Preoperative Corneal Curvature Research Articles

An Interval-Censored Model for Predicting Myopic Regression after Laser In Situ Keratomileusis

A time-varying statistical model was proposed to predict the risk of regression toward myopia after laser in situ keratomileusis (LASIK) and to identify significant predictors within a time frame. A total of 615 eyes of 311 patients derived from a retrospective cohort who underwent LASIK in 2003 were analyzed. Refraction outcomes were recorded at 1 day, 1 week, and 1, 3, 6, 9, and 12 months or longer after LASIK. A cross-validated design was used, to split data into trained (n = 308) and validated (n = 307) data sets. These data sets were used in an interval-censored model to predict the probability of regression toward myopia and to assess the predictors including demographic features and preoperative and postoperative variables. Myopia regression was observed in 164 (26.7%) of 615 eyes during the follow-up period of 12 months or longer after LASIK. Significant predictors for myopia regression after LASIK included preoperative manifest spherical equivalent (P = <0.0001), mean preoperative central corneal curvature (P = 0.001), size of optic zone (P = 0.0043), undercorrection (P = 0.04), and age (P = 0.0734). The risk of regression toward myopia after LASIK increased rapidly within 1 month, slowed down between 1 and 6 months, and became steady after 6 months, regardless of risk group. The risk of myopia regression up to 6 months after LASIK was 21% in average-risk eyes (based on all eyes). The proposed interval-censored model was useful not only for predicting the probability of myopia regression after LASIK but also for identifying the evolution of patients within low, moderate, and high-risk groups.

Factors Associated with the Successful Separation of Corneal Epithelium in Epi-LASIK

Purpose: To determine the incidence and perioperative factors of flap-related complications from Epi-LASIK. Methods: In this study, 122 eyes of 66 patients who had Epi-LASIK using Centurion SESTM epikeratome (Norwood Eye Care, Australia) were enrolled. Associations of pre-operative corneal curvature, white-to-white distance, central corneal thickness, refractive error, dry eye, punctate corneal erosion, pannus, and history of wearing contact lenses with flap-related complications were investigated. To decrease flap-related complications, surgeons pressed patients' eyelids with a speculum during epithelial separation, and the effect of this method was verified. Results: Complete epithelial separation was achieved in 74 eyes (60.6%), incomplete separation in 29 eyes (23.8%), and free epithelial sheet in 19 eyes (15.6%). Thin corneas (P=.041), a history of wearing contact lenses (P=.008), and the duration of contact lens use (P=.003) significantly decreased the incidence of successful epithelial separation. Pressing down the eyelids with a speculum while separating the epithelial sheet increased the incidence of complete separation from 50.6% to 83.8% (P=.003). Conclusions: The risk of flap-related complications from Epi-LASIK may be higher in people who use contact lenses and in people who have thin corneas. The risk can be reduced by pressing the eyelids with a speculum during epithelial separation.

A Predictive Model for Postoperative Intraocular Pressure Among Patients Undergoing Laser in Situ Keratomileusis (LASIK)

The aim of this study was to develop a predictive model based on preoperative variables for estimating postoperative intraocular pressure (IOP) of those eyes undergoing LASIK surgery, to predict the amount of underestimated IOP after LASIK for myopia and myopic astigmatism. Pretest-post-test longitudinal study. Both eyes of 193 eligible subjects who underwent LASIK procedures at the Department of Ophthalmology, National Taiwan University Hospital, from July 2000 to December 2002 for myopia and myopic astigmatism were identified to build up the predictive models. IOPs were measured with noncontact air-puff tonometry. Information on age, gender, preoperative central corneal thickness (CCT), preoperative central corneal curvature (CCK), preoperative spherical equivalent refractive error, and ablation depth was collected and applied for predicting postoperative IOP after LASIK based on linear mixed model. Significant predictors for postoperative IOP after myopic LASIK procedures included age, gender, preoperative IOP, ablation depth, preoperative CCT, and preoperative spherical equivalent refractive errors. The linear mixed model, taking into account these significant preoperative correlates and the correlation of IOPs between both eyes of the same patient, explained 91% of the variation of postoperative IOP. A statistical model was developed for predicting the amount of underestimated IOP after LASIK for myopia and myopic astigmatism, which is of clinical importance to uncover ocular hypertension among patients whose information on postoperative IOP immediately after LASIK is not available.

