Relationship between foot and knee external rotation angle and its determinants in direct anterior approach-total hip...

The direct anterior approach (DAA) for total hip arthroplasty (THA) commonly employs a traction table, which externally rotates the limb to facilitate femoral exposure. Although this foot position is typically standardized, the resulting knee external rotation-which may influence femoral stem anteversion-has not been quantitatively assessed. To our knowledge, no prior studies have systematically evaluated the relationship between intraoperative foot and knee external rotation angles or the extent of inter-individual variability. Understanding this relationship, including its inter-individual variability, is important for achieving accurate and reproducible stem alignment during DAA-THA.

We hypothesized that intraoperative knee external rotation achieved with standardized foot positioning would vary among patients, and that this variability could be associated with anatomical or demographic characteristics.

We retrospectively analyzed prospectively collected data from 128 hips undergoing DAA-THA using a traction table; all procedures were performed by two experienced surgeons. With the foot externally rotated to 150 °, intraoperative knee external rotation was determined fluoroscopically by aligning the posterior femoral condyles; the alignment and the corresponding C-arm rotation scale reading were simultaneously verified by the operating team. Femoral anteversion was determined on a preoperative axial CT slice immediately inferior to the femoral head by a single independent observer. Multiple linear regression assessed associations between knee rotation (dependent variable) and age, sex, height, weight, and femoral anteversion (prespecified predictors). Additionally, patients were stratified into three groups based on femoral anteversion: low (≤10 °), moderate (11-20 °) and high (≥21 °) for group comparisons.

The mean knee external rotation angle was 87.1 ° ± 9.9 °, with notable inter-individual variability. Femoral anteversion was the only significant predictor (β = -0.58, R² = 0.35, p < 0.001); other variables showed no statistical significance. Group analysis revealed that patients with high anteversion had significantly lower knee external rotation angles (80.4 ° ± 10.0 °) than those in the low (91.6 ° ± 8.7 °) and moderate (90.1 ° ± 6.8 °) groups (p < 0.001). No difference was found between the low and moderate groups.

While standardized foot positioning at 150 ° generally produced a clinically useful knee rotation near 90 °, high-anteversion cases frequently exhibited insufficient rotation. These findings emphasize the need for individualized intraoperative assessment. From a clinical standpoint, our findings suggest that relying on intraoperative foot position and visual or manual assessment of knee alignment may be inadequate; confirming knee orientation on a true lateral fluoroscopic view and adjusting stem version to maintain safe combined anteversion may help avoid excessive stem anteversion. This pragmatic approach may improve the reproducibility of stem positioning by accounting for anatomical variability.

III; Retrospective Cohort Study.