ACL Injury and Treatment Overview


The anterior cruciate ligament (ACL) originates from the tibial plateau just medial and anterior to the tibial eminence. The ACL tracts from the tibia superiorly, laterally, and posteriorly to its insertion on the posterior aspect of the medial wall of the lateral femoral condyle. The ACL is composed of 2 bundles, the anteromedial bundle and the posterolateral bundle. The ACL provides 85% of the total restraining force to anterior translation of the tibia. An ACL tear is a common injury that occurs in all types of sports. This injury usually occurs during a sudden cut or deceleration, as it typically is a noncontact injury. The patient states, “I planted, twisted, and then heard a pop.” Before the advent of arthroscopic knee surgery in the early 1970s, ACL tear was often a career-ending injury.


The ACL, like all other ligaments, is composed of type I collagen. The ultrastructure of a ligament is close to that of tendons, but the fibers in a ligament are more variable and have a higher elastin content. Ligaments receive their blood supply from their insertion sites. The vascularity within a ligament is uniform, and each ligament contains mechanoreceptors and free nerve endings that are hypothesized to aid in stabilizing the joint. Avulsion of ligaments generally occurs between the unmineralized and mineralized fibrocartilage layers. The more common ACL tear, however, is a midsubstance tear. This type of tear occurs primarily as the ligament is transected by the pivoting lateral femoral condyle.


In the US: Epidemiologic studies estimate that approximately 1 in 3000 individuals sustains an ACL injury each year in the United States. This figure corresponds to an overall injury rate approaching 200,000 injuries annually. This estimate is low for women because ACL injury rates are estimated to be 2-8 times higher in women than in men participating in the same sports. The average cost for surgical repair of an ACL tear is approximately $11,500. If all ACL injuries were repaired, the associated expenditure for 100,000 procedures would eclipse $2,000,000,000 annually. Internationally: International statistics are not available.
Mortality/Morbidity: Not a single report of mortality was found in 6 different studies examining the morbidity and mortality of ACL repair. The total number of patients in these combined studies was 363. Morbidity was divided into 5 classes. The first class included patients who were able to perform only symptomatic activities of daily living (ADL). The second class included patients who were able to perform all ADL. Patients in the third class were able to perform mildly stressful sports (eg, jogging, swimming, biking, cross-country skiing). The fourth class included patients who were able to perform moderately stressful sports, including baseball, alpine skiing, racquet sports, dance, and lacrosse. The last class included patients who returned to perfect health and were capable of performing very stressful sports such as soccer, basketball, football/rugby, volleyball, gymnastics, and hockey.

Postsurgery status of patients is as follows

Remained class 1 – 3.3% Remained class 2 – 1.4% Attained class 3 – 11.8% Attained class 4 – 17% Returned to class 5 – 66.5%

Race: No known correlation exists between race and occurrence of ACL injuries.

Sex: According to numerous studies, female athletes sustain a greater number of ACL injuries than male athletes. These results are well supported in 2 different papers. The first paper, by Arendt and Dick, showed that female athletes sustained significantly higher incidences of ACL injuries than their male counterparts did when competing in collegiate soccer and basketball. Their data show that women have 2.4 and 4.1 times greater chance of incurring ACL injury when compared with males in soccer and basketball, respectively. A second paper, by Hutchinson and Ireland, reports that women athletes competing in the 1988 Olympic basketball trials sustained 81% of ACL injuries during the trials.

Age: ACL injuries occur most commonly in individuals aged 14-29 years. These years correspond to a high degree of athletic activity.

History: Obtain as much information as possible directly from the patient. The important facts can be clarified by asking questions about the following:


  • Mechanism of injury
  • PainFeeling/hearing a pop
  • Feeling knee give out
  • Ability to continue playing sport
  • Swelling
  • Loss of knee motion
  • History of previous knee injury


Up to 50% of patients with acute knee injuries who report feeling or hearing a snapping or popping sound are found to have ACL injury. A hemarthrosis almost always is present because of the vascular supply to the ACL. When a complete ligamentous tear occurs, pain may begin immediately, followed by resolution. Immediately following injury, minimal effusion or spasm is present, so ACL injury usually can be identified easily. Several hours after injury, effusion and spasm make diagnosis of ACL tear more difficult.

To determine the patient’s normal amount of laxity, examine the uninjured knee first.

