Does implant weight matter? The effect of total knee arthroplasty on thigh mass distribution and estimated hip loading.

PURPOSE: Few studies in the literature have evaluated the effect of weight changes on knee joint biomechanics induced by total knee arthroplasty (TKA). The aim of this study was to evaluate changes in the hip muscular moment and the thigh centre of mass following primary TKA. METHODS: All primary TKAs performed with conventional instrumentation between April and December 2024 were prospectively included. Radiographic measurements were obtained to develop an analytical biomechanical model. The resected bone was collected and weighed intraoperatively, together with the femoral component and implanted cement. An analytical biomechanical model was developed to estimate changes in thigh weight, centre of mass position and hip muscular moment before and after TKA. RESULTS: A total of 70 patients were included in the study. Primary TKA resulted in a statistically significant increase in thigh weight and a distal shift of the thigh centre of mass (p < 0.05). The average increase in implant-related knee weight after TKA was 333.5 g (standard deviation [SD] 51.0 g), while the femoral weight increased by 199.1 g (SD 40.1 g), resulting in an average increase in total thigh weight of 2.8% (SD 0.6%; range: 1.6-4.2%). The analytical biomechanical model showed a statistically significant distal shift of the thigh centre of mass of 6.4 mm (SD 1.5 mm; range: 3.7-9.7 mm) after TKA (p < 0.05). This redistribution of thigh mass was associated with a statistically significant estimated increase in hip muscular moment of 6.3% (SD 1.4%; maximum increase 9.3%; p < 0.05). CONCLUSION: Primary TKA alters thigh mass distribution through an increase in limb weight and a distal shift of its centre of mass. Within the assumptions of this analytical model, these changes result in an increase in estimated hip muscular moment. Further biomechanical and clinical studies are needed to determine the clinical relevance of these theoretical findings. LEVEL OF EVIDENCE: Level IV.