An anatomically based finite element model of patella articulation : towards a diagnostic tool

Abstract

The development of a patient-specific anatomically based 3D model of patella mechanics using finite element (FE) analysis is presented. The tissue geometry was developed from the Visible Human (VH) dataset and includes active and passive soft tissue attachments (quadricep musculotendon units and the patella tendon), attachment surfaces (hip, tibia and femur bones), and articulating contact surface (proximal femur). The development of additional generic geometries of the lower limb are also illustrated (lower limb muscles, knee joint ligaments and the menisci), and other organ geometries (forearm, neck and back muscles) highlighting the goal of developing a reusable generic geometry database as a contribution to the world-wide IUPS Physiome Project. The models were described using high-order cubic Hermite interpolation and customised using a variant of the free-form deformation (EEU) technique (host-mesh, fitting). The continuum was solved using the governing equation of finite elasticity, with the multibody problem coupled through contact mechanics using the ’cross-constraints’ technique. Additional constraints such as incompressibility are also imposed. Passive material properties are taken from the literature and implemented for deformable tissue with a non-linear microstructurally based constitutive relation (the ’pole-zero’ law). Bone and cartilage are implemented using a St-Venant Kirchoff model suitable for rigid body rotations. The surface fibre directions have been estimated from anatomy images of cadaver muscle dissections performed at Auckland Medical School, and active muscle contraction was based on a steady-state calcium-tension relation. The 3D continuum model of muscle, tendon and bone is compared with experimental results from the literature, and surgical simulations performed to illustrate its clinical assessment capabilities (a Maquet’s procedure for reducing patella stresses and a vastus lateralis (VL) release for a bipartite patella). A geometric analysis was also undertaken using a subset of the developed geometry and volume preserving EEU methods to analyse muscle length. This study aims to develop a tool for assessing impaired gait, such as cerebral palsy (CP), before and after surgery. The model and methodology are presented using a sample of healthy and CP affected patient case studies. Finally, the practical issues and limitations of the models are addressed, and future improvements discussed. Various directions for research initiated from this project are proposed and possible applications for the models are highlighted.