Historical Review of Endoscopic Spinal Decompression

Improvements in imaging techniques, along with advancements in optomechanical devices such as 2-mm-thin endoscope, electronic scope, and CCD cameras, and development of new devices such as the high speed drill greatly affected the techniques of minimally invasive endoscopic surgery. In behind the birth of these groundbreaking medical techniques there were engineers who have supported from the beginning. Thus, it is also the history of medical- and-engineering ties between doctors and optical engineers, and past experience becomes a beam of light that shed upon the future. Primum nil nocere (minimize the invasion) was a concept since the days of Hippocrates, and it has been an everlasting theme for surgeons. Then, a paradigm shift occurred, from the times of microscopes to the times of video endoscope where operators can share the live image with others. With these advancements, surgeons were able to understand the anatomical structures and relative positional relationship of each organs through the endoscope and developed their depth perception through two-dimensional video images. Endoscopic spinal decompression that uses high-definition endoscopic images and can treat a wide range of the spinal column enabled to decrease numbers of complications and faster rehabilitation.


History of Endoscopy
Rigid endoscope -semi-flexible endoscope period (1868-1957) (Table 1) The first person to look inside the stomach of a live person using anesophago-gastroscope was a German doctor by the name of Adorf Kussmaul (1822Kussmaul ( -1902 in 1868. Using a metal tube with a length of 47cm and a width of 13mm, the process was extremely painful for the patient. The tip was shaped into an oval, but as it was a rigid endoscope, it was not able to bend at all during examination [1]. In 1887, Nitze and an Austrian technician by the name of Josef Leiter developed an urethro-cystoscope that used a molten platinum glow wire as a light source [4,5] (Figure 1).
The new age of endoscopy arrived in 1932 when Rudolf Schindler invented a flexible gastroscope that had a flexible optic axis, with the help of an optical engineer by the name of Georg Wolf [8]. In 1959, after countless trials, Masaki Watanabe and Tsunekichi Fukuyo invented the the 21 st Watanabe Arthroscope, which enabled not only examinations but also surgery and filming inside the joint [9]  Cold light source video endoscopy period (1952-now) ( Table 2) Fourestier developed the cold light fiberglass in 1952 [10]. The technical advancement of the endoscopy accelerated with the development of the quartz rod lens by Hopkins [11], a lens still used today. This incorporates a new theoretical concept of light transmission called the total internal reflection, and this made it possible for the light to be transmitted inside the endoscopy lens without lowering the luminosity. Hopkins' "Rod Lens" and the flexible light fiber scope enabled the image to be brighter and clearer [12]. In 1970, by using CCD camera that converts light into electrical signals, operations were done by multiple surgeons using TV monitor displays, and the development of the CCD camera accelerated the development and the popularization of the endoscope. Even now, small-sized CCD camera and robots are continued to be in development to miniaturize video endoscopy with clearer resolution.

History of Endoscopic Spinal Decompression Techniques
Each type of endoscopeis categorized depending on the thinness (Table 3) PELD (Percutaneous Endoscopic Lumbar Discectomy) is an

Invention of silicon transistor, 1947
Theoretical publication about fiber optics in Nature, Hopkins, 1954     optical technique innovation expanded from the PN (Percutaneous Nucleotomy) method, and a rigid-type endoscopy called Full endoscopy is used for the spine. It is also used in uniportal bilateral decompression of the thoracic spine OYL resection, and biportal bilateral decompression is also in consideration.
Although tubular surgery with assistant of endoscopy (an approach similar to when using the miscroscope) expanded into a technique suitable for the posterior region [24][25][26], as it is a natural cavity, a suction tube becomes necessary, and there is a limit to the size of the working channel. For the anterior region, both anterior lumbar discectomy and fusion during laparoscopy of aeroperitoneum and retroperitoneoscopy uses endoscope as a support for lateral approach. Also, this technique is used in VATS and applied during the decompression andcorrection of thoracic and thoracolumbar transitional vertebrae.

