The anterior cruciate ligament (ACL) is located within the knee joint and connects the shin (tibia) to the thigh bone (femur). The ACL consists of three separate bundles that have an abundant blood and nerve supply. The native ACL provides structural stability to the knee via its direct connection between the bones, and functional stability by providing information to the nervous system regarding knee joint position (proprioception).
Almost 90% of ACL injuries occur without direct contact to the knee when the knee is slightly bent, typically when decelerating to change direction or when landing on one leg.
At the time of injury, the individual usually experiences significant knee pain, although some ACL ruptures may not be particularly painful. There is often a ‘pop’ or ‘snap’ and a sensation of knee instability as the joint moves out and then back into position. The person is usually unable to continue the activity, or even weight bear, and notices immediate swelling within the joint (i.e. within 2 hours of injury).
Patients with a tibial eminence fracture may be unable to fully straighten their knee (true locking) if the detached bone fragment ‘jams’ the joint. The torn ACL may also become pinched between the bones causing a physical block to movement or a stump impingement reflex, where the hamstrings contract and prevent the knee from fully straightening. ACL ruptures often occurs in combination with traumatic meniscal tears, which may also cause true locking if the tear is displaced.
Once the initial pain and swelling settles, the main complaint in symptomatic patients is knee instability.
For details on the diagnostic accuracy of clinical tests for ACL injury, please visit the statistics section.
Clinical tests that quantify anterior translation of the tibia, or attempt to reproduce the pivot shift phenomenon, are used to assess the integrity of the ACL. Based on the most recent systematic review and meta-analysis of ACL tests, the pivot shift and Lever sign are the best tests for ruling in and ruling out an ACL tear respectively. The diagnostic accuracy of the most commonly used tests are comparable; these tests are described below.
Lachman test: This test is performed with the patient lying supine (on their back), with the involved extremity on the side of the examiner. With the patient’s knee held between full extension and 15° of flexion, the femur is stabilised with one hand while firm pressure is applied to the posterior aspect of the proximal tibia in an attempt to translate it anteriorly.
Anterior drawer test: This test is performed with the patient supine, hip flexed to 45° and knee flexed to 90°. The examiner sits on the patient’s foot, with their hands behind the proximal tibia and thumbs on the tibial plateau. The hamstrings tendons are palpated with index fingers to ensure relaxation of the hamstrings muscles and an anterior force is then applied to the proximal tibia.
It is important to note that the tibia and foot should not be rotated during the anterior drawer test, as this would represent a different clinical test assessing the medial or lateral knee.
Pivot shift test: This test is performed with the patient supine. The leg is picked up at the ankle with one of the examiner’s hands, and the knee is flexed by placing the heel of the other hand behind the fibula over the lateral head of the gastrocnemius. As the knee is extended, the tibia is supported on the lateral side with a slight valgus strain applied to it. Subluxation can be slightly increased by subtly internally rotating the tibia, with the hand that is cradling the foot and ankle. A strong valgus force is placed on the knee by the upper hand. At approximately 30° of flexion, and occasionally more, the displaced tibial plateau will suddenly reduce in a dramatic fashion; this sensation may feel similar to when the patient’s knee gives way.
Lever test: this test is performed with the patient supine. The examiner stands at the side of the patient and places a closed fist under the proximal third of the calf. This causes the knee to flex slightly. A moderate downward force is applied to the distal third of the quadriceps using the other hand, assessing for the patient’s heel lifting off the surface.
The test should be performed on a hard surface; when using a soft surface, the fist can sink down as pressure is applied.
X Rays are often normal in isolated ACL ruptures and are therefore not indicated unless there is a suspicion of knee fracture. A tibial eminence fracture is an avulsion fracture of the ACL’s attachment to the tibia, is more common in younger patients but can occur in skeletally mature individuals. This type of fracture can be seen on X-ray but MRI or CT scans may be required to classify the fracture and plan treatment accordingly.
Although the ACL cannot be directly visualised on X Ray, indirect signs of ACL injury may be evident, which raise the suspicion of ACL injury. A Segond fracture is an avulsion (pull off) fracture of the lateral tibia by the lateral knee soft tissue structures; this type of fracture is synonymous with ACL rupture.
A lateral femoral notch sign is an indentation of the femur seen on X Ray, indicating that the posterior aspect of the lateral tibia has impacted the middle portion of the lateral femoral condyle, in keeping with the pivot shift phenomenon. The lateral femoral notch sign is suspicious of ACL injury and/or a traumatic meniscal tear laterally.
