Department of Orthopaedic Surgery

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Sports Medicine

Anterior Cruciate Ligament Injury (ACL)


The Anterior Cruciate Ligament (ACL) is one of four major ligaments (ligaments connect bone to bone) of the knee joint that coordinate function and promote stability of the knee joint. It runs in a notch at the end of the femur (intercondylar notch) and originates at the back part of the femur (postero-medial aspect of the lateral femoral condyle) and attaches to the front part of the tibia (tibial eminence). In an adult knee, the ACL prevents forward movement of the tibia. It also provides roughly 90% of stability in the knee joint.5

Incidence of Injury

ACL injury has an annual incidence of more than 200,000 cases with ~100,000 of these knees reconstructed annually3. The majority of ACL injuries (~70%4) occur while playing agility sports, and the most often reported sports are basketball, soccer, skiing, and football. An estimated 70% of ACL injuries are sustained through non-contact mechanisms, while the remaining 30% result from direct contact.3,4

ACL injury is most prevalent (1 in 1,750 persons) in patients 15-45 years of age.4 It is more common in this age group in part because of their more active lifestyle as well as higher participation in sports.

More ACL injury cases occur in males due to greater numbers of male sports participants, however females have a higher risk of being injured. NCAA statistics found that female athletes are 2-8 times more likely to sustain an ACL injury playing sports.4,6 Several factors are speculated to increase risk of ACL injury in females including lower extremity alignment (wider pelvis, knee valgus, foot pronation), joint laxity, hamstring flexibility, muscle development, hormonal differences, and ACL size.4,6

Injury Description

The classic ACL injury occurs during a non-contact event usually when decelerating, stopping suddenly, twisting, cutting, or jumping. Oftentimes the patient will hear or feel a “pop” at the time of injury and sometimes they may report brief a hyperextension of the knee joint. Just after the injury patients may be able to continue activity, however most of the time the patient is unable to continue regular activity and a few hours after insult the knee swells considerably.

When the ACL is injured as a result of direct contact, football is often the associated sport. Most often, the knee is subjected to a direct blow to the lateral side and other ligaments are injured in addition to the ACL. The most common multi-ligament injury is the “unhappy triad” that includes the ACL, medial collateral ligament (MCL), and the medial meniscus.

Clinical Evaluation

Patient History
The patient may recount any of the above descriptions as well as a feeling of the knee giving way. Patients involved in sports may describe feeling like they need to “round” their cuts rather than being able to pivot easily on the injured leg. Occasionally the patient may also describe the knee locking up or a catching in the knee.

Physical Examination for ACL Injury
The following tests may indicate ACL injury:

Lachman Test (most sensitive ACL test) {Link to Test}

  • Flex Knee @ 30 degrees and apply anterior force on the tibia.

  • Positive test = laxity or forward displacement (>4mm) of tibia.

  • 98% accurate in predicting an ACL injury7.


Anterior Drawer Test {Link to Test}

  • Similar to Lachman’s test but performed with knee flexed @ 90 degrees.

  • Positive test = laxity or forward displacement of tibia.

Pivot Shift Test {Link to Test}

  • Flex knee with internal rotation and then flex knee while applying a valgus force to the proximal tibia

  • Positive test = clunk/give is felt during knee flexion.

T-2000 Arthrometer test

  • Objective & quantitative method of assessing ACL injury.

  • Max side-to-side difference exceeding 3mm indicates ACL injury.

  • This test is not routinely performed due to the high specificity of the above tests.

Physical Examination for Meniscus Injury {Link to Meniscus}In addition to testing for ACL injury, the meniscus should always be evaluated for tears. The following tests may indicate meniscus injury:

McMurray’s Exam

  • Assess medial & lateral meniscal tears by extending the leg from a flexed position with either external rotation with valgus or internal rotation with varus force, respectively.

  • Positive test = pop/click on extension can be heard.

    Other Positive Signs

  • Tenderness to palpation of medial & lateral joint lines.

    Mechanical block during to range of motion (ROM) testing.

  • Imaging:
    Radiographic imaging and MRIs are used to confirm an ACL injury diagnosis. The current protocol calls for weight bearing anteroposterior (AP), lateral, and 45 degree posteroanterior (PA) X-ray views of the knee. MRIs are invaluable in diagnosing associated articular or mensical injuries.


ACL injuries range from a partial ACL tear to a complete ACL tear to bone avulsions. Partial and complete ACL tears are repaired with sutures or reconstructed using a graft. Bone avulsions are repaired by reattaching the bony fragment.

