The biologic interface between bone and cementless femoral endoprostheses.

Femoral canal membrane tissue from patients who had revision of an uncemented endoprosthesis to a total hip replacement was analyzed and interlocking bone specimens were retrieved from implant fenestrations to compare membrane histology with microscopic bony remodelling. Radiographs of each hip were evaluated for bony adaptations and correlated with implant stability and interface histology. Uncemented canal membranes are composed mostly of mildly cellular dense connective tissue. Significant inflammation is rare, even in the presence of moderate metallic debris. A fibrocartilage interface often occurs above interlocking bone in areas subject to direct compressive loads. A thin membrane helps to distribute loads evenly from the rigid implant to more compliant bone. Dense lines of bone around the implant stem may be a biologic response indicating a steady-state, load-sharing equilibrium; they are not a reliable sign of implant instability. Retrieved interface membranes allow a direct view of the prosthetic-bone junction of human joint implants. In the absence of polyethylene and methacrylate wear debris, the membrane contains quiescent fibrous tissue with little inflammation. Membrane formation appears to be a mechanical adaptation to improve load distribution in the proximal femur. Presence and function of the membrane must be considered in the design and application of future press-fit implants.