LAS VEGAS — Magnetic resonance imaging can help physicians evaluate an anterior cruciate ligament tear and plan surgery to repair an ACL injury, said Dr. Kathryn Stevens at a symposium on emergency medicine sponsored by Stanford University.
MRI identifies additional bony or cartilage injuries—notably, injuries to the posterolateral corner of the knee—that increase the risk of knee instability when coupled with an ACL tear, said Dr. Stevens, a diagnostic radiologist at Stanford (Calif.) University.
“If you don't recognize posterolateral corner structures when you go in to do an ACL repair, the ACL graft is more likely to fail,” she said.
Always look at the posterolateral corner structures on an MRI of the knee prior to surgery, she emphasized. Prompt imaging is helpful because surgery within 3 weeks of injury yields the most effective results, she noted.
The ACL consists of two bundles of dense connective tissue, the anteromedial and the posterolateral bundles. ACL tears typically occur when a person slows down or changes direction quickly and are most common in younger athletes whose sports involve quick starts, stops, and pivots, such as basketball, soccer, and skiing, said Dr. Stevens.
Although MRI is not essential for diagnosing an ACL tear, some physicians opt to order images for confirmation. Diagnostic criteria for an ACL tear that are visible on an MRI include an empty notch on a coronal image, an edematous mass, and a wavy contour to the knee. Indirect signs of an ACL tear that are visible on an MRI include the “kissing contusions” where the edges of the leg bones barely touch. Kissing contusions identify cartilage involvement in ACL tears, especially if the image shows that the posterior horn of the lateral meniscus is no longer covered by the knee.
The posterolateral corner structures consist of three layers. Attachment points for the patellar retinaculum, the iliotibial band, and the biceps tendon are all found in the first layer; the lateral collateral ligament in the second layer; and the arcuate and fabellofibular ligaments in the third layer.
Common posterolateral corner injuries include an arcuate fracture (a tearing away of the fibula at the point where the arcuate ligament attaches to the knee) and a Segond fracture (a tearing away of the meniscotibial part of the lateral capsular ligament). More than 90% of Segond fractures are associated with ACL tears, Dr. Stevens said.
Posterolateral corner injuries are relatively rare but can compromise the success of ACL surgery. Such injuries are receiving more attention, in part because they can be better identified with current imaging techniques, she added.
After surgery, MRI can be used to evaluate graft alignment and to identify instability or infection. When ACL surgery doesn't go well, MRI can identify graft impingement or graft rupture. Graft impingement will show up as increased signal intensity within two-thirds of the graft area; graft rupture will show up as increased signal intensity in the general area where the graft should be, Dr. Stevens said.
Coronal CT and coronal T2 FS images show an avulsion fracture of Gerdy's tubercle (white arrow, left) and an avulsion of the iliotibial band (black arrows, right). Photos courtesy Dr. Kathryn Stevens
