THERMAL AND MECHANICAL STABILITY OF THE LENS CAPSULE

Purpose. Several procedures in cataract surgery carry the risk of high temperature increases in the capsular bag. The present study was unde rtaken to determine the shrinkage temperature of the human lens capsul e and to investigate the effect of temperature on the mechanical behav ior of the lens capsule. Method. Thermal-shrinkage characteristics of the lens capsule were determined during gradual heating of circular sp ecimens (2 mm in diameter) prepared from anterior lens capsules from 2 5 human donors, ranging in age from 20 to 98 years. Uniaxial mechanica l testing was carried out at 22 degrees C, 36 degrees C and 61 degrees C on ring-shaped test specimens prepared from anterior lens capsules from 5- to 6-month-old pigs. Results. The mean shrinkage temperature ( Ts) for the human lens capsule was 51.5 degrees C (range 49.3-54.3) an d the mean shrinkage area in percent of the original area (AST) was 49 % (36-66). T-s was significantly associated with the age of the donors and decreased 0.1 degrees C per year until age 65 after which Ts was found to increase, AS(T) showed no association with age. The mechanica l effect of temperatures below the shrinkage temperature was modest. T he capsule became slightly more extensible with increasing temperature . The effect of temperatures above the shrinkage temperature was an in creased ultimate strain, a reduced ultimate stiffness and a slightly r educed ultimate stress. Conclusion. Thermal stability of the human len s capsule (type IV collagen) seems to be considerably lower than that of fibrous connective tissue (type I collagen). A potential risk of ca psular shrinking has to be taken into account when the capsule is expo sed to thermal stress during cataract surgery.