Corneal Curvature Values Research Articles

A review of corneal diameter, curvature and thickness values and influencing factors*

The cornea is an important ocular structure involved in the mediation of visual perception. It is the principal refractive surface of the eye and vision can be significantly affected by relatively small changes in its structure and parameters. Measurement of corneal parameters is important in the diagnosis and management of ocular diseasessuch as keratoconus and glaucoma, and also in the fitting of contact lenses or with refractive surgery such as Laser-Assisted in situ Keratomileusis(LASIK) and photorefractive keratectomy (PRK). The human corneal diameter, anterior curvature and centre thickness as well as factors influencing them are reviewed in this article. This review will be useful to eye care professionals who routinely measure these parameters when fitting contact lenses and assessing, diagnosing as well as managing corneal and other ocular conditions. (S Afr Optom 2013 72(4) 185-194)

Theoretical image performance with customized aspheric and spherical IOLs – when do we get a benefit from customized aspheric design?

Background & PurposeImplantation of an artificial intraocular lens (IOL) during cataract surgery significantly changes the balance of aberrations in the eye. We demonstrate the theoretical superiority of customized aspheric IOL designs over standard spherical IOLs for different values of corneal curvature, asphericity and axial length. MethodsFor a selected set of corneal surfaces we calculated a best-fit equiconvex spherical IOL. In a second step we customized the IOL back surface to correct the wavefront aberrations of the cornea. Then we calculated a quadric approximation of the IOL back surface to retrieve the aspheric parameters of the customized lens in terms of radius of curvature and asphericity/conic constant. The optical performance of the three IOL models was monitored in terms of lateral ray spread (LRS) at retinal plane for variations of corneal curvatures, asphericity and axial lengths of the pseudophakic eye. ResultsThe LRS of customized aspheric IOLs was significantly smaller compared to that of spherical IOLs (typically between 10 and 25dB). For high aspheric coefficients the LRS increased with spherical IOLs. With aspheric IOLs LRS was higher for steep corneas paired with high aspheric coefficients, mostly due to the fitting error of the quadratic function. For several combinations of corneal curvature and aspheric coefficients the focal point of the aspheric IOL was up to 400 times smaller than the spherical one. ConclusionThis study appeals to the reader for the potential benefit of customized aspheric IOL design instead of the principle of a ‘one size fits all’ aspheric coefficient as used currently in clinical practice. A benefit with customized IOLs is less depending from the axial length and can be achieved with corneas of a moderate prolate aspheric shape with an equal or more negative Q value than the average of -0.22. Longer eyes seem to benefit less than short eyes.

The study of anterior sclera thickness and associated ocular parameters

Background The condition of the sclera is associated with many ocular diseases.The measurement of human scleral thickness in vivo is helpful for us to understand the features of the sclera and related diseases.Objective The present study was to measure the anterior sclera thickness(AST) in patients with senile cataract and to analyze the relationship among AST and other associated ocular parameters.Methods This study was approved by the Ethic Commission of First Hospital of Peking University.Written informed consent was obtained from each individual prior to examination.One hundred and five senile cataract patients were recruited in this study.Central corneal thickness (CCT),corneal curvature (CCV) and axial length were measured using ultrasonic pachymeter,keratometer,and A-scan unit,respectively.The AST was measured at 2 mm posterior to the scleral spur in the temporal meridian using ultrasound biomicroscope (UBM).The differences of CCT,CCV,ocular axial length and AST between bilateral eyes and the different sexes were compared by the Paired test and independent sample t test.The correlations among various parameters were assessed by the Pearson linear correlation analysis.The differences of CCT and AST among different axial length groups were evaluated by one-way ANOVA.Results No significant differences were found in the CCT,CCV,axial length and AST between bilateral eyes (t =0.584,P =0.561 ; t =1.161,P =0.248 ; t =0.140,P =0.889 ; t =0.342,P =0.773).Temporal AST at 2 mm posterior to the sclera spur was (0.589 ±0.051)mm in the right eyes.An insignificant decline in CCT was found in the male group compared with the female group (right eyes:t =0.469,P =0.641 ; left eyes:t =0.465,P =0.643).However,compared with the female group,the increase of axial length,reduction of the mean CCV value and enhancement of the mean AST were observed(right eyes:all P＜0.01 ;left eyes:all P＜0.01).CCV showed a negative correlation with ocular axial length (r =-0.50,P＜0.01),but no significant correlation was found among age,CCT,ocular axial length and AST(P＞0.05).No remarkable differences were found in CCT and AST among the various axial length groups (CCT:F =0.998,P=0.372;AST:F=1.919,P=0.383).Conclusions In senile cataract patients,correlation is not found between AST and CCT;the increase of axial length is not associated with the thinning of the eyeball wall to a certain extent.Differences exist in some ocular parameters between different sexes. Key words: Bio-measurement; Scleral thickness; Central corneal thickness; Axial length; Ultrasound biomicroscopy

