Orthopaedic Surgery - Anterior Cruciate Ligament Injury

Basics

The anterior cruciate ligament (ACL) is essential for maintaining knee stability, particularly during athletic activities that involve running, cutting, pivoting, jumping, landing, and kicking.

The ACL originates from the posteromedial aspect of the lateral femoral condyle and inserts on the anterior portion of the tibial plateau between the intercondylar eminences.

It consists of two principal functional bundles: the anteromedial bundle and the posterolateral bundle.

The ACL is the primary restraint to anterior translation of the tibia relative to the femur and also acts as an important secondary restraint to excessive internal rotation of the tibia.

Injury Pattern by Age

The pattern of ACL injury differs between skeletally immature and mature patients.

In children and adolescents with open growth plates, the ligament itself may remain intact while the injury occurs at the bone-ligament interface. This can produce an avulsion fracture of the tibial spine, where the ACL attachment is pulled away with a fragment of bone.

In skeletally mature adults, injury more commonly produces a midsubstance rupture of the ACL.

Epidemiology

ACL injuries are strongly associated with sports that involve rapid changes in direction, deceleration, jumping, landing, or direct contact.

Commonly associated activities include football, hockey, basketball, lacrosse, gymnastics, wrestling, and volleyball.

ACL injuries occur in both contact and noncontact situations, although many occur without direct impact to the knee.

Risk Factors

Female athletes participating in comparable competitive sports have an ACL injury rate approximately 4–6 times higher than male athletes.

The difference is likely multifactorial and may involve anatomical, hormonal, biomechanical, and neuromuscular factors.

Etiology

Several anatomical features have been associated with increased susceptibility to ACL injury.

These include an increased Q angle, narrowing of the femoral intercondylar notch, and a relatively narrow ACL.

Neuromuscular factors are also important. Landing with inadequate knee flexion together with excessive dynamic knee valgus can substantially increase strain on the ACL.

Noncontact Mechanism

Many ACL tears occur through a noncontact mechanism.

A typical injury occurs when an athlete rapidly decelerates, plants the foot, changes direction, pivots, or lands from a jump.

The foot remains fixed while abnormal rotational and translational forces are transmitted across the knee, producing ACL failure.

Contact Mechanism

ACL rupture may also result from direct trauma.

A classic contact mechanism involves a valgus force applied to the knee combined with external rotation of the tibia, such as a clipping injury during contact sport.

This mechanism may also damage other structures, particularly the medial collateral ligament.

Associated Bone Bruising

Bone bruises or trabecular microfractures occur in more than half of acute ACL injuries.

They are typically found on the posterior aspect of the lateral tibial plateau and near the sulcus terminalis of the lateral femoral condyle.

These opposing contusions are sometimes described as a “kissing contusion” pattern.

They are believed to occur when the posterolateral tibial plateau impacts against the lateral femoral condyle during the injury mechanism that ruptures the ACL.

Meniscal Injuries

Meniscal tears accompany more than half of ACL injuries.

During the acute injury, lateral meniscal tears are generally more common than medial meniscal tears.

In a chronically ACL-deficient knee, repeated episodes of instability place greater stress on the medial meniscus. Consequently, medial meniscal tears become more common in chronic ACL deficiency.

Other Associated Injuries

ACL rupture may occur together with other ligamentous injuries.

The medial collateral ligament (MCL) is more frequently injured than the lateral collateral ligament.

Articular cartilage damage may also occur either at the time of the original injury or later as a consequence of recurrent instability.

Diagnosis

Signs and Symptoms

Patients commonly describe immediate knee pain at the time of injury.

Many report hearing or feeling an audible “pop” within the knee.

Rapid swelling usually develops within several hours because of acute hemarthrosis.

The patient often feels that the knee is unstable and may be unable to continue participating in the activity.

Weight-bearing may initially be painful or difficult.

History

The mechanism of injury should be carefully established.

Important historical features include sudden deceleration, pivoting, landing from a jump, a contact valgus injury, or twisting of the knee while the foot was planted.

The presence of a pop, rapid swelling, immediate instability, and inability to continue sporting activity strongly raises suspicion for ACL rupture.

Physical Examination

A careful physical examination can diagnose the majority of ACL injuries.

Findings in the injured knee should always be compared with those of the contralateral normal knee because baseline ligamentous laxity varies between individuals.

Knee Effusion

Inspection commonly reveals a moderate or large knee effusion.

Effusion can be assessed by compressing fluid from the suprapatellar pouch toward the joint while palpating for increased fluid pressure around the knee.

Patellar ballottement may also help identify a significant intra-articular effusion.

Range of Motion

Full knee extension is commonly limited during the acute stage.

This may result from pain, joint effusion, protective hamstring spasm, or mechanical impingement from the torn ACL stump.

