.²⁴⁷ employed a Zeiss OPMI CS-NC surgical microscope for 5-ALA-induced PpIX fluorescence observation in over 50 glioma resections. The xenon light source had strong light intensity, which yielded high tumor fluorescence. The electromagnetic filter switcher enabled the easy introduction of the emission filter without disturbing the surgical flow. Vital tumor regions and tumor-normal boarder was easily recognized with the help of microscope-mounted fluorescence. Acerbi et al.³⁹⁶ performed surgeries for grade IV gliomas in 15 patients using the Zeiss OPMI PENTERO microscope with BLUE 400 or YELLOW 560 for visualization. 5-ALA-guided resection of gliomas using BLUE 400, which is a fluorescence kit of Zeiss with a maximum excitation of 494 nm and maximum emission at 521 nm, was performed in the first several cases. Then, another fluorescence kit YELLOW 560, which excites in 460 to 500 nm and observes in 540 to 690 nm, was employed in the rest of the patients. Both kits allowed good visualization of tumor, but YELLOW 560 offered improved visualization of the nonfluorescent normal tissue in the fluorescence mode, as well as a better quality of video reproduction. The contrast-enhanced tumor was completely removed in 75% of the patients, and the remaining patients had a mean tumor resection of 90.5%. Twenty biopsies at the tumor-normal margin in five patients showed a high sensitivity (91%) and specificity (100%) of fluorescein tumor identification. Richter et al.²⁷⁴ combined a Leica M720 OH5 surgical microscope with a handheld fluorescence-spectroscopy probe (HHF-probe), which detected the 5-ALA-PpIX fluorescence for tumor identification and FGR. The system was evaluated in 18 operations on 16 patients with suspected high-grade gliomas, and the HHF-probe was compared with FL400, which is a commercial fluorescence module of Leica. Results showed better sensitivity of the probe than a normal FGR surgical microscope, especially with low fluorescence ratios, which is beneficial for complete tumor removal. White et al.³⁹⁷ developed a microscopic navigation system combined with the Viewsite Brain Access System (VBAS) technique for deep brain lesions and employed this system in high-grade glioma surgery. After craniotomy, the microscope with the navigation system was registered to the position and placed over the exposed surgical site. The crosshairs seen through microscope eyepieces were used to target the deep lesion, and the VBAS was carefully inserted into the brain for a smooth deep tumor dissection with minimal associated morbidity. Göker and Kırış²⁹ evaluated the safety and feasibility of sodium fluorescein-guided surgery with Zeiss OPMI PENTERO 900 surgical microscope and YELLOW 560 filter in pediatric brain tumor surgery. The 23 pediatric patients had different intracranial pathologies, including seven gliomas, six supratentorial tumors, four intraventricular tumors, two infratentorial tumors, two pineal tumors, one clivus tumor, and one tumor with supra and infratentorial extensions in the current series. The fluorescence was helpful for tumor identification in 20 cases, and total resection was achieved in 11 cases. Figure 24 shows the tumor differentiation with YELLOW 560.

Fig. 24

Intraoperative view of fluorescein-guided pediatric brain tumor surgery: (a) under microscope white light illumination and (b) fluorescein enhancement under YELLOW 560 filter.²⁹

Fig. 25

AR aneurysm surgery with image injection of the patient’s head: (a) image injection of the patient’s head in 2D, (b) image injection of the patient’s head in 3D, and (c) image injection of a right middle cerebral artery bifurcation aneurysm in 3D and of the underlying bony sphenoid ridge in 2D.⁵⁷

Fig. 26

Intraoperative ICG videoangiography during arteriovenous malformation surgery. (a, d) Surgical view and (b, c, e, f) ICG videoangiography at the time ofoperation. Intraoperative ICG videoangiography disclosed the arteriovenous malformation clearly (b, 7 seconds after cerebral circulation was visualized by ICG; c, 30 seconds after that). After detection of the nidus, ICG videoangiography indicated no filling of the nidus and to-and-fro filling of the drainer (e, 7 seconds after cerebral circulation was visualized by ICG; f, 30 seconds after that; arrows indicate the drainer).

