Biomechanical characteristics of the human anterior lens capsule in relation to age

Abstract

Purpose: The purpose of the study was to investigate the influence of age on the biomechanical properties of the human anterior lens capsule.

Methods: The material comprised 67 lens capsules obtained from human donors ranging in age from 7 months to 98 years. Test specimens were prepared from the anterior lens capsule as tissue rings by means of excimer laser technique using a metal ring (mask) to shape the laser output (outer diameter = 3.2 mm, width = 100 microns). Capsular thickness was measured under microscope as the difference in focus between microspherules placed on the outer and inner surfaces of the capsule. The rings were slipped over two pins connected to a motorized micropositioner and a force transducer, respectively, and stretched at constant speed until rupture, with continuous recording of load and elongation.

Results: Capsular thickness was associated significantly with age of the donors and increased gradually (1.2% per year) until age 75, after which a slight decrease was observed. The elastic response curves showed a high degree of nonlinearity and were influenced markedly by age. Ultimate strain decreased 0.5% per year (range, 108% to 40%). Ultimate tensile strength decreased 1% per year (range, 17.5 N/mm2 to 1.5 N/mm2), and ultimate elastic stiffness (tangent modulus) decreased 0.9% per year (range, 44.8 N/mm2 to 4.4 N/mm2), whereas elastic stiffness corresponding to a specific strain level (30%) increased until age 35, after which a slight decrease was observed.

Conclusions: Aging of the human anterior lens capsule is associated with a progressive loss of mechanical strength. The young capsule is strong, tough, and highly extensible, whereas the older, thicker capsule is less extensible and much more brittle, and it has a markedly reduced breaking strength.