Anterior Cruciate Ligament (ACL) Injury
Definition
An Anterior Cruciate Ligament (ACL) injury is a sprain or tear of the ACL, which is one of the major ligaments in the knee (1). The ACL is the primary restraint to excessive anterior translation of the tibia relative to the femur and provides rotational stability to the knee (2). Injuries are graded by severity, with a Grade 3 sprain representing a complete tear of the ligament, leading to knee instability (3).
Epidemiology
In Malaysia, ACL injuries are a significant concern, particularly among young and active individuals. A study on athletes who underwent ACL reconstruction between 2015 and 2020 found that the majority of cases were in males (83.2%), with a mean age of 27.1 years. Complete tears were the most common type of injury (87.8%), and concomitant meniscal injuries were present in 61.1% of cases (4). The most frequent causes of injury are non-contact pivoting movements common in sports like football, basketball, and netball (5). [Specific prevalence data for ACL injuries in the general Malaysian population was not found in recent national health surveys; the available data is primarily focused on athletic cohorts undergoing surgery and should be interpreted with this in mind]. Globally, the annual incidence of ACL injuries is estimated to be around 68.6 per 100,000 person-years (4).
Pathophysiology
The ACL runs diagonally in the middle of the knee, originating from the medial aspect of the lateral femoral condyle and inserting on the anterior intercondylar area of the tibia (2). It is composed of two main bundles: the anteromedial and posterolateral bundles, which work together to control stability (6).
An injury typically occurs through two main mechanisms:
Non-contact pivoting: This is the most common cause, where an athlete suddenly decelerates, changes direction, or lands from a jump. This movement creates a combination of valgus force (knee collapsing inward) and external rotation of the tibia on a planted foot, exceeding the tensile strength of the ACL, which is approximately 2100 N (6, 7).
Direct contact: A direct blow to the knee, often to the lateral side, can also cause a tear (7).
Once torn, the ACL has a very limited capacity to heal on its own due to its intra-articular location, where synovial fluid prevents clot formation and subsequent healing processes (6). This leads to chronic instability, which, if left unmanaged, increases the risk of secondary damage to the menisci and articular cartilage, predisposing the patient to early-onset osteoarthritis (8).
Clinical Presentation
The classic presentation involves a patient reporting a traumatic knee injury during a sporting activity.
Diagnostic Clues:
The history often includes a "popping" sound or sensation at the time of injury, followed by immediate, significant pain and swelling (haemarthrosis) within a few hours (7). A key feature is a feeling of the knee "giving way" or buckling, especially with twisting movements (5).
Common Symptoms (>50%):
Audible "pop" or crack at time of injury (~70%) (7)
Severe pain immediately following the injury (5)
Significant knee swelling (haemarthrosis) developing within 6-12 hours (7)
Feeling of instability or the knee "giving way" (5)
Restricted range of motion, particularly loss of full extension (3)
Less Common Symptoms (10-50%):
Inability to bear weight on the affected leg (5)
Tenderness along the tibiofemoral joint line (3)
⚠️ Red Flag Signs & Symptoms:
Locked knee or inability to fully extend the knee: This may suggest a displaced meniscal tear (e.g., a bucket-handle tear) that requires urgent assessment.
Signs of neurovascular compromise: Such as a cold, pale foot, paraesthesia, or diminished distal pulses. This could indicate a knee dislocation, which is a surgical emergency.
Associated fractures: Indicated by bony tenderness, especially over the fibular head (suggesting peroneal nerve risk) or tibial plateau.
Complications
Complications can be categorized based on timing and nature:
Acute:
Associated Injuries: Up to 50% of ACL tears are associated with other injuries, most commonly meniscal tears (especially the lateral meniscus in acute tears and medial in chronic cases) and injuries to other ligaments like the Medial Collateral Ligament (MCL) – part of the "unhappy triad" (ACL, MCL, and medial meniscus tear) (2, 9). Bone bruises on the lateral femoral condyle and posterolateral tibial plateau are seen in over 80% of cases on MRI (9).
Chronic:
Knee Instability: Recurrent episodes of the knee giving way during daily or sporting activities (8).
Secondary Meniscal and Cartilage Damage: Chronic instability leads to increased shear forces across the joint, causing progressive damage to the menisci and articular cartilage (8).
Post-Traumatic Osteoarthritis: The risk of developing knee osteoarthritis is significantly higher in an ACL-deficient knee, regardless of whether it is managed surgically or non-surgically, though instability may accelerate this process (10).
