Computerized Videokeratography Research Articles

Intraocular Lens Power Calculations in Eyes with Prior Corneal Refractive Surgery

Accurate intraocular lens (IOL) power calculation is crucial to meet the high expectations of patients undergoing cataract surgery. With current technological advances, IOL power calculation in normal eyes is relatively straightforward. However, it is problematic in eyes that have undergone corneal refractive surgery. There are two major causes of error in IOL calculations in these eyes: 1) inaccurate corneal power measurements obtained from standard keratometers or computerized videokeratography (CVK), and 2) incorrect estimation of effective lens position (ELP) calculated by most 3rd or 4th generation IOL power calculation formulas.

Comparison of Axial and Tangential Topographic Algorithms for Contact Lens Fitting After LASIK

Computerized videokeratography (CVK) has become the standard method for measuring the corneal curvature after refractive surgery. However, few resources exist on the use of CVK data for postoperative contact lens fitting, and no recommendations exist on the selection of the best topographic algorithm relevant to gas-permeable (GP) lens fitting. This study assessed the optimal use of topographic data to fit spherical GP contact lenses on patients who have undergone laser in situ keratomileusis. A retrospective analysis of CVK maps from eight post-LASIK eyes fitted with spherical GP lenses was performed in the Contact Lens Service of the Department of Ophthalmology at Case Western Reserve University and University Hospitals of Cleveland. Axial and tangential maps from the Dicon CT 200 (version 3.50) or Humphrey Atlas (version A8) topographer were analyzed. Averaged dioptric curvatures from different locations (inferior, superior, and temporal) at various distances (2.0 mm, 2.5 mm, 3.0 mm, 3.5 mm, and 4.0 mm) from the vertex normal of each eye were compared with the base curves of the prescribed GP contact lenses. One-way analysis of variance, Pearson correlation analysis, and paired t tests showed that the best topographic predictors of a successful GP base curve were the average curvatures at the 4.0 mm distance on the axial maps (r=0.8078, P=0.05) and at the 2.0 mm distance on the tangential maps (r=0.9738, P=0.0002). The mean dioptric powers of the GP base curve, axial map 4.0 mm curvature, and tangential map 2.0 mm curvature were 41.50 diopters (D), 42.65 D, and 42.67 D, respectively. To simplify and guide GP fitting after LASIK, the average dioptric curvature 4.0 mm from the vertex normal on axial maps or 2.0 mm from the vertex normal on tangential maps are the best predictors of accurate GP base curve selection.

Comparison of visual results in initial and re-treatment laser in situ keratomileusis procedures for myopia and astigmatism

PURPOSE: To report the results of initial and re-treatment laser in situ keratomileusis procedures for myopia and astigmatism. METHODS: A total of 1161 consecutive inital and retreatment laser in situ keratomileusis procedures in 697 patients performed between September 19, 1996, and June 20, 1998, were retrospectively analyzed. Baseline and postoperative best spectacle-corrected visual acuity, uncorrected visual acuity, spherical and cylindrical refractions, computerized video keratography, and biomicroscopy were measured or performed on each eye. RESULTS: Of 1,071 eyes, 900 (84%) underwent a single-laser in situ keratomileusis procedure, and 171 of 1,071 eyes (16%) underwent one or two re-treatment procedures. The preoperative mean ± SD spherical equivalent was −5.09 ± 3.15 diopters (range, −0.75 to −14.38 diopters) and the cylinder was 0.97 ± 1.09 diopters (range, 0.00 to 3.25 diopters) in the group that underwent a single laser in situ keratomileusis treatment. The preoperative mean ± SD spherical equivalent was −6.26 ± 3.04 diopters (range, −1.75 to −12.88 diopters) and the cylinder was 1.53 ± 1.11 diopters (range, 0.00 to 3.50 diopters) in the group that underwent re-treatment. Before re-treatment, 110 eyes (64.3%) in the group that underwent re-treatment achieved 20/40 or better uncorrected visual acuity and 23 eyes (13.5%) achieved 20/25 or better. Three months after re-treatment, 155 eyes (90.6%) achieved 20/40 or better, and 80 eyes (46.8%) achieved 20/25 or better. In the single-procedure group, uncorrected visual acuity at 3 months revealed 20/40 vision or better in 856 eyes (95.1%) and 20/25 vision or better in 572 eyes (63.6%). In this study group, 23.6% of eyes with a preoperative spherical equivalent greater than −6.0 diopters underwent re-treatment compared with 11.8% of eyes with a preoperative spherical equivalent of −6.0 diopters or less. CONCLUSIONS: Laser in situ keratomileusis appears to be an effective procedure for mild, moderate, and severe myopia. In eyes with residual refractive error, re-treatment laser in situ keratomileusis procedures can result in good visual outcomes.

