The clinical presentation is consistent with an ACL tear. The ACL extends form the anterior intercondylar area of tibia to posteromedial femur.
History and physical exam are the primary modalities to diagnose an ACL tear. A history of a pop followed by an immediate effusion is highly suspicious of an ACL tear. On physical exam a positive anterior drawer test occurs when the tibia moves anterior to the femur on manipulation with significant increased laxity compared to the other side.
van Eck et al. performed a meta-analysis and found that that while the sensitivity of the anterior drawer test is only 38%, it has a high specificity at 81%. The Lachman test - performed in a similar fashion to the anterior drawer, but with the knee flexed at 20-30 degrees - proved to have the highest sensitivity at 81% and the same specificity of 81%.
Cimino et al. describe different modalities for evaluating a torn ACL and emphasize that evaluation of the ACL should be performed immediately after an injury if possible, but is often limited by swelling and pain. They argue that when performed properly, a complete knee examination is more than 80 percent sensitive for an ACL injury. They further explain how specific exercises aimed at stabilizing the knee can be effective prophylaxis against ACL injury.
The video is an animation that illustrates the anatomic positioning of the main stabilizing ligaments of the knee.
Answer 1: This describes the attachments of the posterior cruciate ligament.
Answer 3: This describes the attachment of the patellar tendon to the tibia.
Answer 4: This describes the attachments of the lateral collateral ligament.
Answer 5: This describes the attachments of the medial collateral ligament.
Cimino F, Volk BS, Setter D. Anterior cruciate ligament injury: diagnosis, management, and prevention. Am Fam Physician. 2010 Oct 15;82(8):917-22. Review. PubMed PMID: 20949884.
PMID:20949884 (Link to Abstract)
van Eck CF, van den Bekerom MP, Fu FH, Poolman RW, Kerkhoffs GM. Methods to diagnose acute anterior cruciate ligament rupture: a meta-analysis of physical examinations with and without anaesthesia. Knee Surg Sports Traumatol Arthrosc.
PMID:23085822 (Link to Abstract)