Association of height, body weight, age, and corneal diameter with calculated intraocular lens strength of adult horses

To determine whether differences exist in the calculated intraocular lens (IOL) strengths of a population of adult horses and to assess the association between calculated IOL strength and horse height, body weight, and age, and between calculated IOL strength and corneal diameter. 28 clinically normal adult horses (56 eyes). Axial globe lengths and anterior chamber depths were measured ultrasonographically. Corneal curvatures were determined with a modified photokeratometer and brightness-mode ultrasonographic images. Data were used in the Binkhorst equation to calculate the predicted IOL strength for each eye. The calculated IOL strengths were compared with a repeated-measures ANOVA. Corneal curvature values (photokeratometer vs brightness-mode ultrasonographic images) were compared with a paired t test. Coefficients of determination were used to measure associations. Calculated IOL strengths (range, 15.4 to 30.1 diopters) differed significantly among horses. There was a significant difference in the corneal curvatures as determined via the 2 methods. Weak associations were found between calculated IOL strength and horse height and between calculated IOL strength and vertical corneal diameter. Calculated IOL strength differed significantly among horses. Because only weak associations were detected between calculated IOL strength and horse height and vertical corneal diameter, these factors would not serve as reliable indicators for selection of the IOL strength for a specific horse.

Differences of corneal curvature measured by Pentacam and corneal topography.

Objective To compare the differences of keratometry determined by Pentacam and corneal topography.Methods Preoperative measurement of corneal cuvature was prospectively obtained in 200eyes of 100 patients with ametropia by Pentacam and corneal topography.The ages of the patients were between 17 and 42 years old(average age:22.6 years old).The maximum,minimum and mean values of corneal curvature were recorded.Paired t test was used to analyze the difference of them.Results The maximum values of corneal curvature was(44.18 ± 1.49)D for Pentacam and(44.06 ± 1.35)D for corneal topography(P ＜ 0.01),and the minimum values of corneal curvature was(43.01 ± 1.33)D for Pentacam and(43.17 ± 1.28)D for corneaI topograpby,(P ＜0.01),but the mean values of corneal curvature was (43.58 ± 1.38)D for Pentacam and(43.62 ± 1.30)D for cormeal topography(p ＞ 0.05).Conclusion Both of Pentacam and corneal topography can be used for measuring keratometry,but Pentacam is more accurater. Key words: Corneal cuvature; Pentacam; Corneal topography

Comparison of central corneal thickness, corneal curvature and axial length values in pseudoexfoliation cases with or without cataract

