Peripheral Steepening Research Articles

Importance of accurately assessing biomechanics of the cornea.

This article summarizes the state-of-the-art in clinical corneal biomechanics, including procedures in which biomechanics play a role, and the clinical consequences in terms of error in estimating intraocular pressure (IOP). Corneal biomechanical response to refractive surgery can be categorized into either stable alteration of surface shape and thus visual outcome, or unstable biomechanical decompensation. The stable response is characterized by central flattening and peripheral steepening that is potentiated in a stiffer cornea. Two clinical devices for assessing corneal biomechanics do not yet measure classic biomechanical properties, but rather provide assessment of corneal deformation response. Biomechanical parameters are a function of IOP, and both the cornea and sclera become stiffer as IOP increases. Any assessment of biomechanical parameters must include IOP, and one value of stiffness does not adequately characterize a cornea. Corneal biomechanics plays a role in the outcomes of any procedure in which lamellae are transected. Once the corneal structure has been altered in a manner that includes central thinning, IOP measurements with applanation tonometry are likely not valid, and other technologies should be used.

Changes of asphericity of posterior corneal surface after corneal refractive surgery

To investigate the changes of the asphericity (Q-value) of the posterior corneal surface with different corneal diameters after corneal refractive surgery. One hundred and fifteen myopic eyes of 115 subjects undertaken corneal refractive surgery (59 LASIK and 56 EPI-LASIK) were enrolled in this prospective study. The Q-value of the posterior corneal surface for patients with different corneal diameters (6, 7, 8 and 9 mm) was measured with Pentacin pre- and post-operatively. The correlations between Q-value, Q change (ΔQ), and the mean preoperative spherical equivalent refraction (SE), central corneal thickness (CCT), central ablation depth (AD) and AD/CCT were investigated. The mean Q-value of the posterior corneal surface in 115 preoperative eyes with corneal diameters at 6, 7, 8 and 9 mm was 0.08 ± 0.16, -0.05 ± 0.13, -0.15 ± 0.12 and -0.26 ± 0.11, respectively. The Q-value had a statistically significant positive shift 1 month after surgery in eyes with 6, 7 and 8 mm corneal diameters (LASIK: t(6mm) = -2.076, P = 0.042; t(7mm) = -2.873, P = 0.006; t(8mm) = -2.180, P = 0.033; EPI-LASIK: t(6mm) = -2.859, P = 0.006; t(7mm) = -3.901, P = 0.000; t(8mm) = -3.510, P = 0.001). The mean Q-value diminished significantly 6 months compared to 1 month after surgery, and had an obvious tendency back to its preoperative level. No statistically significant correlation between Q and SE or CCT was found in the preoperative eyes. Also, no correlation between ΔQ and SE, CCT, AD and AD/CCT was found after surgery. The shape of the posterior corneal surface in myopic eyes was more prolate as corneal diameter increased, which was similar to the anterior corneal surface. The posterior corneal surface showed central flattening and peripheral steepening (oblate shift) at early stage either post-LASIK or post-EPI-LASIK, and then returned to its original state over time.

Use of Intralimbal Rigid Gas-Permeable Lenses for Pellucid Marginal Degeneration, Keratoconus, and After Penetrating Keratoplasty

To discuss the results of fitting Dyna Intra-Limbal lenses (Lens Dynamics, Inc., Golden, CO) (DIL) for pellucid marginal degeneration (PMD), keratoconus, and after penetrating keratoplasty (PK). The charts of patients fitted with DILs between January 2003 and December 2004 were retrospectively reviewed. Ocular diagnosis, indication for DIL, flat and steep curvatures by corneal topography, and age at the time of initial fitting were noted. The DIL data included initial base curve, power, and the number of changes made in parameters during the follow-up. The outcome data included visual acuity and the duration of follow-up and lens wear. Complications and complaints were also noted. Twenty-seven eyes of 22 patients were reviewed. Fourteen eyes had PMD; seven had keratoconus; and six eyes had undergone PK. The mean age of patients was 52.7 +/- 13.1 years. The mean follow-up was 8.9 +/- 7.4 months. Nine (33.3%) eyes achieved 20/20 visual acuity; 13 (48.1%) eyes achieved 20/25 to 20/40; and five (18.5%) eyes achieved 20/50 to 20/70. Fifteen (55.6%) eyes achieved visual improvement (two lines or more). The mean number of refits was 1.1 +/- 0.9. No infection or neovascularization was noted; a corneal abrasion occurred in one eye. Good wearing time was achieved in 18 (66.7%) eyes of 13 patients, who were still wearing DILs at the last follow-up. DILs may be a good alternative in selected patients with flat central and superior corneas and inferior peripheral steepening.

