The Lachman Test is a physical examination technique used to assess the integrity of the knee’s internal structures. It serves as a diagnostic tool in orthopedic evaluations. This manual test provides healthcare professionals with insights into potential injuries, specifically those affecting a key ligament within the knee joint. Its effectiveness makes it a common initial step in diagnosing certain knee conditions.
Purpose of the Lachman Test
The primary purpose of the Lachman Test is to evaluate the anterior cruciate ligament (ACL) in the knee. The ACL is a strong band of tissue connecting the thigh bone (femur) to the shin bone (tibia), stabilizing the knee joint. Its main function is preventing the tibia from moving too far forward relative to the femur.
This test is performed when an ACL injury is suspected, common in sports requiring sudden stops, pivots, or direct impacts to the knee. Injuries can range from a stretched ligament to a complete tear, leading to knee instability, difficulty with movement, and an increased risk of further joint damage. The Lachman Test helps determine the extent of ligament laxity, guiding decisions for further diagnostic imaging or treatment. It is considered more sensitive and reliable than some other knee stability tests, particularly for acute injuries.
How the Test is Conducted
Performing the Lachman Test involves a precise sequence of steps. The patient lies on their back with leg muscles relaxed, especially the hamstring. The knee being examined is bent to an angle between 20 to 30 degrees. Slightly rotating the leg outward can help relax surrounding muscles, aiding reliable results.
The examiner positions one hand to stabilize the patient’s lower thigh. The other hand grasps the upper part of the shin bone, with the thumb placed on the tibial tuberosity, a bony prominence below the kneecap. While maintaining thigh stability, a firm force is applied to pull the shin bone forward. This maneuver assesses the degree of forward movement of the tibia relative to the femur.
Understanding the Test Results
Interpreting the Lachman Test results relies on two observations: the amount of forward movement of the shin bone and the quality of the “end-feel” when the movement stops. A healthy ACL will prevent excessive forward translation of the tibia, resulting in an abrupt stop known as a “firm endpoint.” This firm resistance indicates the ligament is intact and limits joint motion.
Conversely, a “positive” Lachman Test suggests a torn ACL. This is indicated by increased forward movement of the tibia without a firm stop, often described as a “soft” or “mushy” endpoint. An anterior translation of more than 2 millimeters compared to the uninjured knee is considered a positive sign of an ACL tear. Healthcare professionals compare the tested knee to the patient’s uninjured knee to identify subtle differences in laxity and confirm findings. In some cases, specialized instruments, such as a KT-1000 arthrometer, can provide an objective measurement of tibial movement in millimeters.