The Effect of Two-Zone Concentric Bifocal Spectacle Lenses on Refractive Error Development and Eye Growth in Young Chicks

Abstract

Purpose.: To characterize the effects on refractive error development and eye growth in young chicks of two-zone concentric lens designs, which differentially affect the defocus experiences of central and peripheral retinal regions. Methods.: Monocular defocusing lenses were worn for 5 days from 17 days of age. Four two-zone concentric lens designs (overall optical zone diameter, 10 mm) combining plano with either −5- or +5-D power were used. Lens designs were as follows: (1) +5 D center (+5C), (2) +5 D periphery (+5P), (3) −5 D center (−5C), and (4) −5 D peripheral (−5P), with plano in periphery for all C-designs and in the center for P-designs. Five central zone diameters (CZDs) were tested, ranging from 2.5 to 6.5 mm in 1-mm increments. Plano, +5- and −5-D single-vision (SV) lenses were used as the control. A minimum of six birds were included in each lens group. Results.: For the two-zone lenses, the P designs (i.e., peripheral defocus) had greater effects than the C designs (i.e., central defocus) on both on-axis eye growth and refractions. All but the 6.5-mm CZD +5P lens induced larger changes than the +5SV lens. The +5C lenses with CZD less than 5.5 mm had little effect. The two-zone −5-D lenses had less effect than the −5SV lens, and only the 6.5-mm CZD lens of the −5C series had a significant effect. Conclusions.: The results demonstrate that peripheral defocus can influence both peripheral and central refractive development. The inhibitory effect on axial eye growth of the +5P lenses opens the possibility that appropriately designed concentric lenses may control the progression of human myopia.