The anterior cruciate ligament (ACL) is one of the most commonly injured knee ligaments, with 80-90% of cases occurring without direct contact to the knee. The classic signs/symptoms of ACL rupture include pain and a popping sensation at the time of injury, immediate swelling and subsequent knee instability when pivoting/twisting.
Nearly half of all individuals with ACL ruptures can cope without surgical intervention, but those with recurrent instability typically undergo reconstruction of the ligament. 81% of patients return to some form of sport after ACL reconstruction; 65% return to pre-injury level of activity, but only 55% return to competitive sports. After ACL surgery, the risk of a second ACL injury is greater in both knees but more likely to occur on the un-operated side. Osteoarthritic changes are common after ACL injury, whether the ligament has been reconstructed or not.
The anterior cruciate ligament (ACL) is located within the knee joint and connects the shin (tibia) to the thigh bone (femur) (images 1-2). The ACL consists of three separate bundles (anteromedial-medial, anteromedial-lateral and posterolateral) that have an abundant blood and nerve supply. The posterolateral and anteromedial bundles are thought to provide more stability with the knee straight and bent respectively. The native ACL is thought to provide 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).
Images 1-2: front and back views of the knee showing the ACL. From Kennedy et al (2013).
Almost 90% of ACL injuries occur without direct contact to the knee, typically when decelerating to change direction or when landing on one leg. Recent studies indicate that non-contact ACL injuries are most likely to be caused by a combination of knee movements when the knee is slightly bent.
Koga et al (2010) propose the following mechanism for non-contact ACL rupture. When valgus load is applied to the knee, the medial collateral ligament becomes taut and compression occurs in the outside (lateral) compartment (image 3). The geometry of the lateral compartment, in combination with an anterior ‘pull’ on the tibia created by contraction of the quadriceps (image 4), causes the lateral femoral condyle to translate backwards (posteriorly) down the slope of the lateral tibial plateau (image 5); this movement produces relative internal rotation of the tibia on the femur. Combined valgus, anterior displacement and internal rotation of the tibia ruptures the ACL and once this restraint to anterior tibial translation is disrupted, the medial (inside) femoral condyle also displaces posteriorly on the tibia (image 6). Posterior translation of the medial femoral condyle is observed as external rotation of the tibia and is considered a consequence rather than the cause of ACL injury. The movement occurring at the knee during this mechanism of injury is referred to as a ‘pivot shift phenomenon’.
Images 3-6: proposed mechanism of non-contact ACL injury.
This proposed mechanism of injury is supported by cadaveric and imaging studies but the exact mechanism of injury is still debatable. A recent study suggests that these movements preferentially strain the ACL when the knee is in shallow knee flexion (approximately 25°).
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 characteristic ‘pop’ and a sensation of knee instability as the joint moves out and then back into position; this may be demonstrated by the individual using their fists (the ‘two fists’ sign).
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). Swelling that develops within this time period indicates bleeding within the knee joint (haemarthrosis) due to injury of a structure with an abundant blood supply; approximately 50% of patients presenting to accident and emergency with a haemarthrosis have an ACL rupture. X-ray is indicated if there is a tense haemarthrosis, to confirm or exclude knee fracture.
Patients with a tibial eminence fracture (image 7-8) 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 rupture 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 is knee instability during pivoting or twisting movements, although approximately 50% of patients with ACL rupture do not have recurrent instability if they engage in appropriate physiotherapy. ACL deficient patients that do not have knee instability are referred to as ‘copers’, while ‘non-copers’ typically describe a lack of trust in their knee or feel the knee giving way during some of their usual everyday activities.
For details on the diagnostic accuracy of clinical tests for ACL injury, please visit the statistics section.
Based on MRI and surgical findings, up to 50% of ACL injuries are isolated, with most injuries occurring in combination with traumatic meniscal tears or injury to other ligaments. ACL injuries often occur when the knee dislocates; 18% of knee dislocations involve vascular injury, which can become limb or life threatening. As a priority, it is therefore important to perform a thorough vascular assessment in cases of known or suspected knee dislocations.
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, but the Lachman test has been previously overestimated: 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 (video 1).
A positive test, indicating disruption of the ACL, is one in which there is proprioceptive and/or visual anterior translation of the tibia in relation to the femur with a characteristic “mushy” or “soft” end point. This is in contrast to a definite “hard” end point elicited when the anterior cruciate ligament is intact.
Video 1: Lachman test
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 (video 2).
Increased tibial displacement compared with the opposite side is indicative of an ACL tear.
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.
Video 2: anterior drawer test.
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 (Video 3).
Rationale: the pivot shift test attempts to sublux the lateral tibial plateau forward on the lateral femoral condyle. With the thigh relaxed and leg supported below the knee with slight valgus force, the lateral tibial plateau may sublux (partially dislocate) anteriorly in an ACL deficient knee. With an intact medial collateral ligament, strong valgus force impinges this subluxed tibial plateau against the lateral femoral condyle, jamming the two joint surfaces together, and preventing easy relocation (reduction). As the tibia is flexed on the femur the iliotibial band becomes a knee flexor, pulling the tibia backwards into its normal position, assisted by the geometry of the lateral knee compartment. This pivot shift phenomenon replicates the mechanism of injury proposed for non-contact ACL injury.
Video 3: pivot shift test (original description).
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 (video 4).
