Periprosthetic tibial bone mineral density changes after total knee arthroplasty

Background and purpose - Total knee arthroplasty (TKA) may cause postoperative periprosthetic bone loss due to stress shielding. Bone also adapts to mechanical alterations such as correction of malalignment. We investigated medium-term changes in bone mineral density (BMD) in tibial periprosthetic bone after TKA. Patients and methods - 86 TKA patients were prospectively measured with dual-energy X-ray absorptiometry (DXA), the baseline measurement being within 1 week after TKA and the follow-up measurements being at 3 and 6 months, and at 1, 2, 4, and 7 years postoperatively. Long standing radiographs were taken and clinical evaluation was done with the American Knee Society (AKS) score. Results - The baseline BMD of the medial tibial metaphyseal region of interest (ROI) was higher in the varus aligned knees (25%; p < 0.001). Medial metaphyseal BMD decreased in subjects with preoperatively varus aligned knees (13%, p < 0.001) and in those with preoperatively valgus aligned knees (12%, p = 0.02) between the baseline and 7-year measurements. No statistically significant changes in BMD were detected in lateral metaphyseal ROIs. No implant failures or revision surgery due to tibial problems occurred. Interpretation - Tibial metaphyseal periprosthetic bone is remodeled after TKA due to mechanical axis correction, resulting in more balanced bone stock below the tibial tray. The diaphyseal BMD remains unchanged after the initial drop, within 3-6 months. This remodeling process was related to good component survival, as there were no implant failures or revision operations due to tibial problems in this medium-term follow-up.