The anterior cruciate ligament (ACL) originates from the lateral femoral intercondylar notch and inserts into the tibial spine. It is composed of anteromedial and posterolateral bundles that lie parallel or more vertical to the roof of the intercondylar notch (Blumesaat’s line). Because the ACL courses at 55 degrees to the main magnetic field, it is subject to the magic angle phenomenon, especially at the tibial insertion [10]. It also interfaces with the anterior horn of the lateral meniscus which creates an intermediate signal speckled appearance to the meniscus, that can extend to the surface and should not be mistaken for a tear.

ACL tears can be partial or complete. The diagnosis is based upon clinical exam with the aid of imaging. Partial tears may affect one of the two bundles and can be subtle. Complete tears cause disruption of the all of the fibers and are most common within the central tendon but closer to the femoral attachment. Avulsion fractures more commonly occur in children and young adolescents and are usually located distally at the insertion on the tibial plateau.

Depending on the mechanism of injury there will be a different pattern of bone contusions on the MR images. The classic pattern related to valgus stress is associated with contusions of the lateral femoral condyle and posterior lateral tibial plateau, and in some cases there is an additional contusion in the posterior medial tibial plateau [10, 11]. A deepened condylopatellar sulcus in the lateral femoral condyle greater than 2 mm can be the result of impaction with this type of injury and is also visualized on radiographs as a clue to the presence of an ACL injury [11, 12] (Fig. 4). Hyperextension can cause ACL or posterior cruciate ligament (PCL) tears. Contusions associated with this mechanism of injury are located along the anterior aspect of the femur and tibia around the knee joint. A third mechanism of ACL injury is varus stress with an avulsion of the tibia below the plateau, resulting in the Segond fracture. These fractures are easier to see on conventional radiographs but can be found on MRI in the region of the attachment of the iliotibial band and anterolateral ligament on the tibia. Because they are avulsions, there is little reactive marrow change in the tibia.

All three orthogonal planes can compliment each other to evaluate the ACL. Slight external rotation of the knee is favored. Various planes parallel and perpendicular to the ACL have also been suggested in more difficult cases. New 3D volumetric FSE images will allow thin section reconstructions in the desired plane [10, 13].

Disruption with high signal intensity of the normal fibrillar low signal intensity structure of the ACL is the primary MRI sign. The ACL may also lie at a more acute angle or can be displaced anteriorly causing a locked knee. The secondary anterior translation of the knee where the posterior margin of the tibia lies anterior to the posterior margin of the femur can be measured with MR sagittal images at the lateral femoral condyle, and is considered abnormal when it is greater than 5 mm [10]. Partial tears are more difficult to diagnose. It is helpful to follow each bundle in multiple planes. The anteromedial bundle is more often disrupted than posterolateral bundle [14]. In some situations, these partial tears are repaired or reconstructed if there is resultant knee instability.

Chronic ruptures of the ACL may be obvious with an empty notch sign signifying an absent ACL. An old ACL tear might be called if the alignment of the ACL is at an acute angle to the roof of the intercondylar notch. The torn ACL fibers may adhere to the PCL. A thickened ACL or one that lacks striation is a clue to an old injury. On the other hand, sometimes the ACL tear heals with fibrous tissue and may look normal on MRI. Despite a normal MR appearance, the ligament may be associated with laxity resulting in clinically detectable knee instability that requires ACL reconstruction. Anterior translation of the tibia can help to identify some of these cases. Most ACL tears are reconstructed with a patellar or hamstring autograft. In some circumstances the torn ACL will heal without surgery and the patient will be able to return to athletic activity [15].

The comma-shaped posterior cruciate ligament originates from the medial aspect of the intercondylar notch of the femur and inserts into the posterior tibia. Tears are most often seen in the proximal third of the PCL, and can be intrasubstance, partial or complete (Fig. 5). Avulsions of the PCL are also seen at the tibial plateau. The mechanisms of injury of the PCL include a direct anterior force on a flexed knee as may be seen in an auto crash involving a passenger in the front seat. Hyperextension and severe valgus stress may also affect the PCL. The PCL has even greater potential than the ACL to heal spontaneously [10, 16]. The MRI often looks normal in these cases. Thickening of the PCL can be a sign of an old injury.

Medial ligaments include the medial collateral ligament (MCL) complex and components of the medial retinaculum. There are three layers of tissue that comprise these ligaments. The medial collateral ligament extends from the medial femur to the proximal tibial diaphysis approximately 7 cm below the tibial plateau. The medial patellofemoral ligament (MPFL) is part of a retinacular complex that goes from the patella to the femur, meniscus, and tibia. The MPFL extends from the medial patella to the adductor tubercle of the femur where it blends in with the MCL. The MPFL stabilizes the medial patellofemoral joint. These anteromedial structures have three layers. The superficial component (layer I) is composed of the crural fascia that includes the sartorius and anteriorly is fused with layer II forming the medial retinaculum and the medial patellofemoral ligament. The medial layer, layer II corresponds to the superficial layer of the medial collateral ligament (MCL) and posteriorly is fused with layer III forming the posterior oblique ligament (POL) of the medial collateral ligament complex. The deepest layer (layer III) includes the joint capsule, meniscopatellar ligament and meniscofemoral and meniscotibial (coronary) ligaments. A tibial collateral ligament bursa lies between the superficial and deep layers of the MCL [10, 17–21].

The MCL is the most commonly injured ligament of the knee. Trauma to this ligament is usually related to valgus stress and external rotation, a mechanism of injury that may also injure the ACL and the posterior horn of the medial meniscus, especially at its meniscocapsular attachment.

The simplest description of MCL sprain and tear involves three different grades. Grade I represents a sprain or small partial tear of the superficial layer of the MCL without knee instability. On MRI there is associated T2 hyperintensity medial to but not involving the ligament. Grade II represents a partial tear produced by moderate valgus instability that involves deep and superficial layers on MRI and demonstrates T2 hyperintensity and morphologic disruption of some of the fibers of the MCL without complete discontinuity or avulsion. Grade III represents a complete tear, with discontinuity of the fibers and instability (Fig. 6). This MR classification system does not completely correlate with surgical findings and is based mainly on the superficial layer of MCL. Treatment of MCL tears is usually conservative. Surgery is considered when there is a complete tear of the MCL, especially if there are associated injuries of the posteromedial corner involving the posterior oblique ligament. Distally the MCL may tear at the tibial attachment and lie superficial to the pes anserine tendons. In this situation, the interposition of the tendons, may block healing of the tear, similar to a Stener lesion of a Gamekeeper’s thumb, and would also require surgery. Chronic injuries of the MCL produce thickening of the ligament on MRI. Chronic injury may also be associated with ossification of the ligament that is better seen on radiographs, and has been termed a Pellegrini Stieda lesion [10, 18, 21].