Effects of misalignment during corneal topography.

PURPOSE: To evaluate consequences of misalignment during corneal topography. SETTING: Maisonneuve-Rosemont Hospital, Montreal, Quebec, Canada. METHODS: Twenty-nine healthy subjects were enrolled in a study of 2 types of misalignment during Orbscan topography acquisition. The first was rotation, which was defined as angular deviation of the subject's visual axis away from the central fixation target, and the second was translation, defined as off-center displacement of the joined half slits on the computer screen during alignment by the technician. The effect of vertical and horizontal misalignments, by rotation or translation, was assessed. Study parameters included simulated keratometries (SimKs), mean corneal power (mean power), and corneal apex (R) and the asphericity factor (Q) values. Inferior-superior (I-S) and temporal-nasal (T-N) indices were calculated. Differences in parameter values between aligned and misaligned positions were studied. A mathematical model was also developed to simulate rotational and translational misalignment and consolidate clinical experimentation. RESULTS: The mean SimK values were slightly but significantly affected by rotation. The I-S value increased significantly with downward rotation and decreased with upward rotation of the eye, while T-N values increased significantly with temporal rotation and decreased with nasal rotation. The R and Q values increased with rotation. Overall, the topography parameters were minimally affected by translation. Comparable results were obtained with the theoretical model. CONCLUSIONS: Rotational misalignment of the eye during acquisition influenced topography more than the translational misalignment during focusing by the technician. Rotational misalignment induced corneal asymmetry on elevation and curvature maps as well as pseudokeratoconus patterns.

Topographic screening of donor eyes for previous refractive surgery.

PURPOSE: To determine whether donor eyes had previous refractive surgery using Orbscan (Bausch & Lomb Surgical) corneal topography. SETTING: Lions Eye Bank of Oregon, Portland, Oregon, USA, and Maisonneuve-Rosemont, Hospital, Montreal, Quebec, Canada. METHODS: Orbscan corneal topographies of 50 donor eyes from the Lions Eye Bank of Oregon were obtained; 10 eyes had previous refractive surgery (6 laser in situ keratomileusis, 2 photorefractive keratectomy, 2 radial keratotomy) to correct myopia, and 40 had not had surgery. Algorithms based on corneal anterior and posterior elevations and anterior tangential curvature were developed: The difference in curvature (DC) was based on the difference in the mean anterior tangential curvature between central and midperipheral areas; difference in elevation (DE) represented the difference between the anterior and posterior central elevations. Receiver-operating characteristic (ROC) curves for each algorithm were obtained, and sensitivity values at fixed specificities were calculated. RESULTS: The mean area under the ROC curve, which corresponds to the probability of correctly identifying the presence of a previous refractive surgery, was 0.853 +/- 0.079 (SE) for DC and 0.933 +/- 0.057 for DE. The DC algorithm resulted in a sensitivity of 80% for a specificity of 87.5%, and DE yielded a sensitivity of 90% for a specificity of 92.5%. There was a strong correlation between the value of the DE and DC algorithms and the amount of previous refractive surgery (DC: r = 0.84, P = .008; DE: r = 0.76, P = .028). CONCLUSION: The results led to a proposed criteria-based system using Orbscan corneal topography to screen eye-bank eyes for previous refractive surgery.

Amniotic membrane transplantation and fibrin glue in the management of corneal ulcers and perforations: a review of 33 cases.

PURPOSE: To evaluate the efficacy of amniotic membrane in corneal ulcers refractive to conventional treatment and amniotic membrane with fibrin glue in corneal perforations. METHODS: Amniotic membrane transplantation (AMT) was performed in 33 eyes from 32 patients for corneal ulcers refractive to conventional treatment. Fourteen ulcers were perforated and received fibrin glue and amniotic membrane. Ulcers were divided into 3 groups: neurotrophic or exposure, autoimmune, and other etiology. RESULTS: Overall success was observed in 80% (27/33 eyes) of the cases, with success rates of 87.5% (14/16 eyes), 70% (7/10 eyes), 85.7% (6/7 eyes) in groups 1, 2, and 3, respectively. The ulcers healed in a mean time of 3.6 +/- 1.6 weeks and the follow-up was 14.8 +/- 9.9 months. Failure was noted in 6 eyes with severe neurotrophic keratitis, Stevens-Johnson syndrome, ocular cicatricial pemphigoid, and Acanthamoeba keratitis. Grafts with fibrin sealant showed a success rate of 92.9 % (13/14 eyes) compared to 73.7% (14/19 eyes) for amniotic grafts alone. In patients with severe limbal damage, a success rate of only 20% (1/5) was observed. CONCLUSIONS: AMT is a viable option in the treatment of nonhealing corneal ulcers of various depth and etiologies. Perforations up to 3 mm can be safely managed by fibrin glue and AMT. These techniques lead to rapid reconstruction of the corneal surface and can give a good final functional result or allow keratoplasty to be done in more favorable conditions.

Topography-based screening for previous laser in situ keratomileusis to correct myopia and hyperopia.

PURPOSE: To develop a screening tool based on corneal topography to detect previous myopic and hyperopic laser in situ keratomileusis (LASIK). METHODS: Clinical data from three private clinics were analyzed in a university setting. A total of 476 topographies (Orbscan II) were randomly selected (1 topography per patient): 338 from unoperated corneas, 81 from corneas that had LASIK to correct myopia, and 57 from corneas that had LASIK to correct hyperopia. The LASIK procedures were performed using a Technolas excimer laser (217C or 217Z) and a Hansatome microkeratome. The first set of algorithms (VESm and VESh) calculated the volume summation between the anterior corneal surface and the best-fit sphere in the central and mid-peripheral regions. The second set of algorithms (DCm and DCh) calculated the mean anterior tangential curvature difference between the central and mid-peripheral regions. RESULTS: The features VESm and DCm for the screening of a myopic LASIK yield, respectively, sensitivity rates of 92.7% and 95.1% and specificity rates of 94.9% and 96.5%. The features VESh and DCh for the screening of a hyperopic LASIK gave sensitivity rates of 93.1% for both and specificity rates of 94.8% and 97.1%. The performance of those features was superior to clinical assessment using topography color maps. CONCLUSIONS: Criteria based on Orbscan II corneal topography are proposed for the detection of previous hyperopic and myopic LASIK performed with a Technolas excimer laser (version 217C or 217Z).