Corneal Power Estimation Using Orbscan II Videokeratography in Eyes With Previous Corneal Refractive Surgeries

PURPOSE: To report three cases of corneal power estimation for intraocular lens power calculation using Orbscan II videokeratography in eyes with previous corneal refractive surgeries. CASE SUMMARY: In three eyes of three patients with previous corneal refractive surgeries, corneal power values were respectively measured at three, four, five, six mm-diameter zones of total mean, axial, tangential, and optical maps using Orbscan II videokeratography. Then, intraocular lens power values were calculated via the SRK/T formula. After cataract surgeries, back-calculated corneal power (BCK) values were estimated from post-phacoemulsification refraction data, and compared with those measured at three, four, five, six mm-diameter zones of each map in Orbscan II videokeratography. The postoperative refractive values after cataract surgeries were achieved within 1.5D of the target refraction in all eyes by using five mm total axial power and four mm total optical power for intraocular lens power calculation. Orbscan II parameters including three mm, four mm total axial power, and three mm total optical power were the least different from the BCK (0.69+/-0.49D, 1.08+/-0.54D, and 1.10+/-0.44D, respectively). CONCLUSIONS: If historical data are not available, Orbscan II videokeratography can be useful for estimating corneal power for intraocular lens power calculations in patients with previous corneal refractive surgeries.

Intraocular Lens Power Calculation for Cataract Surgery in Eyes with Previous Radial Keratotomy

PURPOSE: To compare the accuracy of the intraocular lens (IOL) power calculation of clinical history method, with contact lens overrefraction method or computerized videokeratography method in the eyes with previous radial keratotomy (RK) METHODS: The medical records of 2 patients (3 eyes) who had previous RK, and recent phacoemulsification with posterior chamber lens implantation were retrospectively reviewed. All surgical procedures were performed by one surgeon. The power of implanted IOL was calculated by using clinical history method and SRK/T formula. Keratometric (K) value was measured with contact lens overrefraction and computerized videokeratography method before cataract extraction. Ideal keratometric value was calculated from the final postoperative spherical equivalent and the power of implanted IOL retrospectively, and then compared to K value of each method. RESULTS: Mean differences between the ideal K value and K value was 1.48 diopters in computerized videokeratography method, 2.54 diopters in clinical history method, and 3.65 diopters in contact lens overrefraction method, respectively. CONCLUSIONS: Unintentional hyperopia can be decreased by intentional postoperative myopia and obtaining K value by the computerized videokeratography method in cataract surgery of the eyes with previous radial keratotomy.

Keratometry and Computerized Videokeratography in Determining Intraocular Lens Calculation

The most important factor in intraocular lens power calculation is the axial length, followed by the corneal power measurement. In measuring corneal power, conventionally used keratometry evaluates only four indivisual points on the central 3mm of the corneal surface. But coinputerized videokeratography (CVK) is a new technology which maps inside and outside of the central 3mm zone using twenty or seventy hundred data maps. We studied 27 eyes of 26 patients having phacoemulsification and posterior chamber lens implantation. We calculated corneal power using keratometry and CVK. Using the SRK II and Holladay, we obtained intraocular lens power with keratometric value and five values derived. from CVK. 6 weeks later, we evaluated actual postoperative refractive errors and predictive values for each pararneters. Keratometric datas were lower than CVK datas in mean absolute error and standard deviation between actual and predicted postoperative refractive errors and were higher in percentages of cases with power prediction errors < OR =l.0, < OR =2.0 and < OR =3.0 diopters for each formula. And the average keratornetric datas were 0.25mm flatter than CVK. Conventional keratometry may provide more accurate corneal curvature value than the CVK in intraocular lens power calculation.

The Corneal Topographic Changes in Sutureless Cataract Surgery

In order to compare the visual outcome and amount of surgically induced outcome between 3.1mm clear corneal incision group and 6.0mm scleral tunnel incision group in sutureless cataract surgery, 55 eyes(44 patients) were evaluated retrospectively. Clear corneal incision group and scleral tunnel incision group was consisted of 35 eyes and 20 eyes respectively and all patients were followed up by 2months postoperatively. Twenty two eyes(65%) from clear corneal incision group and 14 eyes(70%) from scleral tunnel incision group showed the uncorrected visual acuity of 20/40 or better at postoperative first day. As measurement of surgically induced astigmatism corneal topography using CVK(computerized videokeratography) was applied. The corneal flattening was significantly more prominent at the distances of 0.75, 1.5, and 2.5mm along the 90-degree meridian at one week postoperatively in clear corneal incision group than scleral tunnel incision(p0.05). The change of surgically induced astigmatism between one day and eight weeks was within the range of one diopter in both groups. There was moderate corneal edema along the incisional meridian at one week and recovered to preoperative levet at four weeks postoperatively in clear corneal incisional group. Conclusively, clear corneal incision in catarad surgery showed early visual rehabilitation and small surgical astigmatism such like scleral tunnel incision.

Classification of Corneal Topography and Analysis of Astigmatism based on Computer-Assisted Videokeratography

We evaluated the corneal topography of 109 eyes which have 1 diopter or more astimatism, with computer-assisted videokeratography(Eye Sys CAS). There was 11% with round pattern, 17.4% with oval pattern, 49.5% with symmetric bow-tie pattern, 19.3% with assymmetric bow-tie pattern, 2.8% with irregular pattern. 70% of subjects had right and left eyes classified into the same group. There was statistically significant difference among the patterns for keratometric astigmatism and difference among the groups which were classified by the location of the area of highest refractive power for keratometric and total astigmatism. The mean angular separation between flat and steep meridian was 89.2 degrees and ranged from 78.61 degrees to 100.67 degrees. The mean residual astigmatism was 0.547 diopter. In the with-the rule astigmatism type, when the total astigmatism less than 1.89 diopter, residual astigmatism neutralize the corneal astigmatism, but when the total astigmatism increase more than 1.89 diopter, residual astigmatism tend to increase the total astigmatism.