MRI has high diagnostic accuracy for ACL injury but the diagnostic ability of MRI is not dissimilar to clinical tests, and MRI findings alone should not be used to guide treatment. If an MRI scan has been performed, the ACL may appear torn with bone marrow oedema in the posterolateral tibia and lateral femoral condyle in keeping with a pivot shift mechanism of injury. The tibia may be translated anteriorly, indicating a lack of support from the ligament. MRI is useful for identifying associated injuries (e.g. traumatic meniscal tears, cartilage injury), which may influence management in certain clinical presentations (e.g. locked knees).
The management of an ACL injury is determined by the type of injury and the presence or absence of additional injuries. This section discusses the management of an isolated ACL injury.
As soon as practical after ACL injury, measurements of strength and jump performance of the uninjured leg should be recorded to determine the individual’s estimated pre-injury capacity (EPIC). EPIC can then be used as a baseline reference to guide eventual return to sports after ACL injury.
Tibial eminence fractures:
The management of tibial eminence fractures is determined by the type of fracture. Type I injuries are typically managed without surgery, immobilising the knee in slight flexion for 4-6 weeks. Type III-IV fractures are usually managed surgically, but there is a lack of consensus regarding the management of type II injuries.
To date, the highest quality evidence indicates that approximately 50% of patients with an isolated ACL rupture do not require ACL reconstruction if they engage in appropriate physiotherapy rehabilitation, with approximately one third of patients demonstrating MRI evidence of ACL healing. Based on these studies, the current recommendations are that conservative management should be trialled before surgical reconstruction of the ACL. ACL reconstruction is indicated for recurrent instability, despite attempting appropriate rehabilitation.
Conservative management of ACL injury can be divided into separate phases, as described below. Progression through phases is guided by pain/discomfort and swelling whilst monitoring for evidence of knee instability. If ACL reconstruction is indicated, continuing with rehabilitation before surgery (pre-habilitation) has been shown to improve outcomes after surgery. Before proceeding to ACL reconstruction, it is recommended that the patient should have no/minimal knee effusion, full knee extension, knee flexion within 10° of the unaffected side, and at least 80% strength symmetry on dynamometry.
Goals: Restore impairments related to knee joint pain, swelling/effusion and range of motion (ROM). Walking aids should be used until the patient can walk without a limp.
Once joint swelling has resolved, full knee movement is restored and there is a normal gait pattern, the patient can progress to phase 2.
Goals: Restore strength and neuromuscular response
Strength training utilises single and multi-joint movements, with progressive loading as tolerated by the patient.
Neuromuscular exercises include balance and proprioception exercises, perturbation training, progressively increasing the difficulty of the exercises. Explosive and plyometric exercises are initially performed using two legs, progressing to single leg as competence allows.
Post-operative rehabilitation following ACL reconstruction has traditionally been based on graft healing times, with certain activities restricted until specific timeframes have elapsed. More recently, greater importance has been placed on achieving specific criteria before progressing rehabilitation; therefore, current guidelines are both time-based and criteria-based.
The following ACL reconstruction protocol is based on published clinical practice guidelines (download 1).
The following timeframes are based on the earliest recommendations and assume other criteria (e.g. swelling, range of motion, strength) have been achieved; patients should liaise with their physiotherapist to guide progression through phases.
Weight bearing: immediate weight bearing is allowed unless instructed otherwise, but patients should use crutches until they can walk without a limp.
Driving: braking response times have been shown to be normal at two weeks for the left knee and six weeks for the right knee following ACL reconstruction (using autograft); patients should contact their motor insurance company to confirm they are insured to drive.
Cycling: cycling with no resistance on an exercise bike can commence once the patient has enough knee range of movement to rotate comfortably on the pedals. Cycling outdoors is recommended no earlier than eight weeks after surgery.
Jogging: treadmill jogging can commence once the patient has no/minimal pain and effusion despite adequately loading the knee, full range of motion and >70% limb symmetry on strength (quadriceps and hamstrings) and jump testing.
Swimming: breaststroke can be commenced from week 12 after surgery.
Return to sports
Definitive return to sport criteria for a safe return to sport/activity following ACL injury are lacking. Full, on-field sports specific rehab should be successfully completed before attempting to return to game situations, but other recommendations have been proposed, as described below.
Time: timeframes for ACL injury will depend on whether or not the patient has undergone surgery. In non-surgically managed patients, graft healing times do not need to be taken into consideration and a case study has been published reporting a Premier League football player that was ready for first team selection eight weeks after ACL rupture.
A recent study showed that all patients that returned to pivoting sports within 5 months of ACL reconstruction re-injured their knee. Between 6-9 months, the rate of re-injury reduced by 51% for every month return to pivoting sports was delayed but after 9 months, the rate of re-injury did not reduce further. Muscle strength, neuromuscular control and maturation of the graft may not be optimal until approximately two years after surgery, prompting some experts to suggest full return to high risk sporting activities should be delayed until this time point.
