The empty roof sign: A new indirect magnetic resonance imaging sign of posterior cruciate ligament injury

PURPOSE: Posterior cruciate ligament (PCL) injuries can pose diagnostic challenges. Magnetic resonance imaging (MRI) is an important diagnostic tool, yet direct visualization of partial or chronic PCL tears is limited, and reliable indirect imaging signs are not well established. The aim of this study was to describe a novel indirect MRI sign of PCL injury and to evaluate its occurrence in a retrospective case series of patients with and without intact PCL.

METHODS: The 'empty roof sign' (ERS) was defined as an indirect MRI finding on sagittal T2-weighted fat-suppressed images, characterized by a fluid-filled triangular gap between the ACL and the intercondylar notch roof. In a retrospective case series, patients with arthroscopically confirmed partial or complete PCL injuries who underwent surgery were compared to patients with arthroscopically intact PCL who underwent surgery for reasons other than knee ligament reconstruction. Preoperative MRIs were reviewed for the presence of the ERS. Diagnostic performance was assessed using arthroscopy as the reference standard.

RESULTS: The retrospective review of 20 patients with arthroscopically confirmed partial or complete PCL injuries demonstrated a positive ERS on the preoperative MRI in 80% of cases. Notably, all patients with partial PCL injuries exhibited the ERS despite preserved PCL morphology on MRI. In a comparator case series of 22 patients with intact cruciate ligaments, the ERS was present in only 9% of cases. The ERS showed a sensitivity of 80% and specificity of 91%, with a positive predictive value of 89% and a negative predictive value of 83%, yielding an overall accuracy of 86%. The difference between the two case series cohorts was statistically significant ( p  < 0.001).

CONCLUSIONS: Findings from this case series suggest that the ERS may serve as a promising marker of PCL insufficiency, particularly in partial PCL injuries where the ligament appears morphologically intact on the conventional MRI.

LEVEL OF EVIDENCE: Level IV, retrospective case series.