Evaluating an ACL injury
The patient with an ACL injury is likely to describe feelings of knee instability. Although the other ligaments of the knee may be intact, the ACL is a primary stabiliser and when it is torn the knee feels as if it can no longer be trusted. The surgeon will confirm any laxity via a number of subjective and objective tests.
Many hands-on tests have been described, but most knee surgeons will use the Lachman test, the Anterior Drawer Test and the Pivot Shift test, and then may turn to an objective measurement using a KT-2000 machine or a KIRA app.
Subjective Tests
Lachman Test
The Lachman Test
In the Lachman test, the patient lies on his/her back on the examination couch. The examiner stands alongside the knee, facing the patient, and gently takes the knee into both hands, with the upper hand holding the thigh firmly just above the knee, and the lower hand holding the lower leg firmly with the fingers behind the leg and the thumb on the tibial tuberosity (the bony lump just below the kneecap).The knee is bent to 20-30 degrees. The lower leg is pulled forwards while the thigh is held firmly in position, and the examiner feels for any forward movement of the lower leg.
Anterior Drawer Test
The Anterior Drawer Test
With the patient lying on his/her back, the leg is bent to 90 degrees. The surgeon sits alongside the patient, preventing the patient's foot from moving by applying weight on it with his upper thigh.
With the surgeon's thumbs on the front of either side of the upper part of the shin bone, and the fingers around the back, the surgeon pulls the tibia forward, assessing the forward movement and comparing it with the normal side.
Pivor Shift Test
The Pivot Shift test cannot be easily illustrated, but you will see an example towards the end the video below.
The Pivot Shift test is really a 'compound' test, evaluating the anterior cruciate ligament integrity as well as the integrity of another stabiliser called the antero-lateral ligament. The patient lies on his/her back with the hip flexed to 30 degrees. The surgeon stands alongside, and may bend the patient's knee to 20 degrees and tuck the foot under the surgeon's arm to hold it stable. One hand rests lightly on the outer aspect of the knee, and the other holds the lower leg and controls the process. The knee is slowly flexed and gently inwardly rotated. If the test is positive, the surgeon sees or feels a sudden pivot of the tibia bone on the outer side due to structural instability.
Objective Testing
Objective Testing using a KIRA
The anterior drawer is not subjectively always that easy to evaluate, and several devices are on the market that allow a more objective evaluation of laxity. The most commonly used is called a KT-2000.
In this illustration you see an app-based device called a KIRA, which we tend to find useful particularly in children.
Generally the patient's story and a simple clinical evaluation will clinch the diagnosis of an ACL tear, but MRI and arthroscopy may be necessary to fully evaluate the site and nature of the damage.
MRI Evaluation
An MRI scan is a series of flat images such as one might get if the knee were solid and sliced into 3 mm slices. These slices can be in any plane, but we commonly look at three - sagittal (from the front of the limb through to the back), coronal (from the side of the limb through to the other side) and transverse (across the limb, like slicing a sausage).
Here you see a coronal view, which is the best for evaluating disruption of the ACL. An MRI differs from an X-ray in being able to show both the bone and the soft tissues, and clearly highlights the interfaces between soft tissue layers.
Here you can see the three bones of the knee from a slice right in the middle of the joint - the patella (kneecap) is the small bone on the left, and the femur (above) and tibia (below) are the thighbone and shinbone respectively.
Instead of a strong band of ligament reaching from tibia to femur in the middle of the joint, you see only a diffuse wavy-looking structure, because this ACL is torn. It has, in fact, also stuck onto the PCL ligament, and this adds to the strange appearance.
Arthroscopic evaluation
Arthroscopy, or keyhole surgery of the joint, offers additional diagnostic information:
- how much of the original ligament still remains
- how much of the original ligament is still healthy
- whether the ligament is torn off from the femur (or maybe the tibia), and if so whether or not any sliver of bone came off at the same time
- whether the sheath of the original ligament is still intact
Of course, these days the same surgical event is used for both the evaluation and for the repair procedure itself.
COURSE ON
ACL REPAIR IN CHILDREN
This course looks at those particular issues related to ACL tears in children.