Comparison of corneal centering in photorefractive keratectomy

Translation model for CW chord to angle Alpha derived from a Monte-Carlo simulation based on raytracing

Background

The Chang-Waring chord is provided by many ophthalmic instruments, but proper interpretation of this chord for use in centring refractive procedures at the cornea is not fully understood. The purpose of this study is to develop a strategy for translating the Chang-Waring chord (position of pupil centre relative to the Purkinje reflex PI) into angle Alpha using raytracing techniques.

Methods

The retrospective analysis was based on a large dataset of 8959 measurements of 8959 eyes from 1 clinical centre, using the Casia2 anterior segment tomographer. An optical model based on: corneal front and back surface radius Ra and Rp, asphericities Qa and Qp, corneal thickness CCT, anterior chamber depth ACD, and pupil centre position (X-Y position: Pup_X and Pup_Y), was defined for each measurement. Using raytracing rays with an incident angle I_X and I_Y the CW chord (CW_X and CW_Y) was calculated. Using these data, a multivariable linear model was built up in terms of a Monte-Carlo simulation for a simple translation of incident ray angle to CW chord.

Results

Raytracing allows for calculation of the CW chord CW_X/CW_Y from biometric measures and the incident ray angle I_X/I_Y. In our dataset mean values of CW_X = 0.32±0.30 mm and CW_Y = -0.10±0.26 mm were derived for a mean incident ray angle (angle Alpha) of I_X = -5.02±1.77° and I_Y = 0.01±1.47°. The raytracing results could be modelled with a linear multivariable model, and the effect sizes for the prediction model for CW_X are identified as Ra, Qa, Rp, CCT, ACD, Pup_X, Pup_Y, I_X, and for CW_Y they are Ra, Rp, Pup_Y, and I_Y.

Conclusion

Today the CW chord can be directly measured with any biometer, topographer or tomographer. If biometric measures of Ra, Qa, Rp, CCT, ACD, Pup_X, Pup_Y are available in addition to the CW chord components CW_X and CW_Y, a prediction of angle Alpha is possible using a simple matrix operation.

Prediction of CW chord as a measure for the eye's orientation axis after cataract surgery from preoperative IOLMaster 700 measurement data

BACKGROUND

The angles alpha and kappa are widely discussed for centring refractive procedures, but they cannot be determined with ophthalmic instruments. The purpose of this study is to investigate the Chang-Waring chord (position of the Purkinje reflex PI relative to the corneal centre) derived from an optical biometer before and after cataract surgery and to study the changes resulting from cataract surgery.

METHODS

The analysis was based on a large dataset of 1587 complete sets of preoperative and postoperative IOMaster 700 biometry measurements from two clinical centres, each containing: valid data for pupil and corneal centre position, the position of the Purkinje reflex PI originated from a coaxial fixation target, keratometry (K), axial length (AL), anterior chamber depth (ACD), lens thickness (LT), central corneal thickness CCT, and horizontal corneal diameter W2W. The Chang-Waring chord CW was derived from pupil centre and Purkinje reflex PI analysed preoperatively and postoperatively, and a multilinear regression model together with a feedforward neural network algorithm was set up to predict postoperative CW chord from preoperative CW chord, K and biometric distances of the eye.

RESULTS

The Y component of CW chord shows a slight shift in the inferior direction in both left and right eyes, before and after cataract surgery. The X component shows some shift in the temporal direction, which is more pronounced preoperatively and slightly reduced postoperatively but with a larger variation. The change in CW chord from preoperative to postoperative shows a slight shift in the superior and nasal directions. Our algorithms for prediction of postoperative CW chord using preoperative CW chord, keratometry and biometry as input data performed with a multilinear regression and a feedforward neural network approach were able to reduce the variance, but could not properly predict the postoperative CW chord X and Y components.

CONCLUSION

The CW chord as the position of the Purkinje reflex PI with respect to the pupil centre can be directly measured with any biometer, topographer or tomographer with a coaxial fixation light. The mean Y component does not differ between right and left eyes or preoperatively and postoperatively, but the mean temporal shift of the X component preoperatively is slightly reduced postoperatively, but with a larger scatter of the values.

Distribution of angle kappa measurements with Orbscan II in a population-based survey

PURPOSE

To determine the mean angle kappa and its determinants in the population of Tehran, Iran.

METHODS

In a cross-sectional survey with random cluster sampling, a total of 442 participants aged >14 years were selected from 4 municipality districts of Tehran for Orbscan acquisitions. Exclusion criteria were history of eye surgery for refractive errors, cataract or glaucoma, and use of topical medication or any type of contact lens at the time of the study. Mean angle kappa in different age and gender groups and its association with other factors was assessed. Considering the high correlation between the right and left eyes, only results of the right eyes are presented.

RESULTS

After applying exclusion criteria, 800 eyes (399 right eyes and 401 left eyes) were examined. Mean participant age was 40.6±16.8 years (range: 14 to 81 years), and 38.8% of eyes were from men. Mean angle kappa was 5.46±1.33° in total; 5.41±1.32° in men and 5.49±1.34° in women (P=.558). It decreased significantly with age; 0.015°/year (P<.001). In individuals with myopia, emmetropia, and hypermetropia, the mean value was 5.13±1.50°, 5.72±1.10°, and 5.52±1.19°, respectively (P=.025); the post-hoc test indicated this was due to the difference between emmetropes and myopes.

CONCLUSIONS

According to our results, angle kappa reduces with age, and the inter-gender difference is not significant. Largest angle kappas were seen among individuals with emmetropia. Angle kappas were larger in the hypermetropic population compared to the myopic population.

Changes in Higher Order Aberrations and Contrast Sensitivity after Implantation of a Phakic Artisan Intraocular Lens

PURPOSE

To evaluate higher-order aberrations (HOAs) and contrast sensitivity (CS), and investigate their relationship, after implantation of a phakic Artisan intraocular lens (IOL) for high myopia.

METHODS

A prospective study including 25 eyes of 15 patients with high myopia (greater than -8.00 D) who underwent phakic Artisan IOL implantation was carried out. Uncorrected visual acuity (UCVA), best corrected visual acuity, manifest refraction, and pupil diameter were measured at baseline and at 1 and 3 months postoperatively. HOAs were measured by Wavescan(R) (VISX Inc., Santa Clara, Calif., USA) from the 3.0 mm pupil entrance, and CS was measured using the VCTS(R) 6500 (Vistech consultants, Inc., Dayton, Ohio, USA) under photopic conditions. The area under the log contrast sensitivity function (AULCSF), defined as the integration of the fitted third order polynomial of the log CS units between the fixed limits of 0.18 (1.5 CPD) and 1.25 (18 CPD) on the log spatial frequency scale, was calculated.

RESULTS

UCVA LogMAR improved from 1.58 at baseline to 0.22 at 3 months, and spherical equivalent refraction was -0.77 +/- 0.34 D at 3 months. The total HOAs had increased slightly at 1 month. CS had decreased significantly at all cycles per degree (p < 0.05) at 1 month, but returned to baseline values at 3 months. Postoperative AULCSF did not correlate with total HOAs under photopic conditions, but correlated with UCVA LogMAR at 1 month (p = 0.037).

CONCLUSIONS

Phakic Artisan IOL implantation for the correction of high myopia resulted in a small increase of HOAs under photopic conditions. At 1 month, CS was decreased, but returned to baseline at 3 months under photopic conditions. Postoperative AULCSF did not correlate with total HOAs at 1 month.