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The anterior cruciate ligament (ACL) is one of the most commonly injured knee ligaments, with approximately 80% of cases occurring without direct contact to the knee. The classic signs/symptoms of ACL injury include pain, a popping sensation, immediate swelling and knee instability on side to side or rotational movements. The Lachman test is of high diagnostic value both to rule in or out an ACL injury, while a positive Pivot Shift test can only be used to rule in an ACL injury.

Nearly half of ACL injured patients can cope with physiotherapy intervention but those with recurrent instability require surgical 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 an intra-articular structure (located within the knee joint), connecting the tibia (shin bone) and the femur (thigh bone). The ACL consists of three separate bundles (Otsuma et al) that have an abundant blood and nerve supply. The ACL is thought to provide structural stability due to its direct connection between the bones, and functional stability by providing information to the nervous system regarding knee joint position.

  • Concentric: the muscle shortens as it contracts, causing the knee to straighten.
  • Isometric: the muscle does not change length as it contracts; therefore, no movement occurs at the knee.
  • Eccentric: the muscle lengthens as it contracts, controlling the knee as it bends.


The ACL is the main restraint to anterior (forward) displacement of the tibia, providing approximately 85% of the total support (Butler et al, 1980). More anterior shear force is exerted on the tibia at low knee flexion angles (<30°), which in turn places more strain on the ACL. The ACL also resists adduction (varus), abduction (valgus), internal and external rotation of the tibia; internal tibial rotation has been shown to place more strain on the ACL than external rotation (Quatman et al, 2010). Therefore, any forceful movements into these positions, or hyperextension (over-straightening) of the knee, could potentially injure the ACL.

Almost 80% of ACL injuries occur without direct contact to the knee (Renstrom et al, 2008) and video analysis or patient questionnaires suggest these non-contact injuries usually occur when decelerating to change direction or landing on one leg (Griffin et al, 2000, Walden et al, 2015). Recent studies (Koga et al, 2010, Kiapour et al, 2014, Kim et al, 2015) indicate that non-contact ACL injuries are most likely to be caused by a combination of movements, with the knee in a slightly bent position (Quatman et al, 2014). Koga et al (2010) propose the following mechanism for non-contact ACL injury.

When valgus load is applied to the knee, the medial collateral ligament becomes taut and compression occurs in the lateral (outside) compartment. This compressive load, in combination with an anterior ‘pull’ on the tibia created by contraction of the quadriceps, results in the lateral femoral condyle translating posteriorly (backwards) on the tibia; this movement produces relative anterior translation and internal rotation of the lateral tibia on the femur. The combined valgus, internal rotation and anterior displacement of the tibia results in ACL rupture and once this restraint to anterior tibial translation is disrupted, the medial (inside) femoral condyle also displaces posteriorly on the tibia. 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’.

This proposed mechanism of injury is supported by cadaveric studies (Kiapour et al, 2014: Kiapour et al, 2016), and MRI studies investigating bone bruise patterns (Kim et al, 2015), plus imaging findings often associated with ACL injury as described below (Herbst et al, 2015: Grimberg et al, 2015). The exact mechanism of injury is still debatable, with a recent study suggesting that valgus may be the most important movement when the knee is in shallow knee flexion (approximately 25°) (Quatman et al, 2013).


At the time of injury, the patient usually experiences significant knee pain. Patients often describe a characteristic ‘pop’ (Wagemakers et al, 2010) and a sensation of knee instability as the joint moves out and then back into position; this may be demonstrated by the patient using the ‘two fist sign’.

The patient is usually unable to continue the activity, or even weight bear (Swain et al, 2014), and notices immediate swelling within the joint (i.e. within 2 hours of injury) (Wagemakers). Swelling that develops within this time period indicates bleeding within the knee joint (haemarthrosis); an ACL rupture is the most common cause of haemarthrosis due to its abundant blood supply (Sarimo, Olsson). X Ray is indicated for patients with a positive Ottawa Knee Rule (link to Ottawa Knee Rule) or a tense haemarthrosis, to confirm or exclude bone injury.