Limb symmetry: Limb symmetry Index (LSI) uses the unaffected leg as a reference to identify deficits in strength and hopping/jumping ability following ACL injury and reconstruction. A LSI of greater than 90% is recommended on muscle strength testing (quadriceps and hamstrings) and hopping/jumping performance before considering return to play. However, LSI may over-estimate the function of a reconstructed knee, as strength deficits have also been demonstrated in the unoperated leg following ACL injury.
If available, strength and hopping measurements taken before the injury can be used for reference to guide return to sports. If this information has not been recorded, the estimated pre-injury capacity (EPIC) may be a more suitable guideline than the LSI, as strength deficits are less likely to have occurred in the uninjured side so soon after injury. However, the EPIC method relies on measurements being recorded before surgery, which may not available.
Electromechanical dynamometry is the gold-standard method for measuring quadriceps and hamstrings strength. In addition to an LSI >90%, isometric peak quadriceps torque value >3Nm/kg is desirable as this is associated with positive self-reported knee joint function after ACL reconstruction.
Isokinetic testing allows strength to be quantified through range, and can be performed at specific speeds (e.g. 60 or 180 degrees per second); peak torque values will be lower with higher angular velocity.
Numerous single-leg hop/jump tests have been described but the ability of these tests to predict future injury is unknown. Hopping in a horizontal direction (e.g., hop for distance, triple hop, triple cross-over hop and 6-metre timed hop) has traditionally been used to determine whether a patient has restored adequate limb symmetry but recent evidence indicates that horizontal hopping may not be an accurate measure of knee function.
Vertical jumping demands more even contributions of the hip, knee and ankle during propulsion and landing and may better reflect knee function. Countermovement jumps and drop-jump metrics, on either double or single leg, can be analysed using force plates, contact mats and the My Jump 2 app.
It is important to determine whether the individual feels ready to return to normal sporting activities. The ACL return to sport after injury (ACL-RSI) questionnaire measures the individual’s emotions, confidence in performance and risk appraisal in returning to sport after ACL injury. The ACL-RSI app and can be downloaded for free on iOS devices from the App Store by clicking the download button.
According to the highest quality evidence currently available, approximately 50% of non-elite athletes can cope without surgical management of an ACL rupture with no significant difference in activity levels, further surgery or osteoarthritic changes on X-ray between groups 5 years after injury.
81% of surgically reconstructed ACL patients return to some form of sport; 65% return to their pre-injury level of pivoting sports and 55% return to competitive level sport. 83% of elite athletes return to pre-injury level of sport. Nearly 1 in 4 young athletes who sustain an ACL injury and return to high-risk sport will sustain another ACL injury (on either the injured or uninjured side), most likely early in the return-to-play period.
Adult patients with ACL injury may develop symptoms and signs of knee osteoarthritis within 10 years of the index injury. At 20 year follow up, X Ray evidence of knee osteoarthritis occurs in up to 68% of conservatively managed ACL ruptures and 80% ACL reconstructed patients. The incidence of arthritic findings on X-ray is similar for hamstrings and patellar tendon grafts, while medial meniscal injury/meniscectomy increases the risk of knee osteoarthritis following ACL injury.
What happens to the ‘gap’ where the ACL graft was taken from?
The tendon ‘gap’ can refill with new tendinous tissue but it may take more than a year for the new tendon to recover most of its biomechanical properties. Some hamstrings tendon gaps do not refill, but this is not related to hamstrings muscle strength.
What happens to the new ACL?
The tendon graft undergoes a process called ‘ligamentisation’. The remodelling phase of the graft usually occurs between 6-24 months after surgery but may take even longer. The new ACL most closely resembles the native ACL around two years after reconstruction.
What is the risk of the ACL being re-injured?
Numerous risk factors have been associated with re-injury. While exact figures vary between studies, these studies consistently show that those who have previously injured their ACL are more likely to sustain a second ACL injury than those who have not. On average, the risk of second ACL injury is slightly greater on the un-operated knee (8%) while graft failure rate is 7%.
Patients under the age of 25 that return to higher level activity are at more risk of sustaining a second injury, most often during the early stages of return to play. Females are more likely to re-injure their ACL compared with males and individuals that have failed return to sport criteria are more likely to sustain further injury to their knee.
Written by: Richard Norris, The Knee Resource
Reviewed by: Timothy E Hewett, MD
Director Mayo Clinic Biomechanics Labs and Mayo Sports Medicine research. Professor of Orthopaedics, PM&R, Physiology & Biomechanical Engineering.
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