Form deprivation myopia: elastic properties of sclera

Thinning of the posterior sclera may imply that stretching and/or weakening of the sclera plays a role in axial elongation of myopic eyes. We investigated the elastic stress-strain properties of sclera from developing tree shrew eyes made myopic by monocular deprivation (MD) of form vision. Five days of MD induced a relative myopia (mean ± SEM) of −5.6 D ± 0.6 D (retinoscopy) and a vitreous chamber elongation (deprived minus control) of 106 ± 14 μm, n = 10 (ultrasonography). Posterior scleral test samples (2 mm wide) cut from myopic eyes were significantly thinner than their contralateral eye controls (149 ± 4 μm versus 164 ± 4 μm, n = 10, P < 0.01) when measured with a force-controlled micrometer. However, posterior sclera from control eyes was significantly thicker than that from age-matched normal eyes (164 ± 4 μm versus 149 ± 3μm, n = 10, P < 0.01). Under uniaxial tension, posterior scleral samples from myopic eyes failed at 18% lower load (162 g versus 198 g) and extended approximately 25% more than controls at a load corresponding to 20 mm Hg intraocular pressure. These differences were largely accounted for by the differences in scleral thickness. Finite element modelling of tree shrew eyes using the material properties summarised above, implies that simple elastic stretching of the sclera accounts for less than 20% of the observed difference in axial length between myopic and contralateral control eyes.