Anterior Cruciate Ligament Injury

Updated: Feb 26, 2021
  • Author: Matthew Gammons, MD; Chief Editor: Sherwin SW Ho, MD  more...
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Practice Essentials

Anterior cruciate ligament (ACL) 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 ACL secondary to twisting, valgus stress, or hyperextension all directly related to contact or collision. The image below depicts a ruptured ACL.

MRI displaying a ruptured anterior cruciate ligame MRI displaying a ruptured anterior cruciate ligament.

Signs and symptoms

Symptoms of an acute ACL injury may include the following:

  • Feeling or hearing a “pop” sound in the knee

  • Pain and inability to continue activity

  • Swelling and instability of the knee

  • Development of a large hemarthrosis

See Clinical Presentation for more detail.


Most ACL injuries may be diagnosed through a careful history emphasizing mechanism of injury coupled with a good physical examination.

Lachman test

The Lachman test is the most sensitive test for acute ACL rupture. It is performed with the knee in 30° of flexion, with the patient lying supine. 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.

Pivot shift test

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. It is performed with the leg extended and the foot in internal rotation, and a valgus stress is applied to the tibia.

Anterior drawer test

The anterior drawer test is performed with the knee at 90° flexion, with the patient lying supine. There is an attempt to displace the tibia forward from the femur. If there is more than 6 mm of tibial displacement, an ACL tear is suggested. This test is not very sensitive and has been found to be positive in only 77% of patients with complete ACL rupture.


MRI has a sensitivity of 90-98% for ACL tears. MRI also may identify bone bruising, which is present in approximately 90% of ACL injuries.

An MRI allows the physician to confirm an ACL tear, but it should not be used as a substitute for a good history and physical examination.

See Workup for more detail.


Nonoperative treatment

Nonoperative treatment may be considered in elderly patients or in less active athletes who may not be participating in any pivoting type of sports (eg, running, cycling). Arthroscopy may also be considered for persons who are poor candidates for reconstruction but have a mechanical block to range of motion.

Surgical intervention

Generally, the recommendation is that surgical intervention be delayed at least 3 weeks following injury to prevent the complication of arthrofibrosis. The methods of surgical repair may be categorized into 3 groups: primary repair, extra-articular repair, and intra-articular repair.

Primary repair is not recommended except for bony avulsions, which are mostly seen in adolescents. Because the ACL is intra-articular, the ligamentous ends are subjected to synovial fluid, which does not support ligamentous healing.

Extra-articular repair generally involves a tenodesis of the iliotibial tract. This may prevent a pivot shift but has not been shown to decrease anterior tibial translation.

Intra-articular reconstruction of the ACL has become the criterion standard for treating ACL tears.

See Treatment and Medication for more detail.



Anterior cruciate ligament (ACL) 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 ACL secondary to twisting, valgus stress, or hyperextension, all directly related to contact or collision. The MRI image below shows a ruptured ACL.

MRI displaying a ruptured anterior cruciate ligame MRI displaying a ruptured anterior cruciate ligament.

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 and 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. [1, 2, 3, 4, 5, 6]

Clinical studies

A randomized, prospective study by Wipfler et al comparing bone-patella-bone (BTB) autografts to hamstring tendon (HT) grafts at 9 years demonstrated significantly better International Knee Documentation Committee (IKDC) scores in the HT group, with no significant differences in laxity, tunnel widening, or any other parameters. [7]

Leys et al also found equivalent IKDC scores and better long-term outcomes (radiological evidence of osteoarthritis, level of activity, knee motion and single leg hop test) in HT versus BTB in a long-term cohort study following patients for 15 years postoperatively. The HT group had higher ipsilateral graft rupture rates (17% versus 8%), but lower contralateral ACL injury (12% versus 26%). [8]

One study compared the clinical outcomes of ACL reconstruction with hamstring tendon autograft versus irradiated allograft. The results found the rate of laxity with irradiated allograft was higher than that with autograft (32.3% vs 8.3%, respectively) in the 67 patients studied. Statistically significant differences were noted between the groups in the Lachman test (P = .00011), anterior drawer test (P = .00016), pivot-shift test (P = .008), and KT-2000 arthrometer assessment (P = .00021); the anterior and rotational stabilities decreased significantly in the irradiated allograft group. No significant differences were found between the 2 groups in functional and subjective evaluations, and activity level testing; however, patients in the irradiated allograft group had a shorter operative time and a longer duration of postoperative fever. [9]

Another study evaluated the outcome of anatomic double-bundle anterior cruciate ligament reconstruction with hamstring tendon autografts in both women and men. After a 2-year postoperative evaluation, the results noted that the assessment results for ligament laxity were approximately identical in both groups. [10]

The results from another study noted that 11 years after anterior cruciate ligament reconstruction, both hamstring and patellar tendon autografts provided good long-term outcomes and stability. However, a positive result on the pivot-shift (1+) test was significantly more frequent in the patellar tendon group, as was the rate of osteoarthritis. [11]

Geib et al compared intermediate-term outcomes of ACL reconstruction by bone-patellar tendon-bone (BPTB) with the outcomes associated with quadriceps tendon with a bone plug (BQT) and quadriceps tendon without a bone plug (QT). They found that QT and BQT produced results equivalent to those of BPTB autograft in arthroscopically assisted ACL reconstruction. When compared with BPTB autograft, the quadriceps tendon autograft showed significantly better results, with less anterior knee pain (4.56% vs 26.7%), less anterior numbness (1.5% vs 53.3%), a higher percentage of arthrometer measurements showing a side-to-side difference of 0 to 3 mm (88% vs 68%), and better extension (mean loss, 0.55º vs 2.77º). [12]