Biomechanics of Corneal Refractive Surgery

From London Vision Clinic, London, United Kingdom (Reinstein); and the Departments of Ophthalmology and Biomedical Engineering, The Ohio State University, Columbus, Ohio (Roberts). I n this issue of the Journal of Refractive Surgery, we introduce a Biomechanics Section, which will feature one to two original articles per issue, highlighting current work devoted to investigating the biomechanics of the cornea. A new era is dawning in biomechanical research that comes with the ability to not only measure corneal biomechanical properties in vivo,1,2 but also determine the internal structure of the cornea layer by layer.3,4 This is analogous to the early days of topographers or wavefront sensors, when the devices and their capabilities were not well understood. Optical aberrations initially provided an explanation for the patient with visual complaints, despite excellent visual acuity. Ultimately, wavefront customization grew from the ability to objectively document image quality on an individual basis. Similarly, individual variations in biomechanical properties may offer the ability to initially explain, and ultimately predict, corneal response to a surgical procedure, leading to the next generation of biomechanical customization. Some of the evidence pointing to the impact of corneal biomechanical properties on surgical outcomes lies in the measurement of intraocular pressure (IOP), both before and after refractive surgery. It is well known that measured IOP is reduced, on average, following a refractive procedure. It has been assumed that this is the result of reduced curvature and thickness in myopic procedures. However, Chang and Stulting5 performed a retrospective review of over 8000 myopic LASIK patients, and determined that although measured pressure was reduced on average by approximately 2 mmHg, the range of change was approximately 10 to 15 mmHg. Every patient in this population had reduced thickness and curvature, and yet almost half of them had an increase in measured IOP. Clearly, the artifact in IOP measurement cannot be explained by thickness alone, and “correction” of measured IOP postoperatively using a linear correction factor based on thickness is problematic.6 This leads to the conclusion that refractive surgery likely alters the fundamental biomechanical properties of the cornea. Reinstein et al have demonstrated that after LASIK, peripheral thickening of the stromal layer occurs outside of the zone of tissue ablation,4 and this has been corroborated by topography studies demonstrating peripheral steepening after LASIK.7 Therefore, as our understanding of corneal biomechanics improves, surgical outcomes may also improve.8-11 The Biomechanics Section is initiated in this issue with two original articles that present fi nite element based models of corneal behavior. Deenadayalu et al offer a model that predicts central fl attening and a subsequent hyperopic shift after lamellar fl ap creation that supports clinical data in the literature. Fernandez et al offer a model of corneal response to transplantation, including lamellar and penetrating procedures. Both articles advance our knowledge of corneal behavior.

