eMedicine Specialties > Sports Medicine > Knee

Anterior Cruciate Ligament Injury

Author: John D Hubbell, MD, Consulting Surgeon, Department of Orthopedic Surgery, Southampton Hospital
Coauthor(s): Evan Schwartz, MD, Director of Orthopedic Surgery, St John's Queens Hospital, New York Medical College; Assistant Professor, Department of Surgery, Albert Einstein School of Medicine
Contributor Information and Disclosures

Updated: Mar 7, 2006

Introduction

Background

Based on statements found in the Orthopaedic Knowledge Update 7 regarding the increased incidence of knee ligament injuries, the author proposes that this incidence may be associated with the current emphasis on fitness. These injuries are most often a result of low-velocity, noncontact, deceleration injuries and contact injuries with a rotational component. Contact sports also may produce injury to the anterior cruciate ligament (ACL) secondary to twisting, valgus stress, or hyperextension all directly related to contact or collision.

When matched for activities, a greater prevalence for ACL injury is found in females compared with males. Approximately 50% of patients with ACL injuries also have meniscal tears. In acute ACL injuries, the lateral meniscus is more commonly torn; in chronic ACL tears, the medial meniscus is more commonly torn. The only study on the prevalence of ACL injuries in the general population has estimated the incidence as 1 case in 3,500 people, resulting in 95,000 new ACL ruptures per year. 

The importance of the ACL has been emphasized in athletes who require stability in running, cutting, and kicking. The ACL-deficient knee has also been linked to an increased rate of degenerative changes and meniscal injuries. For these reasons, approximately 60,000-75,000 ACL reconstructions are performed annually in the United States.

For restoration of activity and stability, the expected long-term success rate of ACL reconstruction is between 75-95%. The current failure rate is 8%, which may be attributed to recurrent instability, graft failure, or arthrofibrosis.

Treatment options must be tailored to a patient's preoperative level of activity. The following activity levels are based on the International Knee Documentation Committee:

• Level I includes jumping, pivoting, and hard cutting.
• Level II is heavy manual work or side-to-side sports.
• Level III encompasses light manual work and noncutting sports (eg, running, cycling).
• Level IV is sedentary activity without sports.

Nonsurgical treatment may be considered for patients who participate in level III or IV activities; all others should be considered as candidates for surgery. In addition, consider surgical consultation on any young athlete due to potential complications from recurrent instability.

Frequency

United States

An estimated 200,000 ACL-related injuries occur annually in the United States, with approximately 95,000 ACL ruptures. Approximately 100,000 ACL reconstructions are performed each year. The incidence of ACL injury is higher in people who participate in high-risk sports such as basketball, football, skiing, and soccer. When the frequency of participation is considered, a higher prevalence of injury is observed in females over males, at a rate 2.4-9.7 times greater for females.

International

No international incidence is reported in the literature.

Functional Anatomy

The knee joint develops as a cleft between mesenchymal rudiments of the femur and the tibia. This occurs around the eighth week of fetal development. The cruciate ligaments appear as condensations of vascular synovial mesenchyme at the same time.

By fourteen weeks gestation, the ACL and posterior cruciate ligament have divided; both have a functional blood supply, which is mainly derived from the middle geniculate artery. The inferomedial and lateral genicular arteries also provide blood supply through the fat pad.

The ACL is composed of densely organized, fibrous collagenous connective tissue that attaches the femur to the tibia. The ACL is composed of 2 groups, the anteromedial and posterolateral bands. During flexion the anterior band is taught while the posterior is loose; during extension, the posterolateral band is tight, while the anterior band is loose.

The ACL attaches to bone through a transitional zone of fibrocartilage and mineralized cartilage. On the femur, the ACL is attached to a fossa on the posteromedial edge of the lateral femoral condyle. The tibial insertion is located in a fossa that is anterior and lateral to the anterior tibial spine. The tibial attachment is noted to be somewhat wider and stronger than the femoral attachment.

The ACL is intracapsular and extrasynovial. It courses anteriorly, medially, and distally as it runs from the femur to the tibia.

The ACL receives nerve fibers from the posterior branch of the posterior tibial nerve. The main function is believed to be proprioception, providing the afferent arc for postural changes during motion and ligament deformation.

Sport Specific Biomechanics

The ACL is the primary (85%) restraint to limit anterior translation of the tibia. The greatest restraint is in full extension.

The ACL also serves as a secondary restraint to tibial rotation and varus/valgus angulation at full extension. Since the relationship between the tibia and femur provides little bony stability, the ligamentous structures must provide stability. When the ACL is injured, a combination of anterior translation and rotation occurs.

The average tensile strength for the ACL is 2160 N. This is slightly less than the strength of the posterior cruciate ligament and approximately half as strong as the medial collateral ligament (MCL).