According to the results of a study by Marchant et al, computed tomography is the most reliable imaging modality for evaluation of ACL bone tunnels, as proven by superior intraobserver and interobserver testing results, when compared with results obtained with MRI and radiographs. According to the authors, radiographs and MRIs were not reliable even for identifying the presence of a bone tunnel. Intraobserver kappa scores for tibial cross-sectional area using CT, radiographs, and MRI were 0.66, 0.5, and 0.37, respectively. Interobserver kappa scores for tibial cross-sectional area using CT, radiographs, and MRI were 0.65, 0.39, and 0.32, respectively. [13]

According to a study of National Football League players by Brophy et al, a history of meniscectomy, but not ACL reconstruction, shortens the expected career of a professional football player, but a combination of ACL reconstruction and meniscectomy may be more detrimental to an athlete's durability than either surgery alone. In their study, 54 athletes with a history of meniscectomy, 29 with a history of ACL reconstruction, and 11 with a history of both were identified and matched with control subjects. Isolated meniscectomy reduced the length of career in both years (5.6 vs 7.0; P = .03) and games played (62 vs 85; P = .02). Isolated ACL surgery did not significantly reduce the length of career in years or games played. Athletes with a history of both surgeries had shorter careers in games started (7.9 vs 35.1; P < .01), games played (41 vs 63; P = .07), and years (4.0 vs 5.8; P = .08) than athletes with a history of either surgery alone. [14]

Lyman et al found that although ACL reconstruction appears to be a safe procedure, the risk of a subsequent operation on either knee is increased among younger patients and those treated by a lower-volume surgeon or at a lower-volume hospital. According to the authors, patients were at increased risk for readmission within 90 days after surgery if they were older than 40 years, sicker (eg, had a preexisting comorbidity), male, or operated on by a lower-volume surgeon. Predictors of subsequent knee surgery included being female, having concomitant knee surgery, and being operated on by a lower-volume surgeon. Predictors of a subsequent ACL reconstruction included age less than 40 years, concomitant meniscectomy or other knee surgery, and surgery in a lower-volume hospital. [4]


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 14 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 the posterolateral bands. During flexion, the anterior band is taut, while the posterior band 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).



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 et al found a 100-fold increase in the incidence of ACL injury in college football players when compared to the general population.


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. One study has determined that ACL laxity does not vary with the menstrual cycle, thus dismissing this possible etiology.

Femoral notch stenosis

Femoral notch stenosis is 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.


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.



United States data

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. A study by Gornitzky et al found that there is an approximately 1.6-fold greater rate of ACL tears per athletic exposure in high school female athletes than males. [15]



Patients treated with surgical reconstruction of the ACL have long-term success rates of 82-95%. Recurrent instability and graft failure is seen in approximately 8% of patients.

A study by Brophy et al found increased rates of chondral damage in patients who underwent partial meniscectomy compared to patients with a previous meniscal repair or no history of meniscal injury. [16]

Knee scores of those treated nonoperatively have fair/poor results up to 50% of the time. As many as 40% of patients treated nonoperatively have no episodes of giving way. The knee scores in this group may be too sensitive, not accurately representing the clinical situation.

Patients with ACL ruptures, even after successful reconstruction, are at risk for osteoarthrosis. The goal of surgery is to stabilize the knee, decrease the chance of future meniscal injury, and delay the arthritic process.

In a retrospective study of 135 young athletes (average age, 14 years) who underwent ACL reconstruction, Anderson et al found that the timing of surgery was related to the risk of secondary knee injuries. Compared with patients who had surgery within 6 weeks after an ACL injury, those who underwent surgery 6-12 weeks after injury had a 1.45 times higher risk of lateral meniscus injury, and those who underwent surgery more than 12 weeks after injury had a 2.82-fold increased risk. Risk for medial meniscal tears was 4.3 times higher when surgery was delayed at least 6 weeks. Other risk factors for secondary knee injuries were younger age, resumption of participation in sports before surgery, and earlier episodes of knee instability. [17]


The current failure rate for ACL reconstruction is approximately 8%. The 3 major categories of failure in an ACL reconstruction are (1) arthrofibrosis (due to inflammation of the synovium and fat pad), (2) pain that limits motion, and (3) recurrent instability, secondary to significant laxity in the reconstructed ligament. These factors may be related to the surgical procedure (eg, malpositioned tibial or femoral tunnels, misplaced hardware, inadequate notchplasty).

  • Anterior placement of a tibial tunnel may result in graft impingement. If a tunnel is placed too posteriorly on the femoral side, the posterior cortex of the femur may be violated.

  • A graft also may fail due to a lack of incorporation, secondary to rejection or stress shielding.

  • Trauma from re-injury or aggressive rehabilitation also may cause graft failure. The incidence of graft re-rupture is approximately 2.5%.

Other complications include patella fractures and patella-tendon rupturesReflex sympathetic dystrophy, postoperative infection, and neurovascular complications are rare (each accounting for less than 1% of complications). The rate of postoperative deep venous thrombosis is approximately 0.12%.