N. Ziebarth

Role of the lens capsule on the mechanical accommodative response in a lens stretcher.

PURPOSE To determine whether changes in elastic properties of the lens capsule ex vivo with age contribute to the forces necessary for accommodation. METHODS Postmortem human (n = 22; age average: 41 +/- 17 years; range: 6-7) and cynomolgus monkey (n = 19; age average: 7.7 +/- 1.8 years, range: 4.2-10) tissues including the lens, capsule, zonules, ciliary body, and sclera were mounted in an optomechanical lens-stretching system. Starting at 0 load, the sclera was symmetrically stretched to 2 mm in 0.25-mm steps at a speed of 0.1 mm x s(-1). The load and lens diameter were measured at each step. The lens contents were removed through a mini-capsulorhexis. The stretching cycles were repeated on the empty capsular bag. The forces necessary to stretch the natural lens and empty bag were quantified as a function of age and compared. RESULTS The force needed to stretch the empty lens capsule was independent of age (human, 2.6-34.9 g/mm [25.2-342.7 mN/mm]; monkey, 8.2-21.3 g/mm [80.3-208.6 mN/mm]). The ratio of the force necessary to stretch the empty lens capsule to the force necessary to stretch the natural lens decreased with age in the human and monkey lenses (P = 0.003, P = 0.72, respectively). CONCLUSIONS The mechanical properties of the empty lens capsule assessed ex vivo in a lens stretcher remain constant with age, suggesting that the changes in elasticity of the lens capsule do not play a significant role in presbyopia. In young eyes, the lens capsule determines the force necessary to stretch the whole lens. The age-related increase in force needed to stretch the lens is due to changes in the lens contents.