Corneal Topographic Changes in a Case of Limbal Conjunctival Carcinoma

To describe topographic changes in a case of limbal conjunctival carcinoma. Corneal topography was performed before and 3 months after excision. Preoperative topography revealed a localized flattening (37.1 diopters [D]) adjacent to the tumor and extending to the corneal apex, causing a 3-diopter asymmetric with-the-rule astigmatism. Preoperative corneal curvature was abnormally flat in all hemimeridians (average 40.8 D). During surgical excision, no corneal infiltration was found. Three months later, the astigmatism was 0.3 D, and corneal curvature had increased to normal values in all hemimeridians (45.5 D in the tumor area; average of all hemimeridians 45.5 D). Corneal steepening after tumor excision measured by topography almost doubled that observed in refraction. The similarity with the patterns induced by pterygia, the lack of corneal infiltration, and the complete reversal of astigmatism after excision support the theory that tear pooling at the tumor apex is responsible for secondary astigmatism and diffuse corneal flattening. However, the irregularity induced by the tumor may affect the precision of topographic measurements.

Quality of vision and patient satisfaction after LASIK.

The purpose of this article is to review the literature and find characteristics that lead to improved patient satisfaction and better quality of vision. Flatter preoperative corneal curvature is a risk factor for starbursts after laser-assisted in situ keratomileusis (LASIK). Pupil size has not been found to be correlated with night vision symptoms. Wavefront-guided ablations reduce higher-order aberrations in comparison with traditional LASIK. Night vision symptoms are correlated with younger age, greater correction/increased ablation depth, enhancement, and decreased ablation diameter. Contrast sensitivity has been found to initially decrease after LASIK, returning to baseline 6 to 12 months postoperatively. LASIK has quickly become the refractive procedure of choice around the world. Quality of vision and patient satisfaction after LASIK can be difficult to assess because of the many variables that must be considered to accurately measure these endpoints. Preoperative characteristics such as: increased patient age, decreased corneal toricity, or increased pupil size reduce patient satisfaction. Intraoperative factors like decentration, ablation-zone size, active eye tracking, and wavefront guided ablations affect quality of vision. Finally, postoperative factors such as night vision symptoms, reduced contrast sensitivity, and re-treatment can lead to a decline in patient satisfaction. Eliminating or limiting these variables may lead to increased patient satisfaction and higher quality of vision after LASIK.

Impact of preoperative corneal curvature on the outcome of penetrating keratoplasty in keratoconus.

To assess the impact of the patient's preoperative corneal curvature on the refractive outcome after penetrating keratoplasty (PK) in keratoconus before and after suture removal. In this retrospective cross-sectional clinical study, 236 keratoconus patients (mean age 37 +/- 11 years) were divided into four groups based on their preoperative keratometric (K-) readings: group 1, <50 diopters (D) (n = 24); group 2, <60 D (n = 52); group 3, > or =60 D (n = 101); group 4, irregular corneal shape with unmeasurable K-reading (n = 59). An 8.0/8.1-mm central round PK was performed using 193 nm Meditec excimer laser trephination along metal masks with eight "orientation teeth/notches." A 16-bite double running cross-stitch suture was applied in all cases. Postoperative examinations were performed before removal of the first suture (ie, 12 months) and after removal of the second suture (ie, 18 months). The outcome measures included central power (C-power), keratometric astigmatism (AST), surface regularity index (SRI), surface asymmetry index (SAI), spherical equivalent (SEQ), refractive cylinder (Cyl), and best corrected visual acuity (BCVA). In addition, the regularity of Zeiss keratometry mires was classified semiquantitatively (0, regular; 1, mildly irregular; 2, moderately irregular; 3, unmeasurable). RESULTS Before/after suture removal, median C-power was 43.4/43.3 D; AST was 3.0/3.0 D; SAI was 0.6/0.6; SRI was 0.9/0.9; Cyl was 2.5/2.5 D; BCVA was 0.7/0.7. After suture removal, the percentage of regular keratometry mires increased from 37% to 61%. Comparisons among the four groups revealed no significant differences for any parameters tested either before or after suture removal. CONCLUSIONS With laser trephination and a double running suture, the refractive and visual outcome of PK for keratoconus seems to be independent on the patient's preoperative corneal curvature or irregularity. Suture removal did not effect an increase of corneal astigmatism but did increase the proportion of regular keratometry mires. Thus, our policy of not performing PK in keratoconus eyes before the patient becomes contact lens intolerant is well supported.