Pseudoexfoliation (PE) is an age-related condition characterized with production and progressively accumulation of extracellular fibrotic material in ocular tissues. In this study, it was aimed to compare central corneal thickness in terms of average keratometric values and axial length in control group and eyes with PE with or without cataracts. Files of the cases who were admitted to Gaziantep University, Medical Faculty, Ophthalmology Clinic between January 2004 and February 2011, and diagnosed as PE were analyzed retrospectively. All patients were performed a detailed ophthalmologic examination including ultrasonic pachtmetry, automatic keratorefractometry and ultrasonic biometry measurements. Study group was consisted of a total 159 cases with PE (115 cases with cataract and 44 cases with no cataract) and 60 healthy subjects (control group). Mean central corneal thickness value of the cases with PE was 537.20±5.24 microns, axial length values was 23.31±1.03 mm, and mean keratometric values was 44.06±0.24 diopter, while these values for control group were 556.23±13.21 micron, 23.12±0.98 mm, and 44.28±0.65 diopter, respectively. When PE cases and controls were compared, the difference was significant (p0.05). Mean central corneal thickness value, axial length, and mean keratometric value of the cases with PE cataract were 541.93±48.29 micron, 23.07±1.32 mm, and 44.59±1.29 diopter, respectively. These values were 540.28±4.34 micron, 23.31±1.03 mm, and 44.06±0.24 diopter for the cases without PE cataract. A significant difference was not detected between the cases with and without cataract in terms of average central corneal length, axial length, and keratometric values (p>0.05). Cornea was seen to be thinner in cases with pseudoexfoliation compared to control group. Central corneal thickness measured in these subjects was not detected to be different in terms of average keratometric values and axial length in eyes with or without cataract.

Biometry of the Cornea in Myopic Chinese Patients

To develop a close-fit mathematical model of corneal anterior and posterior topography and investigate the variation of corneal topography with biometric parameters in a group of young adult myopic Chinese patients. Corneal topography data were acquired from 112 eyes of 61 myopic patients (mean age: 24.10±3.66 years). Videokeratoscopic images were recorded for anterior and posterior corneal surfaces using an Orbscan IIz topography system (Bausch & Lomb), and elevation data were fit to ellipsoid, hyperboloid, and paraboloid equations. Axial length, anterior chamber depth, refractive error, corneal curvature, central corneal thickness, and white-to-white distance were also measured and statistically analyzed. The ellipsoid equation provided the best fit for the anterior and posterior corneal surfaces in all patients. Corneal asymmetry slightly improved the accuracy of the mathematical model and reduced errors of fit with the clinical data. Strong evidence of right and left corneal symmetry was obtained in the form of similar corneal curvature, shape factor, and thickness values. The refractive error showed strong correlation with axial length (r=-0.586, P<.001) but not with corneal curvature or anterior chamber depth, indicating that myopia may be related to scleral stretching and globe elongation rather than changes in corneal curvature or anterior chamber depth. Axial length appears to be the main morphological parameter related to myopia. Myopic corneas took the form of a rotationally asymmetric prolate ellipsoid that gradually flattened towards the periphery. The study also provided evidence of symmetry between an individual's right and left corneas.

Ultra B2 - Promoção de ligações covalentes do colágeno corneal (Corneal cross-linking) no tratamento de ceratocone: resultados preliminares

