Why did we undertake this study?
Posterior cruciate ligament (PCL) injuries of the knee are difficult to diagnose. MRI and clinical findings can be subtle, especially in partial or chronic injuries, leading to a frequent delayed or missed diagnosis. For the patient, this diagnostic uncertainty can mean persistent complaints and inability to perform daily activities despite several imaging and clinical examinations that appear normal at first glance, but it may be associated with long-term impairment of knee function and poor outcomes.
While we already had conducted a study to describe an uncommon triad of clinical signs in partial PCL injuries, we wanted to complement existing diagnostic methods for PCL injuries, to both help clinicians to recognize these tears more reliably and to allow patients to be diagnosed more quickly. We therefore had a closer look at their MRI to improve the radiological detection of PCL injuries.
What is the specific question we wanted to answer?
The research question came from our clinical experience. We had the impression that a lot of the patients with PCL injuries had an unusual course of the anterior cruciate ligament (ACL) on sagittal MRI thereby creating a space above the ACL, which we termed “empty roof”. To confirm this, we built a study to verify whether this sign represented a possible indirect MRI indicator of a PCL injury and determined how frequently this sign appeared in patients with confirmed PCL tears and whether it appeared in a series of uninjured knees.
What did we find?
The study found that the empty roof sign was consistently observed in patients with PCL injury, especially in partial tears which are amongst the most difficult tears to diagnosis. To the opposite, the empty roof sign was rarely present in uninjured knees. These findings suggest the empty roof sign could be a useful indirect marker of PCL damage on MRI.
What are the implications of our findings?
These preliminary results show that the empty roof sign has promising diagnostic value for PCL injuries. Future studies are needed to understand whether this sign could in the future serve as an additional imaging feature when evaluating the knee and reduce the likelihood of missed or delayed diagnoses of PCL injuries. They should validate this sign in a larger patient population, assess its reliability and how it may be integrated into routine MRI assessment protocols. The discovery of the empty roof sign could however have important implications both for the orthopedic surgeons and radiologists. Earlier and accurate detection of PCL injuries would help to ensure that patients receive before chronic instability develops.
Authors:
Magosch A, Mengis N, Ibañez M, Nührenbörger C, Mouton C, Hoffmann F, et al. The empty roof sign: a new indirect magnetic resonance imaging sign of posterior cruciate ligament injury. J Exp Orthop. 2026;13:e70805.
Dr. med. Amanda Magosch, Médecin en voie de spécialisation (MEVS), Service de Médecine du Sport et de Prévention / Service de Chirurgie Orthopédique