Prognosis
The prognosis for an ACL injury depends heavily on the patient's activity level, associated injuries, and management strategy. With appropriate treatment (surgical or non-surgical) and dedicated rehabilitation, many patients can return to a high level of function. However, return to pre-injury sporting levels can be challenging. Studies suggest that while over 80% of patients return to some form of sport after ACL reconstruction, only about 65% return to their pre-injury level, and even fewer (around 55%) return to competitive sports (11). The main factors influencing a poorer prognosis are the presence of concomitant meniscal or cartilage damage and the development of chronic instability (10, 11). Re-rupture rates after reconstruction are estimated to be around 5-10% (5).
Differential Diagnosis
Meniscal Tear: This is a key differential due to the shared symptoms of knee pain, swelling, and mechanical symptoms like locking or catching. However, an isolated meniscal tear may not present with the profound instability or "pop" heard in a complete ACL rupture. The McMurray test can help identify a meniscal tear, but MRI is often required for definitive diagnosis (12).
Patellar Dislocation/Subluxation: Consider this diagnosis, especially in younger females, as it also presents with acute pain, swelling, and a feeling of instability. However, the instability is related to the patellofemoral joint, and patients often describe the kneecap "slipping out of place." Tenderness is typically maximal over the medial patellar retinaculum, and a positive patellar apprehension test is a key distinguishing feature (12).
Multi-ligament Knee Injury (including PCL/MCL/LCL tears): This is a critical differential, particularly after high-energy trauma. The presence of varus or valgus instability on examination, or a positive posterior drawer test, points towards other ligamentous involvement. A knee dislocation should always be suspected in severe multi-ligament injuries, which constitutes a vascular emergency (2).
Tibial Plateau Fracture: This should be considered if the patient is unable to bear weight and there is significant haemarthrosis. It is often caused by high-energy trauma or in older individuals with osteoporosis. Points against this diagnosis would be the absence of significant bony tenderness on examination and no fracture line on plain radiographs, although occult fractures may require CT or MRI for detection (9).
Investigations
Immediate & Bedside Tests
Clinical Examination: A thorough examination of both knees is the most critical immediate step. Key tests include:
Lachman Test: This is the most sensitive test (sensitivity ~85%) for an acute ACL tear. It is performed with the knee at 20-30° of flexion to assess for increased anterior tibial translation with a soft or absent endpoint (the action), which is crucial for making a clinical diagnosis of ACL rupture (the rationale) (13).
Pivot-Shift Test: This test is highly specific (>95%) but can be difficult to perform in an acutely painful and swollen knee due to muscle guarding. It replicates the patient's feeling of instability (the action) by demonstrating the dynamic subluxation and reduction of the tibia that occurs with an ACL-deficient knee (the rationale) (13).
Anterior Drawer Test: Performed at 90° of flexion, it is less sensitive than the Lachman test in the acute setting because haemarthrosis and hamstring spasm can mask the instability (13).
Diagnostic Workup
First-Line Investigations:
Plain Radiographs (AP, Lateral, and Skyline views): These are essential to rule out associated fractures (the action), such as a tibial spine avulsion (common in skeletally immature patients) or a Segond fracture (a small avulsion of the lateral tibial plateau that is pathognomonic for an ACL tear) (the rationale) (9).
Gold Standard:
Magnetic Resonance Imaging (MRI): MRI is the gold standard for confirming the diagnosis and evaluating associated injuries. It has a high sensitivity (>95%) and specificity (>95%) for detecting ACL tears, meniscal damage, collateral ligament injuries, and bone bruising (the rationale), which is critical for comprehensive surgical planning (the action) (14).
Monitoring & Staging
Arthroscopy: While MRI is the primary diagnostic tool, diagnostic arthroscopy remains the ultimate "gold standard" for direct visualization of the ACL and other intra-articular structures (the rationale). It is now rarely used for diagnosis alone but is performed at the time of surgical reconstruction (the action) (12).
Management
Management Principles
The management of an ACL injury is individualized and focuses on restoring knee stability, optimizing function, preventing secondary injuries, and allowing a return to desired activities, guided by a shared decision-making process between the clinician and patient (15).
Acute Stabilisation (The First Hour)
Initial management follows the P.R.I.C.E.R principle:
Protection: "Protect the knee from further injury by using crutches to offload the joint (the action), which is vital to prevent exacerbation of the tear or damage to other structures (the rationale)."
Rest: "Advise cessation of sporting activity immediately (the action) to minimize further injury (the rationale)."
Ice: "Apply ice packs for 15-20 minutes every 2-3 hours (the action) to induce vasoconstriction, which helps reduce swelling and alleviate pain (the rationale)."
Compression: "Use a compression bandage (the action) to limit the development of haemarthrosis and provide symptomatic relief (the rationale)."
