Abstract
Purpose: :
The loss of accommodative amplitude in the adult human eye is correlated with a number of age-dependent changes in the crystalline lens, such as continued lens growth and decrease in central lens radii of curvature. The three-dimensional relationship between the lens and ciliary muscle within the anterior segment is also altered with aging, leading to the geometric (and modified geometric) theory of accommodation and accommodative loss. While this theory is not inconsistent with the idea of the lens growing stiffer with increasing age, it does not rely necessarily on it to explain presbyopia. This study examines the potential contribution of lens stiffening to accommodative loss.
Methods: :
Our inverse biomechanical analytic model of lens accommodation, which correlates changes in lens shape with the pattern of forces applied to the lens, has been modified and upgraded to consider more realistic models of the lens than were possible when it was first developed. Input variables for a small accommodative change include lens age and accommodative state (and thus shape), plus the two Young’s Moduli of Elasticity and the two Poisson ratios in the equatorial and polar directions. Values for the Moduli were derived from the work of RF Fisher (1971). In one set of analyses, the Moduli were held constant over an age range of 20-60 yr, and optimal results were determined for each age. In another analysis, values for the Moduli were increased with increasing age and the optimal results determined.
Results: :
If the Young’s Moduli are held constant over the age range, the optimal Poisson ratios - 0.43 and 0.47 in the equatorial and polar direction for these values - are also conserved. However, the total forces acting upon the lens decrease with increasing age, particularly after age 40, a contradictory result. In contrast, when the Moduli are increased with increasing age, very slightly between ages 20 and 40, and more steeply beyond 40, there are appropriate age-related increases in the forces acting on the lens. The Poisson ratios associated with the optimal results vary with age, but appear to cluster around 0.45-0.46.
Conclusions: :
A variety of aging processes in the lens are bimodal, changing slightly up to 40-50 yr and more steeply beyond that. Fisher’s elasticity data appear to follow this general pattern. The results from their use suggest that stiffening of the lens is only a minor factor in the loss of accommodative ampitude. Since, however, there is a steep increase in the elastic moduli beyond this age range, it may well be that lens stiffening might result from presbyopia.
Keywords: presbyopia • accommodation • aging