Antonio Calossi

Corneal asphericity and spherical aberration.

PURPOSE To summarize the various values of asphericity in different notations and present how corneal asphericity, corneal curvature, and entrance pupil diameter influence the longitudinal spherical aberration of the anterior corneal surface. METHODS After the conversion factors between the different asphericity notations were described, finite ray tracing through a conic section that models the anterior cornea profile was performed. The anterior cornea was given a range of curvatures and asphericities and a range of entrance pupil diameters. RESULTS If the value of asphericity remains constant, longitudinal spherical aberration increases with the square of the entrance pupil diameter. If the pupil diameter remains fixed, the spherical aberration becomes a function of the value of asphericity, the refractive index, and the radius of curvature. If the refractive index, pupil diameter and asphericity are considered constant, the spherical aberration will decrease if the corneal surface flattens and increase as the cornea becomes steeper. In this way, with the same shape factor and with the same starting apical radius, longitudinal spherical aberration became a function of the surgically induced refractive change. With equal curvature, the longitudinal spherical aberration becomes negative if the surface is more prolate than perfect Cartesian oval; it will become positive if it is less prolate, spherical, or oblate. CONCLUSIONS A conversion chart for corneal asphericity notations with the corresponding spherical aberration and a diagram reporting values of asphericity necessary to maintain the physiological value of the corneal spherical aberration after refractive procedures may be useful tools in corneal surgery.

A new formula for intraocular lens power calculation after refractive corneal surgery.

PURPOSE When calculating the power of an intraocular lens (IOL) with conventional methods in eyes that have previously undergone refractive surgery, in most cases the power is inaccurate. To minimize these errors, a new IOL power calculation formula was developed. METHODS A theoretical formula empirically adjusted two variables: 1) the corneal power and 2) the anterior chamber depth (ACD). From the average curvature of the entrance pupil area, weighted according to the Stiles-Crawford effect, the corneal power is calculated by using a relative keratometric index that is a function of the actual corneal curvature, type of keratorefractive surgery, and induced refractive change. Anterior chamber depth is a function of the preoperative ACD, lens thickness, axial length, and the ACD constant. We used our formula in 20 eyes that previously underwent refractive surgery (photorefractive keratectomy [n = 6], laser subepithelial keratomileusis [n = 3], laser in situ keratomileusis [n = 6], and radial keratotomy [n = 5]) and compared our results to other formulas. RESULTS Mean postoperative spherical equivalent refraction was +0.26 diopters (D) (standard deviation [SD] 0.73, range: -1.25 to +/- 1.58 D) using our formula, +2.76 D (SD 1.03, range: +0.94 to +4.47 D) using the SRK II, +1.44 D (SD 0.97, range: +0.05 to +4.01 D) with Binkhorst, 1.83 D (SD 1.00, range: -0.26 to +4.21 D) with Holladay I, and -2.04 D (SD 2.19, range: -7.29 to +1.62 D) with Rosas method. With our formula, 60% of absolute refractive prediction errors were within 0.50 D, 80% within 1.00 D, and 93% within 1.50 D. CONCLUSIONS In this first series of patients, we obtained encouraging results. With a greater number of cases, all statistical adjustments related to the different types of surgery should be improved.

Efficacy and Acceptability of Orthokeratology for Slowing Myopic Progression in Children: A Systematic Review and Meta-Analysis

Background. To evaluate the efficacy and acceptability of orthokeratology for slowing myopic progression in children with a well conducted evidence-based analysis. Design. Meta-analysis. Participants. Children from previously reported comparative studies were treated by orthokeratology versus control. Methods. A systematic literature retrieval was conducted in MEDLINE, EMBASE, Cochrane Library, World Health Organization International Clinical Trials Registry Platform, and ClinicalTrials.gov. The included studies were subjected to meta-analysis using Stata version 10.1. Main Outcome Measures. Axial length change (efficacy) and dropout rates (acceptability) during 2-year follow-up. Results. Eight studies involving 769 subjects were included. At 2-year follow-up, a statistically significant difference was observed in axial length change between the orthokeratology and control groups, with a weighted mean difference (WMD) of −0.25 mm (95% CI, −0.30 to −0.21). The pooled myopic control rate declined with time, with 55, 51, 51, and 41% obtained after 6, 12, 18, and 24 months of treatment, respectively. No statistically significant difference was obtained for dropout rates between the orthokeratology and control groups at 2-year follow-up (OR, 0.79; 95% CI, 0.52 to 1.22). Conclusions. Orthokeratology is effective and acceptable for slowing myopic progression in children with careful education and monitoring.

