ABSTRACT
Accommodation has until recently been explained by the Helmholtz hypothesis. This hypothesis holds that passive anteroposterior thickening of the lens and relative curvature changes in the anterior and posterior lens surfaces result from zonular relaxation with ciliary muscle contraction (Fig. 1). Presbyopia is likewise described as the loss of accommodation due to decreasing elasticity of the lens fibers and capsule (1,2). Recent work (3,4) suggests a very different model of accommodation. Morphological changes in the lens with accommodative effort are seen as the result of active rather than passive interactions. The three components of the ciliary body-the longitudinal, radial and circular fibers-act in concert to increase tension in the equatorial zonules while decreasing tension in the anterior and posterior zonules. The result is an active elongation of the lens diameter with peripheral thinning and central thickening due to dynamic internal volume changes (Fig. 2). The net result is increased plus refracting power of the eye.
