Stress Shielding in Cemented Hip Implants Assessed from Computed Tomography

Abstract

Background: Aseptic loosening is the major cause of revisions for hip replacement. This mode of failure is often caused by stress shielding. Stress shielding in the femur occurs when some of the loads are taken by the prosthesis and shielded from going to the bone. There is little information regarding the stress shielding among cemented hip implants.Purpose: The purpose of this study is to investigate the effect of stress shielding on the proximal femur with a femoral prosthesis.Methods: A patient had undergone open reduction and internal fixation (ORIF) due to a comminuted reversed oblique fracture of the right intertrochanteric hip. ORIF had failed and was converted to bipolar hemiarthroplasty. CT scans were performed on both the right and left hips. Housefield units were determined by using the probe tool. By using equations formulated by Carter and Hays, Linde et al., various parameters such as apparent density, Young's' modulus and ultimate strength were calculated. The results were compared to that a native hip. Results:The hip with the cemented implant had a significant increase in the apparent density, Young's modulus and ultimate strength, when compared to the left hip. In addition, it was found that the right hip had a higher strain energy density than that of the left.Interpretation: It has been concluded the most stress shielding occurred in the calcar region of the femur. The instances of stress shielding have been extensively reported for non-cemented or direct bone to implant constructs, this paper reports stress shielding in cemented implants supported by imaging data and biomechanical calculations carried out at the bone-cement-metal interface.