LATE MECHANICAL STABILITY OF THE PROXIMAL COATED AML PROSTHESIS

Micromotion of a bone-ingrown anatomic medullary locking (AMI.) femora l component was evaluated in a specimen retrieved from a 66-year-old m an who died 4 years following total hip replacement. Radiographic sign s suggested bone ingrowth into the proximal porous coating, but a radi olucent line surrounded by a sclerotic line was present around the dis tal stem. Both torsional and axial loading to physiologic levels elici ted minimal micromotion at the proximal bone-prosthesis interfaces. Ho wever, medial-lateral micromotion at the distal tip was found to be as high as 44 mum with axial load. An AML femoral component of similar d esign was inserted into the opposite normal femur using the cementless press-fit technique originally used for the implanted specimen. Micro motion proximally was much greater in the freshly implanted systems in response to both torsional and axial loading than in the bone ingrown specimen. However, distal medial-lateral micromotion was much greater for the bone-ingrown stem. These findings show that bone ingrowth pro vides rigid fixation of the implant to bone; however, flexibility of t he femur causes increased micromotion of the femur around the smooth d istal stem despite initial tight distal fit.