Physicians and other medical professionals are usually able to make the diagnosis of an ACL tear by the patient's history and physical exam.

Patient interview. A doctor will ask the patient about how the accident or injury happened, symptoms, the pattern of knee pain, and how symptoms affect lifestyle.

Physical exam. A doctor will examine the patient’s knee, noting any swelling, tenderness and pain points, and range of motion.

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A person with an ACL tear will have difficulty bearing weight on the affected leg and the knee will show swelling within the joint. Physical examination tests specifically designed to test the integrity of the ACL are particularly useful when they are also performed on the other (healthy) knee for comparison.

  • Lachman’s sign is the most accurate ACL injury test. It is performed with the athlete on his or her back with the affected leg relaxed, and the examiner holding the leg with the knee bent at 30 degrees of flexion. With one hand on the thigh for stabilization, a pull forward on the calf will show an increase in motion and soft endpoint compared to the other knee if the ACL is ruptured.
  • Other tests for the ACL include the pivot-shift and the anterior drawer tests. Caution must be exercised if the examination occurs after significant swelling has occurred because this may reduce their accuracy.

Examination of the knee is also performed to evaluate for concomitant injuries to other structures, such as the meniscus and the medial collateral ligament (MCL). These structures can be evaluated by palpating the joint lines and placing the knee through its full range of motion.

  • For the meniscus, a test called McMurray’s sign is performed by flexing the knee and then internally and externally rotating the tibia while extending the knee.
  • An inward (valgus) force on the knee while it is flexed at 30 degrees may induce pain or laxity if the MCL is injured.
  • Pain upon palpation, or pressing down on, the bony structures around the knee may increase suspicion for a fracture

Medical imaging. A plain X-ray of the knee is imperative if an ACL tear is suspected because of the risk of fracture. An MRI of the knee is utilized to confirm the diagnosis, evaluate for other injuries to the knee, and help plan for surgery.