Perform the Lachman test

This test is performed with the knee in 30° of flexion with the patient lying supine.Using one hand on the anterior aspect of the distal femur and a second hand behind the proximal tibia, attempt to displace the tibia forward from the femur.A positive Lachman occurs when no endpoint is encountered. The degree of excursion may also indicate an ACL tear. Another test to detect ACL tear is the anterior drawer.
Perform this test with the knee at 90° of flexion with the patient lying supine.Place both hands behind the proximal tibia and attempt to displace the tibia forward from the femur.If there is more than 6 mm of tibial displacement, ACL tear is suggested.The anterior drawer test is not very sensitive and has been found to be positive in only 77% of patients with complete ACL rupture.
Causes: No one single cause accounts for this injury. ACL injuries can be related to extrinsic factors and intrinsic factors. Numerous studies document the fact that poor levels of conditioning correlate directly with increased levels of injury. Research also has demonstrated that improved conditioning results in reduced numbers of injuries.

Body and Movement Factors

The first 2 factors, body movement and positioning, play a big role in ACL injuries.Noyes et al have demonstrated that most ACL injuries (ie, 78%) occur without contact. Most of these injuries occur upon landing after a jump. The Noyes study involved only women basketball players, but the capacity of the knee to plant and turn or absorb the shock of a jump is relevant to both men and women in all sports.

Muscle Strength

Muscle strength is the last of the extrinsic factors that affects the ACL. The hamstring is an ACL agonist working in concert with the ACL to prevent anterior tibial translation. Conversely, the quadriceps acts as an antagonist to the ACL, generating force that promotes anterior tibial translation. Ideally, a balance exists between these opposing forces to protect the knee; however, the quadriceps averages 50-100% greater muscle strength than the hamstring.Strength coaches often emphasize quadriceps strengthening and ignore hamstring strengthening, further exacerbating the inequality.Several intrinsic factors can contribute to ACL injuries.

Joint laxity is one of these factors. Significant controversy surrounds this topic, as published studies are contradictory about whether or not increased laxity contributes to ACL injuries. Acasuso-Diaz et al and Kibler et al concurred that a strong relationship exists; however, Godshall and Jackson et al maintain that ACL laxity does not predispose to ACL injury.The Q angle is the acute angle between the line connecting the anterior superior iliac spine, the midpoint of the patella, and the line connecting the tibial tubercle with the same reference point on the patella. Theoretically, larger Q angles signal increases in the lateral pull of the quadriceps muscle on the patella and put medial stress on the knee. Shambaugh et al studied 45 athletes and found that the average Q angles of athletes sustaining knee injuries were significantly larger than the average Q angles for players who were not injured. Because lower extremity alignment cannot be altered, no recommendation can help minimize the athlete’s risk of ACL rupture; however, the dynamic position of the tibia can be improved with internal rotation exercises for the tibia (eg, medial hamstrings).A narrow intercondylar notch may be a predictive factor for ACL rupture. According to various reports, athletes who sustain ACL injuries often have narrow notch widths compared to fellow athletes with uninjured knees. The notch width index (NWI), defined by Souryal et al, is “the ratio of the width of the intercondylar notch to the width of the distal femur at the level of the popliteal groove on a tunnel view radiograph.” Another study by Souryal et al established that NWI measurements fall along a Gaussian curve, indicating that measurement is reproducible. Results showed that athletes sustaining ACL injuries had the lowest NWI. The critical NWIs were calculated as 1 standard deviation below the gender-dependent mean. Athletes falling into this critical range, according to data reported, are 26 times more susceptible to ACL injuries than other athletes.

Other Problems to be Considered (Differential Diagnosis):

Patellar dislocation/fractureKnee dislocationFemoral, tibial, or fibular fractureMeniscal knee injury

Imaging Studies:

MRI of the knee usually is recommended prior to surgery for evaluation of the other ligaments and the menisci, as findings can influence the treatment plan. MRI helps the surgeon to have the correct equipment at hand prior to beginning the surgery.Sensitivity of MRI has been shown to be greater than 95% and specificity approximately 98%, with positive predictive value of 95% and negative value of nearly 99% with fast spin-echo techniques.
Procedures: Perform aspiration of any large hemarthrosis under aseptic conditions, if indicated, to alleviate patient discomfort. Presence of fat globules suggests an intra-articular fracture. Radiographs may demonstrate an avulsed fragment just lateral to the tibial plateau. This type of fracture, referred to as a Segond fracture, represents an avulsion of the middle third of the lateral capsule from the tibial plateau and also is referred to as the lateral capsule sign.


The key to successful treatment of the ACL tear is proper and early rehabilitation. Preoperative and postoperative rehabilitation programs are similar initially. Restoration of motion, swelling control, and strength are the goals of each.

