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A positive Lachman's sign is a clinical test used to assess the integrity of the anterior cruciate ligament (ACL) in the knee. When a healthcare provider performs this test, they stabilize the femur with one hand and pull the tibia forward with the other hand. A significant amount of forward movement (anterior translation) of the tibia compared to the femur, accompanied by a lack of firm end feel, indicates that the ACL may be damaged.

This test is particularly sensitive for ACL injuries because it relies on the differential movement between the tibia and femur when the ACL is compromised. The Lachman's test is preferred over other tests, such as the anterior drawer test, because it is typically less painful and more reliable in assessing ACL integrity.

In contrast, the other options refer to different injuries: medial meniscus injury would present with joint line tenderness and possibly a positive McMurray's test, knee dislocation usually involves observable deformity and more extensive ligamentous injury, and a positive sign for posterior cruciate ligament damage often uses a different test, such as the posterior drawer test.