Patient satisfaction and visual symptoms after laser in situ keratomileusis

Purpose To examine preoperative risk factors for decreased satisfaction and symptoms of glare, halos, and/or starbursts after laser in situ keratomileusis (LASIK). Design Retrospective, case-control study. Participants Participants were 604 previously myopic patients who underwent LASIK at least six months prior to completing the questionnaire. Methods Questionnaires about satisfaction and night vision symptoms were mailed to 2100 patients from the Northeastern Eye Institute, UCLA, and the University of Pittsburgh. Preoperative and postoperative ocular variables were collected from the medical charts of the 841 subjects who returned the questionnaire. Main outcome measures Responses to various satisfaction and night vision symptom questions were compared to demographic and preoperative ocular variables using multivariate logistic regression. Results The questionnaire return rate was 42.9%. The mean age for all subjects was 43.0 ± 10.6 years, and 63% of the subjects were women. Overall, 97% of the subjects would recommend LASIK to a friend. Among subjects who would not recommend LASIK to a friend, a significantly larger percentage reported that they experienced glare, halos, or starbursts (13 of 16 (81.3%) vs. 206 of 388 (53.1%), χ 2 = 4.9, p = 0.03). Also, subjects with flatter preoperative minimum corneal curvature values were more likely to report that they would not recommend LASIK to a friend (OR = 0.71, 95% confidence interval (CI) = (0.52, 0.96), p = 0.03). The median score for satisfaction with vision was 100%, or the maximum value of 127 mm on the visual analog scale. Increasing age was significantly associated with dissatisfaction with vision after LASIK (OR = 1.68, 95% CI = (1.10, 2.56), P = 0.02). Halos were reported by 30%, glare by 27%, and starbursts by 25% of all subjects. Subjects who reported starbursts had a significantly flatter preoperative minimum corneal curvature (43.54 ± 1.60 D versus 43.92 ± 1.69 D, t = 2.15, p = 0.03). Also, subjects who had surgical enhancement were more likely to experience glare, halos, and/or starbursts (OR = 2.14, 95% CI = (1.33, 3.46), p = 0.002). Conclusions Most subjects are satisfied with their vision after LASIK. Our study suggests, however, that certain factors are associated with decreased satisfaction and night vision symptoms after LASIK. Some of these factors include: increasing age, flatter preoperative minimum corneal curvature, and surgical enhancement.