The postoperative rehabilitation program begins as soon as the patient awakens from anesthesia, especially because patients are discharged earlier now compared with previous years. Quadriceps co-contractions are the first exercise to teach patients for maintaining terminal extension. Passive motion is emphasized with active flexion and assisted extension in the sitting or prone position to ensure good leg control (ie, ability to flex the hip and lift the leg against gravity without assistance.) A continuous passive motion machine (CPM) can be used to establish 0-30° of motion immediately postoperatively and progress to 60° of knee flexion by the morning following the operation. The patient then begins gait training with crutches (weight bearing as tolerated), with the knee in an immobilizer. The patient usually can be discharged on the first postoperative day and should be encouraged to avoid crowds, keep the leg elevated when not ambulating, use the crutches at all times for protection, and continue frequent icing. A number of different programs are used by different physical therapists. The therapy program chosen depends on the activity level of the patient and the type of surgery performed, coexistent injuries (meniscal or other ligamentous injury), the surgeon, the insurance policy, and time constraints. The following rehabilitation program is an accelerated program for patellar tendon grafts. Note that the other grafts rehabilitate slightly differently. This rehabilitation program is classified as a goal-oriented approach. The dates listed are not meant to be followed strictly and can be varied by a day or two, depending on the physician or patient’s schedule. On day 3 following surgery, have the patient return to the surgeon for evaluation.

Begin therapy on an outpatient basis, concentrating on gait training and other ambulation activities. The goal is to maintain terminal knee extension and progression toward 90° of flexion. The therapist emphasizes a normal heel-to-toe gait pattern, and the patient may weight bear as tolerated on the involved leg. Continue passive flexion ROM exercises. Have the patient increase quadriceps activity, introducing the partial squat with progression from bilateral to unilateral, placing increased body weight on the extremity involved at no more than 45° of flexion. Continue these exercises for 1 week. Continue the knee immobilizer when ambulating and continue regular icing of the knee. On day 10 following surgery, have the patient return to the surgeon for evaluation. Advance therapy to include wall slide-squats and a stationary bike as tolerated. Place emphasis on terminal extension, progressive flexion, and full weight-bearing ambulation with normal heel-to-toe mechanics. In a controlled environment (no pets, children, or distractions), have the patient begin practicing crutch ambulation while out of the knee immobilizer. The patient should achieve full terminal knee extension and approximately 90-100° of knee flexion.

Three to four weeks after the surgery, the aggressive patient is ambulating with normal gait mechanics. At this point, the knee immobilizer can be removed. Advance the patient’s activity to include loaded squats, swimming, eccentric quadriceps strengthening, bridging with a physioball, and a stair stepper. During this time, the patient can develop tendonitis of the quadriceps tendon or other repetitive use injuries of the lower extremity if the therapist is not observant. Application of ice after each therapy session is very important. Six weeks after surgery, release the patient to light jogging or bicycling.

If the patient is older and has concomitant degenerative joint disease, encourage bicycling. The graft is still very weak at this stage, so advise the patient that it is important not to fall. The patient should jog only on a track or other flat protected surface. At this point, active ROM should be approaching 0-125° with minimal to no joint effusion. Work on balance and proprioception with activity drills. At 3 months, recommend that the patient begin a gradual return to normal activities. At this point, most people do not require bracing, but, occasionally, some athletes request a brace to increase their own comfort level when competing. Significant discussion surrounds the difference between open kinetic chain (OKC) and closed kinetic chain (CKC) exercises during ACL rehabilitation.

The difference concerns the assumption that CKC exercises are safer than OKC exercises because they place less strain on the ACL graft, producing less patellofemoral pain. The second assumption is that CKC exercises are more functional and are equally effective in improving quadriceps muscle force production. A study by Beynnon et al showed no difference in ACL strain characteristics between OKC and CKC exercises. A report by Fleming et al argues that, with improved anatomical placement of the ACL graft, the graft may respond more like the intact ACL during OKC and CKC exercises. Therefore, these 2 articles argue that both types of exercise can be performed safely. With regard to safety, both OKC and CKC exercises can be applied in a manner that minimizes risk of excessive graft strain and patellofemoral compression. Using different knee joint motion excursions for each type of exercise is the key to risk reduction. When OKC knee extensions are performed, limit knee joint motion to more flexed positions. During CKC lower extremity exercises, limit knee joint motions to more extended positions. Patients who are not highly active or athletic or who are minimally symptomatic (or for other reasons) may opt for nonoperative treatment. In these cases, after initial control of pain and effusion, start hamstring and quadriceps activation/disinhibition and protected weight bearing in a hinged brace. As swelling and pain slowly resolve, ROM should return to normal, or nearly normal, parameters. Start exercises that take place in an anterior/posterior plane (eg, stationary cycling). Exercises need to be nonballistic.

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Medical Issues/Complications

The primary goals in treatment of ACL rupture are restoration of function in the short term and prevention of long-term pathologic changes in the knee. Nonoperative treatment is a reasonable approach in patients who are not active athletically.Current research demonstrates that the natural history of untreated complete injuries of the ACL consists of progression of symptomatic instability to recurrent injuries. These injuries damage the menisci and the articular cartilage, eventually leading to osteoarthritis and osteoarthrosis.Complications from ACL surgery generally arise during surgery (see Allograft Reconstruction, ACL-Deficient Knee).

