Application of corneal tomography before keratorefractive procedure for laser vision correction

Ectasia after refractive surgery represents a major concern among refractive surgeons. Corneal abnormalities and preexisting corneal ectasia are the most important risk factors. Corneal topography and central corneal thickness are the factors traditionally screening for in refractive surgery candidates. Study of the anterior surface by Placido topography allows for identification of keratoconus before biomicroscopy. However, this is insufficient for the evaluation of pre-operative refractive surgery. There are cases of ectasia after laser in situ keratomilusis (LASIK) without identifiable risk factors such that there is a need to go beyond the corneal surface. A key requirement is quantifying susceptibility to corneal biomechanical instability and progression to ectasia. Tomographic indices derived from elevation maps and pachymetry spatial variation produce a Belin Ambrosio display final D index (BAD-D index), which has shown better results compared to surface curvature indices for detecting very mild forms of ectasia. A logistic regression formula, integrating age, residual stromal bed, and BAD-D (Ectasia Susceptibility Score, ESS) resulted in a significant improvement in accuracy, leading to 100% sensitivity and 94% specificity for detecting susceptible cases. A comprehensive corneal structural analysis based on corneal segmental tomography can detect susceptible corneas, which increases safety for refractive surgery patients.

[Corneal power after descemet stripping automated endothelial keratoplasty (DSAEK) - Modeling and concept for calculation of intraocular lenses]

BACKGROUND AND PURPOSE

Descemet stripping automated endothelial keratoplasty (DSAEK) is an established treatment option for pathologies of the corneal endothelium. It is typically accompanied with a hyperopic shift in refraction. The purpose of this work is to predict corneal geometry after DSAEK based on model data and to present a concept how to determine corneal power, e.g. for intraocular power calculation to prevent a refractive surprise with a subsequent cataract surgery.

MATERIAL AND METHODS

Based on data of the Kooijman schematic model eye we simulated a microkeratome cut parallel to the corneal front surface for donor trephination to determine the radial thickness profile of the posterior corneal donor lamella. This donor lamella was tension-neutrally adapted to the back surface of the host and the profile of the cornea after DSAEK was derived and characterized by a quadric surface. Comparison with the curvature of the host without and with donor could resample hyperopic shift which was published in literature. A method was shown how to determine corneal power after DSAEK.

RESULTS

From the data of the Kooijman schematic model eye and the donor characteristics central / peripheral corneal thickness was increased by 150 / 250μm due to adaptation of the donor lamella. Geometry of corneal back surface showed a reduced radius of curvature (by about 0.9mm) and a change in conic constant (by about -0.13). Persistent clinically observed hyperopic shift correlates to the change in geometry of the cornea due to adaptation of the donor lamella, which reduces corneal power by 0.88 D.

CONCLUSION

DSAEK leads to a hyperopic shift in refraction, which can be explained by a change in corneal back surface geometry. In case of subsequent cataract surgery, the intraocular lens power should be calculated with consideration of both corneal surfaces rather than keratometry or corneal topography in order to minimize a systematic hyperopic shift due to misinterpretation of corneal power after DSAEK. In case of a Triple-DSAEK, a target refraction of -1.5 D should be chosen in order to safely prevent postoperative hyperopia.

Topographic and Tomographic Indices for Detecting Keratoconus and Subclinical Keratoconus: A Systematic Review

Purpose

To provide an overview of the topographic and tomographic indices developed for detecting keratoconus (KC) and subclinical keratoconus.

Methods

Literature review of studies describing and testing KC indices as well as indices developed for improving the sensitivity of subclinical KC detection.

Results

Several indices, based on anterior and posterior curvature measurements, corneal spatial distribution or posterior corneal elevation have been developed for improving the detection of KC and subclinical KC. However, to date, none of them could reach, alone, sufficient discriminating power for differentiating the mildest forms of the disease from normal corneas. New detection programs, based on a combination of corneal indices, and generated using artificial intelligence emerged recently and helped to significantly improve the subclinical KC detection.

Conclusion

The combination of topographic and tomographic corneal indices has helped to significantly improve the sensitivity of subclinical KC detection. However, combining these morphological indices to wavefront and biomechanical analyses of the cornea will certainly further improve the sensitivity of the future screening tests.

How to cite this article

Smadja D. Topographic and Tomographic Indices for Detecting Keratoconus and Subclinical Keratoconus: A Systematic Review. Int J Kerat Ect Cor Dis 2013;2(2):60-64.

Novel pachymetric parameters based on corneal tomography for diagnosing keratoconus

PURPOSE

To describe pachymetric progression indices (PPI) of the Pentacam HR (Oculus Optikgeräte GmbH) and the concept of relational thickness, and to test their accuracy for differentiating keratoconic and normal corneas compared with single-point thickness values.

METHODS

One hundred thirteen individual eyes randomly selected from 113 normal patients and 44 eyes of 44 patients with keratoconus were studied using the Pentacam HR by acquiring central corneal thickness (CCT), thinnest point (TP), position of the TP and PPI at minimal (PPI Min) and maximal (PPI Max) meridians, and the average (PPI Ave) of all meridians. Relational thickness parameters were calculated as the ratios of TP and CCT and PPI values. Mann-Whitney U test assessed differences in groups for each variable. Receiver operating characteristic (ROC) curves were calculated for all variables and pairwise comparisons were performed.

RESULTS

Statistically significant differences were noted between normal and keratoconic eyes for all parameters (P<.001), except for horizontal position of TP (P=.79). The best parameters, named Ambrósio's Relational Thickness (ART), were ART-Ave (TP/PPI Ave) and ART-Max (TP/PPI Max) with areas under the ROC curves of 0.987 and 0.983, respectively. The best cutoffs were 424 μm and 339 μm for ART-Ave and ART-Max, respectively. Pachymetric progression indices and ART had a greater area under the curve than TP and CCT (P<.001); TP (0.955) had a greater area under the curve than CCT (0.909; P=.002).

CONCLUSIONS

Tomographic-derived pachymetric parameters were better able to differentiate normal and keratoconic corneas than single-point pachymetric measurements. Further studies are needed to evaluate the role of tomography in identifying early forms of ectasia as well as ectasia risk among LASIK candidates.