Elastic constants of the human lens capsule

Abstract

SUMMARY1. A technique is described whereby the elasticity of the human lens capsule has been determined at birth and throughout life. This technique requires three separate determinations: (a) thickness; (b) stress and strain; (c) Poisson's ratio; (a) the capsule was clamped between accurately perforated ground glass plates and its thickness determined by noting the change in depth of focus between Latex spherules adhering to its upper and lower surfaces; (b) the undisturbed capsule was then placed in a specially designed glass distension apparatus and the relationship between volume and pressure recorded when it was distended with isotonic saline. The permeability of the capsule was also measured; (c) in some cases Poisson's ratio was determined by measuring the change of thickness of the capsule and the height to which it rose when distended with isotonic saline at different pressures. An apparatus was designed for this purpose.2. The average thickness of the anterior capsule increases from birth until about the 60th year but thereafter it decreases slightly.3. Poisson's ratio was about 0 47 for both cat and human capsule, and no significant variations with age in human capsule could be detected.4. Corrected volume pressure curves obeyed Hook's law almost to the point of capsule rupture.5. In childhood Young's Modulus of elasticity is about 6 x 107 dyn/cm2 and decreases to 3 x 107 dyn/Cm2 at 60 and 1.5 x 107 dyn/cm2 in extreme old age.6. The ultimate tensile stress was 2-3 x 107 dyn/Cm2 in young capsules and 0 7 x 107 dyn/cm2 in old ones. The maximum percentage elongation was 29 per cent and independent of age. 7. The implications of these findings are discussed in relation to (a) the mechanical properties of the lens capsule; (b) the ageing of the lens capsule and basement membranes; and (c) the decrease in elasticity of the lens capsule as a cause of presbyopia.

The significance of the shape of the lens and capsular energy changes in accommodation

“…Material Six specimens each of rabbit and cat capsules were examined by the method used for the thirty human lens capsules described in the previous paper (Fisher, 1969). From a different source, a further twenty-four human lenses were obtained for profile determinations, their post mortem ages being 24 hr or less.…”

“…The central part of the anterior capsule was distended into a portion of a sphere in the pressure-volume apparatus (Fisher, 1969). The volume of this spherical segment (v) and the pressure (P) required to change it by one microlitre was obtained using values from corrected pressure-volume curves previously described.…”

“…When Young's Modulus of elasticity of human lens capsule was found to have a value over 10 times as great as human aortic tissue (Fisher, 1969) it was thought that the accommodative power of the human lens may reside in the elastic strength of its capsule. To test this hypothesis, the elastic moduli of cat and rabbit lens capsules were determined: they are much lower than in man.…”

“…However, this is not a perfect analogy since the surface tension of a pure fluid is independent ofextension. In the case of the lens capsule, the force required to extend the capsule is proportional to the extension (Fisher, 1969).…”

Variações nas dimensões do cristalino humano de acordo com a idade

“…Quanto ao diâmetro do cristalino, já se observou que esta medida é de aproximadamente 6,3 mm ao nascimento, atingindo 9 mm ao final da segunda década de vida, a partir de quando este diâmetro continua crescendo, ainda que em taxas mais sutis (4,17,(22)(23)(24) .…”