Complications include the following:

  • Extravasation of irrigation fluid during arthroscopy
  • Posterior femoral cortex compromise during endoscopic reaming of the femoral tunnel
  • Paraesthesias along the lateral aspect of the knee
  • Improper handling of the graft (eg, dropping it on the floor)
  • Bruising and/or hematoma formation
  • Blood loss
  • Improper alignment of the tunnels, causing graft impingement
  • Improper graft placement, making the graft too short and, thus, not allowing the knee to reach full terminal extension
  • The main complication of ACL surgery during the postoperative period is rupture of the graft.
  • Careful and conservative physical therapy (PT) during the first 8-12 weeks is important.
  • Another complication that can develop after surgery is failure to achieve full knee extension.

Surgical Intervention

Several options exist for the patient who elects to have surgery. For complete rupture, no local healing response is detectable at the injury site, and a graft must be used to replace the ACL. Today 4 options are used. The first 3 types are autografts using either the central one third of the patellar ligament (considered a bone-ligament-bone graft), the quadruple semitendinosus/gracilis tendon, or the quadriceps tendon. The fourth type of graft is a cadaveric allograft.


The patellar ligament with its bony ends has been a popular ACL replacement because of its high ultimate tensile load (~2300 N), its stiffness (~620 N/mm), and the possibility for rigid fixation with its attached bone graft.By comparison, the dimensions of a round 10 mm quadruple semitendinosus/gracilis tendon graft (hamstring graft) are more comparable to those of the intact ACL, and its ultimate tensile load has been reported as high as 4108 N. The quadruple tendon graft also may provide a multiple bundle replacement graft that better approximates the function of the 2-bundle ACL. Disadvantages of this soft tissue graft include concern over tendon healing within the osseous tunnels and lack of rigid bony fixation. A 1998 study from Japan suggests that aggressive early rehabilitation after an ACL reconstruction using the hamstring graft has more risk of residual laxity than the patellar tendon graft.The quadriceps tendon graft has been shown to have an ultimate tensile load as high as 2352 N. This graft has become an alternative replacement graft, especially for revision ACL surgeries and for patients with multiple ligament injuries in the knee.Cadaver studies have shown that the strength of fixation between the patellar tendon graft and the hamstring graft is equal to approximately 450 N, but the patellar graft can achieve fixation strength as high as 1000 N. Grafts fixed to bone close to the articular surface (ie, patellar tendon grafts) undergo less strain and are stiffer that those grafts fixed outside of drill holes (ie, hamstring grafts).


Allograft tissues are harvested from human donors and typically include either the patellar tendon or the Achilles tendon.Allografts are used for multiple ligament reconstructions and revisions of ligament reconstructions, as well as in patients who are not high-performance athletes. Reduction in tensile strength that occurs with sterilization, however, is a concern, as well as the risk of inflammatory reactions.

Ultimately, the decision of which graft is best is still a matter of contention. The agreement is that the patellar and hamstring grafts are superior to the quadriceps and allograft; however, the decision as to which is the better of the patellar and hamstring grafts is dependent upon which surgeon is operating.

With regard to osteoarthritis, the type of graft does not appear to influence the development of osteoarthritis. In spite of the type of graft, a certain percentage of patients develop osteoarthritis in the reconstructed knee, especially patients with concomitant or subsequent meniscectomy.

Other Treatment (injection, manipulation, etc.):

Some patients, especially those minimally involved in sports, elect not to have surgery and choose bracing. Several custom and off-the-shelf ACL-specific braces are available. Braces are not recommended for those involved in vigorous sports without surgical stabilization.


In a paper published by Hewett et al, a jump training program was recommended strongly. Another paper by Wojtys et al showed that plyometrics and exercises requiring agility, such as running through cones, figure eights, and single leg jumps are proven methods to improve muscle reaction time significantly. Ultimately, physical conditioning and balanced knee strengthening (hamstrings as well as quadriceps) are the keys to reducing the risk of an ACL tear.


Most patients achieve good health and mobility after treatment for ACL injury. They perform ADL without difficulty and return to participation in their previous sporting or recreational activities.

Medical/Legal Pitfalls:

Medical/legal issues from ACL injury and graft replacement generally arise from complications during surgery. Initial misdiagnosis of ACL injury also can be a source of potential litigation. Obtain a complete history from the patient. The mechanism of injury for ACL tear is fairly consistent. A thorough physical examination helps the physician confirm the diagnosis, and an MRI identifies additional possible injuries to other ligaments or cartilage. Potential for a lawsuit arising from improper physical therapy also exists. If the therapist is too aggressive in rehabilitation exercises and rupture of the ACL graft occurs, some patients might consider litigation.


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