Three-Dimensional Finite Element Analysis of Prosthetic Finger Joint Implants

The purpose of this study was to develop high resolution three-dimensional (3D) finite element (FE) models of the Swanson® (No. 2) and NeuFlex® (No. 10) joint implants to: simulate implant function; evaluate stress distributions and bending stiffness of these implants; and assess their comparative potential for fracture and range of motion (ROM) in flexion and extension. Geometric representations of the implants accurate to within 20 μm were achieved using digital laser imaging technology. Images were transferred to ANSYS 5.7 using appropriate interfacing software and 3D FE models of the implants were constructed. Hyperelastic material properties of the silicone elastomers were derived experimentally from uniaxial tensile tests on implant sections. Both implants experienced maximum von Mises stresses at 90° of flexion and minimum stresses at the neutral position of flexion (Swanson: 0°, NeuFlex: 30°). Within the reported functional ROM (33°–73°), the NeuFlex implant exhibited lower maximum von Mises stress and bending stiffness than the Swanson. The Swanson implant, which has a straight hinge, exhibited lower peak stresses and bending stiffness than the NeuFlex for flexion less than 20°. Areas of high von Mises stress for the Swanson implant included the stem–hinge junction and the peripheral zone of the body of the hinge, corresponding to clinical reports of fractures. In the NeuFlex implant, the maximum stress occurred on the dorsal surface of the hinge. Bending stiffness of the NeuFlex implant was modelled to be substantially less than that of the Swanson throughout the functional ROM (33°–73° of flexion). The resting position of the Swanson implant is at 0° of flexion. A moment was required to extend the NeuFlex implant from 30° to 0° of flexion. These results suggest that the NeuFlex may potentially facilitate flexion of the metacarpophalangeal (MP) joint, whereas the Swanson may promote a more extended position of the joint.