Biomechanical measurements of the porcine lens capsule

Biomechanical measurements were carried out on capsular rings prepared from the anterior lens capsule by means of photoablation technique (excimer laser). The outer diameter of the metal ring used to shape the laser output was 3.2 mm and the width (wall thickness) was 100 microns. Thickness of the lens capsule (depth of the tissue ring) was measured with a microscope coupled to an electronic length gauge as the difference in focus between latex spherules placed on the upper and lower surfaces of the capsular rings. For mechanical measurements the capsular rings were mounted around two pins connected to a motorized micropositioner and a force transducer, respectively, and stretched at a constant rate until rupture. Estimates of the precision of the test method were based on differences between measurements of paired eyes from 15 6-month-old pigs. Thickness of the lens capsules was 59 microns (50-66) (median, range). Repeatability of a single measurement was +/- 2 microns for a probability of 95%. Maximum strain as a percentage of the initial length was 78% [68-89] with a repeatability of +/- 6%. Maximum stiffness (elastic modulus) was 23.0 N mm-2 [10.0-31.5] with a repeatability of +/- 4.3 N mm-2, and maximum stress was 4.4 N mm-2 [2.8-7.7] with a repeatability of ./:1.27 N mm-2. There were no significant differences between paired eyes, but a significant variance component between individuals. The experimental set-up described in this paper seems reliable and useful for biomechanical measurements of the lens capsule, in the physiological as well as the surgical function range.