Abstract:
To attain up to 14 D of accommodation, human and other mammals change lens shape from a spherical to a flatter configuration. The sphere is a solid that contains a given volume with minimal surface. Thus, during accommodation the lens must change its volume, its capsular surface area or both. Lenses swell or shrink by gaining or loosing fluid across its capsule when exposed to anisotonic solutions, an indication of high capsular and fibril water permeability. Because the lens is a symmetrical structure around an anterior–to–posterior central axis, its topological properties are similar to that of a torus. We developed a theory that allows volume determination from the lens cross–sectional area (CSA) obtained by photography. The CSA is the area obtained by focusing the camera perpendicular to the anterior–posterior axis of the lens. The area was computed with software from http:/rsb.info.nih.gov/ij/. We calculated the volume of isolated rabbit and bovine lenses in conditions simulating accommodation by forcing changes in volume and shape with osmotic forces. Three measurements were taken: CSA, equatorial length (EL) and anterior–posterior length (APL). Volume (V) was calculated. Rabbit lenses in normal Ringer (simulating accommodation state), hypertonic (un–accommodation) and hypotonic (over–accommodation) gave the following values:
Mechanically stretching a ring of sclera with the ciliary body and zonula intact increased the EL from 10.85 to 11.43 mm and decreased APL from 7.79 to 7.16 mm. Proportionally similar results were obtained in bovine lenses. Osmotic forces, by eliciting fluid flow across the lens surface produce changes in shape, dimension and curvature consistent with those observed during accommodation. Changes in lens CSA previously observed in human and attributed to "fluid compression" could have resulted from fluid movement out of the lens. Thus, accommodation may not only involve changes in capsular surface but also in lens volume. Impediment of fluid flow between aquaporin–rich lens fibers and across lens surface could result in loss of accommodative power.