Association of Noncontact Anterior Cruciate Ligament Injury With Presence and Thickness of a Bony Ridge on the Anteromedial Aspect of the Femoral Intercondylar Notch

Background: Noncontact anterior cruciate ligament injuries are among the most common injuries in sports medicine, and identification of risk factors for noncontact injury is an area of active research. Hypothesis: Evaluation of the femoral notch along the path of the anterior cruciate ligament may elucidate anatomical risk factors previously unseen on conventional images. Study Design: Case-control study; Level of evidence, 3. Methods: Twenty-seven patients with noncontact anterior cruciate ligament injuries were matched to healthy individuals for height, weight, age, and sex in this case-control study. Sagittal magnetic resonance images of the contralateral knee of the injured patients and randomized knees of the healthy controls were digitally transformed for viewing along the plane of the anterior cruciate ligament and evaluated for abnormalities of femoral notch outlet shape. Femoral notch shape was also evaluated by computer-generated surface models of the knee. Results: A bone ridge was observed on the medial side of the anterior notch outlet, and increasing ridge thickness was strongly associated with noncontact anterior cruciate ligament injury (3.87 +/- 2.17 mm in injured and 2.16 +/- 1.80 mm in controls; P = .0014). Anterior and posterior femoral notch outlet stenosis were both significantly associated with noncontact anterior cruciate ligament injury (P = .0008 and .02, respectively), although anterior outlet stenosis became nonsignificant when stratified by the presence of an anteromedial bone ridge. Finally, images directed through the femoral notch with the knee at 45 degrees of flexion provided an unobstructed view for ridge detection that may be utilized in plain radiography. Conclusion: The presence and thickness of an anteromedial bone ridge in the femoral notch has been identified as a potential risk factor for noncontact anterior cruciate ligament injury. A biomechanical injury model involving the femoral notch ridge in anterior cruciate ligament tears is proposed in which the anterior cruciate ligament may be tented and stretched over the anteromedial notch ridge during knee valgus and internal tibial rotation; alternatively, the anterior cruciate ligament may be impinged at the lateral anterior outlet wall during knee valgus and external tibial rotation. Further investigation to determine the origin of the bone ridges and the validity of the proposed injury model is recommended.