Preoperative screening of contact lens wearers before refractive surgery

Purpose To evaluate corneal stability, measured by computerized videokeratography (CVK) after discontinuation of contact lens wear in preoperative refractive surgery candidates. Setting Baylor College of Medicine, Cullen Eye Institute, Houston, Texas, USA. Methods Topographic differences were analyzed in 136 noncontact lens wearers (NCLW) and 76 contact lens wearers (CLW) (18 rigid gas-permeable contact lenses [RGPCL], 58 soft contact lenses [SCL]) using EyeSys CVK after discontinuation of SCL wear for 2 weeks and RGPCL wear for 5 weeks. Results There were no differences in CVK patterns between the NCLW and the CLW groups. There were no statistically significant differences between the 2 groups in topographic symmetry, asphericity, corneal uniformity index, predicted corneal acuity, and irregular astigmatism. The dioptric range for the axial and the profile difference maps in the CLW group was slightly lower than in the NCLW group. Conclusion For patients whose manifest refraction and CVK maps were within 0.5 diopters of earlier values, discontinuation of SCL wear for 2 weeks and RGPCL wear for 5 weeks was adequate for the cornea to return to its baseline topographic state.

Surgical control of late postkeratoplasty astigmatism with or without the use of computerized video keratography: A prospective, randomized study

Objective To assess the effectiveness of computerized videokeratography (CVK) in refining the surgical design and in improving predictability of surgical correction of postkeratoplasty astigmatism. Design A prospective, controlled, randomized, clinical trial. Participants A total of 31 postkeratoplasty eyes, divided into 2 groups (group A, 16 eyes; group B, 15 eyes), with more than 4 diopters (D) of disabling astigmatism were studied. Intervention All eyes were treated with combination of arcuate relaxing incisions and compression sutures. The surgical plan in group A was based on topographic information, whereas in the control group B, the surgical plan was based on information obtained by refraction and keratometry alone. Main outcome measures Change in the surgical plan induced by the CVK information, astigmatism, topographic patterns, and factors associated with outcome were measured. Results In all 16 cases of group A, the use of CVK changed some aspect of the surgical plan. At 12 months after surgery, both groups showed a significant net reduction ( P = 0.001) of baseline astigmatism. However, the reduction (47% and 41% for groups A and B, respectively) did not differ significantly between the two groups. The topographic astigmatism at 12 months measured 4.24 ± 0.71 D in group A and 5.60 ± 0.51 D in group B ( P = 0.139). Significant differences between the two groups at 12 months were seen only for keratometric astigmatism (3.60 ± 0.81 D in group A vs. 5.77 ± 0.52 D in group B, P = 0.035) and refractive astigmatism (2.34 ± 0.37 D in group A vs. 4.88 ± 0.52 D in group B, P = 0.000). The mean vector surgical effect was 91% for group A and 70% for group B. Regular astigmatism patterns had a greater benefit from surgery than irregular patterns ( P = 0.008). Previous refractive surgery was associated with less-favorable outcome ( P = 0.045). Conclusions The current study indicates that the use of CVK provides a benefit compared to keratometry and refraction alone in the planning and outcome of surgical treatment for high postgraft astigmatism.

Radial and staggered treatment patterns to correct hyperopia using noncontact holmium:YAG laser thermal keratoplasty

