How the Test Is Performed
With the patient lying flat and relaxed, the examiner bends the knee slightly (about 20 to 30 degrees). The examiner then stabilizes the thigh while pulling the shin forward. Holding the leg in slight external rotation (outwards) will help relax the IT band.
The test places stress on the ACL. Both the amount of movement (shifting) of the shin bone, as well as the feel of the endpoint of movement (how solid the ligament feels), offer information about the ACL.
Grading
Most examiners grade the results of the Lachman test on two criteria. First, the endpoint, and second, the amount of laxity. When assessing the endpoint, the examiner is feeling for the ACL limiting the amount of shifting of the shin bone.
Typically an examiner will describe the endpoint as either “firm” or “soft.” A firm endpoint implies the ACL is doing its job to limit the amount of movement in the knee joint. A soft endpoint is indicative of the ACL not functioning well and other structures (the secondary stabilizers) limiting the amount of movement in the joint.
The second criteria to assess the Lachman test is the amount of laxity (movement) of the joint. The grading is based on a comparison to the non-injured extremity. Therefore, it is common for your healthcare provider to examine both knees to determine the grade of the Lachman test.
These measurements can be difficult to quantify, and oftentimes different examiners may come up with different results. Some healthcare providers use a tool, called a KT-1000, to perform this test. The KT-1000 has been shown to be a reliable measure that can more precisely measure the grade of the Lachman test.
