Anterior Ligament Tear (ACL)

An ACL tear is one of the most significant and most feared sports injuries -- and one of the most commonly searched knee conditions worldwide. The anterior cruciate ligament (ACL) is a critical stabiliser of the knee joint, and its complete rupture causes a knee that cannot be trusted in pivoting, cutting, or landing movements.

More than 200,000 ACL reconstructions are performed annually in the United States alone, and rates in Asia are rising with increasing sports participation. In Singapore, ACL tears are seen regularly across football, basketball, netball, badminton, touch rugby, and martial arts -- as well as in non-athletic mechanisms such as falls and road traffic accidents.

Anatomy and Function of the ACL

The knee is stabilised by four primary ligaments: the anterior cruciate ligament (ACL), the posterior cruciate ligament (PCL), the medial collateral ligament (MCL), and the lateral collateral ligament (LCL). The ACL and PCL cross each other inside the knee joint -- hence the term 'cruciate' (from the Latin for cross).

The ACL runs from the posterior aspect of the lateral femoral condyle (outer thigh bone) to the anterior tibial plateau (front of the shin bone), passing through the centre of the knee. It has two functional bundles: the anteromedial bundle (taut in flexion) and the posterolateral bundle (taut in extension). Together they control:

• Anterior tibial translation: Preventing the shin bone from sliding forward relative to the thigh bone
• Rotational stability: Particularly internal rotation of the tibia
• Combined pivot stability: The combination of anterior translation and rotation that occurs during pivoting movements

The ACL is crucial for any activity involving a change of direction, deceleration, or landing from a jump. Without it, the knee is unstable in precisely the movements that sports require, and the menisci and articular cartilage are subjected to abnormal forces with each instability episode.

ACL Tear: Grading and What Each Grade Means

ACL injuries are classified into three grades based on the structural integrity of the ligament and the functional stability of the knee. Grade III complete tears are the most common presentation requiring orthopaedic management.

Mechanism of Injury -- How ACL Tears Happen

Non-contact mechanisms (most common)

Approximately 70% of ACL tears occur without direct contact -- the ligament fails from the forces generated by the athlete's own movement. Common non-contact mechanisms include:

• Sudden deceleration followed by a direction change -- the classic ACL mechanism in football and basketball
• Pivoting on a planted foot -- the foot is fixed while the knee rotates
• Landing from a jump with the knee in slight flexion and valgus (knee collapsing inward) -- particularly common in female athletes
• Hyperextension of the knee

Contact mechanisms

• Direct blow to the lateral knee -- forces the knee into valgus, stretching the medial structures and ACL
• Posterior blow to the tibia -- forces the shin bone forward relative to the thigh, loading the ACL

At the moment of injury, most patients describe a distinctive popping or cracking sensation, followed by immediate or rapidly developing pain. The ability to walk off the field immediately after injury does not exclude an ACL tear -- some patients can initially weight-bear before swelling and instability set in.

Symptoms of an ACL Tear

At the time of injury

• An audible or palpable 'pop' at the moment of injury -- described by approximately 50% of patients
• Immediate pain -- often severe, though some patients initially have only mild discomfort
• Rapid knee swelling developing within 1-2 hours (haemarthrosis)
• A sensation that the knee 'gave way' or buckled

In the hours and days after

• Inability or significant difficulty bearing weight
• Loss of range of motion -- particularly full extension, due to haemarthrosis and pain
• Feeling of instability -- the knee feels unreliable or about to give way
• Bruising developing over 24-48 hours

In the weeks after (if untreated)

• Recurrent episodes of the knee giving way -- particularly on stairs, uneven ground, or with direction changes
• Difficulty returning to sport or physical activity
• Secondary swelling with activity
• Gradual loss of confidence in the knee -- patients often unconsciously modify their movement patterns to compensate

Associated Injuries: What Else Is Often Damaged

ACL tears rarely occur in isolation. The forces that rupture the ACL frequently damage adjacent structures simultaneously. Identifying and managing associated injuries is as important as the ACL itself.

• Medial Meniscus Tear: The most common associated injury with ACL tears -- particularly in chronic ACL deficiency. The medial meniscus acts as a secondary restraint to anterior tibial translation and experiences increased load without the ACL. Chronic ACL deficiency causes progressive medial meniscal damage with each instability episode. A meniscal tear is repaired or trimmed arthroscopically at the time of ACL reconstruction.
• Lateral Meniscus Tear: More commonly associated with acute ACL injuries than the medial meniscus in the immediate post-injury period. A posterolateral meniscal root tear is particularly important to identify on MRI as it must be repaired at the time of surgery. Missed lateral meniscus root tears lead to progressive cartilage damage.
• MCL (Medial Collateral Ligament) Injury: The 'unhappy triad' -- ACL, MCL, and medial meniscus injury together -- is a classic combined knee injury from a valgus contact mechanism. Most MCL injuries associated with ACL tears heal conservatively while the ACL reconstruction is deferred until the knee has settled.
• Bone Bruise (Kissing Contusion): The impact forces at the time of ACL injury cause bone bruising to the lateral femoral condyle and posterior lateral tibial plateau -- a characteristic pattern on MRI. Bone bruising resolves over 6-12 months and is not separately treated but may contribute to longer-term cartilage vulnerability.
• Osteochondral Injury: A fracture of the articular cartilage surface and underlying bone -- more common in high-energy ACL injuries. May require arthroscopic treatment (microfracture, mosaicplasty, or osteochondral grafting) if the fragment is large or displaced.
• Posterolateral Corner (PLC) Injury: Combined ACL and PLC injury requires careful surgical planning -- a missed PLC injury leads to failure of ACL reconstruction. PLC reconstruction may be performed simultaneously with or staged after ACL reconstruction. Pre-operative stress imaging and careful clinical examination are required.