Patients with a tibial eminence fracture (link) 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 (Carmont). Clinically, this can mimic a bucket handle tear of the meniscus (link to meniscus injury) and it is important to differentiate these separate injuries.

Once the initial pain and swelling settles, the patient’s main complaint is knee instability during rotational or side-to-side movement (Wagemakers, Swain). However, it is important to note that approximately 50% of patients with ACL rupture do not have recurrent instability if they engage in appropriate physiotherapy (Frobell et al, 2010, 2013); this group of patients are referred to as ‘copers’. ‘Non-copers’ typically describe a lack of trust in their knee or feel the knee giving way during functional tasks.


For more detailed information on the validity and reliability of each test, please click here (link to validity/reliability)

12% of ACL injuries are isolated, with most injuries occurring in combination with meniscal tears or additional ligament injuriesOlsson(Geeslin 2010, 2011). ACL injuries often occur during when the knee dislocates; 18% of knee dislocations involve injury to vascular structures, which can become limb or life threateningMedina. As a priority, it is therefore important to perform a thorough vascular assessment in cases of known or suspected knee dislocations (link to vascular assessment).

Clinical tests that quantify anterior translation of the tibia or reproduce the pivot shift phenomenon are used to assess the integrity of the ACL. The most commonly used 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.


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 intactTorg.(GoPro video 1).

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.


Increased tibial displacement compared with the opposite side is indicative of an ACL tear Malanga et al 2003 (image 3) (GoPro video 2).

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 different structures of the knee (jump to Slocum test/anteromedial drawer).

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. At this point, the patient will jump and exclaim, ‘that’s it!’ Galway and Macintosh 1980

Rationale: the pivot shift test aims 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 reduction (relocation). 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.

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 surfaceLelli.


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 (figure 1a). 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 (figure 1b).

It is important to perform this test on a hard surface to avoid the fist sinking into a soft surface as pressure is applied.


X Rays are often normal in isolated ACL ruptures. 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 (image 1) is an avulsion (pull off) fracture of the lateral tibia by the lateral knee soft tissue structures (Shaikh et al, 2017); this type of fracture is synonymous with ACL rupture. A reverse Segond fracture (image 2) is a similar finding seen on the medial aspect of the tibia, which is suggestive of ACL, posterior cruciate or medial collateral ligament injury (Kose et al, 2017).

A tibial eminence fracture (image 3) is an avulsion fracture of the ACL’s attachment to the tibia and is more common in younger patients, but can occur in skeletally mature individuals.

A lateral femoral notch sign (image 4) 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 tear of the lateral meniscus (Herbst et al).

In ACL deficient knees, the tibia may be translated anteriorly, indicating a lack of support from the ligament (image 5).

MRI has high diagnostic accuracy for ACL injury (link to validity/reliability) but the diagnostic ability of MRI is not dissimilar to clinical tests (Phelan et al, 2015). MRI is useful for identifying associated injuries (e.g. meniscus), which may influence management.

Classification of ACL injuries

IKDC grading scale: the side-to-side difference in joint translation during the Lachman, anterior drawer and pivot shift tests is graded as follows.

Modified Meyers & McKeever classification for tibial eminence fractures (Meyers and McKeever, 1959: Zariczny 1977)


Management of 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; click for multi-ligament (link to multi-ligament) or combined ACL and meniscal injuries (link to meniscus).

Tibial eminence fractures:

The management of tibial eminence fractures is determined by the type of fracture. Type I injuries are managed without surgery, placing the patient in 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.

ACL rupture

To date, the highest quality evidence indicates that 49% of young (18-35), non-elite athletes with an ACL rupture do not require ACL reconstruction if they engage in appropriate physiotherapy rehabilitationFrobell2010,2013. Based on this study, the current recommendations are that conservative management should be trialled before considering surgical reconstruction of the ACL (Smith 2014, Monk et al, 2016). Patients that put themselves, or others, at risk if their knee gives way (e.g. those that work on roofs) may not be suitable for conservative management (BOA blue book).