Fig. 27

Direct view of the nasopharyngeal tumor under a surgical microscope.⁸⁹

Fig. 28

Microscope-mounted intraoperative OCT in ophthalmic surgery: (a) MMOCT setup in posterior lamellar keratoplasty and (b) iOCT improves visualization of all the procedures in descemet membrane endothelial keratoplasty.⁴⁴⁹

Fig. 29

Applications of the DSM in endodontics: (a) removal of the pulp tissue, (b) vertical root fracture detected at $10\times$ magnification with dye, and (c) root end preparation in apicoectomy $16\times$ magnification.⁷³

Microscope-integrated fluorescence-guided surgery was also applied for metastatic brain tumor resection. Okuda et al.³⁹³ performed 38 metastatic brain tumor resections with the aid of yellow fluorescence for visualization, and the results showed the potential to decrease the recurrence rate. Schebesch et al.¹⁰⁹ used a surgical microscope with YELLOW 560 for the resection of 35 malignant brain tumors including gliomas, cerebral metastases, nonmalignant astrogliosis, and postradiation necrosis. The fluorescence signal was accessible very fast and was helpful to improve visualization of tumor margin in 28 cases. It was noted that the fluorescence effect was the most obvious in grade III and IV glioma without previous treatment. Hamamcıoğlu et al.²⁷ employed YELLOW 560 in 30 surgeries of 28 patients with high-grade or metastatic brain tumors. Sodium-fluorescein was found helpful in 29 of 30 operations, and a total resection was achieved in 23 out of 29 operations. Furthermore, Kim et al.³⁹⁰ applied 32 times of intraoperative ICG videoangiography to brain tumor surgery in 23 patients, who had brain tumors including meningioma, metastasis, glioma, and hemangioblastoma. A Zeiss surgical microscope was used with the INFRARED 800 fluorescence module, which is designed for excitation in the wavelength range of 700 to 780 nm and fluorescence visualization in 820 to 900 nm. Benefiting from the high image quality and spatial resolution provided by the microscope, blood vessels were visualized with the ICG videoangiography, and important information about blood patency, blood direction, and small perforating arteries were obtained. It was proven to be extremely useful for decreasing operative morbidity and unexpected deterioration by ensuring safe manipulation of tumor-related and normal brain vessels.

Pituitary tumors happen in the pituitary gland, and transsphenoidal surgery is the most common way to remove the tumor.³⁹⁸ Fatemi et al.³⁹⁹ described their 10-year experience of direct endonasal transsphenoidal approach to the sella with the operating microscope. They had 881 operations in 812 patients with mainly pituitary adenoma, but also Rathke’s cleft cyst, craniopharyngioma, parasellar meningioma, chordoma, and others. The microscope was used for visualization of the nostril during the nasal portion and sphenoidotomy with a hand speculum, the cavernous sinus contralateral to the nostril of approach during adenoma removal, and the residual tumor with angled ring curettes. Besides, the microscope and the handheld speculum were used to explore the nasal cavities for nasal closure. The microscope could provide an effective and minimally invasive route for tumor removal. However, there were times when the microscope offered suboptimal visualization, and then endoscopic assistance could be used. The same group also investigated endonasal microscopic removal of clival chordomas,⁴⁰⁰ which is a skull-based tumor. The microscope was used with frameless surgical navigation in eight cases and endoscopic assistance in four cases for visualization. In addition, the microscope and the speculum were used in the direct endonasal approach, which yielded excellent access to the clivus, medial cavernous sinus, and intradural space anterior to the brainstem. The tumor removal operation using the microscope was proven to be safe and effective with gross total removal or near-total removal in 86% of patients.

4.1.2.

Neurovascular surgery

The surgical microscope is also very useful in surgeries for neurovascular diseases, such as aneurysms and arteriovenous malformation.^{32,34,231,405} The high magnification of the microscope enables safe and confident manipulation of vessels smaller than 3 mm.⁴⁰⁶ Microscope-integrated fluorescence modules are commonly used tools in these applications, as they are able to aid intraoperative visualization of the blood vessels and the patency of blood flow in both arteries and veins. For example, ICG videoangiography can be used for intraoperative diagnosis of sigmoid sinus thrombosis during vestibular schwannoma removal,⁴⁰⁷ and sodium fluorescein can be helpful for solitary brain abscess removal.⁴⁰⁸

4.2.1.