Elevation: "Keep the leg elevated above the level of the heart (the action) to facilitate venous and lymphatic drainage, thereby reducing swelling (the rationale)."
Analgesia: "Prescribe simple analgesics like Paracetamol or NSAIDs (e.g., Ibuprofen 400mg TDS) as needed (the action) to manage pain and inflammation (the rationale)."
Definitive Therapy
The decision between non-operative and operative management depends on the patient’s age, activity level, degree of instability, and associated injuries (15).
Non-Operative Management:
This is typically reserved for lower-demand, less active patients, or those with partial tears without significant instability.
Structured Physiotherapy: This is the cornerstone of non-operative treatment. It focuses on reducing swelling, restoring full range of motion, and strengthening the hamstring and quadriceps muscles (dynamic stabilisers) to compensate for the ligament deficiency (the action). Pre-habilitation (physiotherapy before surgery) is also crucial for optimizing post-operative outcomes in those who do choose surgery (16).
Operative Management (ACL Reconstruction):
This is the treatment of choice for young, active patients who wish to return to pivoting sports or those with persistent, debilitating instability (15).
Timing: Surgery is typically delayed for several weeks (3-6 weeks) after the initial injury (the action) to allow swelling to resolve and knee motion to be regained, which reduces the risk of post-operative arthrofibrosis (stiffness) (the rationale) (5).
First-Line Treatment (Surgical Technique): Arthroscopic ACL reconstruction is the standard procedure. It involves removing the torn ligament and replacing it with a graft. Common graft choices in Malaysia include:
Hamstring Tendon Autograft (Semitendinosus-Gracilis): This is the most common choice due to lower donor site morbidity (less anterior knee pain) compared to BPTB (17, 18).
Bone-Patellar Tendon-Bone (BPTB) Autograft: This was previously considered the gold standard, providing excellent stability with bone-to-bone healing in the tunnels, but is associated with a higher risk of anterior knee pain and kneeling discomfort (17, 18).
Allograft (donor tissue): Less commonly used as a primary graft in young, active patients due to higher re-rupture rates, but can be an option in revision cases or older, lower-demand individuals (15).
Supportive & Symptomatic Care
Knee Brace: A hinged knee brace may be used in the early phases for stability and comfort, but its long-term use is debated (16).
Intra-articular injection: Corticosteroid injections are generally avoided in the acute phase as they can have deleterious effects on cartilage and soft tissues (19).
Key Nursing & Monitoring Instructions
Monitor for signs of significant swelling or escalating pain, which could indicate a compartment syndrome (though rare).
Ensure the patient is competent and safe with crutch use.
Educate the patient on the importance of the RICE protocol.
Post-operatively, monitor for signs of wound infection, deep vein thrombosis (DVT), and ensure neurovascular observations are stable.
Enforce strict adherence to the surgeon-specific post-operative weight-bearing and range-of-motion protocol.
Long-Term Plan & Patient Education
Rehabilitation: A structured, goal-oriented rehabilitation program is critical for success, regardless of the management strategy. It typically lasts 9-12 months after surgery and progresses through phases:
Early Phase (0-6 weeks): Focus on reducing pain and swelling, achieving full knee extension, and activating quadriceps.
Strength & Neuromuscular Phase (6 weeks - 4 months): Emphasize closed-chain strengthening, balance, and proprioceptive exercises.
Sport-Specific Phase (4-9 months): Gradual introduction of running, agility drills, and sport-specific movements.
Return to Sport: This is a criteria-based decision, not just time-based. The patient must have no pain, full range of motion, satisfactory strength (e.g., >90% of the contralateral side), and successfully complete functional tests before a gradual return to sport is permitted, typically not before 9 months post-surgery (11, 16).
Patient Education: Counsel the patient that there is a long-term increased risk of knee osteoarthritis. Emphasize the importance of lifelong adherence to strengthening exercises and maintaining a healthy weight to protect the knee.
When to Escalate
Call Your Senior (MO/Specialist) if:
The patient has a locked knee, suggesting an irreducible meniscal tear.
There is suspicion of a multi-ligament injury or knee dislocation (gross instability, signs of vascular compromise).
The patient has significant bony tenderness suggesting a concomitant fracture.
Post-operatively, if there are signs of surgical site infection, DVT (calf swelling, tenderness), or sudden-onset shortness of breath (pulmonary embolism).
Referral Criteria:
All patients with a suspected complete ACL tear and functional instability should be referred to an Orthopaedic Surgeon for evaluation and discussion of management options.
Referral to a Sports Physician or a Physiotherapist with expertise in knee rehabilitation is essential for guiding non-operative management and post-operative rehabilitation.
References
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