Findings:
On the sagittal T1-W (Image 1a) and sagittal T2-W images, complete transection
of the mid-portion of the ACL (Image 2a, arrows- transected ACL, arrowhead-
PCL) is identified. On the coronal STIR image, the normal ACL is absent, resulting
in an “empty notch” appearance (Image 3a).
Additional imaging findings include: a high-grade MCL injury (Image 4); and osseous contusions of posteromedial and posterolateral tibia (Image 3b, arrow- lateral, arrowhead- medial) and lateral femoral condyle (Image 4a). The PCL and menisci are intact.
Discussion:
Anterior cruciate ligament (ACL) injuries are a common and unfortunate consequence
of athletics thoughout the world. Cruciate ligament injuries do occur in non-athletic
activities, but are much less common. The anterior cruciate ligament is much
more commonly injured then the posterior cruciate ligament (PCL).
The classic and most common contact mechanism of ACL injury is the clipping valgus injury as seen in American football. The second most common contact injury is a hyperextension injury from an anterior impaction injury related to either sports or motor vehicle collisions. The ACL can also be injured when a valgus force is applied to a flexed knee combined with external rotation of the tibia- the so called “pivot shift injury.” This type of injury is non-contact related and is often seen in skiers and sports requiring rapid change in direction such a basketball or soccer.
Clinically, patients present with pain after a knee injury. Swelling secondary to a joint effusion is common and may preclude accurate clinical examination. In the hyperacute setting, a joint effusion may not be present secondary to the extrasynovial location of the ACL. Often an audible “pop” is reported by the patient. On clinical examination, abnormal anterior translation of the tibia may be elicited with the knee flexed 15-30 degrees- the Lachman test.
The T2-W sagittal dataset is the preferred sequence for evaluating the ACL. The coronal and axial images are utilized to confirm or exclude pathology. The femoral attachment, midportion, and tibial attachment are best visualized on the axial, sagittal, and coronal datasets, respectively. The majority of ACL transections occur in the midportion.
Direct and indirect signs have been described for evaluating the integrity of the ACL. A direct sign refers to imaging findings related to the ACL itself. Indirect signs are secondary findings of injury which have a high association with ACL transection. Direct and indirect signs are summarized in Table 1 and Table 2, respectively. The accuracy of MRI for evaluating full-thickness ACL tears is 98%.
| Table 1: Direct Criteria for Complete ACL Tear |
| -Complete midsubstance discontinuity -Abnormal cruciate course (ligament does not attach to its normal posterosuperior femoral site or anteroinferior tibial site) -Abnormal ACL direction (“laying down sign”- ACL is parallel to the tibia related to a proximal avulsion and/or tear) -Midsubstance intracapsular hematoma or pseudomass -Cruciate ligament corrugation or buckling |
| Table 2: Indirect Criteria for Complete ACL Tear |
| -Buckling of the posterior cruciate ligament (a sign of ACL laxity, injury, or tear) -Femorotibial subluxation or translation (MRI “drawer sign”) -Posteromedial and posterolateral tibial contusions -Lateral femoral sulcus terminalis contusion |
Associated injuries are common and include: bone contusions, meniscal tears (lateral more commonly than medial), collateral ligament injuries (medial more commonly than lateral), and posterolateral corner injuries.
References:
1. Kaplan P., Helms C., Dussault R., Anderson M. Major N. Musculoskeletal MRI.
W.B. Saunder Co. 2001. pg117-118.
2. Pomeranz S., Gamuts and Pearls in MRI & Orthopedics. MRI-EFI Publications;
1997.
3. Stoller D., Tirman P., Bredella M., Beltran S., Branstetter R., Blease S.
Diagnostic Imaging Orthopaedics. Amirsys. 2004.
