In-vitro biomechanical evaluation of stress shielding and initial stability of a low-modulus hip stem made of β type Ti-33.6Nb-4Sn alloy.

Stress shielding-related proximal femoral bone loss after total hip arthroplasty occurs because of the different stiffness of metallic alloy stems and host bone. To overcome this, we fabricated a low-modulus cementless hip stem from β-type Ti-33.6Nb-4Sn alloy (TNS). Then we evaluated its stiffness, stress shielding, and initial stability compared with a similar Ti-6Al-4V alloy stem. Stiffness was determined by axial compression and cantilever-bending tests. Thirteen triaxial strain gages measured cortical strain. Stress shielding was defined as the percentage of intact strain after stem insertion. To evaluate initial stability, displacement transducers measured axial relative displacement and rotation. Intact and implanted femurs underwent single-leg-stance loading. Axial stiffness was 56% lower in the TNS stem than in the Ti-6Al-4V stem, and bending stiffness of the TNS stem decreased gradually from the proximal region to the distal region, being ≤ 53% that of the Ti-6Al-4V stem, indicating gradation of Young's modulus. The TNS stem decreased stress shielding in the proximal calcar region (A1: 83%, B1: 85% relative to intact cortical strain) without affecting the proximal lateral region (B3: 53%). The initial stabilities of the stems were comparable. These findings indicate that the TNS stem with gradation of Young's modulus minimizes proximal femoral bone loss and biological fixation, improving long-term stability.