Corneal densitometry and polarization power 20 years after Excimer Laser Photorefractive Keratectomy

Abstract

Purpose : To evaluate in the long-term the corneal densitometry in eyes after photorefractive keratectomy (PRK) and in unoperated eyes and to investigate the de-orientation effect of PRK by observing the cross patterns and polarization power of corneas.

Methods : 90 eyes (57 patients) underwent clinical evaluation 20 years after myopic PRK. The preoperative mean spherical equivalent (MSE) ± standard deviation was -6.82±4.46 D (range: -1.0 to -20.0 D), the mean programmed correction was -6.3±3.53 D (range: -1.0 to -15.0 D).
Anterior, central and posterior corneal optical density was evaluated with the corneal densitometry software on Pentacam in different zones (0-2 mm, 2-6 mm, 6-10 mm, 10-12 mm). The patients were divided in 2 groups. The first group included patients who underwent ablation from 10 to 75 µm, the second group from 76 to 120 µm. The control group included unoperated controlateral eyes (27 eyes) of patients of this study who have undergone PRK in only one eye. Polarizing power and cross patterns were monitored and recorded by the Lumaxis software.

Results : 20 years after myopic PRK, the MSE was -2.59±3.02 D (range: -12.0 D to +1.5 D), with 22% of eyes within 0.5 D and 44% within 1.0 D of intended correction. There was no difference in MSE between 1 and 20 years (P>0.5). The efficacy index was 0.73 and the safety index was 0.99. All corneas were clear 20 years after myopic PRK, with no evidence of ectasia.
Statistically significant differences in corneal densitometry were found in the anterior cornea in the optical zone with 0-2 mm diameter between the first group (29.5±7.73) and the control group (26.9±2.03) (p= 0.020) and between the second group (38.0±9.90) and the control group (26.9±2.03)(p<0.001).
In each study group Lumaxis software showed unchanged polarizing power and cross patterns after PRK.
No difference in polarizing power and cross patterns was found between the operated eyes and the control group.

Conclusions : PRK increases the corneal densitometry that lasts over 20 years but these changes have no influence on the polarizing power and cross patterns after PRK and therefore they have no effect on the visual acuity. The increase in corneal densitometry can explain some variations in corneal power measurement and stiffness that could influence intraocular lens power and intraocular pressure measurements.

This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.