Flexion may also be restricted because of swelling and discomfort.

Restoring full extension is particularly important during rehabilitation.

Posterior Sag Consideration

When assessing anterior tibial translation, the examiner should first ensure that the tibia is not sitting abnormally posteriorly because of an associated posterior cruciate ligament injury.

If posterior sag is present, anterior translation during testing may simply return the tibia to a neutral position and falsely suggest excessive anterior laxity.

Lachman Test

The Lachman test is the most sensitive clinical examination for diagnosing an acute ACL tear.

The knee is positioned in approximately 30° of flexion.

The examiner stabilizes the distal femur and applies an anteriorly directed force to the proximal tibia or calf.

The amount of anterior translation and the quality of the endpoint are assessed.

Lachman Test Interpretation

An intact ACL normally produces a firm endpoint.

An ACL-deficient knee generally demonstrates increased anterior tibial translation together with a soft or poorly defined endpoint.

The difference between the injured and uninjured knees is clinically important.

Guarding During Lachman Testing

The Lachman test can be difficult to perform in the acute setting because pain and anxiety may cause involuntary hamstring contraction.

The examiner should support the leg fully, minimize discomfort, and encourage the patient to relax.

Reducing muscle spasm improves the accuracy of the examination.

Pivot Shift Test

The pivot shift test assesses dynamic anterolateral rotational instability of the ACL-deficient knee.

It evaluates anterior subluxation of the lateral tibial plateau relative to the lateral femoral condyle.

The test can be difficult to perform reliably in an awake patient with an acutely painful knee because guarding interferes with the maneuver.

It is often more informative when performed under anesthesia.

Pivot Shift Technique

The patient lies supine with the knee initially extended.

The tibia is placed in internal rotation while the examiner applies a valgus force to the knee as it is slowly flexed.

In an ACL-deficient knee, the lateral tibial plateau begins in an anteriorly subluxated position.

At approximately 20–40° of knee flexion, the iliotibial band causes sudden reduction of the tibia.

A palpable or visible jerk during this reduction constitutes a positive pivot shift.

Anterior Drawer Test

The anterior drawer test is less reliable than the Lachman test for diagnosing an acute ACL injury.

The patient is positioned with the hip flexed to approximately 45° and the knee flexed to 90°.

The examiner grasps the proximal tibia and applies an anteriorly directed force.

The amount of anterior translation and the firmness of the endpoint are assessed and compared with the opposite knee.

Instrumented Laxity Testing

Objective knee laxity can be measured with devices such as the KT-1000 arthrometer.

These devices quantify anterior tibial translation and are useful in research, postoperative assessment, or when objective documentation is required.

They are not necessary for routine diagnosis in most patients.

A side-to-side difference of more than approximately 3 mm in anterior tibial translation is considered abnormal.

Imaging

Plain Radiographs

Initial radiographic evaluation generally includes anteroposterior, lateral, and tunnel views of the knee.

Plain radiographs do not directly demonstrate the ACL but may reveal associated osseous abnormalities that strongly suggest ACL injury.

Tibial Spine Avulsion

A tibial spine avulsion fracture may be seen in skeletally immature patients.

This represents avulsion of the ACL attachment from the tibial eminence rather than midsubstance rupture of the ligament.

The amount of displacement is important in determining treatment.

Segond Fracture

A Segond fracture is a small avulsion fracture involving the lateral aspect of the proximal tibia.

It is strongly associated with ACL rupture and represents injury around the anterolateral capsular or ligamentous structures.

Identification of a Segond fracture should prompt careful evaluation for an ACL injury.

Other Radiographic Findings

Additional radiographic findings suggestive of ACL injury include avulsion involving the anterolateral ligament and deepening of the lateral femoral sulcus or sulcus terminalis.

These findings reflect the mechanism of impaction and rotational instability associated with ACL rupture.

MRI

MRI is the imaging modality of choice for evaluating the ACL and associated intra-articular injuries.

It can demonstrate the ligament directly and assess accompanying bone bruises, meniscal tears, collateral ligament injuries, cartilage damage, and occult fractures.

MRI has an overall diagnostic accuracy of approximately 95% for ACL injury.

MRI in Children

MRI is also highly effective in pediatric patients.

Reported sensitivity is approximately 96%, with specificity around 97% for diagnosing ACL injuries in children.

It is particularly useful for distinguishing a midsubstance ACL injury from tibial spine avulsion or associated meniscal pathology.

MRI Appearance of ACL Tear

On sagittal MRI, an intact ACL normally appears as a continuous low-signal structure extending from the femur to the tibia.

A tear may appear as discontinuity, abnormal orientation, waviness, or increased signal within the ligament.

Associated bone marrow edema often provides additional evidence of the injury mechanism.

