The Lachman’s test is performed between 20°-30° of knee flexion, which helps to isolate the affect of the ACL providing stability to anterior translation of the knee. It is absolutely essential during this exam that the patient relax as much as possible so that the examiner can determine if there is any increase in motion anteriorly. Thus, in an acute injury, where a patient may be unable to relax entirely, it may not be possible to determine whether the ACL is intact.
One hand of the examiner will secure and stabilize the distal femur while the other hand will secure and stabilize the proximal tibia. A gentle anterior translation force is then applied to the proximal tibia to determine if there is an end point. In general, there should be a minimal, usually 0-3 mm, amount of increased translation. In most patients, one can feel a hard, firm stop which is provided by the ACL. In other patients, it may be difficult to determine this because of guarding. In patients who do not have a firm stop, or endpoint, one should be very suspicious that there is an ACL tear.
In patients who have a significantly increased amount of anterior translation in a Lachman’s test, there must be concern that there is a lack of secondary restraints present which would put a future ACL reconstruction graft at risk for stretching out over time. This lack of secondary restraints can include injuries to the posterolateral corner of the knee, unrecognized posterior cruciate ligament tears, deficiency of the posterior horn of the medial meniscus, or a posterior horn medial meniscus root tear. Other causes may also be present but are less frequent. Thus, when the examiner finds a 3+ or 4+ Lachman’s test, one must be very suspicious that there is a lack of secondary restraints and the cause of this should be investigated further.