Otology

Surgical microscope was first applied in aural surgery²⁰ in 1921, and it has then been utilized for a large portion of otology operations,²⁶ including diagnosis of the tumor in middle ear,⁴¹⁹ dissection of the temporal bone,⁴²⁰ examination of the drum position in atelectatic ears,⁴²¹ and laser surgery for chronic ear disease and otosclerosis treatment.⁴²² Its main advantage is the improved visualization, which results in other benefits including precise surgical manipulation, smaller instruments, and less trauma.²⁶ It enabled the discovery of the minute structures in the middle and inner ear and offered the possibility to master the complicated surgical anatomy of the temporal bone.^26,423

Nowadays, new technologies combined with surgical microscopes are being evaluated for ear surgery to aid the visualization of ear structures. Lankenau and Pau^296,424 investigated using a surgical microscope with microscope-mounted OCT (MMOCT) for anatomical orientation in cochlear implant surgery. The system was able to reveal the cochlear morphology without opening the enveloping membranes. The OCT images supplied information about inner ear structures, showing the potential of providing an orientation guide to precisely localize the scala tympani before opening the fluid-filled inner ear. Cho et al.²⁹⁵ developed an AR system combining OCT with a surgical microscope. The system was able to simultaneously acquire in vivo microscopic and OCT images of middle-ear and inner-ear microstructures. AR technique was utilized to show both images overlaid in the left eyepiece. The system was tested on a mouse and was able to show monolayers of each microstructure in real time.

4.2.2.

Rhinology

The development of surgical microscopes has opened new ways for functional approaches and less radical surgery of rhinology.⁴²⁵ The nasal fossa and paranasal sinus, where the narrow port of entry, deep situation, and complicated anatomy exist, are ideal areas for microsurgery.⁴²⁶ The use of a surgical microscope in nasal surgery was evaluated and proposed to be an alternative to the traditional endoscopic method.⁸⁸ The surgical outcome and the feedback from patients proved that microscopic surgery was well tolerable and superior to other surgical procedures. With the aid of an autostatic speculum, surgeons were able to work bimanually, which was the main advantage. In addition, the widefield stereoscopic vision, magnified operating field, excellent illumination, as well as adequate exposure of nasal and paranasal cavities, allow a more panoramic view and additional precision with good healthy tissue preservation and less surgical complication. Amedee et al. performed microscopic endonasal surgery for the treatment of paranasal sinus⁴²⁷ and chronic sinusitis with polyps.⁴²⁸ In addition to the aforementioned advantages, they also mentioned the easy control of operative hemorrhage with appropriate tools as well as the outstanding opportunity for resident teaching. Teatine et al.⁴²⁹ performed transnasal sinusectomy with either a microscope technique or a combined microscopic and endoscopic technique. The surgical microscope was used in ethmoidectomy and to open the sphenoid sinus and the antral window, while the endoscope was used for cleaning and drainage. Surgeries carried out with the microscope technique had minor complications, and the combination technique resulted in trivial complications. Testa et al.⁴³⁰ coupled a carbon dioxide laser with a surgical microscope for turbinate surgery. The laser was sighted through the microscope and cut the obstructive rhinitis tissue with a small wound. Dietrich et al.⁴³¹ employed a surgical microscope instead of an endoscope in endonasal dacryocystorhinostomy, due to its advantage of increased precision by allowing bimanually work while drilling bone from the frontal process of the maxilla. Kumar et al.⁵ performed preauricular sinus surgery using an operating microscope. The magnification of microscope, used along with other instruments, was helpful to provide a bloodless field and enable precise dissection without any epithelial breach.

4.2.3.