Differential Diagnosis

Important differential diagnoses include osteochondral fracture, osteochondritis dissecans, tibial plateau fracture, meniscal injury, and articular cartilage injury.

Other ligamentous injuries that may mimic or accompany ACL rupture include injuries of the MCL, LCL, and PCL.

Initial Stabilization

During the acute period, the injured knee may be supported temporarily with a splint or knee immobilizer.

Crutches can be used for comfort and to reduce painful weight-bearing.

Prolonged immobilization should generally be avoided because early restoration of motion is important.

The primary early rehabilitation goal is to regain full knee range of motion, particularly full extension.

Acute Symptom Control

Ice and elevation help reduce swelling.

Simple analgesics or anti-inflammatory medication may be used for pain control.

Once tolerated, early active knee motion should be encouraged to prevent stiffness.

General Treatment Principles

Treatment decisions are individualized according to several factors.

Important considerations include patient age, activity level, occupational demands, sporting participation, degree of instability, skeletal maturity, and associated meniscal or cartilage pathology.

Activities involving cutting, pivoting, jumping, and rapid directional change place particularly high demands on ACL stability.

Treatment in Skeletally Mature Patients

Treatment options include structured nonoperative rehabilitation or ACL reconstruction.

Nonoperative treatment may be appropriate for older individuals, relatively sedentary patients, or those who can modify their activities and do not experience significant instability.

Reconstruction is more commonly considered in young or active patients who wish to return to pivoting or cutting sports.

Treatment in Skeletally Immature Patients

Management of ACL tears in children requires consideration of the open growth plates.

Historically, some children were treated nonoperatively until skeletal maturity.

However, prolonged instability may contribute to progressive meniscal and articular cartilage injury.

As a result, contemporary treatment increasingly favors carefully selected surgical reconstruction using techniques designed to minimize damage to the physes.

Tibial Spine Avulsion Treatment

Nondisplaced or minimally displaced tibial spine avulsion fractures in skeletally immature patients can often be managed with closed reduction and immobilization with the knee in extension.

More significantly displaced fractures may require operative reduction and fixation to restore ACL tension and joint congruity.

Activity Modification

Patients should be counseled regarding activities that place high rotational loads on the knee.

Sports requiring cutting, pivoting, sudden deceleration, jumping, and landing are particularly likely to produce recurrent instability in an ACL-deficient knee.

Activity modification may be sufficient for some patients treated nonoperatively.

Functional Bracing

The benefit of routine functional knee bracing after ACL injury or reconstruction remains controversial.

Braces may improve confidence in selected patients but do not reliably substitute for normal ligament function, neuromuscular control, or rehabilitation.

Physical Therapy

Rehabilitation is important whether the ACL injury is treated nonoperatively or surgically.

The program should emphasize early restoration of range of motion, particularly full extension, together with early appropriate weight-bearing.

Progressive strengthening is then introduced.

Strengthening

Closed-chain, weight-bearing exercises are commonly used to strengthen the quadriceps and hamstrings while limiting excessive anterior shear across the knee.

The goal is to restore quadriceps and hamstring function to at least approximately 90% of the strength of the opposite limb before unrestricted return to high-level activity.

Preoperative Rehabilitation

Patients selected for ACL reconstruction should ideally regain full knee range of motion and substantially reduce swelling before surgery.

Operating on a stiff, swollen knee increases the risk of postoperative stiffness and arthrofibrosis.

Reconstruction outcomes are generally better when full or near-full motion has been restored before surgery.

Postoperative Rehabilitation

After reconstruction, rehabilitation focuses on restoring motion, strength, proprioception, balance, and neuromuscular control.

Agility and progressive strengthening exercises are commonly introduced around 6 weeks after surgery, although exact timing depends on the reconstruction technique and rehabilitation protocol.

Later stages include running, jumping, cutting, and sport-specific drills.

Medication

During the acute period, pain may be treated with NSAIDs or acetaminophen.

Routine opioid analgesics are generally avoided when symptoms can be adequately controlled with simpler medications.

Surgical Indications

ACL reconstruction is commonly recommended for active individuals who experience functional instability after an acute tear.

It is also indicated in patients with chronic ACL deficiency when recurrent instability threatens the menisci and articular cartilage.

Associated repairable meniscal injury may strengthen the indication for stabilization.

ACL Reconstruction

Modern ACL surgery usually involves intra-articular ligament reconstruction rather than primary repair of a midsubstance tear.

The torn ACL is replaced with a tendon graft positioned to reproduce the function of the native ligament.

Reconstruction may be performed arthroscopically or with combined open techniques when required.

Graft Selection

Graft choice depends on patient age, activity demands, anatomy, surgeon experience, and individual preference.

Both autografts and allografts can be used.

