Biomechanical contributions to murine lens shape: Confinement, compaction, and residual stresses.

Presbyopia is the progressive loss of near vision with age and affects nearly everyone by age 50. This most common visual deficit is a result of age-related changes in lens shape and refractive index, which dictate the optical power of the fully accommodated lens. Lens shape in the absence of external loads is dictated by the balance of biomechanical forces between the lens and its capsule. These residual stresses arise from differential growth. However, these stresses remain unknown. This study uses the nearly spherical murine lens as a model for elucidating how these residual stresses may be calculated and which experimental parameters must be measured to enable such calculations. Several key concepts arise from the analysis in agreement with recent studies. It is suggested that the lens fiber cells are poroelastic and that fiber cell compaction arises from biomechanical confinement effects of the lens capsule. It is possible to computationally "recapsulate" the uncompacted lens after growth, then estimate the extent to which the capsule compacts the fiber cells and, in turn, the extent to which the fiber cells distend the capsule. The simple biomechanical models presented are capable of predicting residual stresses in line with published experimental measurements, suggesting that they capture the essence of how the lens and capsule push and pull during years of differential growth.