Knee Ligament Anatomy (

Definition: Sprain or tear of the anterior cruciate ligament (ACL).

Mechanism of Injury (Boden 2000)

  • High energy (Motor Vehicle Accident and Contact sports) 
    • Direct blow to the knee causing hyperextension or valgus deformity
    • Can also be seen in sports, as in football with a tackle into the leg with the foot planted
    • Direct blow in MVA, often missed in primary evaluation in multi-trauma cases
  • Low energy (non-contact sports injuries)
    • Sudden deceleration or direction change in a running or jumping athlete
    • Sudden rotation or valgus stress to the knee, with minimal flexion and internal rotation.

Epidemiology (Miyasaka 1991Agel 2016)

  • Most commonly injured knee ligament, 100,000-200,000 ruptures per year 
  • Annual incidence of 1 in 3500 
  • Most commonly from non-contact athletic injuries 
  • Largest number in the US are from football, although those are more often contact injuries
  • More common in women by percentage although not overall number 

Lachman’s Test


  • History
    • Usually report a “popping” sensation at the time of injury (Boden 2000)
    • Acute swelling
    • Knee instability, especially with squatting, pivoting, lateral movement (OrthoInfo 2017)
  • Physical Examination
    • Always compare to the unaffected knee for comparison. (OrthoInfo 2017)
    • Often exam is best immediately after the injury, as swelling and pain may increase after the injury

      Anterior Drawer Test

    • Lachman test
      • Place knee in 30 degrees flexion, stabilize distal femur and pull proximal tibia anteriorly
      • Positive with anterior translation of tibia
    • Anterior drawer test
      • Patient lies supine with knee flexed at 90 degrees, tibia pulled anteriorly
      • Positive with anterior translation of the tibia
      • Always compare to the contralateral side as many people have physiologic laxity
      • Great test in chronic injuries, but performs worse in acute injuries
  • Pivot Shift Test (Video Link)
    • Difficult to perform if patient is guarding. Requires full cooperation and relaxation
    • Start with knee in extension, internally rotate the tibia while placing valgus stress on the knee causing the joint to sublux, then flex the knee, causing a reduction
    • Positive test with a “clunk” of the tibia as it reduces with flexion
  • Lever Test (Video Link)
    • Newly established test (Lelli 2016)
    • The original study found it to be 100% sensitive and specific in partial and complete tears
    • Subsequent studies showed 98% sensitive under anesthesia, 96% without
    • This would make it more sensitive and specific than Lachman, especially for partial tears
    • Further study is needed, but this is a very promising diagnostic test

Likelihood Ratio Comparisons for ACL Physical Exam Maneuvers (Benjaminse 2006, Lelli 2016)

Lachman Anterior Drawer* Pivot Shift Lever
Sensitivity 85 92 24 98
Specificity 94 91 98 100¢
+LR 14.16 10.2 12
-LR 0.16 0.09 0.78 0

*Anterior Drawer values for chronic tear only, does not do as well for acute tear ¢Lever Test values based on 2 studies only, limited data

    • Posterior Collateral, Medial Collateral and Lateral Collateral Ligaments

      Posterior Drawer Test (

      • Concomitant PCL/MCL/LCL injuries are common
      • PCL Injuries
        • ACL exams can be falsely positive with PCL injury, as the knee may have a posterior lag, and the return to neutral can be confused for an anterior translation
        • Posterior drawer test is similar to anterior drawer but with posterior translation of the tibia
    • MCL/LCL injuries
        • Assessed with gradual varus/valgus stress to the knee
        • Always compare to the contralateral knee as many patients, particularly children, may have some baseline laxity
        • Valgus Stress Video
        • Varus Stress Video


  • Always important to note if the injury is isolated or associated with damage to another structure, such as the cartilage, meniscus, or other ligaments (common in 50% of ACL injuries)
  • Grade 1 sprain: mild damage to the ligament, slightly stretched but able to keep knee joint stable
  • Grade 2 sprain: the ligament is stretched enough to be loose, otherwise known as a partial tear.
  • Grade 3 sprain: complete tear of the ligament, into two pieces, with knee joint instability.


