Changes in scleral creep rate during compensation for a minus power spectacle lens in tree shrew

Academic Article

Abstract

  • Purpose. Visually controlled modulation of the mechanical properties of the sclera may play a role in regulating axial elongation rate in the mammalian eye. In tree shrew and several other species, a minus power spectacle lens induces a transient increase in axial elongation rate that matches the axial length to the altered focal plane. We measured scleral creep rate (an index of extensibility by a constant force) in tree shrews during compensation for a minus power spectacle lens to learn whether creep rate is modulated in parallel with the rise and fall in axial elongation rate. Methods. Starting 24 days after natural eyelid opening, tree shrews wore a goggle frame that held a -5 D lens over one eye (treated eye) while the fellow eye served as a control. After 2, 4, 11, or 21 days of lens wear (n=3 each group) retinoscopy and A-scan ultrasonography were performed under anesthesia and atropine cycloplegia. A 3 mm wide strip of posterior sclera was then removed from each eye and creep rate (% elongation/hr) was measured under 1, 3, and 5 grams of tensile force (1 gram ≈12 mm Hg IOP). Results. Treated eyes compensated for the lens by 11 days of lens wear [(treated-control) ret, -5.36±.88 D; vit cham, +.09±.02 mm] and then maintained the difference. During the first 11 days of lens wear, axial elongation rate in the treated eyes increased rapidly and then dropped back to approximately the level of the control eyes. Similarly, creep rate in the treated eyes rose and fell during the first 11 days. The creep rate in treated eyes was significantly higher than in control eyes at 4 days (p<.05, paired t) when the axial elongation rate was the highest. Creep rate in control eyes at 11 and 21 days was not significantly different from age matched normals eyes. Conclusions. During compensation for a -5 D lens, scleral creep rate rises rapidly and then returns to normal levels approximately in parallel with the rise and fall in axial elongation rate. These data suggest that visually controlled modulation of the mechanical properties of sclera may participate in the control of axial elongation rate by making the posterior sclera more or less susceptible to expansion by intraocular pressure.
  • Authors

    Author List

  • Siegwart JT; Norton TT
  • Volume

  • 37
  • Issue

  • 3