Anterior Ciliary Sclerotomy with Silicone Expansion Plug Implantation: Effect on Presbyopia and Intraocular Pressure

Until recently, accommodation has 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 surface 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. Recent work 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. The important difference between the Helmholtz model and the Schachar model is that the latter suggests a more active interaction between the ciliary muscle and the lens-zonule complex. An interaction in which active effort by the ciliary muscle results not simply in passive relaxation of the lens-zonule complex but a more complicated active differential response of different zonular types resulting in morphological changes in the lens. If this recent model of accommodation is correct, presbyopia may not be explained by simple sclerosis of the lens fibers and capsule (as previously understood). Rather, the decline in accommodative power of the eye may be due to the inability of the lens equator to expand into the posterior chamber. Thornton described this as “a crowding” of the lens in the posterior chamber as the lens grows.