Approximately 50% of ACL injuries occur with injuries to other structures in the knee. The meniscus (50% of multi-structure injuries), medial collateral ligament (30%), or articular cartilage (30%) are the most frequently concurrently injured structures.3 The lateral meniscus is more often damaged than the medial meniscus during an isolated acute ACL injury.

Operative vs. Non-Operative Treatment

Treatments for ACL injuries include surgical and non-surgical options. The decision to treat ACL injuries with operative measures is based on several factors with concurrent meniscal damage, activity level, and type of employment at the top of the list.

General Guidelines for Surgical Treatment

  • 4hrs per week of participation in Level I or II activities (click here to see chart)

  • KT-2000 difference greater than 5mm

  • Unstability or giving out episodes

  • Associated meniscal or lateral ligament injury

Active infections in the knee joint and loss of range of motion (ROM) in the knee secondary to arthrosis or adhesions are contraindications for surgical treatment.

Non-surgical treatment can be used for patients with little or no athletic participation, patients unwilling to participate in postoperative rehabilitation procedures, and patients with unrealistic goals. However, if non-operative treatment fails, the patient and physician can discuss the option of operative treatment.

Non-Operative Treatment

After acute injury, initial steps should be taken to reduce swelling by applying the established RICE principles (Rest, Ice, Compression, and Elevation). Early efforts should be made to regain full ROM, especially extension, and walking with full weight bearing is encouraged 7-10 days after injury. Usually within 1-3 weeks acute swelling has reduced and full ROM is established.

Once full ROM is established, an aggressive rehabilitation program focusing on strengthening and endurance should be followed. The goal of these exercises is to change the quadriceps/hamstring strength ratio from a normal 3:2 ratio to a 1:1 ratio.

When strength is approximately 70% of normal, the rehabilitation program should incorporate a proprioceptive / balancing component using balance boards. The goal of this training is to increase stability in the knee joint.

The last step of the rehabilitation program should assess which activities create pain, discomfort, or instability in the knee joint and actively modify one’s life style to avoid such activities.

Operative Treatment Timing and Options


Current protocol calls for patients to regain full ROM, reduce swelling, and have quadriceps control prior to surgery. Usually patients are involved in a preoperative therapy program and attainment of full ROM can take 2 weeks or longer. It has been speculated that full ROM prior to surgery can decrease postoperative rehabilitation time. It is very rare that ACL surgery is performed within 2 weeks after initial injury.

Operative Options

Operative procedure for ACL injury is an ACL reconstruction using a graft and is typically done using arthroscopic methods. The surgeon makes small incisions so that a pencil shaped instrument with a camera and lighting system attached can penetrate inside the knee joint and see the inside structures. The inside of the knee can be seen on a TV screen and the surgeon will perform surgery through several small incision points around the knee.

Graft Options

Graft Harvest Sites

There are three common types of graft harvest sites:

+ patellar tendon
+ hamstring tendon
+ quadriceps tendon

Allo or Auto

In addition to harvest site, another difference in grafts is autograft versus allograft. Allografts typically come from cadaver donors while autografts are harvested from the patient undergoing ACL reconstruction. Each graft has advantages and disadvantages (click here for a comparison chart).

While the patellar tendon graft has fallen out of favor, it is important to note that no one graft has been proven to be superior.8

The surgeon and patient must together decide which option is best for that patient.

Postoperative Treatment

After ACL reconstruction surgery a comprehensive rehabilitation program must be followed to regain full knee joint function

Long Term Expectations

ACL reconstruction surgery has a 90% success rate in terms of knee stability, patient satisfaction, and return to full activity.9 ACL reconstruction seems to protect the menisci from further injury and slow degenerative changes in the knee joint.2

The re-rupture rate of a reconstructed ACL is very low, one long-term study reported a 2.6% rupture rate at a mean of 2.5 years after surgery.12 After sustaining one ACL injury, risk of subsequent ACL injury in either the graft or the other leg increases substantially from 1 in 3,000 to 1 in 50.12

Patients who opt out of ACL reconstructive surgery may experience further injury to the knee joint. ACL deficient patents are at higher risk for later meniscectomy, 20% over the 5 years following ACL injury.2 Also, 70% of ACL deficient patients have signs of osteoarthritis in the knee, and ACL reconstruction can reduce the rate of osteoarthritis.11


  • Generalized complications such as infection, neurovascular injury, and thromboembolic disease are extremely rare (0.2-0.48%)3.

  • Deep vein thrombosis is another low probability (.12%)3 complication.

