Quantifying ACL Testing
Quantifying ACL Testing
The Anterior Cruciate Ligament (ACL) accounts for over a quarter million injuries per year, and that doesn’t count the number of re-injuries. In addition to identifying the mechanism of injury and symptoms such as popping or audible sound, immediate swelling, and instability in weight bearing, clinical tests are very important. These tests include the Lachman, anterior drawer, and Lelli test. In spite of the presence of a variety of tests, 74% of acute ACL injuries are misdiagnosed in the emergency department.
For decades, the classic Lachman’s test has been considered the gold standard. It involves again translating the tibia on the femur. With the knee in ~20° of flexion, the distal portion of the femur is stabilized proximal to the knee and the tibia is translated anteriorly. However, there are some problems with the performance of the Lachman test….3 problems to be exact.
- Protective muscle guarding of the hamstrings can restrict that anterior translation.
- Too much knee flexion can cause the posterior horn of the meniscus to act as a wedge and prevent translation of the tibia.
- There is often a mismatch between the size of the clinician’s hands and that of the patient’s leg circumference. Most clinicians have a hand span of somewhere in the vicinity of six to nine inches. Yet a lot of individuals who suffer ACL injuries will have thigh circumferences that exceed 18 to 23 inches. That creates a significant problem in being able to stabilize proximally with enough force to be able to generate a linear translation of the distal segment.
The Mobil–Aider Device
Designed to address all of these pain points as well as provide a quantitative value on the linear translation of the knee, use of the Mobil–Aider device for the Lachman test is displayed. The contoured attachments distribute the contact forces over the anterior surfaces and the straps help to stabilize the device on any size knee. The bolster positions the knee in ~20° of flexion. The device is aligned with the joint line of the knee, the proximal segment is stabilized, and the distal segment is translated anteriorly. With the translation, the magnitude of the motion is displayed on the digital screen. Comparison with radiographs revealed a 6.9mm anterior translation with a reading of 7.1mm on the Mobil–Aider. That is less than a 2% margin of error in the testing of the device. These data is available here.
Prone Lachman Test
Performing ACL testing in the prone position can allow gravity to be of assistance in anterior translation and stabilizing the thigh it not a challenging. Once in prone with the knee off the edge of the plinth and the lower leg supported, there are three different ways to apply the anterior translation: one-hand, two-hand, and forearm.
This position affords several advantages. As stated, gravity can assist with the motion. There are no issues with hand versus leg size. The likelihood of a false positive is decreased due to the elimination of tibial sag with a PCL injury. However, there is still no way to quantify millimeters of motion. By donning the yellow attachment on the Mobil–Aider, one can capitalize on all the advantages of the prone position and also quantify the magnitude of translation.
So what is the criteria for a (+) Lachman test (classic or prone)? Quantitatively, a difference of 5 mm between involved and uninvolved knees is considered (+). Yet, there is also a qualitative component to the assessment. The ACL should abruptly stop the anterior translation of the tibia on the femur. If this does not occur, one should suspect an ACL injury.
Anterior drawer test
This is another ACL test that places the knee in 90° with the foot on the table. The clinician’s fingers are wrapped around the posterior knee to confirm the hamstrings are relaxed. The thumbs are placed on the anterior tibial plateau and the tibia is anteriorly translated on the femur.
There are a number of causes of false negatives of the anterior drawer. This can include hemarthrosis and active synovitis that prevents putting the knee in the 90° test position, protective knee guarding of the hamstrings restricting anterior translation, and posterior horn medial meniscus wedging against the medial femoral condyle that can block anterior translation of the tibia. The advantage is the test is easy to perform regardless of hand and leg size.
The test appeared in the literature in the last decade and uses a lever system to assess the integrity of the ACL. The patient is positioned in supine with the hip and knee extended. A foam roll is placed under the proximal third of the tibia. The clinician exerts a downward force to the distal third of the femur. With an intact ACL, the proximal tibia translates anterior, and as a result of the lever system, the foot then comes off the table. This is a negative test. If the ACL is not functioning, then a positive test would be the failure of the tibia to translate on the femur, and the heel of the involved leg would not rise off the table. Lelli et al (2014) reported 85% of ACL injuries are from the femoral attachment so it made more sense to manipulate the femur than the tibia.
The statistical overview of the ACL tests is displayed below:
Although the metrics for this test are fairly good, they are still binary tests, either their positive or negative. The clustering of the Lelli, pivot shift, anterior drawer, and Lachman has a 67% to 91% accuracy in the ACL diagnosis when all of these tests are (+). For more information on the Mobil–Aider and ACL testing, please download iOrtho+ Premium Mobile App or visit https://iortho.xyz/
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