Measuring Posterior Tibial Slope: A Comparison Using a 10-cm Anatomic Axis Versus 15-cm Anatomic Axis.

Background: Accurate measurement of posterior tibial slope (PTS) is for understanding knee biomechanics and stability, as well as in planning surgical procedures. The validated methodology for measuring PTS requires outline visualization of 15 cm distal to the proximal tibia, a cutoff that is not always met on standard lateral knee radiographs.

Purpose/hypothesis: The goal of this study was to critically evaluate whether using a 10-cm anatomic axis on conventional lateral knee radiographs is comparable with the established 15-cm standard. The authors hypothesized that a shortened 10-cm anatomic axis would yield comparable results to the conventional 15-cm method.

Study Design: Cohort study (diagnosis); Level of evidence, 3.

Methods: Lateral knee radiographs were reviewed until a cohort of 100 patients was identified where ≥15 cm of the tibial shaft distal to the joint line could be measured. PTS was calculated using both the conventional 15-cm proximal anatomic axis (15-PTS) and the proposed modified 10-cm axis (10-mPTS). Measurement reliability and agreement were quantitatively assessed via intraclass correlation coefficients (ICCs) and Bland-Altman analysis, respectively. Paired tests and linear regression were performed to evaluate the equivalence of methods. Additionally, a second set of 100 lateral knee radiographs was randomly selected to determine a percentage of radiographs that could be measured ≥15 cm below the joint line.

Results: There was a statistically significant congruence between the 10-mPTS and 15-PTS techniques (ICC, 0.92; < .001). The mean PTS discrepancy was statistically significant at 0.94°, with a 95% CI ranging from 0.78° to 1.0°. Both methods demonstrated high inter- and intrarater reliability (ICCs > 0.90). On 61% of standard lateral knee radiographs queried, we were unable to visualize 15 cm of the proximal tibia.

Conclusion: The 10-mPTS method exhibits <1° of deviation compared with the conventional 15-PTS approach, establishing it as a practical and reliable alternative for assessing PTS on lateral knee radiographs, particularly when visualization of 15 cm distally is not possible.