  • Graft misplacement complications due to the graft not placed anatomically can lead to motion problems, impingement, and graft failure. Careful attention to detail during surgery must be observed to avoid these complications.1

  • Other complications include knee stiffness (5-25% incidence)3 anterior knee pain (10-20%)1, Patellar tendonitis (20% in 1st year, then rare afterwards)1, Patella fracture (.35-1.8%)1. Back to TOP

ACL Injury in Children

Incidence ACL injuries in patients younger than 14 years vary from 3.4% to 10%. However, sports are becoming increasingly more competitive at younger ages, thus ACL injury incidence is expected to increase among the younger population.

Treatment Initially, an ACL injury in children is treated non-operatively, using a similar treatment plan as described above, especially in children with widely open growth centers. With patients who fail conservative, non-operative treatment operative treatment must be considered because recurrent episodes of pivoting cause cartilage and mensical damage, which can lead to early degenerative changes.

Special considerations must be made when deciding whether or not to move forth ACL reconstructive surgery in children. There is possibility of interrupting and/or arresting normal bone growth that can result in significant leg length differences. Physeal sparing ACL reconstruction can be performed for younger patients.

New Developments in ACL Reconstruction

While ACL reconstructions have allowed patients to return to high level sports and prevent meniscus tears, the surgery still has room for improvement. Recent studies of patients that had ACL reconstructions 10 years prior still develop early osteoarthritis. There has been a lot of interest in ACL augmentation or double bundle ACL reconstruction. These are surgical techniques that are aimed to preserve or replace the normal ACL in a more anatomic fashion.

ACL Augmentation

With ACL augmentation surgery, we will preserve as much of the original ACL as possible while performing the reconstruction. When the ACL is torn, the two bundles in the ACL may have varying degree of injuries. If one of the bundles is still functional, we will preserve that bundle while reconstructing the other one. The attempt is to preserve the patient’s ACL so that it can heal towards the graft to have better control and healing of the reconstruction.

Double Bundle ACL Reconstruction

There are two bundles to the ACL, namely the AM (anteromedial) and PL (posterolateral) bundles. Historically, only one graft is used and the anatomical position of one bundle of the ACL is reconstructed. Two or double bundle ACL reconstructions are performed to reconstruct both bundles of the ACL. This operation technically is more demanding and is indicated for selected patients.

The sports medicine group here at UCSF is actively conducting longitudinal outcomes analysis following these operative procedures. The group collaborates with one of the leading MR imaging centers in the world (MQIR) to have early diagnosis of cartilage injuries and determine how well the ACL reconstruction restore normal knee function.


      1. Chapman MW. Chapman’s Orthopaedic Surgery. 3rd Edition, Volume 3, 2001; 2348-2388.
      2. Daniel DM, Stone ML, Dobson BE, et al. Fate of the ACL-Injured Patient. A prospective Outcome Study. American Journal of Sports Medicine.1994; 22:632-644.
      3. American Academy of Orthopaedic Surgeons, July 2007, Anterior Cruciate Ligament Injury: Surgical Considerations, (July 11, 2008).
      4. Griffin LY. Noncontact Anterior Cruciate Ligament Injuries: Risk Factors and Prevention Strategies. Journal of the American Academy of Orthopaedic Surgeons. 2000;8:141-150.
      5. ehealthMD, July 2004, What is the Anterior Cruciate Ligament?, (July 12, 2008)
      6. Hewett TE, Myer GD, Ford KR. Anterior cruciate ligament injuries in female athletes: Part 1, mechanisms and risk factors. American Journal of Sports Medicine. 2006 Feb; 34(2):299-311.
      7. Hardaker WT Jr, Garrett WE Jr, Bassett FH 3rd. Evaluation of acute traumatic hemarthrosis of the knee joint. South Med J. 1990 Jun; 83(6):640-4.
      8. Goldblatt JP, Fitzsimmons, SE, Balk, E, Richmond, JC. Reconstruction of the anterior cruciate ligament: meta-analysis of patellar tendon versus hamstring tendon autograft. Arthroscopy 2005; 21:791.
      9. West RV, Harner CD. Graft Selection in Anterior Cruciate Ligament Reconstruction. Journal of the American Academy of Orthopaedic Surgeons. 2005;13:197-207.
      10. Gillquist J, Messner K. Anterior Cruciate Ligament Reconstruction and the long term Incidence of Gonarthrosis. Sports Medicine. 1999; 27:143-156.
      11. Garrick JG, Requa RK. Sports and Fitness Activities: The Negative Consequences. Journal of the American Academy of Orthopaedic Surgeons. 2003;11:439-443.
      12. Shelbourne DK, Gray T. Anterior Cruciate Ligament Reconstruction with Autogenous Patellar Tendon Graft Followed by Accelerated Rehabilitation: A Two-to Nine-Year Followup. American Journal of Sports Medicine. 1997; 25;786.

Written by Jennifer Kim Reveiwed & Updated by Joe Smith, January 2009