A biomechanical model for evaluating the performance of accommodative intraocular lenses

Abstract

Accommodation alters the shape of the eye lens to change focus from distant to near vision. This function declines with age in the development of presbyopia and most people experience a near total loss of accommodative ability by 55 years. Currently, there are no surgical procedures that correct presbyopia, but considerable work has been done in the development of accommodative intraocular lenses (AIOLs) implanted during cataract surgery. Despite these efforts, AIOLs only restore ∼ 20% of youthful accommodative amplitude and they suffer from high rates of visually-debilitating fibrosis. An important design tool that is lacking that could aid in improving AIOL designs is modeling. Herein, we addressed this need through the development of a fully 3-D finite element model that was used to predict the behavior of a dual-optic AIOL implanted within the post-surgical lens capsule. Models of the native human lens were developed to identify the stress-free configuration of the lens capsule needed to accurately predict the accommodated state of the lens and the configuration of the zonular traction needed for the disaccommodated state. The AIOL model demonstrated the functional importance of implant stiffness and predicted an approximately linear relationship between zonular traction magnitude and axial displacement of the optics. To our knowledge, this is the first model that can be used to gain insights into AIOL efficacy. It provides a foundation for continued development of a predictive tool that could ultimately improve AIOL designs that seek to restore youthful accommodative function.

Keywords: Accommodation; Biomechanics; Computational modeling; Hyperelastic; Intraocular lens; Lens capsule.

Fig. 1.

Finite element models of the (A) native lens in the fully accommodated state and (B) AIOL in the reference configuration. The native lens is shown with a cutaway view to allow visualization of the capsule (grey), lens cortex (outer yellow), and lens nucleus (inner yellow).

Fig. 2.

Diagram of the native lens model with labels for the eight geometric parameters used for calibration. These parameters included: total lens thickness (TT), lens radius (R_eq), nuclear radius (R_nuc), anterior cortical thickness (ACT), nuclear thickness (NT), posterior cortical thickness (PCT), and anterior and posterior curvatures (C_ant, C_post). Red dots indicate the nodal locations used to compute some of the metrics.

Fig. 3.

Initial configurations of the native lens fiber model (prior to introducing the lens capsule). The geometric parameters of each configuration in the initial state were calibrated to achieve the geometry of the lens with the capsule in the accommodated state. These configurations included: (A) modeling both the anterior and posterior cortex as ellipse segments with varying semi-minor axes, (B) modeling both the anterior and posterior cortex as spline segments, and (C) modeling the anterior cortex as an ellipse segment and the posterior cortex as a spline segment. Light grey indicates the lens cortex and dark grey indicates the nucleus.

Fig. 4.

Configurations of the zonular traction applied to the native lens model. The magnitude of each configuration was calibrated to achieve the geometry of the native lens in the disaccommodated state. These configurations included: (A) a single uniform traction applied to the anterior, central, and posterior regions of the lens capsule, (B) separate tractions applied to the anterior and posterior regions, (C) separate tractions applied to the anterior, central, and posterior regions with a discontinuous area, and (D) separate tractions applied to the anterior, central, and posterior regions with a continuous area. Models are shown in the loaded (disaccommodated) state.

Fig. 5.

Optimized finite element model of the 30-year lens. The model is shown in the (A) fully accommodated state (using Configuration 3 for the initial lens fiber geometry) and (B) fully disaccommodated state using Configuration III and (C) Configuration IV for application of the zonular traction.

Fig. 6.

Optimized finite element model of the 65-year lens in the accommodated state. The model is shown with respect to the (A) native (lens capsule and fibers) and (B) post-surgical (lens capsule and AIOL) states.

Fig. 7.

Change in thickness of the AIOL from the accommodated to the disaccommodated state as a function of stiffness and zonular traction. (A-D) Top and side views of the post-surgical lens model in the disaccommodated state for (A,C) Configuration III and (B,D) Configuration IV of the zonular traction. The models shown utilize the intermediate AIOL stiffness (18 MPa). Axial displacement of the implanted AIOL versus percentage of traction magnitude for (E) Configuration III and (F) Configuration IV at AIOL stiffnesses of 12, 18, and 24 MPa.