The Lachman test is a physical examination maneuver used by clinicians to assess the stability of the knee joint. It is a common orthopedic test performed when a patient presents with a suspected ligamentous injury following trauma. This test provides real-time information about the knee’s internal mechanics. It helps guide the need for further imaging or specialized intervention.
The Specific Injury Targeted
The Lachman test is specifically designed to evaluate the functional integrity of the Anterior Cruciate Ligament (ACL). The ACL is one of the four main ligaments in the knee, connecting the femur (thigh bone) to the tibia (shin bone). Its primary function is to resist the tibia from sliding too far forward beneath the femur, a movement known as anterior tibial translation. The ligament also helps restrict excessive rotation, contributing to the knee’s overall stability.
ACL injury is common, particularly among athletes in sports involving sudden stops, cutting, and changes in direction. These non-contact mechanisms, along with awkward landings or knee hyperextension, can cause the ligament to stretch or tear. When the ACL is torn, it compromises the knee’s stability, which the Lachman test is intended to reveal.
Executing the Examination
The Lachman test is performed with the patient lying on their back (supine). The affected knee is gently flexed to a slight angle, typically between 20 and 30 degrees. This minimal bend relaxes the hamstring muscles and the posterior capsule, which could otherwise mask an ACL tear.
The examiner stabilizes the distal end of the femur with one hand to prevent movement. The other hand grasps the proximal tibia, just below the knee joint line. A gentle but firm anterior force is then applied to the tibia, attempting to pull it forward relative to the stabilized femur. This action isolates the role of the ACL as the primary restraint to forward movement.
Understanding the Findings
Interpreting the Lachman test depends on two factors: the amount of forward movement of the tibia and the quality of the “endpoint.” A “positive” result suggests an ACL injury and occurs when there is increased anterior translation of the tibia compared to the healthy knee. A side-to-side difference of more than two millimeters often indicates injury.
The second factor is the feel of the endpoint, which is where the tibia stops moving forward. In a healthy knee, the ACL provides a solid, abrupt stop, described as a “firm” or “hard” endpoint. A positive test is characterized by a “soft” or “absent” endpoint, where the tibia yields gradually with no distinct halt. This suggests the ligament is torn and no longer provides mechanical resistance.
Laxity, or looseness, is often graded to quantify the severity of the injury. Grades commonly range from mild (Grade 1) to severe (Grade 3) based on the degree of excessive movement. The Lachman test is considered the most reliable clinical examination for diagnosing acute ACL tears because the slight knee flexion minimizes protective muscle guarding that can interfere with other stability tests.