Teaching Case

Patient presented with right knee joint instability without any history of trauma. The symptoms were more pronounced while walking down the stairs, with an onset of 5 years prior to imaging. On physical examination, both the pivot shift and the Lachman’s tests were positive for anterior cruciate ligament (ACL) tear.

The axial T2-w, coronal T1-w and sagittal PD-w MR images (Fig. 1-3), showed a bony outgrowth at the medial aspect of the lateral femoral condyle projecting into the intercondylar notch. The ACL was horizontally displaced and torn. Moderate joint effusion was also evident.

A. The ACL is a stabilizer of the knee that resists anterior translation and secondarily resists varus and valgus forces. ACL tear usually occurs following a quick deceleration, hyperextension or rotational injury. Very rarely, the ACL may be torn due to its impingement against the intercondylar notch.
B. An acute ACL tear following trauma is characterized by pain, haemarthrosis and instability. At the time of injury, a popping sound or sensation may occur. The role of MRI is to explore the presence of ACL rupture and in addition to show any associated findings such as meniscal tears and osteochondral injuries.
C. The MRI findings in ACL tears include discontinuation, swelling, and horizontal course [1]. Non-traumatic ACL tears may result from a mal-positioned screw after ACL reconstruction and in non-operated patients from intecondylar notch disorders such as narrowing, enthesophytes and congenital bony outgrowth [2].
D. A torn ACL in a young symptomatic adult should be treated with arthroscopic reconstruction. MRI is necessary to confirm the ACL tear and reveal any additional pathology. In our case, it showed in addition the presence of a bony outgrowth, which allowed the proper treatment planning that included removal of the extra bone.
E. ACL rupture in non-injured patients may result from chronic impingement causing ligamentous degeneration. MRI is the imaging method of choice for exploring this underlying cause.