In an intact knee, the creation of a complete lever by the ACL allows the downward force on the quadriceps to more than offset the force of gravity, the knee joint rotates into full extension, and the heel rises up off the examination table. With a partially or completely ruptured ACL, the ability to offset the force of gravity on the lower leg is compromised and the tibial plateau slides anteriorly with respect to the femoral condyles. In this case, the heel will not lift off the surface.
It is important to perform this test on a hard surface to avoid the fist sinking into a soft surface as pressure is applied (video 4).
Video 4: lever test performed on a soft and hard surface in a patient with an arthroscopically confirmed ACL rupture.
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 (image 7-8) 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.
Images 7-8: X-ray and MRI images of a tibial eminence fracture.
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 (images 9-10) 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.
Images 9-10: X-ray and CT scan images of a Segond fracture.
A lateral femoral notch sign (images 11-12) 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.
Images 11-12: X-ray and MRI scan images of a lateral femoral notch sign.
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 (image 14) with bone marrow oedema in the posterolateral tibia and lateral femoral condyle (image 15) in keeping with a pivot shift mechanism of injury. The tibia may be translated anteriorly (image 16), 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).
Image 13-16: MRI images of a normal ACL (between white lines), ruptured ACL (ligament not clearly visible), bone marrow oedema (white arrows) and anterior tibial translation.
Increased displacement of the tibia, as compared to the uninvolved side, is indicative of a partial or complete tear of the ACL. During the Lachman and anterior drawer tests, laxity with an end point is suggestive of a partial tear, while laxity with no end point indicates a full-thickness rupture of the ACL.
IKDC grading scale: the side-to-side difference in joint translation during the Lachman, anterior drawer test and pivot shift test is graded as follows.
|Test||Normal||Grade 1+||Grade 2+||Grade 3+|
|Test||Normal||Grade +||Grade ++||Grade +++|
Tables 1-2: IKDC grading scale.
|Type I||No displacement of the fragment from its bed of origin|
|Type II||Partial displacement of the fragment anteriorly but still good apposition of a large portion of the avulsed fragment (hinged posterior cortex)|
|Type IIIa||Complete displacement of the avulsed fragment from its bed and no bone apposition of the fragment.|
|Type IIIb||Complete displacement and rotation of fragment|
|Type IV||Completely displaced and comminuted fragment|
Table 3: Modified Meyers and McKeever Classification of tibial eminence fractures.
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, placing the patient in a plaster of Paris with the knee slightly flexed 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 49% of patients with an isolated ACL rupture do not require ACL reconstruction if they engage in appropriate physiotherapy rehabilitation (videos 5-6), with 58% of patients demonstrating MRI evidence of ACL healing five years post-rupture. Based on this study, the current recommendations are that conservative management should be trialled before considering surgical reconstruction of the ACL. However, conservative management may not be appropriate for patients that put themselves or others at risk if their knee gives way (e.g. those that work on roofs). 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 the patient reacts adversely to an intervention, this should be adjusted accordingly. If ACL reconstruction is indicated, continuing with exercises demonstrated in video 7-8 before surgery has been shown to improve outcomes after surgery. Before proceeding to ACL reconstruction, it is recommended that the patient has no/minimal knee effusion, almost full range of motion and at least 80% quadriceps and hamstrings limb symmetry on strength testing using 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 is eliminated, 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, open and closed kinetic chain exercises, concentric, eccentric and isometric muscle contractions. Eventually, strength exercises are performed using a low number of repetitions with maximum effort, 3-4 sets per session, 2-4 sessions/week on non-consecutive days.
Neuromuscular exercises include balance and proprioception exercises, perturbation training, progressively increasing the difficulty of the exercises. Explosive and plyometric exercises are performed initially using two legs, maintaining lower limb alignment, soft landings and then progressed to single leg exercises.
Videos 5-8 demonstrate examples of exercise that have been used in the conservative, pre and post-operative management of ACL injured patients.
Video 5-6: conservative and post-operative ACL exercises described by Frobell et al (2010).
Video 7-8: pre-operative ACL exercises described by Eitzen et al (2010).
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 van Grinsven et al (2010) and van Melick et al (2016). Rehabilitation is divided into three phases, with specific goals and criteria to progress to the next phase (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 un-operated 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.
Isometric electromechanical dynamometry is the gold-standard method for measuring quadriceps and hamstrings strength. In addition to an LSI >90%, peak quadriceps torque value >3Nm/kg (torque divided by the patient’s body weight) is desirable as this is associated with positive knee joint function after ACL reconstruction. Isometric testing with hand-held dynamometry (HHD) is a cheaper and more portable method but issues occur when the assessor is unable to resist the force exerted by the patient. Pull-dynamometry overcomes these issues but this method has not been adequate evaluated; a prospective study is currently being conducted to determine the reliability and validity of pull-dynamometry for measuring quadriceps strength following 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 speeds.
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. For example, approximately 90% of work during the propulsion phase when hopping for distance is provided by the hip and ankle; more work is done by the knee on landing but patients can compensate with hip flexion.
Vertical jumping demands more even contributions of the hip, knee and ankle during propulsion and landing and may better reflect knee function. Countermovement jump (video 9), single-leg jump (video 10) and drop-jump (video 11) metrics can be analysed using force plates, contact mats and the My Jump 2 app.
Video 9-11: vertical jumps (countermovement jump, single leg jump, single leg drop jump).
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, 49% 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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