Supine lateral radiographs at 90° of knee flexion have a similar diagnostic accuracy for chronic posterior cruciate ligament injuries as stress radiographs

Purpose: This study aimed to evaluate whether supine lateral radiographs (SLRs) could replace stress radiographs for diagnosing chronic posterior cruciate ligament (PCL) injuries and identifying combined PCL injuries (defined as PCL injury with medial collateral ligament or posterolateral ligament complex injury). Methods: In this retrospective study, both SLRs at 30° and 90° of knee flexion (30/90 SLRs) and Telos stress radiographs of patients with chronic PCL injuries (n = 38) and only 30/90 SLRs of healthy controls (n = 84) were taken. Injured-to-normal differences on 30/90 SLRs and stress radiographs were assessed. Correlation analysis was performed to evaluate injured-to-normal differences on 30/90 SLRs and stress radiographs in patients with chronic PCL injury. Subgroup analysis was performed to compare injured-to-normal differences on 30/90 SLRs and stress radiographs between the isolated and combined PCL injury groups. Receiver operating characteristic curves based on 30/90 SLRs were calculated to determine the cut-off value for diagnosing chronic PCL injury and identifying combined PCL injury. Results: Injured-to-normal differences on both 30 SLRs (3.1 ± 3.6 vs 1.6 ± 1.2, P = 0.019) and 90 SLRs (7.5 ± 3.5 vs 1.2 ± 1.0, P < 0.001) were significantly greater in patients with chronic PCL injuries than in healthy controls. Further, 90 SLRs had a highly accurate diagnostic value for chronic PCL injuries (area under the curve 0.958). The cut-off value for diagnosing chronic PCL injuries based on 90 SLRs was 3.0 mm (sensitivity, 94.7%; specificity, 92.9%). Injured-to-normal differences on 30/90 SLRs were significantly correlated with those on stress radiographs. The correlation coefficients were 0.397 (P = 0.014) for 30 SLRs and 0.605 (P < 0.001) for 90 SLRs. The cut-off value for diagnosing combined PCL injuries based on 90 SLRs was 9.6 mm (area under the curve 0.72). Conclusions: The diagnostic accuracy of 90 SLRs for chronic PCL injuries was similar to that of stress radiographs. Therefore, the 90 SLRs are reliable alternative method to assess the posterior knee laxity when the stress radiographs are not available. Level of evidence: Level IV, case series.