"Topography-guided treatments are only useful in cases where the epithelium irregularity is regular," said Professor Dan Z. Reinstein (London Vision Clinic, London, UK) when discussing his study results during his presentation at the annual ESCRS meeting in Vienna, Austria. With his colleagues, Mr Timothy Archer and Dr Marine Gobbe, Prof. Reinstein investigated the impact that irregular epithelial thickness profiles have on topography- or wavefrontguided custom repair of refractive surgery complications.

Irregular irregularities

Using Artemis (ArcScan Inc.) very highfrequency digital ultrasound scans, Prof. Reinstein and colleagues measured the individual corneal layers in a series of eyes, which all had irregularly irregular astigmatism after undergoing corneal refractive surgery. As it is known that the epithelium will try to remodel itself to smooth the corneal front surface, the epithelial thickness profile was used as a surrogate for the stromal surface shape and irregularity. The epithelial thickness profile was subtracted from the corneal front surface elevation to calculate the stromal surface elevation, from which the actual stromal surface topography could be derived.

"When the epithelium is highly irregular the corneal front surface topography is not representative of the stromal surface because of epithelial compensation. If we calculate the stromal surface topography, the flattest and steepest parts can be quite different in power from that of the corneal front surface topography," Prof. Reinstein revealed.

Prof. Reinstein went on to explain that if you do not take the stromal surface into consideration then an irregularity will remain. "Until you take into account the stromal surface shape and not the corneal front surface topographic shape, we will never achieve the most effective repair tools," he continued. "These epithelial thickness profile changes can also apply to cataract surgery IOL calculations post-LASIK because the epithelial power will be different depending on the procedure that has been performed; the epithelium will sit on the stromal surface as a positive lenticule after a myopic ablation whereas after hyperopic ablation there will be a negative epithelial lenticule covering the stromal surface. Two postop myopic LASIK and hyperopic LASIK eyes may have exactly the same K's, but completely different gradient optics: the epithelial power being the major differentiating factor. I am convinced that gradient optics through the epithelial-stromal interface is the missing link to achieving the most accurate IOL power calculations after corneal refractive surgery. None of the current methods of increasing sophistication, such as raytracing through the cornea can address this epithelial power inaccuracy as they all only take into account the front and back surface shape of the cornea."

Case studies

Prof. Reinstein used two case studies to iterate his point. One of these was a patient who had undergone radial keratometry (RK) including inferior/superior trapezoidal incisions for presbyopia correction. In this instance Prof. Reinstein and colleagues found that the epithelial thickness profile was very irregular in thickness with up to 35 µm variation within the central 4 mm corneal diameter with the thinnest epithelium (40 µm) occurring over the two regions of trapezoidal incisions and the thickest epithelium (up to 75 µm) centrally. In this case, the epithelial thickness profile was masking a significant proportion of the stromal irregularity from front corneal surface topography, meaning that this proportion of the stromal irregularity would not be taken into account by a topographyguided ablation algorithm. Therefore, the optimal treatment plan was to perform an Artemis assisted transepithelial PTK procedure to target the component of the stromal irregularity compensated for by the epithelium while modelling the refractive shift and minimizing the amount of stroma to be removed.