Anterior Cruciate Ligament (ACL) Tear

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

KEY IMAGES

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

1. 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
2. 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.