Posterior Tibial Slope Measurement With the Axis Estimate Method Is Not Inferior to the Established Proximal Anatomic Axis Method, and Both Differ From Cortical-Based Measurements.

Abstract

To evaluate whether simpler alternative methods, using a tangential line along the (1) posterior tibial cortex (posterior cortical reference [PCR]), (2) anterior tibial cortex (anterior cortical reference [ACR]), and (3) axis estimate (AE), can yield posterior tibial slope (PTS) measurements comparable in reliability and reproducibility to the proximal anatomic axis (PAA) method. This was a retrospective review of patients aged 18 to 40 years who underwent lateral knee radiographs, followed by magnetic resonance imaging scans within 30 days for pain and suspected internal derangement (between January 2018 and January 2022). Lateral radiographs were required to include at least 10 cm of the tibia distal to the joint line. Patients were grouped as follows based on magnetic resonance imaging findings: anterior cruciate ligament tear, posterior cruciate ligament tear, or no cruciate ligament tear. Each measurement was performed by 2 independent raters. Inter-method, inter-rater, and intrarater reliability was assessed using the intraclass correlation coefficient (ICC). A total of 155 patients met the study criteria: 73 with no cruciate ligament tears, 70 with anterior cruciate ligament tears, and 12 with posterior cruciate ligament tears. Compared with the PAA method, the AE method showed no difference (10.3°, P = .09), whereas the ACR method (11.7°, P < .0001) and PCR method (7.5°, P < .0001) differed significantly from the PAA method. All methods had moderate reliability (ICCs, 0.5-0.74). The inter-method ICC was excellent for AE and PAA (ICC, 0.87) and was lowest for PAA and PCR (ICC, 0.47). The reliability of the AE method was not significantly different from that of the established standard PAA method; thus, it may serve as a suitable substitute in the clinical setting. The ACR and PCR methods were significantly different from the PAA method. Although the PAA and PCR methods had similar inter-rater reliability, the PCR method may underestimate PTS and therefore may not be interchangeable with the PAA method. The PAA method requires several steps to measure PTS, which can be time-consuming and prone to variability. A simpler and equally reliable method such as the AE method may increase utilization and maintain performance.