The anterior and posterior cruciate ligaments are two of the other major ligaments of the knee. Their job is to maintain the anteroposterior relationship of the distal femur and proximal tibia to each other. This job is made a bit trickier by the fact that it needs to be done at all times during flexion or extension.

The anterior meniscus lies lateral to the posterior cruciate. A nice mnemonic for remembering these relative positions is: LAMP, which stands for Lateral Anterior - Medial Posterior.

The posterior cruciate is a relatively simple band of tissue and attaches proximally on the internal aspect of the medial femoral condyle and runs distally to where it attaches to the posterior eminence of the tibia. It generally lies in the sagittal plane, and can usually be seen in its entirety on a single sagittal MR slice.

The anterior cruciate ligament is usually not a simple linear band. Instead, it is normally fan shaped, as shown below. It attaches proximally at the internal aspect of the lateral femoral condyle, and runs distally to its broad attachment to the anterior tibia and the anterior aspect of the tibial spine.

The anterior cruciate ligament usually lies in a plane 10 - 15 externally rotated to the sagittal plane. Therefore, if one obtains images in this plane, one can image the ACL in over 95% of the time.

The anterior meniscofemoral ligament of Humphrey produces a localized anterior thickening of the PCL. It usually looks like a small dark lump, such as seen below. It may occasionally be seen as separate from the PCL and mimic an intraarticular body.

Medial knee pain in a runner - sagittal MR image of the knee shows a tear (bright linear signal intensity, arrow) of the posterior horn medial meniscus.

Diagnosis:

ACL Tear

Discussion:

Sagittal T2 weighted and proton density images which reveal complete tear of the ACL with mild increase in marrow signal in the adjacent osseous structures. The bone bruises, as evidenced by increased signal within the marrow, is likely caused by anterior subluxation of the tibia at the time of tear of the ACL accompanied by impaction of the middle portion of the lateral femoral condyle against the posterior portion of the lateral tibial plateau. Signal intensity abnormalities are probably secondary to edema, hemorrhage, and microfracture.

The anterior cruciate ligament runs obliquely within the lateral aspect of the intercondylar notch, attaching to the inner aspect of the lateral femoral condyle proximally and to the anterior aspect of the intercondylar eminence of the tibia distally. An anterior cruciate ligament-deficient knee allows the tibia to undergo subluxation anteriorly relative to the femur. This is the MR equivalent of the anterior drawer sign.

Frontal radiographs of the knee which reveal a small fleck of bone adjacent to the lateral aspect of the lateral tibial plateau. This "lateral capsular sign" is a simple radiographic finding visible on the routine AP knee x-ray, which indicates severe lateral capsular injury and should alert the examiner to the possibility of ACL tear. The fleck of bone represents an avulsion of the meniscotibial portion of the middle one-third of the lateral capsular ligament. It most commonly implies a disruption of the capsular ligament and the ACL. A-B Radiographs. C. MRI confirms the diagnosis of ACL tear.