• The function of the ACL is primarily to resist anterior tibial subluxation
  • Injuries to the ACL involve traumatic disruption of the normal ligament anatomy, almost always from excessive tensile force
  • In layman’s terms, an ACL deficiency is associated with a "trick knee"


  • The ACL traverses the knee joint from the tibial spines to the medial aspect of the lateral femoral condyle in the notch
  • Composed of 2 fascicles : anteromedial and posterolateral
    • Anteromedial fibers are tight in flexion and limit anterior translation of tibia on femur
    • Posterolateral fibers are tight in extension and limit anterior translation and ER
  • Functions
    • Primary function is to prevent anterior displacement of the tibia on the femur
    • Secondary function
      • Restraint to internal rotation
      • Restraint to varus and valgus stresses
    • Screw home motion occurs around ACL axis
    • Contains mechano receptors with proprioceptive role


  • The pathology associated with an ACL injury includes
    • Functional instability; the deficit associated with the absence of the normal ACL
    • Arthrosis; secondary to the traumatic damage inflicted on the knee during the initial injurious episode
  • The ACL tears when the tibia subluxates a distance greater than the elastic limit of the ligament
  • In the absence of an ACL, the normal knee mechanics are disrupted and the tibia can subluxate forward
  • There can be a sense of instability as the tibia moves from a subluxated to a reduced position
  • There are secondary injuries seen with ACL disruptions, including
    • Tears of the menisci
    • Collateral ligament (most often medial) injury
    • Impaction injury to the articular surface; most often the terminal sulcus on the lateral femoral condyle 


  • The ACL is thought to experience isolated forces during cutting sports
  • Simple consideration of the knee anatomy would indicate that the tibia is also stabilized by muscular force, whose absence may lead to ligament injury (from loss of protection)
  • Accordingly, one could argue that relative muscular weakness is a cause of ACL injury. This is seen in the anecdotal experience of ski injuries
    • Many of these injuries occur in the afternoon when muscle fatigue has built up
    • The fatigued hamstring muscle may not resist anterior translation of the tibia, as it normally would; placing an undo amount of stress on the ACL, leading to a tear


  • It is thought, but unproven that a strengthening program may reduce the incidence of ACL tear

Nutritional factors

  • Increased BMI may increase the risk of ACL injuries due to the increased force placed on the ligament

Natural History

  • ACL injuries occur, it is estimated, in excess over 100,000 times a year
  • More frequently occurring in women. This may be due to
    • Hormonal effects on the strength of the ligament
    • Differences in male, female notch geometry
    • Strength differences 

Clinical Presentation

  • In 1845, Amédée Bonnet described the 3 essential signs of an ACL rupture:
    • A snapping noise
    • Hemarthrosis
    • Loss of function
  • Many people who have ruptured their ACLs will report having literally heard the classic "pop"
  • There is often an acute hemarthrosis
  • Sometimes a bloody effusion can be so large that it has no room for a fluid wave
    • Side to side asymmetry is the clue to its presence
    • Aspiration with a 16 gauge needle will confirm the diagnosis and relieve the patient
  • The injury is variably painful. The ligament injury itself is thought not to be painful, but rather pain may be present depending on the extent of impaction injury at the same time
  • Instability is not often the presenting complaint
    • Rather, it is a late finding once the patient has regained motion and attempted to participate in sports again
    • Instability as an initial presenting complaint is usually a sign of intra-articular cartilage disruption: the instability is from buckling and reflexive quadriceps inhibition
  • Objective physical findings in the setting of an acute ACL disruption include
    • Large effusion
    • Positive Lachman test
    • Positive anterior drawer test
    • >5 mm total displacement or >3 mm difference compared to normal side is diagnostic
    • Signs of associated injuries, if present
  • 50% of all acute ACL tears are associated with meniscal tears
    • Acute ACL tears are more associated with lateral meniscus tears
    • Chronic ACL tears are more associated with medial meniscus tears

Differential Diagnosis

  • Acute Injuries to other parts of the knee can have similar presentations and causes to an ACL tear
  • A tear or injury to other ligaments in the knee, such as the medial collateral ligament, lateral collateral ligament, posterior cruciate ligament, or injury to the lateral meniscus or medial meniscus can present with acute knee pain and swelling, similar to an ACL tear
  • Patellar dislocation should also be considered
  • Direct contact injuries often result in several ligaments being injured at once, including cruciate and collateral ligaments. Meniscus tears, injuries to the joint capsule, articular cartilage, and bone contusions can also occur
  • Perhaps the best known injury associated with an ACL tear is the "terrible triad" which results from a blow to the lateral knee and results in the tearing of the ACL, MCL and medial meniscus
  • It is probably best not to think about differential diagnosis, but additional diagnosis 

Imaging and Diagnostic Studies


  • Imaging studies acutely are often normal
  • There may be a "Segund" sign: a linear avulsion off the lateral tibia
  • Plain radiographs can show tibial plateau fractures and can suggest possible ACL tears, but cannot diagnose them


  • The primary diagnostic test used to diagnose ACL rupture
    • Sensitivity 96%
    • Specificity 98%
  • It is less effective in the setting of partial tears
  • A partial tear demonstrates disruption of some fibers of the tendon, but one can’t be sure if the fibers left are "good enough"
  • A complete tear shows discontinuity of the entire tendon
  • The novice is cautioned to not mistake hemorrhage within the ligament for the ligament itself
  • Likewise, an empty notch is a sign of a torn ACL, even though the "torn edges" are not visible
  • One also must look for signs of bone bruising and impaction injury, along with associated meniscal or collateral ligament injuries
  • The presence of a "kissing contusion" proves that a subluxation has taken place


  • Patients without complaints and without instability need no treatment
    • Some patients are able to control potential tibial subluxation with their own muscular forces, i.e. they can compensate for the lost ligament
    • Other patients are unstable only during certain activities, which they may be willing to give up
  • All patients considering treatment should undergo a therapeutic regimen to insure full range of motion and strengthening
  • It may be reasonable to delay surgery, as some patients may do fine without it
  • While waiting for surgery, the patient is at an elevated risk for re-injury, e.g. meniscal tears
  • The decision to observe and treat only those who need it, may lead to fewer reconstructions (as compared to imminent surgery for all)
  • At the present time, ACL disruptions are thought not to be reparable (The results in the past have been disappointing)
  • If surgery is needed, it is in the form of reconstruction
    • Best performed with biological material
    • Use of artificial ligaments has been disappointing
    • Details of surgical reconstruction are listed elsewhere  

Pearls and Pitfalls

  • It is reasonable to say that the questions to be answered by history and physical are simply** Does this person have an injury worthy of MRI?
    • Do I need to protect the patient with an immobilizer or crutches until follow up?