Elasticity of the lens capsule as measured by osmotic swelling.

Abstract

As an alternative to purely mechanical methods, optical tracking of passive osmotic swelling was used to assess mechanical properties of the ocular lens capsule. Although limited by being a single measurement on a heterogeneous tissue, osmotic swelling provides a quantitative assessment of the stiffness of the lens capsule without requiring dissection or manipulation of the lens. A simple model was developed accounting for the permeability of the lens fiber cells and capsule to water, the concentration of fixed charges in the fiber cells, and the capsule’s resistance to the swelling of fiber cells. Fitting the model solution to experimental data provided an estimate of the elastic modulus of the lens capsule under the assumption of linear isotropic elasticity. The model was developed with the porcine lens to provide validity and was extended to a mouse model with X-linked Alport Syndrome, the most common form of the human disease that results in the absence of a collagen IV monomer normally present in the lens capsule. The calculated elastic moduli for the porcine lens is comparable to previously reported moduli of elasticity for the porcine lens capsule at small strains (<10%), and a slight increase with hypotonicity is consistent with the nonlinear mechanical behavior of the lens capsule. The calculated elastic moduli for the mouse lenses were similar between wild type and Alport and are comparable to a reported modulus of elasticity for rat lens capsules at small strains. The mouse lens modulus of elasticity showed a similar response to bath concentrations as the porcine lenses, increasing with hypotonicity. However, the difference in the tendency to rupture of the Alport and wild type lens capsules were statistically significant; for lenses that reached 14% strain in the equatorial direction, the Alport lenses had a greater tendency to rupture. This work will be extended to investigate the temporal effects of Alport syndrome on the elastic modulus and rupture mechanics of lens capsules. Osmotic challenge overcomes the size limitations of previously employed techniques for measuring the elastic modulus of the lens capsule and can provide insight into the properties of basement membranes through its application to other mutant mice.