Purpose: To compare the effects of two treatment patterns in the correction of hyperopia by noncontact holmium:YAG laser thermal keratoplasty (LTK).Setting: Divisione Oculistica, Ospedale S. Gerardo, Monza, Italy.Methods: Using two treatment patterns, we performed noncontact LTK in one session in 16 eyes of 8 patients with isometropic hyperopic refractive errors; mean preoperative subjective cycloplegic refraction was +4.90 diopters (D) ± 1.17 (SD). The treatment consisted of 24 spots in three concentric rings of eight spots each; ring diameters were 6.0, 7.0, and 8.0 mm, respectively. Each spot received seven pulses of laser energy at 30 mJ/pulse. We treated one eye of each patient with a radial pattern (the spots of the three rings aligned on the eight semimeridians) and the fellow eye with a staggered pattern (the spots of the contiguous rings at 22.5 degrees from each other). Follow-up at 1, 15, 30, 90, 180, and 360 days included subjective cycloplegic refraction, uncorrected (UCVA) and spectaclecorrected visual acuity (SCVA), computerized videokeratography (CVK), and Scheimpflug camera examination.Results: One year postoperatively, the mean subjective cycloplegic refraction was +2.75 ± 1.6 D in the eyes treated with the radial pattern and +3.40 ± 1.6 D in those treated with the staggered pattern; the mean change in subjective cycloplegic refraction was 2.15 and 1.50 D, respectively. Mean UCVA improved by five lines in the radial group and by four lines in the staggered group. Mean SCVA returned to preoperative levels by day 15 in the radial group and at 1 year in the staggered group; at 1 year, SCVA improved by one line in the radial group and remained unchanged in the staggered group. No eye lost one or more lines of SCVA. Refractive astigmatism was essentially unchanged in both groups. Scheimpflug photography and CVK indicated larger and more uniform corrected zones in the radial group.Conclusions: Radial and staggered patterns effectively corrected low hyperopia, although both were subject to a certain amount of regression. The radial pattern produced faster postoperative recovery of SCVA and demonstrated greater refractive stability.

Corneal relaxing incision to aid contact lens fitting for post-penetrating keratoplasty patients

Computerized video keratography has greatly enhanced our understanding of post-penetrating keratoplasty (PK) corneal topography. Tight and loose suture points, irregular areas of the graft-host junction, and asymmetric flat and steep areas can be clearly defined by corneal mapping. Frequently, corneal transplant eyes show irregular, asymmetric, nonorthogonal astigmatism caused by uneven wound healing and suture closing. Corneal relaxing incisions offer a viable alternative to returning the post-PK cornea to a more symmetrical topography. Corneal topographical mapping can provide the appropriate shape information necessary to guide the placement of corneal relaxing incisions on the corneal surface.

Natural history of central topographic islands following excimer laser photorefractive keratectomy

Purpose: To assess the incidence and natural history of central corneal topographic islands following excimer laser photorefractive keratectomy (PRK) and photoastigmatic refractive keratectomy (PARK).Setting: A dedicated corneal diseases and refractive surgery unit within a National Health Service Trust ophthalmology unit in the United Kingdom.Methods: Corneal topographic analysis was performed in a prospective study of 100 eyes of 75 consecutive patients who had PRK. All PRK/PARK excimer laser photorefractive procedures were performed by two surgeons observing a standardized protocol using a VISX 20/20 excimer laser. Mean preoperative myopic error was 5.54 diopters (D) ± 3.44 (SD). Corneal topographic analysis was performed on all eyes preoperatively, 1 week postoperatively, and monthly thereafter for a minimum of 6 months or until central islands, if present, resolved. All patients had a minimum 12 months follow-up.Results: Postoperatively, 29 eyes (29%) demonstrated central corneal topographic islands of greater than 3.00 D topographic power by computerized videokeratography (CVK). All central islands were identified in the first 4 weeks postoperatively. In all cases the differential dioptric power, created by the central islands within the ablation zone, decreased rapidly; within 6 months, 26 (90%) central islands had fully resolved without further treatment, and the remaining 3 (10%) resolved within 1 year of photorefractive surgery. The occurrence of central islands was related to higher preoperative myopic spherical equivalent (P = .01), greater attempted laser correction (P = .01), and greater projected depth of ablation (P = .01) (Student's two-tailed t-test).Conclusions: Central corneal topographic islands occurred in a significantly higher proportion of eyes having excimer laser photorefractive surgery than previously believed. The islands were associated with decreased unaided vision, reduced best spectacle-corrected acuity, and other troublesome visual symptoms; however, the central islands, along with their associated visual effects, usually resolved without surgical intervention within 6 months postoperatively.