Full endoscopic spine surgery
In 1975, Hijikata used a contrast needle to enlarge the surrounding cavity and set up a standard cannula (inner diameter 3.0m, outer diameter 3.5mm, 2.6mm) under fluoroscopic guidance [34,35].
This led to the devisal of PN method (Percutaneous Nucleotomy), a method to extract the nucleus pulposus of the intervertebral disc postlaterally. Kambin used a 4mm Craig cannula. In 1988, Pravis Kambin suggested that a triangular area surrounded by the exiting root, traversing nerve root, and the end plate of the lower vertebral body is safe for endoscopic spine surgery [36][37][38]. Currently, technique that fully utilizes this triangle by conducting until the pedicle is popularized (Figure 3) [27]. Onik resected the intervertebral disc using a mechanical suction tube in which the outer diameter was 2.5mm [39]. Foraminoscope (Field Lens of 6 degrees) designed by Hans-Jorg Leu was utilized for 21 years until 2012 [40]. YESS system created by Tony Yeung consists of a 2.5mm surgical instrument, washing channel, multi channel used for rod lens, and an egg-shaped endoscopy. In particular, the inside-out concept suggested by him was safe and easy to perform even for beginners, as decompression and extraction of the spinal cord herniation is done gradually by going into the avascular area of the intervertebral disc [41,42]. Contrary to this, the outside-in method proposed by Hoogland is complicated and needs much practice to control hemorrhage, as this method directly decompresses the intervertebral disc herniation by resecting parts of the pedicle and the zygapophysial joint transforaminal [43]. The strong endoscopic technique performed by Rutten et al. enabled decompression to be done even to the lateral canal and the hidden zone of MacNab [44][45][46][47]. Now, decompression techniques using endoscopy for wide areas of the spine, from the cervical to the lumbar area and from the anterior to the posterior area, is being popularized. Furthermore, it is started to be applied in decompression techniques of the anterior/posterior areas of the cervical spine and yellow ligament resection of the thoracic spine [48]. Trans spinous process [13] Translaminar [14][15][16][17][18] Transpedicular [19] Transforaminal [20,21] Transiliac [22] Transsacral [23] Decompression classification from pathological condition

Tubular surgery with assistant of endoscopy
In 1997, Forely and Smith performed a surgery where they extracted a herniation in the intervertebral disc by inserting an 18mm diameter microendoscopy (MED) [31,32]. Furthermore, vertebral arch resection was done by using a technique called the Destando syste [49] and Easy GO.

Laparoscopic (retroperitoneoscopic) lumbar decompression surgery technique
Decompression and fixation under pneumoperitoneum started in 1991 [28]. Furthermore, retroperitoneoscopy by lifting was performed. It became possible to decompress by dividing or moving the psoas major muscle anteriorly [33]. Also, at one point, operation performed anteriorly was done actively for thoracic spine herniation and degenerative diseases [29].

Future direction
Until now, popularization and advancements of the endoscopic spinal surgery was affected by the ability of the operator to understand the anatomical structure and the relative positional relationship of each organs under endoscopy, and the ability to see the depth perception from a 2-dimentional video monitor. Also, acquiring handeye coordination under magnified images and organ sensation such as bone, ligament, and nerves, and leading technology such as the AI Navigation robotics will become the gold standard for endoscopic spinal decompression. In particular, decompression techniques such as the full endoscopic technique will likely be developed through meta-analyses and randomized clinical trials more in the future for lesions where the diameter is small and microscope cannot be used [50].
Improvements of the endoscopy in recent times are mesmerizing, decreasing the chance of approach-induced disabilities by making and sustaining a cavity using a dilator and using a high-resolution VTR image endoscope. Evolving from the traditional intervertebral disc resection surgery using a microscope, the current method can reach the lesion precisely by substantially thinning the working channel and still have a similar performance as the traditional method.
The goal is utilizing the limited cavity as much as possible and obtain a clear visionary field to complete the surgery as fast and safe as possible. With the education system being standardized, minimally invasive techniques will provide a safe and nice treatment method for the people in the 21 st century.