Scheimpflug topographical changes after Femtosecond LASIK for mixed astigmatism – theoretical aspects and case study

Objective: To evaluate the corneal topographical changes after Femtosecond-LASIK surgery in eyes with mixed astigmatism. Methods: We present the analysis of the corneal Scheimpflug topographies of a patient treated with Femtosecond-LASIK technique for bilateral mixed astigmatism. Results: Three-dimensional reconstruction maps and differential anterior curvature maps were used to demonstrate the ablation profile and its stability in time. Conclusions: Visual and refractive results were very good after surgery, being topographically confirmed by the corneal reshaping which was performed as planned, the achieved ablation being stable during the one-year follow-up period.


Introduction
Astigmatism is the condition of refraction in which the rays of light coming from a point source cannot produce a point on the retina [1,2]. Cornea is the major source of astigmatism in the optical system as it is responsible for about 74% of the total dioptric power of a normal eye [3]. The optical power of the mixed astigmatic eye is different in two principal meridians, perpendicular to one other, one meridian being myopic and the other being hyperopic [4]. The aim of corneal refractive surgery in mixed astigmatism is to reshape the cornea, flattening it in the myopic meridian and steepening it in the hyperopic meridian [5].
After the corneal refractive surgery, changes in the corneal shape and curvature can be evaluated by using a variety of devices based on Placido-disc systems and elevation analyzers [6]. The Schwind Sirius  (Schwind Eye-Tech-Solutions GmbH&Co, Germany) is a device that combines a Scheimpflug camera with a Placido disc corneal topographer [7]. Placido-based videokeratoscopy measures the corneal reflection of mires (circles of light) of known radius, the corneal power being estimated mathematically. Rotating Scheimpflug camera system uses the slit illumination to obtain an optical section that is captured in a side view, the camera being oriented according to the Scheimpflug principle, in order to create sharp images from anterior corneal surface to depth [8]. Data obtained after corneal scanning are converted to computerized color scale maps [6].
The axial (or sagittal) map is the most commonly used map for routine screening, as it easily classifies the normal and abnormal corneas and differentiates between spherical, astigmatic or irregular corneas. Due to the Placido rings configuration and to the axial acquisition which intersects with the instrument axis, the sagittal map fails to describe the true shape and power of the peripheral cornea [6,9].
The tangential map (also called "instantaneous radius of curvature") has a better accuracy in the evaluation of the peripheral changes in shape and curvature but has the tendency to reveal excessive details that are not always clinically relevant. It may be very useful in detecting mild corneal changes that could not be detected by the sagittal maps [6,9].
The three-dimensional reconstruction maps, available in both sagittal and tangential acquisitions, offer an overall view and a better understanding of the real corneal shape with steep and flat areas [9]. Normal corneas are prolate, being steeper centrally and flatter peripherally, with a medium anterior refractive power of 43.00-43.50 diopters [10]. Fig. 1 shows the three-dimensional tangential anterior and sagittal anterior configuration of a normal spherical cornea.  Postoperative examinations were carried out on the first day following the surgery and then after one, six and twelve months. We evaluated the uncorrected distance visual acuity (UDVA), the manifest refraction and we performed slit-lamp examinations at each visit. Except for the first postoperative day, topographies were achieved at each follow-up visit.

Results
Postoperative results were good, with full recovery of uncorrected vision and a manifest refraction very close to emmetropia. The keratometry was constant over the follow-up period ( Table 1). We further present the corneal topographies performed pre and postoperatively and their analysis regarding the corneal shape and curvature changes occured after the refractive surgery and their stability over the follow-up period. Fig. 2 and 3 demonstrate the results of the calculated ablation we have performed for the right eye and the left eye respectively. The preoperative tangential anterior map was on the upper-left and the one-month postoperative tangential map was on the upper-right. As the color scale was the same for both topographical aquisitions, we were able to directly compare the two scans on a differential map, visualizing the ablation profile. The relative shape of the corneas was changed with flattened areas in the myopic meridians and steepened areas in the hyperopic meridians. As a result of the surgery, the corneas theoretically looked as if they were spheres. The corneal shape changing result can be better understood in the three-dimensional reconstruction, shown in Fig. 4 for the right eye and in  Two postoperative corneal topographical maps were also compared in order to demonstrate the ablation stability in time. The postoperative differential maps for both eyes were achieved by substracting the 12-months postoperative map from the one obtained at the 1-month follow-up visit. The postoperative differential maps displayed on the bottom of Fig.  6 (for the right eye) and Fig. 7 (for the left eye) show exactly the changes that had occured in every corneal point during the 1-year follow-up. The ablation profile was stable, with unsignificant changes of the radius of curvature inside the optical zone.