Induced myopia associated with increased scleral creep in chick and tree shrew eyes.

PURPOSE To investigate the role of scleral creep in the axial elongation of chick and tree shrew eyes with induced myopia. METHODS Form-deprivation myopia was induced with a diffusing occluder worn over one eye. Scleral samples from the posterior pole and equatorial regions of myopic, contralateral (control), and age-matched normal chick and tree shrew eyes were loaded in vitro with a force of 5 g for 20 minutes while creep extension was monitored. The elastic behavior of sclera from myopic, control, and normal chick eyes was also compared. RESULTS In both chick and tree shrew, posterior and equatorial scleral samples from myopic eyes had significantly (P < 0.05) greater creep extensions than equivalent samples from control and normal eyes (n = 10, each group). Among individual tree shrews the difference in creep rate between the sample from the myopic eye and that from the control eye correlated with vitreous chamber elongation (r = 0.746, P < 0.05) and development of myopia (r = 0.792, P < 0.01) in the deprived eye. No such association was found in the data from chicks. The elastic properties of chick sclera were unaffected in form-deprivation myopia. CONCLUSIONS In chick and tree shrew, form-deprivation myopia is associated with increased creep rate of posterior and equatorial sclera. In tree shrew, the correlation between increased scleral creep rate and vitreous chamber elongation in myopic eyes supports the hypothesis that induced changes in the axial length of the mammalian eye are mediated by changes in the creep properties of the sclera.