Mechanical Axis Deviation Angle Versus Hip-Knee-Ankle Angle in Total Knee Arthroplasty: Correlation and Reliability

This study aimed to evaluate the relationship between the Mechanical Axis Deviation Angle (MAD-A) and Hip-Knee-Ankle angle (HKA) in total knee arthroplasty (TKA), and to compare their inter-observer reliability.

MAD-A and HKA measurements were obtained for 108 consecutive knees (57 patients, age, 63.0 ± 4.2 years) undergoing primary TKA from February 2018 to February 2019. Measurements were taken preoperatively and 6 weeks postoperatively using long-leg radiograph, and 3 days postoperatively using CT scanogram. MAD-A was measured as the angle between the femoral mechanical axis (line from the center of the femoral head to the center of the knee) and the overall limb mechanical axis (line from the center of the femoral head to the center of the ankle). This angle specifically quantified the angular deviation of the femoral mechanical axis from the ideal limb mechanical axis. Three independent observers performed measurements for reliability testing.

Bland-Altman analysis revealed consistent differences between measurements, with MAD-A demonstrating lower values compared to HKA (mean differences - 1.4° to - 6.2°), representing approximately 43-54% of HKA values. MAD-A and HKA showed strong positive correlations at all time-points (r > 0.97, p < 0.001). Inter-observer reliability was superior for MAD-A (ICC > 0.99) compared to HKA (ICC 0.966-0.988). Mean HKA improved from - 11.5° ± 6.0° to - 2.6° ± 2.6° at 6 weeks, while MAD-A improved from -5.3° ± 2.8° to - 1.2° ± 1.1° (both p < 0.001).

MAD-A demonstrated strong correlation with HKA and superior inter-observer reliability in assessing coronal alignment in TKA. These findings suggest that MAD-A could serve as a complementary tool to HKA in alignment assessment. Further research is needed to establish normative values across knee pathologies.