How is an ACL Tear diagnosed?

Diagnosis combines clinical examination with MRI. An experienced orthopaedic surgeon can diagnose a complete ACL tear with high confidence on clinical examination, but MRI is essential to confirm the diagnosis, assess the degree of injury, and identify associated injuries that will influence surgical planning.

• Clinical Examination: Lachman test (90%+ sensitivity for ACL tear): the examiner stabilises the femur and applies an anterior force to the tibia with the knee in 20-30 degrees of flexion -- increased laxity with a soft endpoint indicates ACL rupture. Anterior drawer test and pivot shift test provide additional confirmation.
• Magnetic Resonance Imaging (MRI): The definitive investigation. Confirms ACL tear location, completeness, and integrity of the stump. Identifies meniscal tears, bone bruising, cartilage damage, and other ligament injuries. Essential for surgical planning. Should be performed in all significant knee injuries with haemarthrosis.
• X-ray: Does not show the ACL (soft tissue), but identifies associated bony injuries: tibial plateau fractures, Segond fractures (pathognomonic lateral capsular avulsion associated with ACL tears), femoral condyle fractures, and avulsion injuries of the tibial spine.
• CT Scan: Used for complex bony injuries, tibial spine avulsions, or detailed assessment of tunnel positioning in revision ACL cases. Not routine for primary ACL assessment.
• Arthroscopy: Direct visualisation of the ACL and all intra-articular structures at the time of surgery. Confirms the diagnosis, characterises associated meniscal and cartilage injuries, and allows simultaneous reconstruction.

Surgery vs Conservative Management: Who Needs an Operation?

The decision to reconstruct the ACL versus manage it conservatively depends on multiple factors: the patient's age and activity level, the degree of instability, the status of the menisci and cartilage, and the patient's goals and occupation.

ACL reconstruction is generally recommended for:

• Active patients who wish to return to pivoting sports (football, basketball, netball, badminton, touch rugby, martial arts)
• Young patients (under 40) with complete ACL tears and functional instability
• Patients with associated meniscal tears requiring repair -- meniscal repair healing rates are significantly better when performed with concurrent ACL reconstruction
• Patients with symptomatic knee instability affecting daily activities
• Professional or competitive athletes at any age

Conservative management may be appropriate for:

• Older, sedentary patients with complete ACL tears who are willing to accept activity modification
• Grade I and many Grade II tears where the knee remains stable
• Patients with medical comorbidities making surgery higher risk
• Children with open growth plates -- timing and technique of reconstruction requires careful planning to avoid physeal injury

ACL Reconstruction: The Surgical Procedure

Arthroscopic ACL reconstruction is a minimally invasive procedure performed under general or spinal anaesthesia. The torn ACL cannot be sutured back together -- it requires replacement with a tendon graft that is secured in tunnels drilled in the femur and tibia and gradually incorporates over 12-24 months (graft ligamentisation).

The surgical steps

• Examination under anaesthesia confirms the diagnosis and assesses all ligaments
• Diagnostic arthroscopy inspects all compartments -- meniscal tears are addressed at this stage
• Graft harvest: The chosen tendon is harvested through a small separate incision
• Tunnel preparation: Bone tunnels are drilled in the tibia and femur at the anatomical ACL footprint positions, using fluoroscopic and arthroscopic guidance
• Graft passage: The graft is threaded through the tibial tunnel, across the joint, and up the femoral tunnel
• Graft fixation: The graft is secured with interference screws, cortical buttons, or a combination depending on the graft type and surgeon preference
• Wound closure: Two to three portal incisions plus the graft harvest site. Most patients go home the same day or after one overnight stay.

ACL Rehabilitation -- The Road Back to Sport

Rehabilitation after ACL reconstruction is as important as the surgery itself. The graft undergoes a biological process of ligamentisation -- transformation from dead tendon tissue into a functional ACL -- over 12-24 months. During this period, the graft is vulnerable to re-rupture if loaded excessively, particularly in the first 9 months.

Modern ACL rehabilitation is criteria-based rather than purely time-based: progression to each phase depends on achieving specific strength, range of motion, and functional targets, not just on the number of weeks elapsed since surgery.

Preventing ACL Tears

While contact ACL injuries are difficult to prevent, non-contact ACL injuries -- the majority -- can be significantly reduced with targeted training:

• Neuromuscular training: structured warm-up programmes (FIFA 11+, PEP, Sportsmetrics) incorporating landing mechanics, balance, and plyometrics -- reduce ACL injury rates by 50-80% in high-risk sports
• Landing mechanics: practise landing with knees bent and over the toes, not collapsed inward. Avoid straight-legged landings.
• Hamstring strengthening: strong hamstrings are the primary dynamic co-stabilisers of the ACL. Nordic hamstring curls have the strongest evidence for injury prevention.
• Proprioception training: single-leg balance, perturbation training, and wobble board exercises improve the neuromuscular response that stabilises the knee during unexpected perturbation
• Adequate warm-up: cold muscles are less able to respond to unexpected loads
• Fatigue management: ACL injury rates increase significantly in the final quarter of a match -- fatigue reduces neuromuscular control
• Appropriate footwear: shoes with appropriate traction for the playing surface; excessive traction increases rotational forces on the planted foot