Corneal topographic changes after noncontact holmium:YAG laser thermal keratoplasty to correct hyperopia

Purpose: To analyze the changes in corneal curvature induced by laser thermal keratoplasty (LTK) to correct hyperopia.Setting: Cullen Eye Institute, Baylor College of Medicine, Houston, Texas.Methods: We performed LTK on nine hyperopic eyes using a noncontact holmium: YAG (Ho:YAG) laser. Five eyes received a single ring of eight spots at the 6 mm zone (Group A); four received a second ring of eight at the 7 mm zone (Group B). Computerized videokeratography (CVK) was obtained preoperatively and 1, 30, 90, 180, and 360 days postoperatively. We calculated the net dioptric changes in the following CVK values: corneal curvature at the 1, 3, 5, and 7 mm zones; effective corneal refractive power (Eff RP); and spherical equivalent of subjective manifest refraction (SE SMR). We classified difference maps according to the pattern of induced change.Results: At 1 year, steepening at the 1, 3, 5, and 7 mm CVK zones was 0.5 diopter (D), 0.6 D, 0.1 D, and −0.42 D, respectively, in Group A and 1.5 D, 1.5 D, 1.1 D, and 0.54 D, respectively, in Group B. Effective corneal refractive power increased 0.6 D in Group A and 1.5 D in Group B. Mean change in SE SMR was -0.6 D in Group A and −1.4 D in Group B. Most regression occurred during the first 90 days. Difference maps showed five bow-tie, two irregularly irregular, one semicircular, and one homogeneous patterns.Conclusions: Noncontact Ho:YAG LTK produced peripheral corneal flattening and central corneal steepening. A greater change in curvature was produced using a two-ring treatment.

Computerized videokeratography and keratometry in determining intraocular lens calculations

Purpose: To compare the accuracy of standard keratometry and computerized videokeratography (CVK) in determining intraocular lens (IOL) power calculations.Methods: Using the EyeSys Corneal Analysis SystemTM, we prospectively obtained CVK maps on 75 eyes of 69 patients scheduled to have phacoemulsification with implantation of a posterior chamber intraocular lens. Using manifest refraction obtained at 6 weeks postoperatively, we optimized the calculations for the Hoffer Q, Holladay, and SRK/T formulas for standard keratometric and the following six CVK values: average curvatures at the 1 mm, 2 mm, and 3 mm zones, the keratometric equivalent at the 3 mm zone, and the Stiles-Crawford weighted averages over the 3 mm and 6 mm zones. The accuracy of these parameters was determined by calculating the mean absolute error and percentage of patients with accuracy within ≤ 0.5 diopter (D), ≤ 1.0 D, and ≤ 2.0 D.Results: Keratometrically derived data were slightly more accurate than the CVK-derived values. The average difference in mean absolute error between the keratometric and CVK values was 0.13 D for the Hoffer Q formula, 0.11 D for the Holladay, and 0.08 D for the SRK/T.Conclusions: In this population of patients, we found the CVK-derived corneal curvature values to be slightly less accurate than standard keratometry in predicting IOL power. However, CVK provides important corneal curvature data for IOL calculations in patients with abnormal or surgically altered corneal surfaces.

Comparison of the accuracy of computerized videokeratography and keratometry for use in intraocular lens calculations

We compared the accuracy of keratometry and computerized videokeratography (CVK) for use in intraocular lens calculations. We studied 48 eyes of 45 patients having phacoemulsification and posterior chamber lens implantation. Computerized videokeratography was performed with the EyeSys Corneal Analysis System™ (ECAS™). Using the SRK II, SRK/T, and Holladay formulas, we evaluated predictive accuracy calculated with keratometric values and four values derived from ECAS measurements. For each formula, the use of one of the CVK parameters resulted in lower mean absolute errors between actual and predicted postoperative refractive errors and higher percentages of cases with power prediction errors <0.5 and <1.0 diopters. Computerized videokeratography may provide a more accurate corneal curvature value than keratometry for use in intraocular lens calculations.

Computerized videokeratographic analysis of corneal topographic changes induced by sutured and unsutured 4 mm scleral pocket incisions

We used computerized videokeratography to analyze corneal topographical changes in 39 eyes of 39 patients who had phacoemulsification and posterior chamber lens implantation through 4 mm incisions. In one group of patients, the incisions were closed with a single vertical mattress suture; in the other group, no sutures were used. Computerized videokeratography was performed preoperatively and at one and five to six weeks postoperatively; corneal curvatures were assessed at radial distances of 0.75, 1.5, 2.5, and 3.5 mm along the 0-, 90-,180-, and 270-degree semimeridians. At one week postoperatively, there was slightly more steepening along the 90- and 270-degree semimeridians (up to 0.9 diopters [D]) and slightly more flattening along the 0- and 180-degree semimeridians (up to 0.6 D) in the sutured group. At six weeks postoperatively, all mean semimeridional changes were less than 0.5 D. Computerized videokeratography demonstrated that sutured and unsutured 4 mm incisions induced clinically small corneal topographic changes and detected local variations in corneal curvature that may be due to sutures, cautery, and other operative factors.