  • Plain Radiographs
    • Cannot evaluate ligamentous injuries
    • Often performed after acute knee trauma to rule out bony injury but usually, unnecessary
  • MRI
    • Most commonly used in the US
    • Performance characteristics (Crawford 2007)
      • Sensitivity 86.5%
      • Specificity 95.2%
      • (+) Likelihood ratio: 18.02
      • (-) Likelihood ratio: 0.14
    • Typically performed on outpatient basis
  • Ultrasound
    • More common in Europe
    • Performance Characteristics (Skovgaard 2000)
      • Sensitivity 88%
      • Specificity 98%
      • (+) Likelihood ratio: 44.0
      • (-) Likelihood ratio: 0.12

ED Management

  • There is no need for emergency intervention in an isolated ACL injury
  • Supportive Care
    • Rest, Ice, Compression, and Elevation
    • Analgesia
    • Crutches for comfort if needed
    • Knee immobilizer brace if unstable for comfort in the acute phase
      • Long term use of knee immobilizer associated with muscle atrophy and stiffening of joint leading to prolonged recovery
      • Patients will likely be switched to hinged brace by orthopedics on follow up
  • Referral to orthopedic surgery
    • May obtain delayed MRI
      • no indication for urgent MRI
      • Image quality may improve as swelling decreases
    • Surgical options: Patellar tendon graft, hamstring tendon graft, and allograft

Prognosis (Ardern 2014)

  • Prognosis depends on surgical management.
    • Young, active patients typically get surgical repair
    • 81% of reconstructed patients returned to some athletic activity
    • 65% regain preinjury levels of competition
    • 55% high level athletes return to competition
  • Increased risk for osteoarthritis (although how much is controversial)

Take Home Points

  • ACL tear is a diagnosis that can be made of physical exam. Learn multiple exam maneuvers to increase diagnostic accuracy.
  • Always check for concurrent injuries to other structures in the knee such as bones and other ligaments if you suspect an acute ACL tear.
  • Acute management is RICE, analgesia, and referral to orthopedics

Read More

Ortho Info: Anterior Cruciate Ligament (ACL) Injuries

Ortho Bullets: ACL Tear

Spindler KP. Clinical practice. Anterior cruciate ligament tear. N Eng J Med. 2008;359(20):2135. PMID: 19005197


Miyasaka, KC. The Incidence of Knee Ligament Injuries in the General Population. Am J Knee Surg. 1991; 4:43. PMC: 3037119

Agel, J. Collegiate ACL Injury Rates Across 15 Sports: National Collegiate Athletic Association Injury Surveillance System Data Update (2004-2005 Through 2012-2013). Clin J Sports Med. 2016; 26 (6):518-523. PMID: 27315457

Lelli, A. The “Lever Sign”: A New Clinical Test for the Diagnosis of Anterior Cruciate Ligament Rupture. Knee Surg Sport Trauma Arthro 2016 ;24(9):2794-7. PMID: 25536951

Boden, BP. Mechanisms of Anterior Cruciate Ligament Injury. Orthopedics. 2000;23(6):573. PMID: 10875418

Benjaminse, A. Clinical diagnosis of an anterior cruciate ligament rupture: a meta-analysis. J Ortho Sports Phys Therapy. 2006;36(5):267. PMID: 16715828

Crawford, R. Magnetic resonance imaging versus arthroscopy in the diagnosis of knee pathology, concentrating on meniscal lesions and ACL tears: a systematic review. Br Med Bull 2007;84:5-23. PMID: 17785279

Skovgaard Larsen, LP. Diagnosis of acute rupture of the anterior cruciate ligament of the knee by sonography. Eur J of Ultrasound. 2000;12(2):163. PMID: 11118925

Ardern, CL. Fifty-five per cent return to competitive sport following anterior cruciate ligament reconstruction surgery: an updated systematic review and meta-analysis including aspects of physical functioning and contextual factors. Br J Sports Med. 2014;48(21):1543-1522. PMID: 25157180