Late mechanical stability of the proximal coated AML prosthesis

Micromotion of a bone-ingrown anatomic medullary locking (AML) femoral component was evaluated in a specimen retrieved from a 66-year-old man who died 4 years following total hip replacement. Radiographic signs suggested bone ingrowth into the proximal porous coating, but a radiolucent line surrounded by a sclerotic line was present around the distal stem. Both torsional and axial loading to physiologic levels elicited minimal micromotion at the proximal bone-prosthesis interfaces. However, medial-lateral micromotion at the distal tip was found to be as high as 44 microns with axial load. An AML femoral component of similar design was inserted into the opposite normal femur using the cementless press-fit technique originally used for the implanted specimen. Micromotion 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 provides rigid fixation of the implant to bone; however, flexibility of the femur causes increased micromotion of the femur around the smooth distal stem despite initial tight distal fit.