Overview

Anterior cruciate ligament (ACL) tear is one of the most common knee injuries, particularly in young athletes involved in pivoting sports (soccer, basketball, skiing). ACL injuries account for ~50% of all knee ligament injuries, with an incidence of ~68 per 100,000 annually, more common in females due to anatomical and biomechanical risk factors. ACL tears cause instability, impaired performance, and predispose to early osteoarthritis if untreated.

Anatomy & Physiology 

• ACL origin: Posterior aspect of lateral femoral condyle.
• Insertion: Anterior intercondylar area of tibial plateau.
• Function: Restricts anterior translation of tibia relative to femur; stabilises knee during rotation and cutting movements.
• Bundles: Anteromedial (tight in flexion) and posterolateral (tight in extension).

Aetiology and Risk Factors

Aetiology

• Sudden deceleration, pivoting, or change in direction.
• Landing from a jump with knee in valgus/rotation.
• Direct blow to lateral knee causing valgus stress.
  

Risk Factors

• Female sex (smaller notch width, hormonal effects, neuromuscular control differences).
• High-risk sports: soccer, basketball, netball, skiing.
• Poor conditioning or hamstring–quadriceps imbalance.
• Previous ACL injury.
  

Pathophysiology 

1. High-energy force → valgus + external rotation or hyperextension.
   
2. ACL fibres fail → partial or complete tear.
   
3. Immediate hemarthrosis from ligament vascular supply.
   
4. Knee instability → recurrent giving-way episodes.
   
5. Long-term: meniscal tears, chondral damage, osteoarthritis.

Clinical Manifestations

• “Pop” at injury, immediate swelling within 2–3 hours (hemarthrosis), inability to continue playing.
• Instability (“giving way”), difficulty pivoting.
  

Examination

• Positive Lachman test (most sensitive).
• Positive Anterior drawer test.
• Positive Pivot shift test (most specific).
• Reduced ROM due to swelling.

Diagnosis

• Clinical: History + exam (Lachman = gold standard).
• MRI: Imaging of choice; sensitivity >95%, shows associated meniscal or chondral injuries.
• X-ray: Rule out fractures (Segond fracture = pathognomonic for ACL tear).
  

Differential Diagnosis

Treatment

• Acute management: RICE (rest, ice, compression, elevation), analgesia, knee brace, physiotherapy.
  
• Non-operative: Activity modification + rehab (quads/hamstring strengthening). Suitable for low-demand or older patients.
  
• Operative: Arthroscopic ACL reconstruction (hamstring/patellar tendon autograft). Indicated in young, active patients, recurrent instability, or associated meniscal/chondral injury.
  
• Rehabilitation: Gradual return to sport after 9–12 months, with neuromuscular training.

Complications & Prognosis

Complications

• Recurrent instability if untreated.
• Secondary meniscal tears.
• Early osteoarthritis.
• Arthrofibrosis (post-surgical).
• Graft failure after reconstruction.

Prognosis

• Good with surgical reconstruction + rehab
• High return to sport but increased long-term OA risk.

References

1. Griffin LY, Albohm MJ, Arendt EA, et al. Understanding and preventing noncontact ACL injuries. Am J Sports Med. 2006;34(9):1512–32.
   
2. van Eck CF, Schkrohowsky JG, Working ZM, et al. Clinical examination in the diagnosis of ACL tears: a meta-analysis. Knee Surg Sports Traumatol Arthrosc. 2013;21(9):1989–96.
   
3. Frobell RB, Roos HP, Roos EM, et al. Treatment for acute ACL tear: 5-year outcome of the randomized controlled trial. N Engl J Med. 2010;363:331–42.
   
4. Filbay SR, Grindem H. Evidence-based recommendations for rehabilitation after ACL reconstruction. Curr Rev Musculoskelet Med. 2019;12(2):289–96.
   
5. Sanders TL, Maradit Kremers H, Bryan AJ, et al. Incidence of ACL tears and reconstruction: a 20-year population-based study. Am J Sports Med. 2016;44(6):1502–7.