The Lachman test is a widely used diagnostic tool for knee injuries. It serves as a physical examination technique that helps healthcare professionals evaluate the stability of the knee joint. This common procedure provides insights into the condition of internal knee structures.
Purpose of the Lachman Test
The Lachman test is designed to assess the integrity of the anterior cruciate ligament (ACL) in the knee. The ACL is a strong band of connective tissue that connects the thighbone (femur) to the shinbone (tibia), which maintains knee stability by preventing forward shinbone movement and contributing to rotational stability. Injury to the ACL often occurs during activities involving sudden stops, changes in direction, jumping, or direct impact to the knee, commonly seen in sports. When the ACL is torn or sprained, it can lead to instability, difficulty moving the knee, and an increased risk of further joint damage. The Lachman test evaluates the ACL’s ability to restrain anterior tibial translation, making it a primary maneuver for diagnosing suspected ACL injuries.
How the Test is Performed
Performing the Lachman test involves specific steps. The patient lies on their back (supine) with the injured knee positioned in slight flexion, typically between 20 to 30 degrees; this angle helps relax other knee structures and isolate the ACL. The examiner may place a towel under the patient’s knee or use their own knee to maintain this flexion. The examiner then stabilizes the patient’s thigh (femur) with one hand, often just above the knee joint, and with the other hand, gently grasps the upper part of the shinbone (proximal tibia), placing their thumb on the tibial tuberosity. While keeping the thigh stable, a gentle but firm forward (anterior) force is applied to the shinbone, attempting to move it forward relative to the thighbone. Patient muscle relaxation is important for accurate results.
Interpreting the Results
The interpretation of the Lachman test relies on evaluating the amount of forward movement of the shinbone and the quality of the “endpoint” felt by the examiner. A healthy ACL prevents excessive forward movement of the shinbone, resulting in a distinct, firm stop or “firm end-feel,” indicating an intact ligament. A “positive” Lachman test, suggesting a potential ACL injury, is characterized by excessive anterior translation of the shinbone relative to the thighbone, often accompanied by a soft or “mushy” end-feel, or even no discernible endpoint. A forward movement of more than 2 millimeters compared to the uninjured knee, or a total anterior translation of 10 millimeters, suggests an ACL tear. The degree of laxity can be graded, with mild (0-5 mm), moderate (6-10 mm), and severe (11-15 mm) indicating increasing levels of injury.
Why the Lachman Test is Preferred
The Lachman test is often preferred over other clinical tests for ACL injury, such as the anterior drawer test, due to its higher sensitivity and accuracy, particularly in acute injury settings. Its effectiveness stems from the knee’s position during the examination, typically at 20-30 degrees of flexion, which helps minimize tension in the hamstring muscles that can otherwise mask ACL instability and lead to less accurate results. The Lachman test is recognized as one of the most sensitive tests for ACL integrity, demonstrating high intra-rater and inter-rater reliability. Its ability to provide immediate insights into the ligament’s status without complex equipment makes it an important initial step in assessing suspected ACL injuries. While its diagnostic accuracy can be influenced by factors like significant swelling, pain, or muscle guarding, comparing the results to the uninjured knee helps account for natural variations in joint laxity among individuals.