Reverse-Geometry Gas-Permeable Lens Design for Pellucid Marginal Degeneration

To report a case of treating a patient with pellucid marginal degeneration (PMD) by using a reverse-geometry gas-permeable lens design. A 38-year-old Hispanic man was referred for having reduced visual acuity secondary to distorted corneas. The patient was diagnosed with PMD after an extensive slitlamp examination showed a thinning of the inferior peripheral cornea in both eyes. Corneal topography (Orbscan II) was performed to help confirm the diagnosis. A Reverse Aspheric Ortho Focus (RAOF) gas-permeable lens was fitted on this patient. The Orbscan II corneal topography showed distinct peripheral steepening, a pronounced astigmatic pattern (greater in the left eye than in the right), and thinner corneas in the inferior periphery than centrally. The fluorescein pattern of the RAOF 5 lenses showed central alignment, good edge lift 360 degrees, mid peripheral bearing with 0.5 mm vertical movement on blinking, and good centration. The patient's distance visual acuity with the lenses was 20/20-1 in the right eye and 20/20 in the left. The Orbscan II was an important tool in making the diagnosis of early PMD, because few clinical signs were observed. Not only does the Orbscan II provide the clinician with a topographic map of the cornea, but it also measures the corneal thickness, which aided in the diagnosis of this patient. In addition, the posterior float measurement provided by the Orbscan II may be instrumental in making a differential diagnosis. A reverse-geometry gas-permeable lens provided the patient with improved peripheral fit over conventional designs, adequate comfort, and optimal visual acuity.

The Topography of the Normal Cornea

This article is the fourth in a series addressing the use and clinical applications of corneal topography. Before using this technique to diagnose abnormalities of corneal shape, it is vital to have a good understanding of the normal corneal shape and its natural variations. The shape of the normal cornea is complex, being radially asymmetric and aspheric, but can for many practical purposes be considered spherocylindrical or part of an ellipse. For descriptive purposes, the corneal surface is arbitrarily divided into four zones. The position of any feature can be given by its distance from the centre and the meridian on which it lies. The corneal centre can be defined in four ways, each with different applications. The topography of a normal cornea can have one of a variety of shapes, which lie on a spectrum from round to oval, through bow tie, to irregular. Fellow eyes can be strikingly similar, and commonly show mirror symmetry. Small changes in corneal astigmatism occur throughout life: from ‘with-the-rule’ in childhood, through spherical in middle-age, to ‘againstthe-rule’ in later years. Short-term variations due to lid pressure, reduced tear evaporation or the menstrual cycle are rarely of clinical significance. Of much greater importance are the changes that can arise from contact lens wear. Central flattening, peripheral steepening, radial asymmetry and irregular astigmatism may persist for months after removing hard lenses. A normal cornea may appear abnormal on topography due to poor fixation, centring or focusing; or tear film irregularities.

Progressive Hyperopia After Radial Keratotomy - An Explanation?

OBJECTIVE: To study the corneal stability after radial keratotomy. STUDY DESIGN: Measurement of corneal topography after intraocular pressure (IOP) increase up to 40 mmHg to demonstrate the instability of corneas after radial keratotomy. SETTING: Department of Ophthalmology, University of Hamburg, Germany. PATIENTS: Thirty patients after radial keratotomy, ten emmetropic, ten myopic and ten hyperopic subjects as control group. MAIN OUTCOME MEASURES: A 180° tilting table was used to increase intraocular pressure (IOP). A TMS photokeratoscope was used to measure corneal topography before and after pressure increase. RESULTS: Owing to the rapid IOP increase, each image of normal subjects showed a peripheral and central steepening of the corneal curvature during tilting. A peripheral steepening and a central flattening of the corneal curvature could be demonstrated in each patient after radial keratotomy. CONCLUSION: A weakened cornea after radial keratotomy is exposed to a permanent stretching stress. This stress in a tissue with weak stability and prolonged wound healing causes a permanent progressive hyperopia.

Corneal Topography and Radial Keratotomy

ABSTRACT: Computer scanning of twelve-ring photokeratoscopy yields corneal curvature measurements at multiple loci. In order to understand corneal changes resulting from radial keratotomy (RK), scanning of preoperative and postoperative photokeratographs was performed. Radial keratotomy flattens most of the cornea, but the greatest degree of flattening occurs centrally. Radial keratotomy also changes the nature of corneal asphericity. Instead of the peripheral flattening one observes with the normal cornea, peripheral steepening occurs after radial keratotomy. In comparing the effects of different optical zone sizes, the area of greatest activity is represented by rings one through five.