Patients with a torn anterior cruciate ligament (ACL) will typically present with immediate swelling and pain following a twisting mechanism. Patients will commonly report a popping sound or sensation at the time of the injury.
The classic mechanism of injury causing an ACL tear is a non-contact injury after a sudden pivot with the leg planted – think of a skier who twists his or her torso with their ski fixed in the snow (see question 11).
The knee may fill with blood (forming a hemarthrosis). This has two effects on diagnosis. First, the fluid may artificially stabilize the knee. This false stability is enough to mask the tear on examination. The second is that aspiration may help make the diagnosis, not only by removing the false stabilizer, but by merely looking at the fluid. In a nutshell: red is bad. A hemarthrosis after a twisting injury is likely to represent either an ACL tear, a meniscal tear, a chondral (cartilage) fracture or a patella dislocation. At the least, hemarthrosis after a twisting injury should prompt an x-ray, confirmation that the knee is not locked and that patella is located, and urgent referral to a musculoskeletal expert.
Lachman Test: this is the most sensitive physical exam maneuver for detecting a torn ACL. It is performed by stabilizing the femur while the tibia is pulled forward (see Figure 1). A positive test is signified by excessive forward translation without a firm endpoint, indicating disruption to the ACL. Because the Lachman test is assessed by the perceived firmness of the endpoint - an intact ACL will stop anterior translation more suddenly - experienced examiners will perform the test more accurately.
(Note that some patients with a posterior cruciate ligament tear can have a falsely positive Lachman test. That’s because with posterior cruciate ligament laxity, the tibia may sag, that is, rest in posteriorly displaced position relative to the femur. When the examiner translates the tibia forward from this abnormal position as part of the Lachman test, an abnormal amount of anterior translation may be perceived.)
Anterior drawer test: This test assesses whether there is an abnormal amount of anterior displacement when a force is applied to the flexed knee. It is similar to the Lachman test but is less accurate because a varying degree of anterior translation is normal and whether what the examiner is perceiving represents an abnormal amount may be difficult to tell. For this reason, it is also important to compare injured and uninjured knees when performing both the Lachman and anterior drawer tests (which can often be done by comparing the unaffected side of the patient you’re examining).
Pivot shift test: This diagnostic maneuver moves the tibia in and out of its normal anatomical position, if and only if the ACL is damaged. The test is performed with the examiner grasping the leg and holding the knee in extension with a slight internal rotational torque to the tibia. In this position, the tibia will subluxate anteriorly relative to the femur if the ACL is torn. The knee is then flexed. In flexion, the knee is in its normal position, even if the ACL is torn. Thus, if the tibia is noted to “clunk” back into position during flexion, the test is positive (for a torn ACL). This test may be difficult to perform in the apprehensive patient, but is a critical component of the physical examination when performed under anesthesia.
Tests for ACL tears can be falsely negative if there is a displaced fragment of either bone or cartilage in the knee or if the hamstrings are resisting the examiner. Also, as noted, the fluid in the knee may provide mild stabilization. Therefore, if there is a tense effusion, the Lachman test may be falsely negative as well.
The classic history for PCL injury is a direct blow to the anterior shin with the knee flexed (by the dashboard in a motor vehicle collision, say) generalized knee pain, a limp, and mild to moderate knee swelling. Patients with injuries to the PCL do not typically describe a popping sensation.
Patients with chronic PCL deficiency do not complain of instability but rather anterior (patellofemoral) knee pain. That is because patients have learned to stabilize their knee by “holding on” to their tibia with active quadriceps force. This quadriceps force loads the patellofemoral joint causing articular degeneration.
Also, because rupture of the posterior cruciate ligament leads to an increase in passive sagittal laxity in the medial compartment of the knee (the tibia is displaced posteriorly relative to the femur), PCL-deficient knees are at risk of subsequent medial meniscal injury and arthritis of the medial of the femoral-tibial joint.
Posterior Drawer Test: This is the most accurate physical exam maneuver and is performed where the knee is flexed at 90 degrees and a posterior force is applied to the anterior proximal tibia, driving it backwards. A positive sign is excessive posterior translation of the tibia relative to the femur with lack of a firm endpoint as compared to the uninjured leg.
In practice, injuries to the ACL and PCL are diagnosed on imaging. While x-rays can and should be obtained to rule out concomitant fracture or avulsion injury (i.e., Segond fracture – a fleck of bone off the anterolateral tibia that is pathognomonic for ACL injury), the gold standard of imaging for definitive diagnosis of both ACL and PCL injuries is an MRI of the knee (see Figure 2) which allows direct visualization of the ligaments.
It may be common thought that MRI is either too expensive or that imaging studies are too heavily relied upon for the diagnosis of knee ligament injury. It is important however, to ascertain imaging for several reasons. First, as alluded to above, the physical exam is neither sensitive nor specific for injury and relies heavily upon the experience of the provider; for management purposes this is not enough. Second, imaging may help find concomitant injuries such as fracture or adjacent soft tissue injury. Third, imaging studies can guide fully informed consent. Lastly, MRI of the knee is highly localized, and is unlikely to lead to incidental findings or overtreatment; therefore the risk to the patient is low. While cost is thought of as prohibitive, and may be for some specific patients, it is typically overestimated compared to true cost.