Architectural changes in the proximal femur following prosthetic insertion: preliminary observations of an animal model

A study was made of the changes occurring in the proximal femur following unilateral total hip replacement in fourteen sheep. Implants were observed 0-12 months post-operatively. Contralateral implants inserted at sacrifice served as controls. The paired specimens were cut into five transverse sections and stereologically analyzed. The following phenomena were seen during the post-operative year: (a) calcar resorption; (b) encapsulation of the prosthetic cement in fibrous sheath; (c) interruption of the trabecular network between the cement-stem system and the cortex; (d) increase in the cross-sectional moment of inertia of the cortical shell. These changes form a consistent pattern of biological responses to the implant, the details of which are important because they suggest failure mechanisms and demonstrate the inadequacy of simple analytical models based on static pictures of the bone-implant composite structure. Failure modes may depend on the initial degree of bone-cement interdigitation. Concentric layers of fibrous tissue surrounded by layers of dense bone were formed where interdigitation was not extensive. Where interdigitation was maximal, a band of bony resorption separated the cement-bone complex from the endosteal trabecular bone. Increased levels of bone-cement interdigitation would not appear to influence the ultimate outcome, but, rather, only the mode of failure of the prosthetic attachment.