Factors that influence outcomes of hyperopic laser in situ keratomileusis

Purpose To analyze the influence of preoperative corneal curvature, postoperative keratometric power, and the amount of correction on the outcomes of hyperopic laser in situ keratomileusis (LASIK). Setting Clínica Baviera, Instituto Oftalmológico Europeo, Madrid, Spain. Methods In this retrospective study, the records of 376 eyes that had LASIK for hyperopia using the Moria LSK-One microkeratome and the Technolas-Keracor 217C excimer laser were reviewed. The results were analyzed by preoperative hyperopia (5 subgroups) and by preoperative (more than and less than 43.0 diopters [D]) and postoperative (more than and less than 48.0 D) mean keratometry. Results A statistically significant keratometry regression was found in the +3.00 to +3.90 D range ( P < .01), a significant decrease in predictability in the +4.00 to +4.90 D range ( P < .05), and a significant worsening in safety in the highest range (+6.00 to +7.90 D; P < .05). Comparative analysis of the ≥+4.00 D and <+4.00 D groups showed statistically significant differences in most measurement parameters. The preoperative keratometry did not influence postoperative results with the exception of poorer predictability in the group of preoperative flat corneas in which a high degree of hyperopia was corrected; ie, spherical equivalents within ±0.50 D were found in 40.4% and 61.0% of cases with flat and steep corneas, respectively ( P < .05). The efficacy and safety in eyes that achieved a postoperative keratometry >48.00 D did not differ significantly from the efficacy and safety in eyes that had a lower final keratometric power. Conclusions The factor that negatively influenced the outcome of hyperopic LASIK was the degree of hyperopia corrected. Preoperative keratometry did not significantly influence the postoperative results, and postoperative keratometry >48 D did not result in significant worsening of visual results when the attempted correction was less than +4.00 D.

A New Predictive Formula of Anterior Chamber Depth for Pseudophakia

To develop a new predictive formula of the anterior chamber depth(ACD) of the pseudophakic eyes for more accurate intraocular lens(IOL) power calculation with the Ray Tracing method. We studied a series of 149 phacoemulsificated eyes with capsulorhexis and in-the-bag fixation of IOL. ACD was measured with a Jaeger type depth measuring unit which was attached to a slit-lamp biomicroscope. We developed the formula with 3 variables: preoperative anterior corneal curvature with keratometer, and ACD and crystalline lens thickness with preoperative ultrasonography. We named the derived formula "formula H". We compared formula H with Olsen's predictive formula for the ACD(Olsen 92, Olsen 95). Mean absolute error between the predicted and postoperatively measured ACD was 0.174 +/- 0.133, 0.280 +/- 0.235, 0.229 +/- 0.200 mm in formula H, Olsen 92, and Olsen 95, respectively. Formula H was significantly more accurate than both Olsen formulas(p < 0.001, p < 0.05, respectively). The accuracy of IOL power calculation can be improved with the new formula using the Ray Tracing method, using the new ACD predictve formula.

Corneal Topography and Postoperative Refraction After Cataract Phacoemulsification Following Radial Keratotomy

We present one case in which phacoemulsification was performed seven years after radial keratotomy (RK). A 55-year-old military police officer had undergone successful bilateral RK for the correction of myopia seven years before he developed a cataract in his left eye. Pre-RK keratometric and refractive data and post-RK myopia reduction were not available. We relied upon corneal topography to measure corneal refractive power. We took the Effective Refractive Power (EffRP) index from EyeSys Holladay's Diagnostic Summary and used SRK-T formula for IOL calculation. A-scan axial length readings were consistent and reliable (AL = 26.0 mm). Aiming at postoperative emmetropia, we implanted a +20D PC IOL (A cost. = 118) was implanted. The lens was expected (SRK-T formula) to give a -1.35D postoperative refraction. After uneventful cataract surgery, corneal topography showed significant corneal instability with central corneal flattening in the first postoperative weeks, mild central corneal steepening at week 6, and return to preoperative corneal curvature at week 23. One year after cataract extraction, the patient's spherical equivalent is +1.12D, showing a prediction error of about 2.5 diopters.

Corneal Epithelial Wound Healing After Excimer Laser Photorefractive and Photoastigmatic Keratectomy (PRK and PARK)

To record morphologic changes that occur in corneal endothelial cells during daily wear of rigid, gas-permeable fluorocarbon contact lenses during 3 years. Nineteen new wearers and 32 previous wearers were fitted with daily-wear fluorocarbon contact lenses. Photographic images of the central corneal endothelium and central corneal thickness measurements were recorded at baseline, 2 months, and 1, 2, and 3 years after fitting. In the new wearers, an increase in the coefficient of variation of cell area was present by 2 months after lens fitting and increased further over the subsequent 3 years. The increase in coefficient of variation of cell area was correlated with the oxygen transmissibility of the lens. Endothelial cell density was decreased at 2 and 3 years. Corneal thickness did not change. In the previous lens wearers after 3 years, there were no significant changes from baseline in any morphologic value. Corneal thickness was decreased significantly. Although oxygen transmissibility in fluorocarbon contact lenses is improved over previous hard and soft lenses, polymegethism still was induced within 2 months by these lenses, and morphologic changes present from past lens wear did not improve during 3 years of daily wear.

