Anterior Cruciate Ligament (ACL) Tear
- Most commonly injured of the major knee ligaments
- Seen in athletes and nonathletes
- ACL is a stabilizer of the knee that resists anterior translation and secondarily resists varus and valgus forces
- Patients with ACL injury have variable knee instability that may limit even ordinary daily activities
- Long-term morbidity is common with sequelae including osteoarthritis and secondary meniscal tears
- Routine knee MRI includes sagittal, coronal, and axial planes combined with fluid-sensitive fat-suppressed and non-fat-suppressed pulse sequences without contrast; field of view includes the suprapatellar recess at the superior extent and the proximal tibiofibular joint at the distal extent
- Acutely, edema (high T2 signal) and fragments of torn ACL may be seen
- Chronically, ACL may be absent, replaced by fat and scar tissue
- Incomplete tears seen as partial discontinuity or focal high T2 signal
- In sports-related ACL injuries, bone bruises are common in posterior tibia and anterior portion of medial femoral condyle
- Cartilage and osteochondral defects are common, especially along the posterior tibia
Mucoid degeneration, more common in older age groups, may resemble low-grade ACL tears with high T2 signal within its substance
- Mucoid degeneration most commonly occurs along entire central portion of ligament; partial ACL tears tend to involve the periphery and be more focal
- T2 Sag Oblique best for acute injuries
- Sag Oblique PD Fat-Sat best for chronic tears (T2 Sag Oblique may show scar mimicking intact ligament)
- Vahey TN, Broome DR, Kayes KJ, Shelbourne KD. Acute and chronic tears of the anterior cruciate ligament: differential features at MR imaging. Radiology 1991;181:251–253
- Bergin D, Morrison WB, Carrino JA, Nallamshetty SN, Bartolozzi AR. Anterior cruciate ligament ganglia and mucoid degeneration: coexistence and clinical correlation. AJR Am J Roentgenol 2004;182(5):1283-1287.
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