Laryngology

Surgical microscopes can aid visualization during laryngeal surgery, for example, Hazarika et al.⁴³² used a surgical microscope in KTP laser-assisted cricopharyngeal for observation of posterior muscular bar. Welham et al.⁴³³ used a surgical microscope in graft implantation surgery for the treatment of vocal scar and pathologic sulcus vocalis. After exposing the true vocal fold superior surface, the microscope was used in all subsequent procedures, including the incision, a sharp dissection, and graft material implantation. Various imaging techniques have also been evaluated for laryngeal applications. Just et al.²⁹⁸ employed MMOCT intraoperatively in the human larynx. Images of the true vocal folds were acquired, where surgeons identified basement membrane, gelatinous fluid, and regions with remaining fluid chambers. It was noted that MMOCT could describe the microanatomy of the healthy larynx and benign lesions, as well as enable early detection of microinvasive carcinoma. Martin et al.⁴⁶ developed a hyperspectral surgical microscope and captured hyperspectral images of in vivo human laryngeal mucosa of five patients. With the image classification algorithms, they achieved automatic detection of the hemorrhagic polyp and leukoplakia in the vocal cord and showed the potential of this system to provide noncontact optical biopsy.

4.2.4.

Head and neck surgery

Surgical microscopes can be used for the resection of various head and neck tumors, as well as the head and neck endocrine surgeries for thyroid and parathyroid. The advantages of the surgical microscope for head and neck surgeries, except for the magnification, illumination, and stereoscopic view, lie in the safe manipulation of the to-be-resected tissue while achieving good preservation of the normal tissue or nerve.

4.3.1.

Corneal surgery

Ophthalmic surgeries of the anterior segment of the eye require clear visualization of the delicate structures such as the iris and cornea. Besides, safe surgical manipulations such as incision and unfolding demand real-time image guidance. Geerling et al.³⁰⁵ coupled a commercial OCT system with a surgical microscope (Hi-R 900, Moller-Wedel) and tested it for intraoperative visualization of sclera and cornea during lamellar keratoplasty and trabeculectomy. The anterior segment structures such as cornea, conjunctiva, sclera, and iris were seen through the microscope-mounted OCT. During a trabeculectomy, the thickness of the scleral flap could be measured with the MMOCT; and during deep lamellar keratoplasty, the MMOCT was able to show the abnormal cornea and the lamellar interface. Steven et al.^172,449 employed a commercial microscope-mounted SDOCT system (iOCT, OptoMedical Technologies GmbH) with a surgical microscope (Hi-R 900 A NIR, Moeller-Wedel), as shown in Fig. 28(a), and evaluated the system in 26 patients under-going Descemet’s membrane endothelial keratoplasty (DMEK) and 6 patients undergoing deep anterior lamellar keratoplasty (DALK). All procedures in DMEK were benefited by the MMOCT, particularly the visualization of crucial steps such as graft rolling, unfolding, and orientation, as shown in Fig. 28(b). In DSLK, iOCT was helpful to verify trephination depth and needle insertion for identification of the accurate preparation depth, thus to assist in manual trephination. Furthermore, Siebelmann et al.⁵⁸ mounted an SDOCT camera (iOCT, OptoMedical Technologies GmbH, Lübeck, Germany) on a Hi-R Neo 900A NIR surgical microscope (Haag-Streit Surgical GmbH, Wedel, Germany) and evaluated the MMOCT in two Boston keratoprosthesis (KPro) surgery cases.⁵⁸ All parts of the prosthesis and the interfaces between corneal graft and implantation were well visualized. Moreover, the iOCT revealed two gaps that were not visible with a microscope alone.