To present early visual and keratometric results for corneal cross-linking with riboflavin and UV irradiation in patients with progressive keratoconus. Twenty-five eyes of twenty patients (15 males and 5 females) with a progressive keratoconus in the previous 6 months were followed. Unaided visual acuity (UVA), best spectacle corrected visual acuity (BSCVA), spherical equivalent (SEQ), manifest cylinder, and maximal corneal curvature (max K) values were followed at 1, 3 and 6 months. All patients were submitted to corneal cross-linking using riboflavin (vitamin B2) as the photosensitizer and ultraviolet light (UV, wavelength 370 nm). Epithelium was removed with 20% alcohol, cornea was soaked with vitamin B2 for 15 min, and then irradiated with UV light for 30 min, after which a bandage contact lens (BCL) was placed. UVA increased after one month (from 0.15 +/- 0.15 to 0.23 +/- 0.20), and went on increasing at 3 and 6 months, reaching statistical significance (p=0.025 e p=0.037, respectively). BSCVA increased from 0.41 +/- 0.27 to 0.49 +/- 0.29 at month six, without reaching statistical significance at any time point. Progression of keratoconus stopped in all patients, in contrast with progression in all of them in the six-month period prior to the surgery. Max K decreased by more than 2 D (from 53.02 +/- 8.42 to 50.88 +/- 6.05 D), SEQ less that 1 D (from -3.27 +/- 4.08 to -2.68 +/- 3.02 D), while refractive cylinder decreased less than 0.5 D (from -2.29 +/- 1.77 to -1.86 +/- 0.92 D), without reaching a statistically significant difference. None of the eyes lost any line of BSCVA, 12 maintained the preoperative BSCVA, 7 gained one line, 5 gained two lines, and 1 patient gained three lines of BSCVA. Corneal cross-linking with riboflavin and UV light seems to be a safe (no loss of BSCVA) and effective (anatomical and optical properties maintained) procedure, which has shown to stop the progression of the keratoconus: a reduction, although not statistically significant, of the corneal curvature, spherical equivalent and refractive cylinder took place in patients where previous progression of keratoconus had been described.

Corneal Curvature and Axial Length Values in Children with Congenital/Infantile Cataract in the First 42 Months of Life

To evaluate corneal curvature (K(m)) and axial length (AL) of eyes of term-born children aged <3.5 years with uncomplicated congenital cataract and provide age-specific estimates. This was a retrospective review of patients undergoing cataract extraction from January 1994 to December 2006. Exclusion criteria were preterm birth, microphthalmia, microcornea, megalocornea, glaucoma, traumatic or complicated cataract, retinal disease. Keratometric readings were taken with an autokeratometer before surgery. Linear regression with the logarithm of the patient's age as an independent variable was used to model the relationship between the patient's age and biometric and keratometric readings. All unilateral cataractous eyes (n = 69) and a randomly selected eye in bilateral cases (n = 111) were included in the analysis, for a total of 180 eyes. Mean age was 15.5 +/- 11.8 months; mean AL, 20.03 +/- 2.25 mm; mean K(m), 45.07 +/- 3.00 D. K(m) was significantly greater and AL shorter in younger children (P < 0.001). No differences according to sex were found. As a group, eyes from unilateral cataract had significantly longer AL than those from bilateral cases (P = 0.029). In a small subgroup of unilateral cataract patients for which readings from the clear lens eye were available (n = 39), K(m) of the affected eye was significantly greater than that of the fellow healthy eye (P = 0.007). In the first 42 months of age, K(m) and AL are significantly different according to age. These findings have implications for the calculation of intraocular lens power in children.

Intraocular lens power calculations after refractive surgery: Consensus-K technique

To describe a new strategy for intraocular lens (IOL) power calculations after laser in situ keratomileusis (LASIK), determine the accuracy of the method, and compare the results with those of previously described techniques. Emory Eye Center, Atlanta, Georgia, USA. This retrospective comparative series compared eyes having cataract extraction after laser in situ keratomileusis (LASIK) and eyes having cataract extraction without previous surgery (control group) from January 1997 to December 2005. In the LASIK group, 2 strategies were used to determine the appropriate corneal curvature (K) value for IOL calculation: (1) the nonconsensus method and (2) the consensus-K technique. Postoperative outcomes were compared and included refraction, deviation from target refraction, and deviation from the ideal K or ideal IOL value for several techniques. There were 43 LASIK eyes (14 nonconsensus and 29 consensus K) and 50 control eyes. The mean absolute deviation from target refraction in the nonconsensus group (1.47 diopters [D]) was significantly higher than that in the consensus-K group (0.52 D) (P = .02) or control group (0.44 D) (P = .01); the mean was not statistically different between the consensus-K group and the control group (P = .5). Compared with values with the consensus-K technique, the absolute deviation from back-calculated K values was significantly higher (P<.05) with all other K-generating methods tested except the classic refractive history method (0.56 D versus 0.65 D) (P = .4). When compared with the IOL value generated using the consensus-K technique, the absolute deviation from back-calculated IOL values was significantly higher for all other methods (P<.05). The consensus-K technique generated refractive outcomes similar to those in the control group and was better than with all other K- or IOL-generating techniques except the classic refractive history method. The consensus method showed less variability and higher predictability than all other methods tested.

