1. History
- Mechanism of injury: Noncontact pivoting, cutting, deceleration, or landing from a jump (accounts for ~40% of injuries); or direct contact/blow to the planted leg [1-2]
- "Pop": Audible or perceived pop at the time of injury is highly suggestive — a popping sensation has up to 100% specificity and PPV in some pediatric studies [3]
- Immediate swelling: Rapid-onset effusion (within 2 hours) indicates hemarthrosis and strongly suggests significant intra-articular injury [2][4]
- Knee "giving out": Sensation of instability or buckling at the time of injury [2]
- Inability to continue activity: Unable to resume sport or weight-bearing immediately after injury [5]
- Important negatives: Absence of a pop, absence of immediate effusion, and ability to continue activity significantly decrease the probability of ACL tear (LR−: 0.08 when combined with negative Lachman/pivot shift) [6]
2. Alarm Features
- Knee dislocation: Gross deformity, multiligamentous injury — requires emergent vascular assessment (popliteal artery injury in ~10–18% of knee dislocations) [7-8]
- Absent or diminished distal pulses: Suggests popliteal artery injury; emergent vascular surgery consultation [9]
- Inability to perform straight leg raise: Suggests disruption of the extensor mechanism (patellar tendon or quadriceps tendon rupture) [4]
- Open wound over the knee joint: Open fracture or open dislocation — emergent orthopedic consultation
- Neurovascular deficit: Foot drop (peroneal nerve injury), sensory loss, or compartment syndrome signs (pain with passive stretch, tense compartment) [2][4]
- Segond fracture on X-ray: Small lateral tibial avulsion fracture — pathognomonic for ACL tear [2]
3. Medications
- Acute pain management: Acetaminophen and short-course, low-dose NSAIDs are first-line; NSAIDs do not impair ligament healing [2]
- Opioids: Should be used sparingly, if at all [2]
- Avoid: Intra-articular corticosteroid injection in the acute setting (may mask injury severity and impair healing)
- Anticoagulants: Note if patient is on anticoagulation, as hemarthrosis may be more pronounced
- Post-surgical: DVT prophylaxis per institutional protocol after ACL reconstruction [10]
4. Diet
- No specific dietary triggers or restrictions
- Adequate protein intake supports tissue healing and rehabilitation
- Hydration is important during rehabilitation phases
- Weight management: Elevated BMI is a risk factor for ACL injury and may affect surgical outcomes [11]
5. Review of Systems
- Musculoskeletal: Locking, catching (meniscal tear), medial/lateral joint line pain, contralateral knee symptoms, hip or ankle pain
- Neurologic: Numbness, tingling, or weakness in the lower leg/foot (peroneal nerve injury)
- Vascular: Coolness, pallor, or pain in the calf/foot
- Constitutional: Fever, chills (if concern for septic joint in the differential)
- Rheumatologic: Morning stiffness, polyarticular symptoms (if considering inflammatory arthropathy)
6. Collateral History and Family History
- Prior knee injuries: Previous ACL tear (ipsilateral or contralateral) significantly increases re-injury risk — ~32% second ACL injury rate within 2 years in adolescents [12]
- Family history of ACL injury: Genetic predisposition is a recognized risk factor [11][13]
- Activity level and sport: Type and level of sport participation directly influences management decisions [2]
- Social context: Occupational demands, athletic goals, and ability to comply with prolonged rehabilitation
7. Risk Factors
- Female sex: 2–8× higher noncontact ACL injury rate compared with males [1][14-15]
- Young age: Peak incidence at 16–18 years [1]
- High-risk sports: Soccer (highest risk in females), football (highest in males), basketball, lacrosse, skiing [1][11]
- Neuromuscular factors: Poor core control, quadriceps-dominant activation pattern, gluteal weakness, dynamic knee valgus [13][15]
- Anatomic factors: Narrow intercondylar notch, increased posterior tibial slope, increased Q angle, ligamentous laxity [2][13]
- Hormonal factors: Serum relaxin levels, menstrual cycle variations [13]
- Increased BMI [11]
- Prior ACL injury: History of contralateral or ipsilateral ACL tear [12]
8. Differential Diagnosis
- Meniscal tear: Joint line tenderness, locking, catching; often coexists with ACL tear (20–45% lateral, 0–28% medial) [1]
- MCL/LCL sprain: Medial or lateral joint line pain with valgus/varus stress; MCL injury coexists in 19–38% of ACL tears [1]
- PCL tear: Posterior drawer positive; posterior sag sign; typically from dashboard injury or fall on flexed knee
- Patellar dislocation/subluxation: Lateral patellar apprehension, medial retinacular tenderness; common in adolescents [4]
- Tibial plateau fracture: Point tenderness over tibial plateau, inability to bear weight; visible on X-ray
- Tibial spine avulsion: More common in skeletally immature patients; visible on lateral X-ray [12]
