Intact globe inflation testing of changes in scleral mechanics in myopia and recovery

Abstract

The purpose of this study was to examine the effects of myopia-inducing and myopia recovery conditions on the scleral biomechanics of enucleated eyes of young chicks. Enucleated eyes from 5-day old chicks, with fiducial markers attached at 5 locations on the external sclera, were placed in a custom-built chamber filled with phosphate-buffered saline, and subjected to controlled increments in intraocular pressure (IOP). IOP was initially ramped from 15 to 100 mmHg and then maintained at 100 mmHg for one hour, with eyes photographed at a rate of 0.1 Hz over the same period. There were two experimental groups, one in which chicks were monocularly form deprived for four days to induce myopia, and the other in which chicks were allowed two days of recovery from myopia induced by two days of form deprivation. For all chicks, the contralateral (fellow) eyes served as controls. Myopic eyes showed less initial deformation relative to their fellows, while no difference was recorded between recovering eyes and their fellows over the same time frame. With exposure to sustained elevated pressure, eyes in all groups displayed time-dependent changes in creep behavior, which included a linear region of secondary, steady creep. The creep deformation of myopic eyes was significantly higher than that of their fellows, consistent with results of previous studies using uniaxial loading of scleral strips. When allowed only 2 days to recover from induced myopia, previously myopic eyes continued to show increased creep deformation. Compared to results reported in studies involving scleral strips, our whole globe testing yielded higher values for creep rate. Whole globe inflation testing provides a viable, less anatomically disruptive and readily adaptable method for investigating scleral biomechanics than uniaxial tensile strip testing. Furthermore, our results suggest that elastic stretching does not contribute to the increased axial elongation underlying myopia in young chick eyes. They also confirm the very limited involvement of the sclera in the early recovery from myopia, reflecting the well documented lag in scleral versus choroidal recovery responses.

Keywords: creep; intact globe; myopia; recovery.

Figure 1

Summary of diffuser wearing and measurement schedules for the two groups of chicks involved in the study; one group was continuously form deprived for four days while the other group was form deprived for two days, at which point the diffusers were removed and eyes allowed uninterrupted vision.

Figure 2

Left, orientation of the microbeads attached the surface of a left eye relative to the optic nerve (ON); the beads divide the eye into four segments: the far superior (FS), near superior (NS), near inferior (NI), and far inferior (FI). Right, A schematic diagram of the experimental set-up for inflation testing, which included a PBS-filled, adjustable height column (A), connected by tubing (B), to a needle inserted via the cornea and lens into the vitreous chamber of the eye mounted in the testing chamber (C); marker beads were glued to the outer sclera (D), and an appropriated positioned CCD camera (E) used for recording.

Figure 3

Elongation vs. pressure for the Myopia Group (left) and Recovery Group (right). Filled shapes indicate treated eyes and open shapes indicate control eyes. Error bars represent standard errors.

Figure 4

Creep deformation under a fixed pressure of 100 mmHg, shown as mean elongation, expressed as percent elongation from the original eye length, plotted against time for the Myopia Group (left) and the Recovery Group (right). The black lines represent data from treated eyes and the grey lines, data from fellow control eyes. Error bars indicate standard error.