Anterior Cruciate Ligament Tears in Pediatrics

Abstract & Commentary

Synopsis: Magnetic resonance imaging can be used to accurately diagnose ACL tears in children and adolescents with pain and signs and symptoms of internal derangement of the knee.

Source: Lee K, et al. Anterior cruciate ligament tears: MR imaging-based diagnosis in a pediatric population. Radiology 1999;213:697-704.

The purpose of the study was to evaluate the primary and secondary magnetic resonance imaging (MRI) appearances of injuries to the anterior cruciate ligament (ACL) in young patients and the diagnostic accuracy of MRI. Forty-three children had retrospective review of the MRI studies of the knee for primary and secondary signs of ACL tears as well as meniscal tears and collateral ligament injuries. Interpretation was compared with arthroscopy results.

Primary signs of tear were an abnormal course, abnormal signal intensity, and discontinuity. A Blumensaat angle measurement greater than the mean value in all cases was used as the threshold of diagnosis of ACL tear. (The Blumensaat line is parallel to the roof of the intercondylar notch. The Blumensaat angle is the angle between the Blumensaat line and a line along the distal portion of the ACL.) Abnormal signal intensity and a focal gap defined a tear. The secondary signs were presence or absence of bone contusion, position of the posterior cruciate line, posterior cruciate angle, presence or absence of anterior tibial displacement, and extent of uncovering of the posterior horn of the lateral meniscus.

Primary and secondary MRI findings and associated injuries were assessed for their associations with ACL tears. Nineteen patients (mean age, 14.1 yr) had ACL tears and 24 patients (mean age, 13 yr) had normal ACL. Gender association with ACL tear was significant, with 14 of 20 (70%) of female patients demonstrating tears, while five of 23 (22%) of males had tears. Primary MRI findings of tears were present in all the patients with ACL tears. Sixty-eight percent of patients had more than one finding. The secondary findings in ACL tears had high specificity and poor sensitivity.

Associated injuries included 17 (79%) meniscal tears. Five of 19 (26%) patients had collateral ligament injuries.

ACL injury is reliably diagnosed by observing primary findings: abnormalities of the ligament itself. The Blumensaat angle and abnormal signal intensity were the most sensitive primary findings for the diagnosis of a tear. Ninety-five percent of patients with ACL tears had more than one secondary finding.

Comment by Beverly P. Wood, MD

Older children and adolescents with symptoms of internal knee derangement should be considered for MRI to identify possible ACL injuries. In the presence of an ACL tear, it is highly likely that meniscal injury is also present. The 79% association of meniscal tears with ACL injury is higher than that in adult series. This study examined patients with acute and subacute injuries. It is unclear if chronic tears might show fewer secondary findings. MRI for ACL tears appears to be highly reliable.

Identify the secondary sign most frequently associated with ACL tear by MR imaging.

a. Posterior tibial displacement

b. Bone bruise

c. Meniscal tear