Since microscope-integrated iOCT systems became commercially available, several studies have been evaluating MIOCT in different ophthalmic applications. Cost et al.²⁸⁸ employed a Zeiss surgical microscope and RESCAN 700 for iOCT imaging in eight cases of DMEK surgeries during the host and donor tissue preparation, graft orientation, graft apposition, and tissue interface fluid dynamics. Pasricha et al.²⁸⁹ reported using RESCAN 700 in two cases of Descemet’s stripping automated endothelial keratoplasty (DSAEK). The MIOCT helped surgeons to see the structures past the anterior cornea that was negligible with standard microscope illumination. In addition, surgical maneuvers such as graft insertion, unfolding, tamponade, and attachment were clearly visualized. Kobayashi et al.³⁷ used RESCAN 700 for rapid visualization and rough evaluation of the donor tissue for penetrating keratoplasty (PK), precut DSAEK, and prestripped DMEK donor tissue. Sharma et al.⁴⁵¹ used RESCAN 700 for the management of Descemet’s membrane detachment after DALK. In addition, direct visualization through the microscope was utilized when surgeons injected gas and monitored the height of the detached Descemet’s membrane. Eguchi et al.⁴² performed PK and DALK and used Zeiss Lumera 700 microscope with RESCAN 700 to detect iris incarceration and iridocorneal adhesions intraoperatively. Furthermore, Sharma reported a new surgical technique as MIOCT-guided small-incision lenticule extraction. During the surgery, MIOCT showed the location and thickness of the lenticule, revealed the relationship of the anterior-posterior lamellar planes, and helped surgeons achieve the desired plane of dissection by tracking the instrument tip in the intraoperative images.

4.3.2.

Vitreoretinal surgery

It is considered routine to use the surgical microscope in modern vitreous and retinal surgeries.⁴⁵² For the posterior segment, the ophthalmic surgical microscope, as well as MIOCT, is useful to visualize the morphological changes of the retina, for which typical clinical applications include scleral wound, proliferative retinopathy, vitreomacular traction, myopic foveoschisis, macular hole, retinopathy of prematurity, and retinal detachment.³⁸

Zhang et al.⁴⁵² investigated the feasibility of performing scleral bucklings for retinal detachment with a surgical microscope instead of the routinely used ophthalmoscope. The clear, upright image with controllable magnification enabled easy observation and examination of retinal breaks and degenerations. In the total 342 cases of surgeries performed under the surgical microscope, surgeons found 39 eyes where additional or new retinal breaks were found. Moreover, it was said that the routine use of surgical microscopes could help surgeons protect the vortex and prevent inadvertent scleral perforation. Fleming et al.⁹ developed an AR surgical microscope system for retinal surgery, which fused preoperative fundus image and OCT image with intraoperative video to increase targeting accuracy. Ray et al.⁹⁰ mounted a handheld SDOCT device, i.e. the Bioptigen system (Bioptigen, Inc; InVivoVue Clinic v1.2), onto an ophthalmic surgical microscope. The system was used to image the macular hole and the epiretinal membrane of 25 eyes in 24 patients who underwent macular surgery. It captured images pre-incision, post-hyaloid elevation, post-residual epiretinal membrane peel, and post-internal limiting membrane peel. After image analysis, the intraoperative change of macular hole dimension and retinal thickness that were caused by surgical manipulation could be measured. Grewal et al.⁴⁵³ performed an Argus II implantation with a prototype microscope-integrated SDOCT system. The system was used for the visualization of preretinal tack placement, retinal tack placement, and postretinal tack placement. It was said that MIOCT allowed confirmation of array position placement and confirmed the adequacy of tacking in securing the array. Chen et al.⁴⁵⁴ performed intraoperative MIOCT angiography on a 2-year old kid who underwent vitrectomy and a 7-month-old infant who was diagnosed with familial exudative vitreoretinopathy. Jayadev et al.³⁸ and Kumar et al.⁴⁵⁵ reported using RESCAN 700 as a surgical tool to image surgical maneuvers during vitreoretinal surgery. For example, it has the capability to image the anatomical configuration of the scleral wound, determine residual traction membranes in proliferative retinopathy, assess the strength of vitreomacular adhesions in vitreomacular traction, avoid iatrogenic break of myopic retinae in myopic foveoschisis, and avoid inadvertent nerve fiber break by providing real-time imaging of membrane peeling in the cases of macular holes. For retinal detachment, the MIOCT can detect residual subretinal fluid and completion of fluid air exchange. As for retinopathy of prematurity, it is able to assist surgeons to determine the extent of horizontal and anteroposterior tractional elements.

