Proximal anatomical axes show bidirectional variability: Accurate tibial slope requires mechanical axis measurement

To determine whether posterior tibial slope (PTS) measured to proximal anatomical axes can serve as a reliable proxy for PTS measured to the mechanical tibial axis (mTA).

In patients ≥18 years with failed anterior cruciate ligament reconstruction, medial and lateral PTS were measured relative to the mTA on a lateral lower-leg radiograph, and relative to the proximal tibial anatomical axis (PTAA), anterior tibial cortical axis (ATC) and posterior tibial cortical axis (PTC) on a lateral knee radiograph. Agreement was assessed using correlation coefficients, measurement error and Bland-Altman analysis. Diagnostic accuracy of the conventional ≥12° PTAA-based PTS cut-off for slope-reducing osteotomy was determined.

Fifty-nine knees were analyzed. Correlation between proximal anatomical axes and mTA-based PTS measurements was strong (r ≥ 0.71), but agreement was poor. Mean measurement error for PTAA was 0.1 ± 2.4° (range: -5.7° to 7.6°) and -0.5 ± 2.4° (range: -6.6° to 5.2°), for ATC -3.4 ± 2.6° (range: -8.6° to 5.9°) and -3.7 ± 2.6° (range: -9.9° to 2.7°) and for PTC 3.0 ± 2.6° (range: -2.9° to 10.4°) and 2.7 ± 2.4° (range: -3.6° to 7.8°) for the medial and lateral plateau, respectively. For PTAA, absolute measurement error ≥2° occurred in approximately 50% of knees. Bland-Altman analysis showed 95% levels of agreement ranging from -4.89° to 4.62° (medial) and -5.11° to 4.11° (lateral). Using the ≥12° cut-off on PTAA-based PTS values misclassified approximately one in three knees (19.5% false-positive and 13.2% false-negative) relative to the mTA-based PTS reference values.

Despite strong correlation, proximal anatomical axes-based PTS measurements on routine lateral knee radiographs show substantial, bidirectional measurement error outside clinically acceptable limits of agreement compared to mTA-based PTS measurements on full-length lateral lower-leg radiographs. Single-exposure, full-length lateral lower-leg radiographs should be obtained to measure PTS relative to the mTA when considering slope-reducing osteotomy.

Level IV.