Anterior Cruciate Ligament Tear (Torn ACL)

  • Medical Author:
    Benjamin Wedro, MD, FACEP, FAAEM

    Dr. Ben Wedro practices emergency medicine at Gundersen Clinic, a regional trauma center in La Crosse, Wisconsin. His background includes undergraduate and medical studies at the University of Alberta, a Family Practice internship at Queen's University in Kingston, Ontario and residency training in Emergency Medicine at the University of Oklahoma Health Sciences Center.

  • Medical Editor: William C. Shiel Jr., MD, FACP, FACR
    William C. Shiel Jr., MD, FACP, FACR

    William C. Shiel Jr., MD, FACP, FACR

    Dr. Shiel received a Bachelor of Science degree with honors from the University of Notre Dame. There he was involved in research in radiation biology and received the Huisking Scholarship. After graduating from St. Louis University School of Medicine, he completed his Internal Medicine residency and Rheumatology fellowship at the University of California, Irvine. He is board-certified in Internal Medicine and Rheumatology.

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What are symptoms and signs of a torn ACL?

With an acute injury, the patient often describes that they heard a loud pop and then developed intense pain in the knee. The pain makes walking or weight-bearing very difficult. The knee joint will begin to swell within a few hours because of bleeding within the joint, making it difficult to straighten the knee.

If left untreated, the knee will feel unstable and the patient may complain of recurrent pain and swelling and giving way, especially when walking on uneven ground or climbing up or down steps.

What tests do health-care professionals use to diagnose a torn ACL?

Televised sporting events have allowed the general public to watch how knee injuries occur, often repeatedly in slow-motion replay.

The diagnosis of an ACL injury begins with the care provider taking a history of how the injury occurred. Often the patient can describe in detail their body and leg position and the sequence of events just before, during, and after the injury as well as the angle of any impact.

Physical examination

Physical examination of the knee usually follows a relatively standard pattern.

  • The knee is examined for obvious swelling, bruising, and deformity.
  • Areas of tenderness and subtle evidence of knee joint fluid (effusion) are noted.
  • Most importantly, with knee injury ligamentous, stability is assessed. Since there are four ligaments at risk for injury, the examiner may try to test each to determine which one(s) is (are) potentially damaged. It is important to remember that a knee ligament injury might be an isolated structure damaged or there may be more than one ligament and other structures in the knee that are hurt.
  • In the acute situation, with a painful, swollen joint, the initial examination may be difficult because both the pain and the fluid limit the patient's ability to cooperate and relax the leg. Spasm of the quadriceps and hamstring muscles often can make it difficult to assess ACL stability.
  • A variety of maneuvers can be used to test the stability and strength of the ACL. These include the Lachman test, the pivot-shift test, and the anterior drawer test. Guidelines from the American Academy of Pediatrics suggest the Lachman test is best for assessing ACL tears.
  • The Bachmann test is performed as follows:
    • The damaged knee is flexed to 20-30 degrees.
    • The examiner grasps tibia and puts their thumb on the tibial tubercle (the bump of bone just below the knee where the patellar tendon attaches.
    • The examiners other hand grasps the thigh just above the knee.
    • The tibia is pulled forward and normally, there should be a firm stop if the ACL is intact. If the ligament is torn, the tibia will move forward and there will be no endpoint and it feels mushy.
  • The unaffected knee may be examined to be used as comparison.

It may be difficult to examine some patients when muscle strength or spasm can hide an injured ACL because of the knee stabilization that they can provide.

Knee imaging

Plain X-rays of the knee may be done looking for broken bones. Other injuries that may mimic a torn ACL include fractures of the tibial plateau or tibial spines, where the ACL attaches. This second situation is often seen in children with knee injuries, where the ligament fibers are stronger than the bones to which they are attached. In patients with an ACL tear, the X-rays are often normal.

Magnetic resonance imaging (MRI) has become the test of choice to image the knee looking for ligament injury. In addition to defining the injury, it can help the orthopedic surgeon help decide the best treatment options. However, MRI does not replace physical examination and many knee injuries do not require an MRI to confirm the diagnosis.

Medically Reviewed by a Doctor on 3/18/2016
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