The classic mechanism of injury causing an ACL tear is a non-contact injury after a sudden pivot with the leg planted – think of skiers who twist their torso with their ski fixed in the snow. Patients will commonly report a popping sound or sensation at the time of the injury.
With an ACL tear, the knee may fill with blood (forming a hemarthrosis). This fluid has two effects on diagnosis. First, the fluid, by blocking motion in general, 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 allowing the examiner to look at the fluid. In a nutshell: red is bad. A hemarthrosis after a knee trauma with normal x-rays is likely to represent a consequential injury such as an ACL tear, a meniscal tear, a chondral (cartilage) fracture or a patella dislocation.
The Lachman test is the most specific 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. The quality of the endpoint and the degree of displacement should be compared to the uninjured side for greater accuracy.
Figure 1: Lachman Test: The examiner stands next to the supine patient and grasps the lateral thigh just above the knee with the upper (contralateral) hand to exert resistance (blue line). The examiner holds the tibia medially with the lower (ipsilateral) hand, flexes the knee to 30 degrees, and then applies a jerking force to the tibia, in an attempt to subluxate it in an anterior direction (blue arrow). As shown, this will place tension on the ACL (drawn in red).
Tests for ACL tears can be falsely negative if there is a displaced fragment of either bone or cartilage in the knee (blocking motion) or if the hamstrings are resisting the examiner. Also, as noted, the fluid in the knee may provide mild stabilization. These factors limit the sensitivity of the test in clinical practice.
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.
The posterior drawer test is the most accurate physical exam maneuver. It is performed with the knee flexed to 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 as compared to the uninjured leg.
A PCL tear can also be detected with a so-called sag sign (Figure 2).
Figure 2:A PCL tear is present in the knee at left, demonstrating a sag sign (as illustrated by the green lines). Because the PCL is absent, the tibia, when under the force of gravity alone, will sag posterior to its normal position. The knee at right has an intact PCL and it does not sag.
In practice, injuries to the ACL and PCL are diagnosed on MRI. While x-rays can and should be obtained to rule out a fracture, the gold standard of imaging for definitive diagnosis of both ACL and PCL injuries is an MRI of the knee (see Figure 3) which allows direct visualization of the ligaments.
Figure 3: An MRI showing a torn ACL (red arrow) (Case courtesy of Dr. Hani Salam, Radiopaedia.org, rID: 10536)
Figure 4: An MRI showing a torn PCL (red arrow) Case courtesy of Radswiki, Radiopaedia.org, rID: 11812
Additional Points to Consider
Attending physicians and surgeons are universally fond of berating students to “stop ordering tests and just examine the patient!” So why is it that even experienced orthopaedic surgeons who can perform the Lachman test expertly will still obtain an MRI after confirming the diagnosis of an ACL tear on examination? Are these MRI-ordering attendings rank hypocrites? Or, despite having a positive result on a very specific test on physical exam, are MRI’s for ACL-injured patients reasonable?