UCLA

The wrist and finger joints allow us to position the hand, grasp objects, and perform activities of daily living. Their small size, complex anatomy, and substantial internal loads make reconstruction after disease or injury challenging. Arthrodesis can relieve pain and provide stability, but sacrifices motion at the reconstructed joint. Arthroplasty seeks to relieve pain while preserving motion, but current implants face limitations in load bearing, stability, fixation, and durability. Our objective was to develop novel, long lasting compliant implants that bridge this gap by combining inherent mechanical stability with functional motion and load bearing for daily activities. For active patients with extensive wrist bone loss, we designed and optimized an implant based on two nested cross axis flexural pivots. These provide two degrees of freedom for flexion and deviation, which can combine to allow dart throwing motion. Elastic deformation guides movement and provides inherent mechanical stability without a rubbing or sliding bearing, eliminating a conventional source of implant wear. Finite element analysis, precision manufacturing, mechanical characterization, and cadaveric evaluation established a functional titanium prototype. Mechanical testing demonstrated wrist motion under compressive loads up to 600 N and dart throwing motion at up to 300 N. We then extended this principle to patients affected by finger arthritis who require surgery to relieve pain and restore function. For the proximal interphalangeal joint, we sought to improve stem fixation and lateral stability while preserving the greater flexion needed for functional finger use. We replaced the metal flexures with a cable tensegrity weave embedded in silicone to accommodate one principal degree of freedom within a smaller anatomical envelope. Titanium stems were designed to accommodate porous surfaces for bone ingrowth. Separate acute motion and compression tests demonstrated a 120° unloaded arc and load support above the 200 N development target. Lateral testing of three tensegrity configurations demonstrated stabilizing restraint without collateral tissue. Together, these proof of concept studies show how compliant implant architectures can accommodate the distinct mechanical demands of the wrist and finger joints, offering a promising path toward restoring the function patients rely on in daily life.