Autograft Options

Common autograft choices include bone-patellar tendon-bone graft, four-strand hamstring tendon graft, and quadriceps tendon graft.

Each has specific advantages and potential donor-site complications.

Autografts are frequently preferred in young and highly active patients.

Allograft Options

Allograft tissue may be obtained from structures including the Achilles tendon, quadriceps tendon, patellar tendon, hamstring tendons, anterior or posterior tibialis tendons, and fascia lata.

An advantage is avoidance of autograft harvest and therefore less donor-site morbidity.

Historically, concerns have included disease transmission and altered graft incorporation, although tissue-processing techniques have greatly reduced transmission risk.

Graft Choice in Young Patients

In children, adolescents, and young active adults, current practice generally favors autograft reconstruction.

Allograft reconstruction in younger patients has been associated with a higher risk of graft failure compared with autograft.

Primary ACL Repair

Primary repair is not generally recommended for a typical adult midsubstance ACL rupture.

However, operative reduction and repair or fixation may be appropriate for a displaced tibial spine avulsion fracture because the ligament itself may remain structurally intact.

Reconstruction in Skeletally Immature Patients

The operative technique in children depends on skeletal maturity.

The goal is to restore knee stability while minimizing the risk of growth-plate injury.

Tanner Stage 1

Patients at Tanner stage 1 have substantial growth remaining.

Physeal-sparing reconstruction techniques are generally preferred to avoid crossing the open growth plates.

Tanner Stage 2

Patients at Tanner stage 2 may be treated with selected partial transphyseal techniques depending on skeletal maturity, growth remaining, and surgeon preference.

Care is taken to minimize the amount of physeal injury.

Tanner Stage 3 and Above

Patients approaching skeletal maturity, typically Tanner stage 3 or higher, may undergo more conventional complete transphyseal ACL reconstruction.

The potential risk to the growth plates becomes lower as skeletal maturity approaches.

Follow-Up

Patients should be followed regularly during rehabilitation to assess swelling, range of motion, knee stability, quadriceps and hamstring strength, and progression of functional activity.

Follow-up is particularly important during the early rehabilitation period to identify patients developing stiffness or muscle weakness.

Prognosis

The prognosis after appropriately selected and well-performed ACL reconstruction is generally excellent.

Most patients regain good stability and are able to return to a high level of function.

However, return to sport depends on successful rehabilitation, restoration of strength and neuromuscular control, associated injuries, and psychological readiness.

Consequences of Chronic ACL Deficiency

An untreated ACL-deficient knee may experience repeated episodes of instability.

Over time, this can increase the risk of meniscal tears and articular cartilage damage.

Development of later symptomatic osteoarthritis may also occur, although the relationship between ACL reconstruction and prevention of osteoarthritis remains complex.

Contralateral ACL Injury

Patients who have sustained one ACL tear have an increased risk of subsequently tearing the ACL in the opposite knee.

This risk is particularly important in younger and highly active athletes.

Neuromuscular training and appropriate return-to-sport criteria are therefore important for both knees.

Recurrent ACL Tear

Graft rupture or recurrent ACL injury can occur after reconstruction.

Higher reinjury rates have been reported among athletes involved in high-demand pivoting sports, including football, gymnastics, and soccer.

Young athletes returning to sport at a high competitive level are particularly vulnerable.

Complications of Nonoperative Treatment

Chronic ACL deficiency is associated with an increased risk of complex meniscal injury.

Repeated instability can progressively damage both the menisci and articular cartilage.

There may also be an increased risk of later osteoarthritis, although the degree to which reconstruction prevents this remains controversial.

Surgical Complications

Potential complications after ACL reconstruction include graft failure, graft impingement, quadriceps weakness, patellofemoral pain, infection, and arthrofibrosis.

Rare complications include deep vein thrombosis, nerve injury, vascular injury, compartment syndrome related to arthroscopic fluid extravasation, and complex regional pain syndrome.

Arthrofibrosis

Arthrofibrosis can produce substantial postoperative stiffness, particularly loss of knee extension.

The risk is increased when surgery is performed on a knee that remains swollen and stiff.

Preoperative restoration of motion and careful postoperative rehabilitation are therefore important preventive measures.

Bone-Patellar Tendon-Bone Graft Complications

Harvesting a bone-patellar tendon-bone autograft may result in anterior knee pain or discomfort while kneeling.

Rare complications include patellar fracture and patellar tendon rupture.

Graft selection should therefore consider the patient’s sporting, occupational, and kneeling requirements.

Patient Monitoring

Patients should generally be reassessed at approximately 4–6-week intervals during important phases of recovery.

Monitoring should focus on restoration of knee extension and flexion, quadriceps and hamstring strength, swelling, stability, gait, and functional progression.

Patients who fall behind expected milestones may require more intensive physical therapy to prevent persistent weakness or stiffness.