Distal femoral bone mineral density after total knee arthroplasty: a comparison with general bone mineral density.

The bone mineral density (BMD) of the distal femur may decrease after cemented total knee arthroplasty (TKA) as a result of the stress shielding effect of the femoral component. The purpose of the study was to determine the changes in BMD of the distal femur compared with those of the femoral necks and the lumbar spine after cemented TKA. BMD of two regions of interest in the distal femur, both femoral necks and the lumbar spine was measured with dual-energy X-ray absorptiometry in 10 patients (age range 41-80 years, mean 62 years) with 12 TKAs preoperatively and during follow-up for 1 year after surgery. The hip and spine measurements were performed for comparison to assess if general changes in BMD occurred after TKA. The median decrease in BMD in the region behind the anterior flange of the femoral component was 22% (95% CI: 12%-33%), while the average decrease in the region just above the femoral component was 8% (95% CI: 2%-13%). The difference in change of BMD between both regions before and 1 year after TKA was significant (p = 0.03). We found less than 1% difference in BMD of both femoral necks and the lumbar spine on average between the preoperative and 1 year follow-up measurements (not significant). A significant periprosthetic distal femoral bone resorption occurred after TKA. BMD of the femoral necks and lumbar spine did not differ 1 year after TKA.