Patients with recurrent instability, despite appropriate compliance with conservative management, should be considered for surgery. If surgery is indicated, the patient should continue with these exercises before their operation (link to pre-habilitation) as this has been shown to improve outcomes after ACL reconstruction (Alshewaier et al (2016). Measurements of strength and hopping ability of the un-injured leg before surgery may also be used to determine estimated pre-injury capacity (EPIC), which can then be used as a baseline reference to guide return to sports after surgery (link).

Conservative management/pre-operative rehabilitation:

Conservative management of ACL injury can be divided into separate phases, as described belowFROBELL,EITZEN. 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.

Phase 1:
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 and full ROM restored, the patient can progress to phase 2.

Phase 2:
Goals: Restore strength and neuromuscular response
Emphasise intensive, progressive muscle strength training, plyometric and advanced neuromuscular exercises (Eitzen et al, 2010).

Strength: 6-8 reps maximum effort, 3-4 sets, 2-4 sessions/week.
Single and multi-joint exercises, concentric, eccentric and isometric muscle contractions using open and closed kinetic chain exercises.
Examples: Leg press, leg extension, squats, hamstring curls, hamstrings on gym ball

Plyometrics: double legged exercises, maintaining lower limb alignment, soft landings, progressing to single leg exercises.

Neuromuscular exercises: balance and proprioception exercises, perturbation training, progressively increasing the difficulty of the exercises.
Link to conservative management (Frobell appendix)

Surgical management:

Numerous surgical techniques have been proposed, using tissue from different areas of the patients’ own body (autograft), tissue from other individuals (allograft), or synthetic material. The type and source of ACL graft is dependent on numerous factors and should be discussed with the orthopaedic surgeon.

Pre-operative rehabilitation (pre-habilitation):

Pre-habilitation (link) improves outcomes after ACL reconstruction Failla et al (2016), Grindem (2015), Alshewaier (2016). Ensuring the patient can fully extend their knee before surgery reduces the risk of knee joint stiffness (arthrofibrosis) after surgery. >20% side-to-side deficit in quadriceps strength predicts a deficit at 2 years post-op, whereas patients with >90% strength and hopping performance before surgery report superior knee function after surgery. Therefore, muscle strength (quadriceps and hamstring) and hopping performance (link) should be optimised before surgery (Grindem et al, 2015; Van Melick et al, 2016):

Post-operative rehabilitation:

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 important has been placed on achieving specific criteria before progressing rehabilitation; therefore, current guidelines are both time-based and criteria-based. For specific post-operative timeframes, please click here (link).

The following ACL reconstruction protocol is based on Van Melick (2016). Rehabilitation is divided into three phases, with specific goals and criteria to progress to the next phase.

See ACL phases protocols

Post-op timeframes

Weight bearing: immediate weight bearing is allowed unless instructed otherwise, but patients should use crutches until walking without a limp.
Driving: braking response time has been shown to be normal at two weeks for the left knee and six weeks for the right knee, following ACL reconstruction (using autograft) (Nguyen et al, 2000, Wasserman et al, 2017 DiSilvestro et al, 2017); 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 on the pedals (Van Melick). Cycling outdoors is recommended no earlier than eight weeks after surgery (Van Grinsven).
Jogging: treadmill jogging can commence from week 10 (van Melick), with outdoor jogging from week 13 (van grinsven).
Swimming: breaststroke 12/52 (van grinsven)

Return to sport criteria

Definitive return to sport criteria are lacking regarding a safe return to sport/activityHEGEDUS,2015 following ACL injury. Full, on-field sports specific rehab should be successfully completed, but numerous other recommendations have been proposed.

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 a conservatively managed ACL rupture (Weiler et al, 2015).

Between 6-9 months after ACL reconstruction, the rate of re-injury has been shown to reduce by 51% for every month return to pivoting sports was delayed; after 9 months the rate of re-injury did not reduce further. All patients that returned within 5 months of surgery suffered knee re-injury (Grindem et al, 2016). 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 (Nagelli & Hewitt, 2016).