Long-term follow-up of ultrathin corneas after surface retreatment with phototherapeutic keratectomy

To evaluate visual acuity and long-term stability after phototherapeutic keratectomy (PTK) in patients with corneal thickness less than 400 microm and cornea-related vision problems. Department of Ophthalmology, Istituto Clinico Humanitas, Rozzano-Milan, Italy. This retrospective study comprised 48 eyes that had PTK for complications after photorefractive keratectomy that removed more than 200 microm of tissue. Phototherapeutic keratectomy was performed with the Nidek EC-5000 excimer laser with 10 Hz frequency, intraoperative topography, and masking fluid over an ablation diameter of 10.0 mm. The follow-up was 5 years. The mean preoperative best spectacle-corrected visual acuity (BSCVA) was 0.2 +/- 0.09 (SD) with -2.53 +/- 2.34 diopters (D), and the mean pachymetry was 390 +/- 38 microm. The efficacy parameters were pachymetry and visual acuity at 1 year and the stability parameters, optical refraction, corneal curvature (calculated as the mean curvature over the entire 3.0 mm and 5.0 mm central zones), and pachymetry from 1 to 5 years. At 1 year, the mean BSCVA was 0.6 +/- 0.72 with -2.15 +/- 1.67 D and the mean pachymetry, 341 +/- 40 microm. At 5 years, the mean BSCVA was 0.7 +/- 0.15 with -2.33 +/- 1.12 D and the mean pachymetry, 339 +/- 48 microm. In all patients, there was an improvement of 4 or more Snellen lines. Statistical evaluation of refraction and corneal curvature values at 1 and 5 years indicated no statistically significant differences. After PTK in eyes with ultrathin corneas, there was significant improvement in BSCVA and long-term stability of the optical refraction, corneal curvature, and pachymetry. In selected cases, 10.0 mm ablation zone PTK may be an alternative to penetrating keratoplasty, offering long-term corneal stability.

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.

Central corneal thickness in European (white) individuals, especially children and the elderly, and assessment of its possible importance in clinical measures of intra-ocular pressure.

To ascertain the impact of central corneal thickness (CCT, as assessed by pachometry) and central corneal curvature (as assessed by keratometry) on clinical measures of intra-ocular pressure (IOP, as assessed by tonometry), especially in the young and elderly. Pachometry, keratometry and tonometry were carried out on three groups, namely children aged 5-15 years, adults aged 32-60 years, and elderly individuals aged between 61 and 82 years. For children, ultrasound pachometry was combined with non-contact tonometry (NCT), specular microscopy was used with Perkins tonometry in the adults, and ultrasound pachometry was used with Perkins tonometry for the elderly. Central corneal curvature was assessed by keratometry. The average CCT in children was 0.529+/-0.034 mm (n = 104, +/-S.D.), averaged 0.533+/-0.033 in adults (n=75) and 0.527+/-0.034 mm (n=91) in the elderly. Tonometry values averaged 16.7+/-2.9 mm in children, 13.0+/-3.5 mmHg in adults and 13.6+/-2.5 mm in the elderly group. Central corneal thickness values were not predictably different in relation to central corneal curvature values. Regression analyses indicated that the tonometry values were higher in both children and the elderly who had thicker corneas (and vice versa) (p < 0.003), with the measures increasing by 1.3+/-0.4 and 2.6+/-0.4 mmHg for a 10% difference in CCT in children and the elderly, respectively. For adults, no statistically significant difference in tonometry values could be demonstrated with respect to CCT (<1 mmHg for a 10% difference in CCT), and for no group were the CCT or tonometry values predictably different in relation to central comeal curvature values. The results of these studies, albeit relatively small scale, indicate that in young or elderly individuals with essentially normal IOP and CCT measures, the tonometry values show only small differences with respect to CCT. The slope in the observed relationship was not that different from an average of 1.5 mmHg for a 10% different in CCT, as obtained from a literature analysis over a 30-year period. The magnitude of the effect does not provide evidence that pachometry needs to be routinely performed in glaucoma screening protocols based on tonometry. Notwithstanding, the finding of higher than expected tonometry values should be further investigated, by pachometry, especially in very young children and in the elderly.