A group at Duke University and Duke University Medical Center has done massive work on microscope-mounted OCT imaging. They have demonstrated the use of a surgical microscope with a microscope-mounted SD-OCT prototype for the visualization of vitreoretinal surgical procedures,³⁰⁷ including retinal effects of surgical contact, primary static surgical steps,³⁰⁶ and surgical instrument as well as their intraoperative motions.³⁰⁸ In the study of retinal imaging,³⁰⁶ the MMOCT system acquired in vivo human retinal SDOCT images for observation of the posterior pole. Retinal architecture as well as macula and optic nerve could be well visualized in the B-scans. Nonmetallic instruments were better visualized than metallic ones in the B-scans. Intraoperative manipulations of the retina as well as the retina effects were successfully imaged. The PIONEER study^456,457 evaluated a microscope-mounted portable SDOCT system for both anterior and posterior segment surgeries. DSAEK and vitrectomy with membrane peeling were the most common surgeries performed for anterior and posterior segments, respectively. During pars plana vitrectomy surgery, iOCT could help reveal epiretinal membrane, macular edema, posterior hyaloidal traction, and retinal detachment. It was noted that in 48% of lamellar keratoplasty cases and 43% membrane peeling cases, surgeons were informed of surgical decision making by the MMOCT, and their understanding of underlying tissue configurations was altered. In addition, the microscope afforded iOCT axial stability as well as good control of the X-Y-Z translation movement by a foot pedal.

Furthermore, in a 3-year DISCOVER study,^36,290 an MIOCT was used in ophthalmic surgeries of both anterior and posterior segments. Among three commercially available prototype MIOCT systems, namely Zeiss RESCAN 700, Leica EnFocus, and Cole Eye iOCT, the RESCAN 700 MIOCT system was evaluated with a Zeiss Lumera 700 surgical microscope by multiple surgeons in 244 eyes with anterior segment cases and 593 eyes with posterior segment cases. The Z-tracking and focus control of RESCAN 700 provided good image quality and stability. For imaging of the anterior segment, a standard microscope viewing system was used, while for the posterior segment, the RESIGHT lens system or a contact lens was used. The images were captured to evaluate corneal incision, scleral closure, phacoemulsification groove depth, intraocular lens position, epiretinal membrane, vitreomacular traction, and hyaloid release with triamcinolone and completeness of peel in the macular hole. It was reported that the iOCT information impacted surgeons’ decision making in 43.4% of anterior segment surgeries and altered surgical decision making during the procedure in 29.2% of posterior segment surgeries.

4.4.1.

Endodontics

DSM can be employed diagnostically for multiple applications, such as visualizing the root canal system in fine detail, examining dental caries and insufficient crown margins, assessing marginal integrity of restorations, detecting cracks or microfractures, locating canal orifices, observing sub-gingival defects, and observing complex anatomical situations, etc.^{11,72,114,130,460,462} High magnifications of DSM is beneficial in nonsurgical procedures for identification of canal bifurcations, identification and removal of canal obliteration and calcifications, and obturation.^{11,130,181,459} The color and texture of dentin become apparent under the microscope, guiding practitioners to root canal orifices or allowing safer dentin removal.^114,130,459 Magnification allows endodontics to better visualize pulp chamber^463,464 and canal orifices and improves the efficiency of instrumentation.^76,130 Moreover, surgical microscopes can help with the retrieval of broken instruments in the canal and pinpoint precisely.^72,75

For surgical endodontics, DSM has become one of the state-of-art landmarks and an invaluable instrument,^11,130 especially in difficult treatments where high-level magnification is needed, such as root-end preparations and root-end fillings.^74,465 It is predominately recommended for osteotomy, curettage, apicoectomy, apical preparation, and retro filling.¹³⁰ Other endodontic microsurgery steps carried out under various magnifications including root-end resection and inspection of resected surface, detection of apical perforation, microsurgical flap preparation, and microsurgical suturing were also greatly enhanced by the microscopic approaches.¹³⁰ In addition, the microscope is helpful in endodontic retreatment procedures to identify and remove leftover filling materials, and in nonsurgical perforation repair procedure to place the perforation repair material more precisely.^459,460 The goal of utilizing a microscope in surgical endodontics is to achieve the highest possible precision as well as the maximum protection to healthy tissue.¹³⁰ Figure 29 shows some of the important applications of a dental surgical microscope. DSM and microscopic approaches have completely transformed surgical endodontics from a blindfold work to a vision-based work.^74,182,459 The advantages are rapid wound healing, fewer postoperative pain, and better prognosis. In addition, two reviews of surgical endodontic outcomes^466,467 showed that the utilization of microscopes is associated with significantly better outcomes than loupes.

