The Tibial Sag Radiograph Is a Reliable Radiographic Technique to Quantify Posterior Cruciate Ligament Laxity

Purpose To evaluate the interobserver and intraobserver reliability associated with using tibial sag stress radiographs as a measure of posterior laxity in patients with posterior cruciate ligament injuries. Methods All patients with posterior cruciate ligament injuries who underwent stress radiographic measurements using bilateral tibial sag stress radiographs and Telos stress radiographs between December 2018 and July 2024 were identified from an institutional database. All radiographs were measured by 3 independent blinded observers. The magnitude of posterior laxity was calculated by taking the side‐to‐side difference (SSD) in posterior tibial sag between the injured and uninjured limb. Clinical laxity of injured knees was assessed by a fellowship‐trained sports surgeon. Intraclass correlation coefficients (ICCs) were calculated to assess the interobserver and intraobserver reliability of each stress radiograph technique. Results Among the 74 included knees, there were 26 (35.1%) grade I injuries, 39 (52.7%) grade II injuries, and 9 (12.2%) grade III injuries. The mean age of the cohort was 34.4 ± 12.1 years (range, 14‐67 years), and 54 (73%) were men. The tibial sag method showed good to excellent interobserver reliability (ICC 0.89, 95% confidence interval [95% CI] 0.84‐0.92) in measuring SSD. The Telos method showed moderate to good interobserver reliability (ICC 0.70, 95% CI 0.59‐0.78). The intraobserver reliability of measuring SSD using the tibial sag method was excellent for 2 reviewers (ICC 0.96, 95% CI 0.94‐0.97; ICC 0.95, 95% CI 0.92‐0.97) and good to excellent for 1 reviewer (ICC 0.91, 95% CI 0.86‐0.94). Conclusions Tibial sag radiography is a reliable method that can be used to identify posterior displacement of the tibia relative to the femur as measured by SSD in patients with posterior cruciate ligament injuries. Level of Evidence Level III, diagnostic retrospective comparative study.