Clinical

History

Most ACL injuries may be diagnosed through a careful history emphasizing mechanism of injury coupled with a good physical examination. Remember that a previous ligamentous injury may be the cause of instability. When discussing the history, be sure to document mechanism of injury for this episode and any previous episodes.

• Noncontact injury
• • An audible pop often accompanies this injury, which often occurs while changing direction, cutting, or landing from a jump (usually a hyperextension/pivot combination).
  • Within a few hours, a large hemarthrosis develops.
  • Patients usually are unable to return to play, secondary to pain, swelling, and instability or giving way of the knee.
     
• Contact and high-energy traumatic injuries
• • These injuries often are associated with other ligamentous and meniscal injuries.
  • The classic "terrible triad" (ACL, MCL, and medial meniscus tears) involves a valgus stress to the knee with resultant acute injury to the ACL and MCL; however, the medial meniscus tear is now thought to occur later, as a result of chronic ACL deficiency.

Physical

An organized, systematic physical examination is imperative when examining any joint. Immediately after the acute injury, the physical examination may be very limited due to apprehension and guarding by the patient. The basic examination should include the following:

• The examiner should begin with inspection, looking for any gross effusion or bony abnormality. An immediate effusion indicates significant intra-articular trauma. According to Noyes et al, in the absence of bony trauma, an immediate effusion is believed to have a 72% correlation with an ACL injury of some degree.
• Assess the patient's range of motion (ROM), especially looking for lack of complete extension, secondary to a possible bucket-handle meniscus tear or associated loose fragment.
• Palpation of bony structures may suggest an associated tibial plateau fracture.
• Palpation of the joint lines to evaluate a possible associated meniscus tear. Palpation over the collateral ligaments to suggest any possible injury (sprain) of these structures. Up to 50% of ACL ruptures have associated meniscal injuries; acute injuries are likely to have associated injuries of the MCL and meniscus.
• Ligamentous laxity may be difficult to detect in the acute situation. The Lachman test is the most sensitive test for acute ACL rupture. Since the Lachman test must be performed when the patient is relaxed, it is often better to conduct this test prior to manipulating the painful knee.
• • The knee is placed in a position of 20-30° of flexion. The femur is stabilized with a nondominant hand, and an anteriorly directed force is applied to the proximal calf.
  • The amount of displacement (in mm) and the quality of endpoint are assessed (eg, firm, marginal, soft). Asymmetry in side-to-side laxity or a soft endpoint is indicative of an ACL tear. Although difficult to measure, a side-to-side difference of greater than 3 mm is considered abnormal.
• Other ligamentous tests are less reliable especially for primary care providers who may not have as much experience in using these maneuvers. The pivot shift test is performed by extending an ACL-deficient knee, which results in a small amount of anterior translation of the tibia in relation to the femur. During flexion, the translation reduces, resulting in the "shifting or pivoting" of the tibia into its proper alignment on the femur.
• • The pivot shift test is performed with the leg extended, the foot in internal rotation, and a valgus stress is applied to the tibia.
  • Flexion causes a reduction of the anteriorly subluxed tibia at approximately 20-30°
• The anterior drawer test may be influenced by hamstring spasm in the acutely injured knee; thus, this test is considered the least reliable.
• • This test is performed with the patient supine and the knee flexed to 90°
  • The examiner can sit on the patient's foot and grasp around the patient's calf with both hands.
  • An anterior force is applied, and tibial excursion is compared to the unaffected knee.

Causes

Associated risk factors include the following:

• High-risk sports
• • Based on a study performed by Kaiser Permanente, football, baseball, soccer, skiing, and basketball account for up to 78% of sports-related injuries.
  • Hewson and associates found a 100-fold increase in the incidence of ACL injury in college football players when compared to the general population.
     
• Sex
• • Female athletes are more susceptible to ACL injuries. Studies have shown a 2-fold increase in collegiate soccer players and a 4-fold increase in basketball compared with their male counterparts.
  • Differences may be due to experience, differences in training, different strength-to-weight ratios, limb alignment, joint laxity, muscle recruitment patterns, and notch index but further studies to document a definitive cause are ongoing. A recent study has determined that ACL laxity does not vary with the menstrual cycle, thus dismissing this possible etiology.
     
• Femoral notch stenosis (the ratio of the femoral notch width to the width of the femoral condyles)
• • A notch width index of less than 0.2 is defined as notch stenosis.
  • Individuals with notch stenosis have a higher risk of noncontact ACL injuries.
     
• Footwear: Cleats, which have a predominant grip on the periphery, have a higher coefficient of friction on artificial turf and may result in a higher incidence of ACL injuries.

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