Design of Metacarpophalangeal Single-Piece Joint Prosthesis to Increase Motion in Abduction/Adduction

One of the leading causes of disability is rheumatoid arthritis (RA), especially that of the hand, the metacarpophalangeal (MCP) joint being affected the most. Current single piece prostheses are designed to allow for motion in flexion and extension, but little effort is put towards motion in abduction and adduction. The objective of this study is to determine the effects of web (hinge) joint cross-sectional geometry on stress, strain, shear strain, strain energy, and reaction moment magnitude for an MCP joint that is subject to not only flexion and extension but also abduction and adduction. Using finger bone dimensions from the literature, geometry was produced in ANSYS finite element software. The geometry was assigned a hyperelastic material constitutive model, making the analysis nonlinear. The cross-sectional shape of the hinge was controlled by ellipse dimensions, one for thickness and one for width. Motion boundary conditions were applied to the distal portion of the model resulting in bending at the hinge. The study showed that for flexion/extension motion the von Mises (equivalent) stress, shear strain, and equivalent strain are linearly proportional to the thickness but inversely proportional to the width. The reaction moment and strain energy are linearly proportional to the thickness but exponentially proportional to the width. For motion in abduction and adduction the behavior is opposite; the width acting as the thickness does in flexion/extension motion and the thickness acting as the width does in flexion/extension motion. It was also seen that high levels of shear strain develop on the palmer side of the model, indicating that failure has the most potential to occur in that area.