Model of accommodation: contributions of lens geometry and mechanical properties to the development of presbyopia.

PURPOSE To determine the relative importance of lens geometry and mechanical properties for the mechanics of accommodation and the role of these elements in the causes and potential correction of presbyopia. SETTING Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, New Jersey, USA. DESIGN Experimental study. METHODS Finite element methods and ray-tracing algorithms were used to model the deformation and optical power of the human crystalline lens during accommodation. The mechanical model treats the lens as an axisymmetric object, and the optical model incorporates a gradient refractive index. Using these models, the accommodation of a broad range of lenses with different geometries and mechanical properties were investigated. RESULTS The most significant result was that reshaping the 45-year-old lens to the geometry of the 29-year-old lens, while retaining the mechanical properties, restored the former's accommodation amplitude to 72% to 94% of that of the 29-year-old lens, depending on ciliary body displacement. That is, reshaping can add 1.8 to 3.7 diopters of accommodation. A sensitivity analysis showed that this result was robust over a wide range of mechanical and geometrical properties. CONCLUSION The study results suggest that a significant amount of the loss of accommodation is due to changes in lens geometry.