Clinically relevant stabilizers of the posteromedial and posterolateral knee: normal anatomy, scanning technique, and ultrasound findings in patients with anterior cruciate ligament tear.

Unrecognized and untreated injuries of the posteromedial and posterolateral corners of the knee are more common than previously thought and have been linked to poor outcomes after anterior cruciate ligament reconstruction. Amongst imaging modalities, magnetic resonance is currently referred to as the gold standard for the evaluation of these regions, but has several limitations, in particular in the identification of subacute and chronic lesions. Recent technological advancements and the progressive refinement of linear probes have expanded the potential of High-resolution ultrasound in demonstrating the stabilizers of the posteromedial and posterolateral corners in both normal and pathological cases, and now this modality may be considered as a useful complementary tool for the evaluation of these structures. The aim of this work is twofold: (i) to review, also with the support of dedicated dissections and schematic drawings, the normal anatomy and the biomechanical role of the clinically relevant stabilizers of the posteromedial and posterolateral knee, including the distal expansion of the semimembranosus tendon, the popliteus muscle-tendon unit, and the posterior oblique ligament; (ii) to illustrate the normal ultrasound appearance of these structures and the spectrum of pathological findings that this modality may disclose in patients with anterior cruciate ligament tear. KEY POINTS: Question Injuries of the posteromedial and posterolateral knee worsen the outcome after anterior cruciate ligament reconstruction but are often underdiagnosed on imaging. Findings Ultrasound has potential in diagnosing tears of stabilizers of the posteromedial and posterolateral knee and provides complementary information to MRI about ligament status and continuity. Clinical relevance In subacute and chronic injuries, ultrasound has some advantages over MRI as it can disclose subtle abnormalities that might otherwise be unrecognized, thereby improving diagnostic confidence and patient counselling.