Astigmatic stabilization of 3.0 mm temporal clear corneal cataract incisions

Purpose: To determine the time needed for corneal shape changes to stabilize after a laterally oriented, 3.0 mm, unsutured, single-hinge clear corneal tunnel cataract incision combined with implantation of a foldable acrylic 5.5 mm intraocular lens.Setting: Advanced Vision Care, West Hills, California.Methods: The study comprised 45 patients who had a similar phacoemulsification technique and lens implantation through an unenlarged 3.0 × 2.5 mm self-sealing temporal clear corneal tunnel incision. Changes in corneal curvature were determined by computerized videokeratography (corneal topography) prior to surgery and at 1, 2, and 6 weeks after surgery. Simulated keratometry was ascertained from topography and analyzed with the Lens-Stat software program.Results: Preoperative corneal cylinder, cylinder axis, and central corneal curvature did not change markedly after surgery. By vector analysis, approximately 0.50 diopter (D) of induced cylinder was noted, whereas less than 0.25 D of cylinder change was observed in the surgical meridian. Although statistically similar, there were slight differences between the data at 1 and 2 weeks postoperatively. However, the 2 and 6 week data were indistinguishable, indicating that the incision method used in the study was fully stable within 2 weeks of surgery.Conclusions: The surgical method resulted in stable corneal curvature by 2 weeks after surgery at which time the patient is ready for final spectacle prescription and can be discharged from acute postoperative ophthalmologic care in the absence of complicating factors. The shortened recovery time is beneficial for the patient and has significant potential effects on the global costs of cataract rehabilitation and the reimbursement schemes for postoperative management.

Theoretical and Clinical Effect of Preoperative Corneal Curvature on Excimer Laser Photorefractive Keratectomy for Myopia

Individual clinical and optical variables may influence the effect of excimer laser photorefractive keratectomy. A theoretical model to describe the influence of initial corneal power, astigmatism, and topography on the expected results of photorefractive keratectomy would be useful in identifying those variables that may ultimately improve the predictability of the procedure. Using a mathematical analysis based on the change in sagittal depth of the central ablation zone following photoablation, we predict the effect of initial corneal curvature on the ultimate outcome of a standardized photorefractive keratectomy. Refractive results from the Phase III US Food & Drug Administration clinical trials of photorefractive keratectomy were analyzed to confirm these mathematical predictions. We find that the initial corneal power, theoretically, is not expected to significantly affect the refractive change that results from a given ablation. Similarly, the corneal astigmatism present before photorefractive keratectomy is expected to be only minimally altered by a spherical excimer laser treatment. Clinically, there is no detectable difference in predictability of the procedure amongst groups stratified by initial mean keratometric power. Our analysis provides a methodology to predict the optical effects of photorefractive keratectomy upon the cornea and may be applied to a variety of hypothetical clinical settings. The predicted lack of clinical association between initial corneal curvature and predictability of photorefractive keratectomy is confirmed.

Differences Between Men and Women as Related to Intraocular Lens Implantation

We examined 291 cases of IOL implantation in order to evaluate differences between males and females for several factors. We found that women have shorter axial lengths, older age at time of surgery, greater preoperative corneal curvature, higher power of IOL implanted, more surgically induced astigmatism, and a higher rate of complications. Insignificant differences between the two groups included pre- and postoperative refraction, preoperative astigmatism, visual acuity, type of IOL implanted, corneal thickness, endothelial cell count, and percent of patients requiring discissions.