Comparison of the Optomechanical Response of Human and Monkey Lenses During Simulation of Accommodation in a Lens Stretcher

Abstract

Purpose: : To compare the forces required to change the shape and optical power of human, rhesus, and cynomolgus monkey lenses and their age–dependence.

Methods: : Post–mortem human, cynomolgus and rhesus monkey tissues including the lens, capsule, zonules, ciliary body, and sclera were mounted in an optomechanical lens stretching system designed to simulate the radial pull on the zonules and measure the resulting force, optical power, and equatorial diameter. Starting at zero load, the lenses were symmetrically stretched in step–wise fashion from 0 to 1.75 or 2mm in 0.25 or 0.5mm steps at a speed of 0.5mm/s. The load, lens diameter, and lens power were measured at each step. Experiments were conducted on 28 rhesus monkey lenses (age: 8 months to 13 years), 30 cynomolgus monkey lenses (age: 4 to 9 years) and 6 human lenses (age: 14 to 40 years). The load–diameter and power–load responses were quantified. The effect of age was compared only in rhesus and human lenses because the age range of the cynomolgus lenses was too limited.

Results: : In the unstretched lens, there were no significant differences between the powers, diameters and loads measured in rhesus and cynomolgus eyes. The power of the monkey lenses was significantly higher and the diameter significantly lower than in human lenses. In the physiologically–relevant range, the load–diameter and power–load responses were linear with average slopes of 11.2g/mm and –2.9D/g for rhesus monkeys, 11.3g/mm and 3.5D/g for cynomolgus monkeys and 11.6g/mm and –1.4D/g for human lenses. The force required to stretch the lens and induce a change in lens power increased with age with similar rates in rhesus (0.768 g/mm/year, 0.07D/g/year) and human (0.807 g/mm/year; 0.08 D/g/year) lenses.

Conclusions: : Despite anatomical differences in the size and shape of the lens and accommodative structures, the forces required to change the lens shape of pre–presbyopic human and monkey lenses and their age–dependence are comparable.

Keywords: presbyopia • crystalline lens • optical properties