Cataract is a visible opacity in the lens substance, which, when located on the visual axis, leads to visual loss. Age-related cataract is a cause of blindness on a global scale involving genetic and environmental influences. With ageing, lens proteins undergo non-enzymatic, post-translational modification and the accumulation of fluorescent chromophores, increasing susceptibility to oxidation and cross-linking and increased light-scatter. Because the human lens grows throughout life, the lens core is exposed for a longer period to such influences and the risk of oxidative damage increases in the fourth decade when a barrier to the transport of glutathione forms around the lens nucleus. Consequently, as the lens ages, its transparency falls and the nucleus becomes more rigid, resisting the change in shape necessary for accommodation. This is the basis of presbyopia. In some individuals, the steady accumulation of chromophores and complex, insoluble crystallin aggregates in the lens nucleus leads to the formation of a brown nuclear cataract. The process is homogeneous and the affected lens fibres retain their gross morphology. Cortical opacities are due to changes in membrane permeability and enzyme function and shear-stress damage to lens fibres with continued accommodative effort. Unlike nuclear cataract, progression is intermittent, stepwise and non-uniform.

Equatorial wrinkles, or crenations, have been previously observed around the equator in coronal images of the human ocular lens. However, wrinkles are typically not apparent when the lens is viewed from saggital directions. In the current paper, the existence and geometry of these wrinkles is shown to be consistent with a mechanical model of the isolated lens, in which the capsule is held in a state of residual tension by a spatially uniform internal pressure. The occurrence of equatorial wrinkles is therefore seen to be a mechanical consequence of the spheroidal shape of the lens capsule and an excess intralenticular pressure. New observations are made, on post mortem lenses, on the geometric arrangement of these equatorial wrinkles. These observations indicate a well-defined pattern in which wrinkles exists along meridional lines in the equatorial regions of the lens. A preliminary 'puncture test' is used to demonstrate that the residual stresses within the capsule in the equatorial region of the lens are broadly consistent with the proposed mechanical model of the lens capsule. It is suggested that the presence of equatorial wrinkles may have an influence on the mechanical performance of the capsule during the accommodation process.

SummaryIn previous publications, we found that human cortical and cuneiform opacities are accompanied by changes in fiber structure and architecture mainly in the equatorial border zone between the lens nucleus and cortex. Because the lens cortex and nucleus have different viscoelastic properties in young and old lenses, we hypothesized that external forces during accommodation cause shear stress predominantly at this nucleus‐cortex interface. The location of the described changes suggested that these mechanical forces may cause fiber disorganization, small cortical opacities, and ultimately, cuneiform cataracts.Recently, we tested our hypothesis in a stretching device for anterior eye segments from human donor eyes. Lenses with cortical cataracts showed ruptures at the nucleus‐cortex interface adjacent to the cortical cataracts. These ex vivo experiments indicate that the nucleus‐cortex interface is vulnerable and it can be separated when external forces are applied. In vivo forces from the attempt to accommodate are smaller, but a continuous action during years may induce micro ruptures at this interface which may lead to cortical cataract.