During root canal therapy, microscopes can greatly aid caries removal, calcification removal,⁹¹ canal orifices localization,^464,468 and internal resorption treatment with magnification and illumination.⁴⁵⁹ In a study of 184 cases of necrotic teeth with chronic periapical lesions, the impact of the DSM was evaluated with two groups (using a microscope and conservative), and the microscope was proved to be helpful with a significant statistical difference of outcomes ($p=0.0078$). In addition, it was reported to be extremely useful in removing obstacles from the pulp chamber or root canal system and locating calcified or missed canals.⁴⁵⁸ Wu et al.⁹¹ evaluated the clinical application of the surgical microscope in the management of root canal therapy of 546 root canals. A Zeiss OPMI pico microscope, as well as a rubber dam and an ultrasonic instrument, was used in the therapy. Under the microscope, the calcified canal orifices were located, and the colors of the normal chamber floor and the calcified dentine were identified, which were the first key aspects in the treatment. It also aided the detection of perforation and instrument separation. With the help of the microscope, 33 out of 40 root orifices that were not detected with naked eyes were successfully detected. Karapinar-Kazandag et al.⁴⁶⁹ assessed the ability to detect and negotiate the accessory mesial canals in mandibular molars using either a surgical microscope or loupes. Comparing with loupes, the use of the microscope increased the number of detected accessory mesial canals by 6% in the second molar and 2% in the first molar, while comparing with no magnification, the number increased by 17% in the first molar and 5% in the second molar. The microscope also benefited the negotiation of accessory mesial canals with an 8% increase in the second molar and 2% in the first. Mitsuhashi et al.⁴⁷⁰ investigated the marginal adaptation and microleakage of MTA root fillings under unaided vision, dental loupes, and the surgical microscope. They reported that using the microscope left no gap between the MTA and dentin at any magnification, while a clear gap was revealed in the unaided vision group at $5\times$ to $50\times$ magnification and in the loupe group at $50\times$ magnification. In addition, the filling procedure using microscope was able to block the dye penetration, which was significantly different from the deep penetration using unaided vision.

In a case study of 50 apicoectomies, DSM facilitated each phase of endodontics surgery, and the postoperative evaluation showed a reduced incidence of symptoms and less postoperative pain in the cases treated with microscope.⁴⁷¹ Two surveys of apical surgery using DSM reported 96.8% healing within 1 year, and 91.5% healing in the following 5 to 7 years, well beyond the success rate of conventional apicoectomys.⁴⁷² In endodontic retreatment, especially the critical phase of removing the filling material, the surgical microscope can give a direct view of the root canal thus aid the removal of silver cone.⁴⁷³ High magnification of the microscope is also essential in laser dentistry. During noncontact soft tissue ablation, it provides accurate visualization of laser-tissue interaction to eliminate the risk for accidental interaction of laser and tooth; and for hard tissue laser ablation, it replaces the tactile method and helps determine complete decay removal.¹²⁶

4.4.2.

Periodontics

In a well-established periodontal microsurgical practice, 70% to 80% of the typical periodontal microsurgeries can be done at a magnification of $10\times$ to $20\times$ with a surgical microscope, while the rest can be done with a surgical loupe.¹⁵³ Surgical microscopes may represent a useful tool when particular esthetic demands require maximum precision.⁴⁷⁴ The use of surgical microscopes in periodontal microsurgery⁴⁷⁵ can lead to cleaner incision, closer wound apposition, reduced morbidity, reduced hemorrhage, and less trauma at the surgical site. The magnification reveals the fact that many gentle maneuvers with the naked eye turn out to be gross crushing and tearing of the delicate tissue,^475,476 and also adds precision to the repositioning of tissue by allowing smaller needles and sutures.⁴⁷⁷ Meanwhile, it enables fast wound healing and less inflammation.^122,478

