Anterior Cruciate Ligament (ACL) Tear

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Diagnosis Definition

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

Imaging Findings

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

Pearls

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)

References

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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