Biomechanics of the Human Anterior Lens Capsule

The primary function of the lens of the eye, termed accommodation, is to precisely focus light onto the retina by changing curvature and corresponding refractive power. Investigators have long sought to understand the mechanism of accommodation in terms of interactions of the constituent tissues, which recently has been aided by biomechanical modeling. Such models depend heavily on accurate measurements of tissue mechanical properties and seek to predict stresses and strains. A critical component of the accommodative apparatus is the lens capsule, a bag-like membrane that encapsulates the lens nucleus and cortex and mediates tractions imposed onto this structure by the ciliary body. In addition to this physiologic process during normalcy, the lens capsule also plays a fundamental role in cataract surgery; a procedure that involves three basic steps: a quarter of the anterior lens capsule is removed via the introduction of a continuous circular capsulorhexis (CCC), the lens is broken up and suctioned out, and an artificial intraocular lens (IOL) is placed within the remnant capsular bag. Although novel IOL designs have decreased post-surgical complications, they currently lack the important feature of accommodation. Therefore, mechanical analysis of the lens capsule will allow for an understanding of its interaction with an implant that may further assist in the design of future accommodating IOLs (AIOLs). Here, we report a novel experimental approach to study in situ the regional, multiaxial mechanical behavior of both normal and diabetic human anterior lens capsules. Furthermore, we use these data to calculate material parameters in a nonlinear stress-strain relation via a custom sub-domain inverse finite element method (FEM). These parameters are then used to predict capsular stresses in response to imposed loads using a forward FEM model.