FORM DEPRIVATION MYOPIA - ELASTIC PROPERTIES OF SCLERA

Thinning of the posterior sclera may imply that stretching and/or weak ening of the sclera plays a role in axial elongation of myopic eyes. W e investigated the elastic stress-strain properties of sclera from dev eloping tree shrew eyes made myopic by monocular deprivation (MD) of f orm vision. Five days of MD induced a relative myopia (mean +/- SEM) o f - 5.6 D +/- 0.6 D (retinoscopy) and a vitreous chamber elongation (d eprived minus control) of 106 +/- 14 mu m, n = 10 (ultrasonography). P osterior scleral test samples (2 mm wide) cut from myopic eyes were si gnificantly thinner than their contralateral eye controls (149 +/- 4 m u m versus 164 +/- 4 mu m, n = 10, P < 0.01) when measured with a forc e-controlled micrometer. How ever, posterior sclera from control eyes was significantly thicker than that from age-matched normal eyes (164 +/- 4 mu m versus 149 +/- 3 mu m, n = 10, P < 0.01). Under uniaxial te nsion, posterior scleral samples from myopic eyes failed at 18% lower load (162 g versus 198 g) and extended approximately 25% more than con trols at a load corresponding to 20 mm Hg intraocular pressure, These differences were largely accounted for by the differences in scleral t hickness. Finite element modelling of tree shrew eyes using the materi al properties summarised above, implies that simple elastic stretching of the sclera accounts for less than 20% of the observed difference i n axial length between myopic and contralateral control eyes.