Corneal ray tracing versus simulated keratometry for estimating corneal power changes after excimer laser surgery.

Purpose To evaluate whether the refractive changes induced by excimer laser surgery can be accurately measured by corneal ray tracing performed by a combined rotating Scheimpflug camera–Placido‐disk corneal topographer (Sirius). Setting Private practices. Design Evaluation of diagnostic test. Methods This multicenter retrospective study comprised patients who had myopic or hyperopic excimer laser refractive surgery. Preoperatively and postoperatively, 2 corneal power measurements—simulated keratometry (K) and mean pupil power—were obtained. The mean pupil power was the corneal power calculated over the entrance pupil by ray tracing through the anterior and posterior corneal surfaces using Snell’s law. Agreement between the refractive and corneal power change was analyzed according to Bland and Altman. Regression analysis and Bland‐Altman plots were used to evaluate agreement between measurements. Results The study evaluated 72 eyes (54 patients). The difference between the postoperative and preoperative simulated K values underestimated the refractive change after myopic correction and overestimated it after hyperopic correction. Agreement between simulated K changes and refractive changes was poor, especially for higher amounts of correction. A proportional bias was detected (r = −0.77; P<.0001), and the 95% limits of agreement (LoA) were −0.15 −0.14 × ±0.62 diopters (D). The difference between the postoperative and preoperative mean pupil power showed an excellent correlation with the refractive change (r2 = 0.98). The mean pupil power did not overestimate or underestimate the refractive change. The 95% LoA ranged between −0.97 D and +0.56 D. Conclusion Corneal ray tracing accurately measured corneal power changes after excimer laser refractive surgery. Financial Disclosures Dr. Calossi is consultant to Costruzione Strumenti Oftalmici. Dr. Carones is consultant to Wavelight Laser Technologie AG. No other author has a financial or proprietary interest in any material or method mentioned.

Agreement and reliability in measuring central corneal thickness with a rotating Scheimpflug-Placido system and ultrasound pachymetry.

PURPOSE We compare the agreement and the reliability in measuring central corneal thickness (CCT) using two different technologies. METHOD The right eyes of 35 healthy individuals who had a negative history of ophthalmic disease, or ocular surgery were examined. The CCT was determined sequentially with a rotating Scheimpflug camera (Sirius; CSO), and an ultrasound pachymeter (P-1; Takagi). For statistical analysis, we used the methods suggested by Bland and Altman. RESULTS The mean values of CCT obtained from Sirius, and ultrasound were 537±28μm, and 550±35μm, respectively. There was a high correlation between Sirius and ultrasound (r=0.92; p<0.001), but the difference between the two measurements was statistically significant (t=-5.7; p<0.00001). The precision of Sirius and ultrasound were 9.4 and 15.9μm; repeatability 13.3 and 22.4μm, and coefficient of variation 0.9% and 1.5%, respectively. The intraclass correlation coefficient was 0.97 for Sirius and 0.95 for ultrasound. CONCLUSIONS The average difference between corneal thickness measured with Sirius and ultrasound pachymetry was small but clinically significant. This means that the two instruments cannot be used interchangeably. Sirius showed precision and repeatability almost twice as much as ultrasound pachymetry. Confidence interval of 13.3μm for Sirius can show variations in corneal thickness with an uncertainty value lower than 2.5% in 95% of cases. The simplicity of use, the possibility to obtain pachymetric maps, and less invasiveness make this instrument potentially useful in contact lens practice.