Integration of Total Hip Arthroplasty Following Osseointegration in an Above-Knee Amputee

A 64-yr old female patient presents with a history of right above-knee amputation 13 yrs ago following sarcoma radiation therapy. Eight years ago, she underwent an osseointegration (OI) procedure secondary to difficulty ambulating with traditional socket fit prosthesis. She was able to return to an independent and active lifestyle, ambulating over 15,000 steps a day. Because of worsening hip osteoarthritis, she underwent a right total hip arthroplasty (THA) and OI revision several months after her initial OI procedure, producing a complex biomechanical scenario (Fig. 1).FIGURE 1: Right total hip arthroplasty and osseointegrated femur.The direct skeletal attachment in femur OI fundamentally alters force transmission patterns, creating a rigid system where impact forces transmit directly to the proximal femur and hip joint.1 This can influence biomechanical discrepancies through increased peak forces, altered muscle mechanics, and compensatory gait patterns.1 In normal bone, stress is gradually transferred along the femur length. With an osseointegrated implant, stress shielding occurs where the metallic implant bears a greater proportion of weight compared to the surrounding bone tissue, leading to bone remodeling through diminished density and increased weakness.1 The addition of a THA femoral head and stem alone can change the force distribution and gait pattern via varied femoral offset, center of rotation, leg length, and component positioning.2 With both the THA and OI, the proximal femur must then accommodate two distinct load patterns: the axial and torsional forces from the OI implant traveling longitudinally through the femoral shaft, and the multidirectional forces from the hip joint, including both compressive and shear components.1–3 This dual-implant system demands careful muscle function consideration of both the preexisting biomechanical alterations from OI and the new forces introduced by THA.1–3 The abductor muscles, particularly important for hip stability and gait, may have altered lever arms due to the THA geometry.2 This can affect their mechanical advantage and the overall stability of the hip during walking. The presence of the osseointegrated component may also influence muscle attachment and function along the femur.1 The success of this combination heavily depends on implant compatibility and fixation strategies, particularly given the limited femoral bone stock available after OI implementation.3 This case uniquely illustrates the combination of THA with an osseointegrated prosthesis in the same limb, raising important considerations for those who care for the amputee population as OI becomes more prevalent in amputee care. Further investigation is needed to examine how both implant positioning and spacing influence functional and mechanical performance.