Biomechanical and computer analysis of radial head prostheses.

Radial head fractures can lead to significant upper extremity disability. A comminuted radial head fracture with a medial collateral ligament tear presents a clinical conundrum. The radiocapitellar (RC) joint should be maintained, yet the head frequently cannot be reduced and stabilized. The silicone rubber implant currently available is not biomechanically or clinically satisfactory, and thus there is a need for a more suitable prosthetic replacement for the radial head. We have attempted to design such a prosthesis and have examined prosthetic design with various materials that would best transmit force of the RC joint. Design configurations based on our cadaveric and radiographic measurements were tested with structural finite element method computer analyses. Materials examined included titanium alloy, cobalt-chrome alloy, alumina ceramic, and ultrahigh molecular weight polyethylene (UHMWPE). Metals and ceramic transmitted force at the distal bone and implant interface and strain shielded the proximal radial cortex while UHMWPE distributed load uniformly through the cortex and along the entire bone and implant interface. In addition, load transmission comparisons were made with the intact radial head, with a silicone prosthesis, and with 6 and 10 mm thick UHMWPE prostheses in cadaveric specimens. UHMWPE prostheses transmit more force to the RC joint than the silicone prosthesis and with leads closer to physiologic levels at all flexion angles. UHMWPE prostheses not only transmit much more force than silicone, but because of substantially less deformation under load, also provide more stability to the joint.