Stephen S. Khachikian

An introduction to understanding elevation-based topography: how elevation data are displayed – a review

The increased frequency of refractive surgery and the shift towards the correction of higher‐order aberrations necessitates a more detailed understanding of corneal shape. Early topography systems were based on Placido technology, as this was initially more intuitive for the general refractive surgeon. Newer computerized corneal modelling has increased our knowledge beyond what was previously possible. Elevation‐based systems utilize a direct triangulation technique to measure the corneal surface. Elevation‐based Scheimpflug imaging has advantages in that it allows for the measurement of both the anterior and posterior corneal surfaces. Posterior measurements are often the first indicators of future ectatic disease, in spite of completely normal anterior curvature. Examination of the posterior corneal surface can often reveal pathology that would otherwise be missed if one was relying on anterior analysis alone. Although there is little disagreement in diagnosing clinically evident keratoconus, agreement on what constitutes ‘form fruste’ or preclinical keratoconus remains elusive. The ability of elevation‐based topography to analyse both anterior and posterior corneal surfaces adds significantly to our ability to identify eyes believed to be ‘at risk’. As more knowledge is gained, it is appreciated that a full understanding of the workings of the human eye requires knowledge obtained from more than just one surface.

Long-term stability of the posterior cornea after laser in situ keratomileusis

PURPOSE: To study long‐term changes in posterior corneal elevation after laser in situ keratomileusis (LASIK) using Scheimpflug topography (Pentacam, Oculus, Inc.) in eyes 1 year after LASIK. SETTING: Department of Ophthalmology, Albany Medical Center, and a private practice, Albany, New York, USA. METHODS: One hundred two myopic eyes of 52 consecutive patients presenting for their 1‐year follow‐up were prospectively evaluated using the Pentacam to determine elevation changes to the posterior corneal surface between preoperative and 1‐year postoperative measurements. Changes in posterior elevation were performed by comparing the best‐fit sphere preoperatively and postoperatively with a fixed reference sphere determined by the central 9.0 mm preoperative cornea. Statistical and graphical analyses were performed. RESULTS: One hundred two post‐LASIK eyes (mean correction −4.33 diopters; mean ablation depth 68.70 μm; mean estimated residual bed thickness 327 μm) had a mean posterior displacement of −0.47 μm ± 3.48 (SD) (range −10.0 to +7 μm). The mean follow‐up period was 13.6 months (range 8.8 to 19.3 months). CONCLUSIONS: In this population, no patient had significant forward protrusion of the posterior corneal surface a mean of 14 months after LASIK. The posterior cornea in post‐LASIK myopic eyes was very stable. Contrary to results in previous studies, progressive changes to the posterior corneal surface did not routinely occur after LASIK performed within established parameters.

Posterior Elevation in Keratoconus

Dear Editor: In the September 2008 issue, de Sanctis et al provide novel data on posterior corneal elevation, along with sensitivity and specificity for detecting keratoconus and subclinical keratoconus. The authors report that the posterior elevation measurements were taken as the maximum value above the best-fit sphere within the central 5 mm of the posterior cornea. Although this is a standardized method of measurement, we feel it reduces specificity for the disease process being evaluated. A fixed circular area of 5-mm diameter centered on the posterior cornea incorporates astigmatic elevation into the calculation of average normal elevation. This artificially inflates the normal elevation measurements and reduces their screening utility. When collecting elevation data in this manner (maximum elevation in a 5-mm zone), an attempt must be made to distinguish a normal astigmatic pattern of elevation from abnormal ectatic elevation (Figure 1, available online at http://aaojournal.org). This pattern classification should then be included in the disease determination process. By not doing this, the authors have overlapped normal astigmatic elevation with abnormal ectatic elevation, and limited screening ability. In recent work (Khachikian SS, Belin MW. Normal Values for Corneal Elevation Using the Pentacam Eye Scanner. Presented at: ESCRS Winter Congress, February 2008, Barcelona, Spain), we presented values for anterior and posterior elevation for normal eyes and those with keratoconus. The values were obtained at the corneal apex and thinnest point with a fixed 8-mm, best-fit-sphere calculation zone. These are 2 points that can be easily and consistently identified when looking at elevation maps from different patients. Moreover, the thinnest point measurement has a known association with keratoconus, the disease

What's in a Name: Keratoconus, Pellucid Marginal Degeneration, and Related Thinning Disorders

PURPOSE To discuss the implications of the current nomenclature and use of current diagnostic modalities on the classification and treatment of pellucid marginal degeneration and keratoconus. DESIGN Perspective analysis of the literature. METHODS Analysis of published reports on the various treatment methods for pellucid marginal degeneration and keratoconus, and the technologies used in these studies to support the diagnosis and classification of these ectatic disorders. RESULTS Many studies exploring the different treatment modalities for pellucid marginal degeneration and keratoconus rely mainly on anterior curvature maps to establish the diagnosis of these ectatic disorders, and either do not utilize or disregard information provided by pachymetric maps and posterior elevation maps. In addition, the interchangeability of the nomenclature used to describe these disorders in the literature makes it even more difficult for the clinician to determine the applicability of the results to their patient population. CONCLUSIONS We propose that future studies minimally include anterior and posterior elevation maps, anterior curvature topography, and full pachymetric maps. This will allow practitioners a better understanding of the study population, and allow them to ascertain when and in whom the treatment modality being explored may be applicable.

Scheimpflug photographic diagnosis of pellucid marginal degeneration.

