Influence of the Reference Center on the Calculation of Corneal Higher Order Aberrations

PURPOSE

To evaluate the differences in corneal wavefront measurements in myopic and hyperopic eyes when calculated using three different reference centers and explore possible influencing factors for such differences.

METHODS

Corneal wavefront measurements were performed in myopic and hyperopic eyes using a GALILEI Placido Dual Scheimpflug Analyzer (Ziemer Ophthalmic Systems AG). Corneal higher order aberrations (HOAs), including total, vertical, and horizontal coma-like aberrations, spherical aberrations, and total corneal HOAs through a 6-mm pupil size, were calculated over three different reference center positions: pupil center, corneal vertex, and limbus to limbus. Values were then compared between the myopic and hyper-opic eyes, and correlations with kappa distance and spherical equivalent were tested.

RESULTS

A significant decrease in the level of total corneal HOAs (-0.04 ± 0.05 and -0.12 ± 0.09), total corneal coma (-0.07 ± 0.09 and -0.18 ± 0.12), and horizontal coma (-0.07 ± 0.11 and -0.22 ± 0.11) in myopic and hyperopic eyes, respectively, was found when recalculating from pupil center to corneal vertex centration, whereas a significant increase in the same aberrations was observed from pupil center or corneal vertex to limbus to limbus. Significant correlations were found between the kappa distance and changes in total corneal HOAs, total corneal coma, and horizontal coma from the pupil center to the corneal vertex in both groups.

CONCLUSIONS

Corneal vertex centration generated the lowest level of corneal wavefront error in both groups. The differences in corneal aberrations between the reference centers for calculation were highly correlated with the kappa distance in hyperopic eyes. [J Refract Surg. 2023;39(5):340-346.].

Accuracy of the Preoperative Predicted Percentage of Tissue Altered Calculation in Refractive Surgery Planning for Myopic LASIK

PURPOSE

To determine the reliability of the percentage of tissue altered (PTA) calculation as part of the planning strategy for myopic laser in situ keratomileusis (LASIK) by comparing the estimated PTA provided by preoperative calculation to the postoperative PTA actually achieved in microkeratome-assisted myopic LASIK.

METHODS

This retrospective study included 3,624 eyes of 3,624 patients who underwent mechanical microkeratome-assisted LASIK surgery for myopic correction. The calculated preoperative PTA values based on the planned flap thickness and ablation depth were compared with the actual achieved postoperative PTA using the difference of corneal central thickness postoperatively for assessing the achieved ablation depth and the intraoperative ultrasound-assisted flap thickness measurement. Regression analysis was performed to reveal preoperative parameters that might influence PTA calculation accuracy.

RESULTS

The mean difference between the estimated and achieved PTA was 0.451 ± 3.45% (P < .001) (95% CI: 0.3708 to 0.5322) with a preoperative and postoperative mean PTA of 31.07 ± 4.07% and 31.52 ± 5.78%, respectively. The differences between the achieved and planned maximum ablation depth and flap thickness were 4.32 ± 13.70 µm (P < .001) and -1.61 ± 13.66 µm (P < .001), respectively.

CONCLUSIONS

Although a statistically significant difference was found between the preoperative calculated PTA and actually achieved PTA, the difference in PTA value (less than 1%) was clinically non-significant and indicated a highly reliable metric for preoperative refractive surgery planning. [J Refract Surg. 2022;38(7):422-427.].

Influence of Reading on Smartphone Screens on Visual Optical Quality Metrics and Tear Film Stability

PURPOSE

To evaluate the impact of a prolonged reading session on a smartphone screen on optical quality metrics and tear film stability.

METHODS

This prospective study was conducted in 41 healthy volunteers who were asked to read an article on a smartphone screen for 20 minutes. The following tests were performed before the reading task in this consecutive order and repeated after the reading task in the same order: automated noninvasive tear break-up time, optical quality assessment including Objective Scatter Index (OSI), modulation transfer function, Strehl ratio, and tear film dynamic analysis as follows: vision break-up time (VBUT) as a function of OSI changes within 20 seconds, using a double-pass aberrometer imaging system, and fluorescein tear break-up time (FBUT) measured using the slit lamp.

RESULTS

All break-up time-related parameters (noninvasive tear break-up time, FBUT and VBUT) were significantly reduced after the reading task (P < 0.01). The OSI was significantly worsened after the reading task (P = 0.01), whereas all the other optical quality metrics (modulation transfer function and Strehl ratio) slightly deteriorated, were not statistically significant. A significant correlation was found between the shortening of the FBUT, VBUT, and the worsening of the OSI (r = -0.33, P < 0.05).

CONCLUSIONS

A reading session on a smartphone screen in healthy subjects was found to significantly affect the tear film stability and objectively worsen the retinal image quality.

Optical changes and apparent emmetropization in a patient with a peripheral unilateral lens coloboma

Lens coloboma is a developmental defect resulting from abnormalities of the zonules and ciliary body. It may present as an isolated pathology or be accompanied by anomalies in different ocular structures. We report the case of a 20-year-old man referred for evaluation of anisometropic amblyopia in the right eye. Manifest refraction was -2.25 +3.00 ×35 in the right eye; corrected distance visual acuity, 20/50. Corneal topography revealed regular astigmatism of +2.46 D at 124°, and wavefront aberrometry revealed an irregular internal astigmatism of +6.27 D at 35°. Only after full pupillary dilation was a peripheral lens coloboma observed. This case demonstrates that even minor distortions of clear and normally positioned lenses may lead to amblyopia and raises the possibility that corneal changes may occur developmentally through the process of emmetropization partly to compensate for lenticular astigmatism arising from the coloboma.