Elevated Medial Distal Resection Reduces Bone Density in Valgus Total Knee Arthroplasty.

Insufficiency periprosthetic fracture after total knee arthroplasty is uncommon but serious. In valgus alignment, chronic unloading of the medial femoral condyle may reduce local bone mineral density (BMD), increasing fracture risk after coronal realignment. We retrospectively studied 40 valgus knees that underwent primary total knee arthroplasty and compared BMD at the distal femoral resection plane, measured using quantitative computed tomography, with 40 propensity score-matched varus knees (matched for age, sex, body mass index, side, and degree of deformity). In valgus knees we also simulated distal femoral resection at the sulcus level and at planes 2 mm and 4 mm deeper to examine the effect of resection depth on local bone quality. In valgus knees, BMD within the medial femoral condyle was highest laterally and lowest medially. Greater valgus alignment correlated with lower central BMD, and greater medial resection depth was associated with lower BMD across regions. In simulation, deeper resection levels yielded progressively lower BMD. In matched varus knees, the pattern was reversed, with higher BMD medially than laterally. These findings indicate that the medial distal femur in valgus knees is relatively osteopenic and that additional distal resection preferentially engages lower-density bone. CLINICAL SIGNIFICANCE: Careful control of medial distal femoral resection depth during valgus correction may help lower the risk of early insufficiency fracture.