Abstract
Purpose: :
To capture the natural stress equilibrium between the human capsular bag and crystalline lens in a finite element model.
Methods: :
Finite element models of the human crystalline lens and capsular bag were generated using Abaqus/Standard 6.10. Dimensions and material properties for a 29 year old lens were drawn from published data. No true stress-free reference state exists in the natural lens system so, for this model, the crystalline lens was initially modeled in the disaccommodated shape and the capsular bag in the accommodated shape. This represents a physiologically impossible geometry for the system as a whole, but provides a unique stress-free reference state for each part independently. Surface contact conditions were then manipulated to create an interference fit between the overlapping parts, forcing them to fit together.
Results: :
This approach results in a physiologically realistic equilibrium geometry in which both parts are constantly putting pressure on one another. The initial geometries for the two parts overlap and do not fit together, but once an interference fit is imposed, the model finds a pre-stressed equilibrium position which can then act as the initial state for later models. Von Mises stresses in the equilibrium position are on the order of 0.1 kPa in the crystalline lens and 20 kPa in the capsular bag, and the equilibrium geometry corresponds well with published descriptions of accommodated lens shape. Model behavior in response to cilliary forces is consistent with published experimental data.
Conclusions: :
Modeling the capsular bag and crystalline lens in separate stress-free reference geometries and then enforcing an interference fit between the parts results in an equilibrium state that captures the natural mechanical equilibrium between the two. Including these pre-stresses in a model provides a more physiologically realistic representation of the capsular bag that can be used to further our understanding of the effects of aging, changing tissue properties, and surgical intrusions on accommodation, and to guide development efforts for accommodating IOLs.
Keywords: accommodation • intraocular lens • presbyopia