Lachman sign is positive in:

The Lachman test is the most sensitive test for ACL tears. It is done with the knee flexed at 20 degrees, stabilizing the distal femur with one hand and pulling forward on the proximal tibia with the other hand. With an intact ligament, minimal translation of the tibia occurs and a firm end point is felt. With a torn ACL, more translation is noted, and the end point is soft or mushy. The hamstring muscles must be relaxed during this maneuver to prevent false-negative findings. Ref: McMahon P.J., Kaplan L.D. (2006). Chapter 4. Spos Medicine. In H.B. Skinner (Ed), CURRENT Diagnosis & Treatment in Ohopedics, 4e.