Applications of the surgical microscope in periodontal therapy include root surface debridement,¹⁰⁴ implant surgery,⁴⁷⁹ minimally invasive periodontal regeneration,⁴⁸⁰ and mucogingival surgery.⁴⁷⁴ Visual access to the root surface is determinant for the success of periodontal therapy, and it is more efficient under higher magnification for the removal of residual calculus as well as achieving a clean and smooth root surface.⁴⁸¹ In implant therapy, high magnification provided by the microscope can help the implant site development and placement, achieving higher precision.^73,475 The surgical microscope is beneficial at different stages of implant treatment from implant placement to implant recovery. It also brings more precision to the peri-implantitis management¹⁵³ and aids indirect visualization of the sinus membrane in the sinus life procedure.^153,475 The use of the surgical microscope in periodontal regenerative surgery can increase the capacity to manipulate the soft tissue, resulting in an improved potential for primary closure of the wound to 92% of microsurgery from 70% of regular surgery.⁴⁸² It also allows a smaller incision to gain surgical access and debride the periodontal defect prior to placing the graft.⁴⁸³ Mucogingival surgery is also called periodontal plastic surgery.⁴⁸⁴ For some procedures in periodontal plastic surgery that can be very difficult with naked eyes, such as papilla reconstruction, the DSM can turn it into an easy one by increasing visibility, eliminating unnecessary incision, and facilitating access.^475,485 Besides, it enables minimally invasive techniques that can minimize trauma, allow primary wound closure, and improve facial esthetics.⁴⁸⁶

The use of the surgical microscope has been actively evaluated in root coverage procedure, which is a type of mucogingival surgery for the treatment of gingival recession. It enables excellent visualization and an atraumatic surgical approach in root coverage procedures, which results in satisfying cosmetic outcomes such as better searing and marginal profile.⁴⁷⁴ When the procedure is combined with coronal flaps, the microscope also makes the advancement of flaps easier and tension free.^476,481,487 Bittencourt et al.⁹² compared surgical outcomes of root coverage procedure with subepithelial connective tissue graft with or without a surgical microscope. The test group using the microscope significantly outperformed regarding the average percentage of root coverage, complete root coverage, and patient esthetics satisfaction. Thankkappan et al.⁴⁸⁸ performed root coverage procedures using subepithelial connective tissue graft and collagen membrane with a surgical microscope. The $6\times$ magnification delivered precision and visual acuity, which guaranteed precise control of the gingiva. Burkhardt and Lang⁴⁸⁹ employed a Zeiss OPMI Pro magis DSM for recession coverage at $15\times$ magnification, while the graft and palatal closure ahead of it were performed with a prism loupe at $5\times$. Meanwhile, surgical microscope OPMI FR was used for fluorescence angiography. The microscopic approach was compared in the study with a conventional macrosurgical approach. The results showed that the microscope improved visual and tactile approach, and it enabled the use of finer instruments and suture materials, which reduced tissue damage. In addition, the magnification was able to help split a flap in a well-defined thickness, which resulted in decreased vessel injury. The angiography revealed a better vascularization of microscopic approach than the macrosurgical one.

4.5.1.

Craniofacial surgery

Craniofacial surgery involves the congenital or acquired deformities of the skull, head, face, neck, jaws, and the associated structures, such as craniosynostosis, facial fractures, and cleft palate.⁴⁹⁵ The use of a microscope enables minimally invasive incision, clear visualization of the complicated and delicate surgical site, as well as precise operation. Here, we discuss several applications of the surgical microscope in craniofacial surgeries.

4.5.2.

Microsurgery

Surgical microscope has a great impact on reconstructive microsurgery. Its applications include hand surgery,⁵⁰² head and neck reconstruction using free tissue transfer,¹⁴ nerve repair,⁴⁹² anastomosis and duct reconstruction,^503,504 lymphatic reconstruction,⁸⁶ etc. The microscope can not only aid visualization and precise operation but also reduce surgical complications.