Limb symmetry: Limb symmetry Index (LSI) uses the unaffected leg as a reference to identify deficits in strength and hopping ability following ACL injury and reconstruction. LSI >90% is recommended for muscle strength testing (quadriceps and hamstrings) and hopping performance (link to hop testing) before considering returning to play. Single leg hop for distance predicted self-reported knee function one year after measurements in conservatively managed ACL patients. A limb symmetry index (LSI) greater than 88% corresponded with an 89% probability of having normal outcome measures (IKDC 2000) Grindem (2011).

LSI may over-estimate the function of a reconstructed knee, as strength deficits have been demonstrated in both legs following ACL injury. Therefore, measurements of strength and hopping ability of the un-operated leg before surgery, termed estimated pre-injury capacity (EPIC), may be a more suitable reference standard to guide return to sports (Wellsandt et, al 2017). This method relies on these measurements being recorded before surgery, which may not available.

Hop testing:

Numerous hop tests have been described but the ability of these tests to predict injury is unknown. This section describes the most substantially studied testsHEGEDUS; it is important to note that there are variations regarding the way the test is performed and recorded.

The LSI for distance is calculated by dividing the distance measured on the operated leg by the distance measured on the un-operated leg then multiplying this figure by 100. The LSI for time is calculated by dividing the time recorded on the un-operated leg by the time recorded on the operated leg then multiplying this figure by 100.

Single leg hop for distance: The subject stands on the test leg and performs one hop as far as possible, landing on the same leg. Free leg swing is allowed but hands are placed behind the back. The subjects are instructed to perform a controlled, balanced landing and to keep the landing foot in place (i.e. no extra hops were allowed) until the measurer records the landing position; failure to do so resulted in a disqualified hop. The distance is measured in centimetres from the toe at the push-off to the heel where the subject landsGUSTAVSSON. (Video 1)

Single leg triple hop for distance: The procedure for the single leg hop for distance is repeated but the subject performs three hops as far as possible. (Video 2)

Crossover hop for distance: The procedure for the single leg triple hop for distance is repeated but the subject hops across a line marked on the floor. (Video 3)

6 metre timed hop: (van Grinsven 2010) the subject hops 6 metres on one leg as quickly as possible (with big, powerful hops). Free leg swing is allowed but hands are placed behind the back. The time taken to hop 6 metres is recorded (Video 4).

Psychological readiness:

It is important to determine whether the individual feels ready to return to normal activities. The ACL return to sport after injury (ACL-RSI) measures the individuals emotions, confidence in performance and risk appraisal in returning to sport after ACL injury and the App can be downloaded here (link to ACL-RSI download).


49% of non-elite athletes can cope without surgical management of an ACL rupture (Frobell 2013). At five year follow up, there were no differences in activity levels, further surgery or osteoarthritic changes on X Ray between groups when a hamstring graft was used; bone-patellar tendon-bone grafts showed a significantly increased risk of patellofemoral (knee cap joint) osteoarthritis compared with hamstring grafts and non-surgical patients. Adverse events were higher in the surgical group.

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 (Ardern 2014). 83% of elite athletes return to pre-injury level of sport (Lai et al, 2017).

X Ray evidence of knee osteoarthritis occurs in 50-60% of ACL injured patients (Culvenor et al 2013). Medial meniscal injury/meniscectomy increase the risk of OA following ACL injury (Van Meer et al (2015)

Frequently asked questions

What happens to the ‘gap’ where the ACL graft was taken from?
The tendon ‘gap’ refills with new tendinous tissue. It may take more than a year for the new tendon to recover most its biomechanical properties (Suydam et al (2017).

What happens to the new ACL?
The tendon graft undergoes a process called ‘ligamentisation’. For patellar tendon and hamstrings tendon grafts the remodelling phase occurs between 6-12 months and 12-24 months after surgery respectively (Pauzenberger et al, 2013), (Claes et al, 2011). The new ACL most closely resembles the native ACL around two years after reconstruction (Suomalainen (2011). Vogl et al (2001) HS and B-PT-B, Zaffagnini et al (2007), Gohil (2007) Ge (2015) Li (2012).

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 greater on the un-operated knee.

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. (Wiggins et al, 2016). 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 (Grindem et al, 2015).


Written by: Richard Norris & Daniel Massey, The Knee Resource

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