TiNbSn stems with gradient changes of Young’s modulus and stiffness prevent stress shielding compared to similarly designed stems

Abstract Background The difference between Young’s moduli of the femur and the stem causes stress shielding (SS). TiNbSn (TNS) stem has a low Young’s modulus and strength with gradient functional properties during the change in elastic modulus with heat treatment. The aim of this study was to investigate the inhibitory effect of TNS stems on SS and their clinical outcomes compared to conventional stems. Methods This study was a clinical trial. Primary THA was performed using a TNS stem from April 2016 to September 2017 for patients in the TNS group. Unilateral THA was performed using a Ti6Al4V alloy stem from January 2007 to February 2011 for patients in the control group. The TNS and Ti6Al4V stems were matched in shape. Radiographs were obtained at the 1- and 3-year follow-ups. Two surgeons independently checked the SS grade and appearance of cortical hypertrophy (CH). The Japanese Orthopaedic Association (JOA) scores before and 1 year after surgery were assessed as clinical scores. Results None of the patients in the TNS group had grade 3 or 4 SS. In contrast, in the control group, 24% and 40% of patients had grade 3 and 4 SS at the 1- and 3-year follow-ups, respectively. The SS grade was lower in the TNS group than in the control group at the 1- and 3-year follow-ups (p < 0.001). The frequencies of CH in both groups were no significant difference at the 1- and 3- year follow-ups. The JOA scores of the TNS group significantly improved at 1 year after surgery and were comparable to control group. Conclusion The TNS stem prevented SS at 1 and 3 years after THA compared to the proximal-engaging cementless stem, although the shapes of the stems matched. The TNS stem could prevent SS, stem loosening, and periprosthetic fractures. Trial registration: Current Controlled Trials ISRCTN21241251. https://www.isrctn.com/search?q=21241251 The date of registration was October 26, 2021. Retrospectively registered.