The most important factors for long-term stability of an unconstrained implant are a stable soft-tissue envelope and adequate bone stock to allow for fixation through appositional bone growth. The ideal patients for pyrolytic carbon MP joint replacement are OA and TA patients. In the majority of OA and TA patients, the soft-tissue envelope surrounding the MP joint is stable and often thickened, providing added stability following arthroplasty. In addition, the bone quality is excellent and allows for stable fixation with little room for implant subsidence.
Despite this, RA remains the most common indication for MP arthroplasty at the authors' institution. RA presents the most concerns for the surgeon with regards to soft-tissue impairment and ongoing destruction of the joint capsule, ligaments, and tendons. RA patients also present with varying degrees of osteolysis and osteoporosis. Both of these factors may lead to secondary displacement, migration, or implant subsidence. Patients who have RA and who have soft medullary bone or thin cortical bone and significant destruction or imbalance of the soft tissues should be approached with caution. If there is severe deformity with greater than 80° s of an extension lag or more than 45° of ulnar deviation, the use of standard silicone implants may be necessary. In cases of complete MP joint dislocation with proximal migration of the proximal phalanx, a pyrolytic implant should not be used . Patients who fail arthroplasty because of recurrent subluxation or migration may be salvaged by revising the joint with a constrained silicone implant. Wrist stability should also be achieved before MP joint replacement, particularly in RA patients who have significant intercarpal supination, radial deviation, and ulnar translocation of the wrist. Relative contraindications for pyrolytic carbon implant placement also include loss of extensor function, inadequate dorsal soft-tissue, and evidence of recent infection.
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