A new finger joint prosthesis is being developed for the proximal and distal interphalangeal positions. Currently available "joint spacer" prostheses provide relief from pain and cosmetic improvement, but relatively poor long-term function. The new prosthesis employs a mechanical hinge at the joint. It is fabricated from titanium alloy (6A14V). The hinge mechanism avoids direct metal to metal contact by using high density polyethylene bearings. In vitro tests of the hinge mechanism have passed 75 million cycles of continuous flexure without failure (n = 12). The hinge also incorporates a mechanical limit stop to prevent hyperextension. The hinge mechanism is enclosed in a sealed elastomeric jacket that isolates the hinge from connective tissue ingrowth. The jacket, equivalent to an artificial synovial membrane, has an integrally textured exterior surface designed to promote tissue attachment to the implant to stabilize tissue capsule formation around the joint. To test the in vivo efficacy of the new design, a series of 12 devices were implanted in the knee joint position of adult rabbits. A jacketed prosthesis was implanted on one side, whereas 2 weeks later an unjacketed control was implanted contralaterally. The animals then were maintained for an 8 week period. At sacrifice, the implants were removed, and the response of the surrounding tissues was studied histologically. At the time of implantation, the range of motion of the joints was approximately 100-105 degrees. There was a progressive loss in range of motion observed in both groups. The fibrous tissue capsule around the jacketed implants, however, was significantly reduced in thickness compared with the controls (mean thickness, 1.5 mm vs. 4.5 mm).(ABSTRACT TRUNCATED AT 250 WORDS)