Purpose: We present a case of advanced bilateral pellucid marginal degeneration (PMD) with unilateral hydrops to show the unique properties of Scheimpflug imaging and its ability to display the clinical and topographic findings of PMD not seen on traditional placido-based topography. Methods: Case report and literature review. Results: A 51-year-old man was referred with a presumptive diagnosis of bilateral keratoconus with unilateral hydrops. Complete topographic evaluation showed a band of inferior thinning adjacent to the limbus and a Scheimpflug cross-sectional image consistent with PMD. Conclusions: We show the value of Scheimpflug-based topography and pachymetry maps to accurately depict the unique disease characteristics of PMD.

New technology in corneal imaging.

Technological advances in corneal imaging have lagged behind other areas of medicine, let alone other areas in ophthalmology. Although our retinal and glaucoma brethren have made Optical Coherence Tomography (OCT) imaging the norm, the average general ophthalmologist and corneal subspecialist still routinely relies upon visual inspection and technology developed in the late 1800s (keratometry) to assess the cornea. It speaks volumes about our ophthalmic forefathers when 100-year-old technologies are still the basis for computing the IOL selection in modern, small-incision, multifocal IOL surgery. It was, at least in part, the advent of refractive surgery that speeded up the development of more modern and accurate methods of corneal imaging. Refractive surgery, by altering the relationship between the anterior and posterior cornea, revealed the shortcomings of the geometric assumptions made by all keratometers. This article will cover 3 new, but current techniques of corneal imaging: Scheimpflug Optical Cross Sectioning, which has supplanted Placido-based topography as the standard method of topographic analysis; Optical Coherence Tomography (OCT), that allows detailed visual inspection of the anterior segment; and Confocal Microscopy, which permits a microscopic in vivo inspection of the cornea. These 3 were chosen not only because they represent new technology, but also because they represent usable technology. Scheimpflug-based topography is simple and quick enough to be used as a screening tool in

Thin head and single use microkeratomes reduce epithelial defects during LASIK

PURPOSE To evaluate the effect of microkeratome design and head dimension (flap thickness) on the rate of epithelial defects. METHODS A retrospective review of 4000 LASIK procedures performed between May 2000 and December 2003 was conducted. Intraoperative epithelial defects were identified in 326 cases. The microkeratome design (reusable vs disposable) and head dimensions (100, 130, and 150 micron) were recorded along with age, sex, central ultrasonic pachymetry, keratometry, preoperative refraction (sphere, cylinder, and axis), and Schirmer testing. The chi-square and Student t test were used in data analysis. RESULTS A total of 326 (8.2%) procedures had an associated epithelial defect. With the reusable microkeratome, heads that produced thinner flaps were associated with a lower incidence of epithelial defects (P<.05 for all three groups, 100, 130 and 150 micron head). When compared to a reusable head of the same dimension and to the entire population of reusable heads, the disposable 130 head exhibited a significantly lower rate of epithelial defects (P<.0001). CONCLUSIONS The results of this study support that both microkeratome design and head dimension (and resulting corneal flap thickness) play a role in the formation of epithelial defects. These are surgeon selectable factors, unrelated to technique or patient risk factors, which can be used to reduce the rate of epithelial defects and associated complications.

The Use of Intracorneal Rings for Pellucid Marginal Degeneration

IN THEIR RECENT ARTICLE, KUBALOGLU AND ASSOCIATES reported a favorable result with the insertion of a single 210-degree arc length intrastromal corneal ring (ICR) for the treatment of pellucid marginal degeneration (PMD). They join a number of others advocating the use of ICRs for the correction of PMD. The same month, another rticle appeared describing the use of a high-power toric ntraocular lens for treatment of PMD. The diagnosis of PMD and its distinction from keratoconus has been an ongoing discussion in the cornea and topography arena. For most, this is nothing more than an academic exercise with little clinical significance, because most view these two entities as different points on a clinical continuum.

Bilateral corneal ectasia after laser in situ keratomileusis

Figure 1. A: Right eye percentage of thickness increase compared with the mean and 95% CI in a normal population. B: Left eye percentage of thickness increase compared with the mean and 95% CI in a normal population (PTI Z percentage of thickness increase; SD Z standard deviation). The recent case report by Saad and Gatinel highlights a patient who developed bilateral corneal ectasia 2 years after laser in situ keratomileusis despite normal preoperative parameters. In retrospect, the authors note that there was a large degree of pachymetric asymmetry in the patient prior to surgery. Despite otherwise normal parameters, the pachymetric asymmetry of 20 mm at the thinnest point is proposed as a potential risk factor. In 2008, we published an article documenting the standard variance of pachymetric asymmetry in otherwise normal corneas. Our study of 724 normal patients found that at the thinnest corneal point, the mean asymmetry was 9.0 mm G 8.3 (SD). The mean asymmetry values at the apex and pupil center were similar. In only 5% of the population was the thinnest point of pachymetric difference 23.2 mm.We proposed that pachymetric asymmetry should be evaluated when screening patients for refractive surgery. In the Saad and Gatinel case, the midperipheral pachymetric readings showed a variance up to 40.0 mm. It would be interesting to see the pachymetric progression graphs for this patient. Our paper established only the normal interocular pachymetric variance. Since the frequency of iatrogenic ectasia is very low, case reports such as the one by Saad and Gatinel will help determine the clinical significance of pachymetric asymmetry as a screening tool.