Comparison of changes in manifest refraction and corneal power after photorefractive keratectomy

PURPOSE: To determine which corneal curvature values most closely correlate to change in manifest refraction after excimer laser photorefractive keratectomy. METHODS: In a prospective study at the Cullen Eye Institute, excimer laser photorefractive keratectomy was performed on 27 eyes of 27 patients (mean age, 38.07 ± 6.65 years). Preoperative refractive errors ranged from −2.25 diopters to −8.75 diopters (mean, −5.74 ± 2.09 diopters). Preoperatively and 1 month postoperatively, we determined the spherical equivalent of the subjective manifest refraction (corrected for a 12-mm vertex distance) and measured corneal power using standard keratometry (Bausch & Lomb Keratometer; Rochester, New York) and computerized videokeratography (EyeSys Corneal Analysis System; Premier Laser Systems Inc, Houston, Texas). We collected 15 corneal values: standard keratometry and 14 computerized videokeratography values calculated using the axial, instantaneous, and refractive formulas. All calculations were performed with 1.3375 and 1.376 for the refractive index of the cornea. For each of the corneal values, we subtracted the change in corneal power from the change in manifest refraction and calculated for this difference the means, SDs, correlations, and regressions. RESULTS: Mean differences between change in refraction and change in corneal power were lower when for a refractive index of 1.376 than for 1.3375, were lowest for the most central measurement points, and displayed a high SD. A value of 1.408 for the refractive index would be required to optimize the correlation between change in manifest refraction and effective refractive power of the central 3 mm of the cornea. CONCLUSIONS: For individual patients who have undergone photorefractive keratectomy, changes in corneal values determined by computerized videokeratography or by standard keratometry do not reliably predict change in manifest refraction.

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.

Calculation of Intraocular Lens Power After Radial Keratotomy With Computerized Videokeratography

Because standard methods to determine intraocular lens power are not adequate in eyes that have had radial keratotomy, we undertook this study to evaluate the corneal power derived from computerized videokeratography for use in intraocular lens power calculations. We examined four eyes of three patients who had radial keratotomy and who underwent phacoemulsification cataract surgery with implantation of a posterior chamber intraocular lens. We used a computerized videokeratography-derived corneal curvature value in the Holladay formula for intraocular lens calculations. We determined the ideal intraocular lens power and the keratometric value that would have led to the ideal intraocular lens power from the postoperative refraction at 6.1 +/- 1.1 months after cataract extraction. The ideal keratometric value was compared with the keratometric values derived from computerized videokeratography, standard keratometry, contact lens overrefraction, and refractions before and after radial keratotomy. The postoperative refraction at approximately six months averaged -0.32 +/- 0.63 diopter (range, -0.88 to +0.75 diopter) different than the aim. The mean power in ring 3, which was the closest keratometric value to the ideal, disclosed only 0.09 +/- 0.73 diopter and -0.10 +/- 0.72 diopter of deviation from the ideal keratometric and intraocular lens powers, respectively. One to two weeks after phacoemulsification cataract surgery with implantation of a posterior chamber intraocular lens, the videokeratographic differential map disclosed steepening at the wound site with variable regression by six months in all patients. Results suggest that, after radial keratotomy, using the keratometric value derived from computerized videokeratography in intraocular lens calculations is more accurate than using keratometric values measured by routine methods.

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.