Ocular parameters of Hong Kong-Chinese in relation to RGP contact lens wear

Abstract

The main objective of the current study was to study the initial comfort and corneal changes effected by a custom-made, back polynomial design rigid gas permeable (RGP) lens and a commercially available tricurve RGP lens. In the first part of the current report, the normative values of the ocular dimensions of the cornea of a group of young HK-Chinese were determined. The corneal dimensions investigated included the horizontal visible iris diameter (HVID), the palpebral aperture width and size (PAW and PAS), and the corneal shape (p and r0), which could affect the lens fitting as well as the initial comfort RGP lens wear, and the corneal thickness (CT) and the endothelial cell variables, which could be affected by RGP lens wear. Reliability (in terms of repeatability and reproducibility) of the TOPCON specular microscope SP-2000P and the IMAGEnet system, which were used for determining CT and the endothelial cell variables, was determined prior the main experiment. Initial comfort of a custom-made polynomial RGP lens was compared with that of the tricurve RGP lens monocularly and binocularly. Clinical performance of the two lens designs was also compared after wearing the lenses for six months. The effect of lens wear on the change in vision, contrast sensitivity, the ocular dimensions, corneal integrity, CT, and the endothelial morphology were also reported. RESULTS Based on the intra-class correlation coefficient (ICC), the TOPCON SP-2000P was found to have very good reliability in the determination of the central corneal thickness (CCT) and reasonably good to good reliability in the determination of the peripheral corneal thickness (PCT). The TOPCON SP-2000P and the IMAGEnet system were found to have good reliability in the determination of the endothelial cell density (ECD) and the average cell size (ACS). The reliability was poor to average for the determination of the coefficient of variability in cell size (CV) and the hexagonality, and poorer for the peripheral corneal locations than for the central cornea. The mean values of HVID, PAW and PAS were 11.3+-0.5mm, 22.8+-2.2mm and 9,0+-1.3mm respectively. The mean p and r0 were found to be 0.78+-0.12 and 7.82+-0.26mm respectively along the flattest meridian, and 0.83+-0.15 and 7.64+-0.26mm respectively along the steepest meridian. CT was the thinnest in the central cornea (CCT: 547+-41 um) and thicker in the peripheral cornea. The mean thicknesses at the inferior, temporal superior, nasal superior and the superior cornea were 601+-43um, 613+-42um, 645+-47um, and 647+-43um respectively. ECD of the central cornea was 2923+-240cells/mm2 and was higher in the peripheral cornea (2971-3280cells/mm2). On the other hand, ACS of the central cornea was 344.4+-29.0um2 and was lower in the peripheral cornea (308.7-339.8 um2). The ranges of CV and hexagonality in the five corneal locations were 26.7-31.4% and 63.1-70.0% respectively. CV of the superior cornea was significantly higher while hexagonality of the superior cornea was significantly lower than those of the other four corneal locations. CV and hexagonality of the superior cornea were 31.4% and 63.1% respectively. Negative correlations were found between ECD and ACS, and between CV and hexagonality. Initial comfort provided by polynomial RGP lenses (derived from tricurve lens fits) was not found to be different from those provided by the tricurve RGP lenses. There were also no significant differences found in the clinical performances of the two lens designs after the 6-month study period. CONCLUSIONS Ocular dimensions found in this study were comparable to those reported in previous studies on Chinese eyes. Contrary to our expectation, the polynomial RGP lenses were not found to provide superior comfort compared to the conventional tricurve RGP lenses. The clinical performance of the two lens designs was not significantly different. The results may be due to the fact that the polynomial RGP lens specification was based on the fitting characteristics provided by the tricurve RGP lens to provide a similar tear lens profile (i.e. similar lens fitting) and vision.