The effect of two-zone concentric bifocal spectacle lenses on refractive error development and eye growth in young chicks.

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.