Extramedullary porous coating to prevent diaphyseal osteolysis and radiolucent lines around proximal tibial replacements. A preliminary report.

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Abstract
Nineteen patients who had a primary malignant bone tumor of the proximal end of the tibia were managed with wide resection and implantation of a tibial endoprosthesis with cement, combined with a rotating-hinge knee replacement. In the first ten patients, a ring of porous coating was applied to the part of the tibial replacement that was just proximal to the intramedullary portion of the stem, to encourage ingrowth of bridging bone. No growth of bone into these porous rings occurred, and the rings were not applied to the next nine endoprostheses. During the subsequent follow-up period, progressive osteolysis (resorption of more than two millimeters of the proximal end of the remaining tibial cortex) developed in only one patient (who had a deep infection) in the group of ten patients who had a porous ring, compared with five of the nine patients who did not have a porous ring but in whom osteolysis developed. This difference was significant (p < 0.001). In addition, the tibial osteolysis was associated with the subsequent development of progressive periprosthetic radiolucent lines at the bone-cement interfaces (p = 0.001). We postulated that, in the patients who had an extramedullary porous ring, the periprosthetic fibrous capsule that formed around the extramedullary portion of all the implants became firmly adherent to the ring, thereby sealing off the part of the capsule that was distal to the ring. The particulate wear debris that was contained within the articular portion of the capsule could not reach the distal tibial bone surrounding the prosthetic stem, and debris-incited osteolysis, which leads to loosening, did not develop. If this mechanism is confirmed, it could have important implications for the design of implants that are used in joint replacement.

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