Anterior Cruciate Ligament (ACL) Tear

Risk factors

• Pivoting sports — soccer, basketball, football, skiing, lacrosse
• Female sex (in pivoting sports)
• Narrow intercondylar notch
• Increased posterior tibial slope
• Quadriceps-dominant landing biomechanics
• Generalized ligamentous laxity
• Prior ACL injury (10-25% retear or contralateral tear within 5 years)
• Inadequate neuromuscular training

Pathophysiology

Non-contact mechanism: sudden deceleration, valgus collapse, and tibial internal rotation with the knee near full extension. The ACL fails as anterior tibial translation exceeds its tensile strength. Up to 60% of ACL tears have associated meniscal injury. Acute ruptures more often tear the lateral meniscus, whereas the chronically ACL-deficient knee progressively tears the medial meniscus (its posterior horn is a secondary restraint to anterior tibial translation), so delayed reconstruction with continued pivoting raises medial meniscal risk; chronic ACL deficiency predisposes to early osteoarthritis even after reconstruction.

Clinical presentation

Differential diagnosis

• Meniscus tear — Joint-line pain, slower effusion (12-24 h), mechanical symptoms; often coexists
• MCL sprain — Valgus mechanism, medial pain, pain with valgus stress; isolated tear has smaller effusion
• PCL tear — Posterior tibial translation, dashboard mechanism, positive posterior drawer
• Patellar dislocation — Lateral displacement of patella, hemarthrosis, positive apprehension; can mimic ACL by mechanism
• Tibial plateau fracture — High-energy mechanism, X-ray findings, sometimes subtle (lipohemarthrosis on lateral view)
• Quadriceps or patellar tendon rupture — Inability to extend knee against gravity, palpable defect

Diagnostic workup

Diagnostic algorithm

flowchart TD
  A[Knee twisting injury<br/>pop + immediate effusion] --> B[Knee X-rays<br/>look for Segond fracture]
  B --> C[Lachman test<br/>anterior drawer, pivot-shift]
  C --> D{Lachman<br/>positive?}
  D -->|Yes| E[MRI confirms ACL tear<br/>characterize associated injuries]
  D -->|No, equivocal| F[MRI for definitive dx]
  E --> G[RICE, motion, PT to restore extension]
  G --> H{Active patient<br/>or pivoting sport?}
  H -->|Yes| I[ACL reconstruction<br/>after motion restored]
  H -->|No| J[Non-operative care<br/>+ functional bracing]
  I --> K[9-12 mo rehab to RTS]
  J --> K

Complications

• Persistent instability if non-operative care chosen and patient returns to pivoting sport
• Post-traumatic osteoarthritis (high risk regardless of surgical or non-operative management)
• Meniscal (especially medial meniscal) tear progression with ongoing instability
• Graft failure or retear (~5-15%, higher in young athletes returning to sport)
• Arthrofibrosis if early surgery before motion restored
• DVT/PE (low)
• Postoperative infection
• Contralateral ACL injury (similar high risk)

PANCE pearls

• Immediate large effusion + audible pop + pivoting mechanism is ACL until proven otherwise.
• Segond fracture on plain radiograph is pathognomonic for ACL injury.
• Lachman test is more sensitive than anterior drawer; pivot-shift is more specific.
• Restore full extension and reduce swelling BEFORE surgical reconstruction to minimize arthrofibrosis.
• ACL reconstruction does not prevent post-traumatic osteoarthritis — counsel patients realistically.
• Neuromuscular training programs (e.g., PEP) reduce ACL injury risk in female athletes.
• Unhappy (O'Donoghue) triad = ACL + MCL + medial meniscus from a valgus blow to the planted knee; in acute isolated ACL ruptures the lateral meniscus is torn more often, but the medial meniscus is at greatest risk over time in the ACL-deficient knee.

References

• AAOS 2022 — AAOS Clinical Practice Guideline on Management of Anterior Cruciate Ligament Injuries (2022)
• MOON Group — Multicenter Orthopaedic Outcomes Network (MOON) Knee Cohort — long-term outcomes of ACL reconstruction (Spindler et al., AJSM)
• BEAR Trial — Bridge-Enhanced ACL Repair vs Reconstruction (Murray et al., AJSM 2020)