- Knee dislocation (multiligamentous injury): Gross instability, possible vascular compromise — cannot-miss diagnosis [7][9]
- Quadriceps/patellar tendon rupture: Inability to perform straight leg raise, palpable gap [4]
9. Past Medical History
- Previous knee injuries or surgeries (prior ACL reconstruction, meniscectomy)
- History of patellar instability
- Connective tissue disorders (Ehlers-Danlos, Marfan) — increased ligamentous laxity
- Chronic conditions affecting healing: diabetes, peripheral vascular disease
- Skeletal maturity status in adolescents (open vs. closed physes affects surgical approach) [12]
10. Physical Exam
The following figure illustrates key physical examination maneuvers for knee ligament assessment:
Key findings
- Effusion: Tense hemarthrosis within 2 hours of injury is highly suggestive of significant intra-articular pathology [4]
- Gait: Antalgic or inability to bear weight [12]
- ROM: Typically limited by pain and effusion
Provocative tests (compare to contralateral knee): [1][17-18]
- Valgus/varus stress testing: Assess MCL and LCL integrity [18]
- Posterior drawer/sag sign: Rule out PCL injury [18]
- McMurray test / joint line tenderness: Evaluate for meniscal injury [18]
- Neurovascular exam: Distal pulses, sensation, motor function (peroneal nerve) [2]
- Straight leg raise: Rule out extensor mechanism disruption [4]
11. Lab Studies
- Routine labs are not indicated for isolated ACL tear
- Joint aspiration (if performed): Hemarthrosis confirms significant intra-articular injury; fat globules (lipohemarthrosis) suggest occult fracture
- Pre-operative labs: CBC, BMP, coagulation studies if surgical reconstruction is planned
- Inflammatory markers (ESR, CRP): Only if septic arthritis or inflammatory arthropathy is in the differential
12. Imaging
- X-ray (first-line): AP, lateral, notch, and sunrise views to rule out fracture and dislocation [2]
- Segond fracture: Small avulsion off the lateral tibial plateau — pathognomonic for ACL tear [2]
- Tibial spine avulsion: Especially in skeletally immature patients
- Apply the Ottawa Knee Rule to determine need for radiography [21-22]
- MRI without contrast (gold standard): Sensitivity 97%, specificity 100% for ACL tear [1]
- Also evaluates meniscal tears, chondral injury, collateral ligament damage, and bone bruising [1-2]
- Per ACR Appropriateness Criteria: MRI is "usually appropriate" after radiographs do not show fracture when internal derangement is suspected [21]
- CT: Reserved for complex fracture characterization (e.g., tibial plateau fracture)
- CTA/MRA: Only if knee dislocation with concern for vascular injury [21]
13. Special Tests
- Ottawa Knee Rule: Age ≥55, fibular head tenderness, isolated patellar tenderness, inability to flex to 90°, inability to bear weight (4 steps) → if any positive, obtain X-ray; pooled sensitivity 98.5% for fracture [21-23]
- ACLIS Score (ACL Injury Score): 4 history items (immediate instability, inability to resume activity, sensation of dislocation, pivoting-contact activity); ≥2 items → 95% sensitivity, 88% PPV for ACL tear — useful for ED triage and specialist referral decisions [5]
- KT-1000 arthrometer: Quantitative anterior tibial translation measurement; ≥7 mm at 133 N has high diagnostic accuracy (PPV 97%, NPV 88% in pediatric populations) [3]
- Ankle-brachial index (ABI): If concern for knee dislocation or vascular injury; ABI <0.9 warrants vascular imaging [7]
14. ECG
- ECG is not routinely indicated for isolated ACL tear
- Consider ECG only if:
- Pre-operative evaluation for ACL reconstruction
- Syncope or cardiac symptoms preceded the fall/injury
- Concern for cardiac etiology of collapse leading to knee trauma
15. Assessment
Typical presentation: Young athlete with noncontact pivoting injury, audible pop, immediate effusion, inability to continue activity, and positive Lachman test. [1-2] ACL tears represent >50% of knee injuries and affect >200,000 people annually in the US. [1]
Severity stratification
- Isolated ACL tear: May be managed operatively or nonoperatively depending on patient factors
- ACL + meniscal tear: Higher urgency for surgical evaluation — delayed reconstruction increases risk of secondary meniscal damage [1]
- Multiligamentous injury / knee dislocation: Emergent evaluation for vascular injury [7][9]
Complications to consider
- Secondary meniscal tears with delayed treatment (risk doubles if reconstruction delayed >5 months, 6× if >1 year) [1]
- Long-term osteoarthritis (occurs with or without surgical intervention) [1-2]
- Re-tear or contralateral ACL injury (~32% in adolescents within 2 years) [12]
16. Treatment Plan
Initial stabilization (ED/acute setting)
- RICE protocol: Rest, ice, compression, elevation
- Knee immobilizer or hinged brace for comfort and stability
- Crutches for non-weight-bearing or protected weight-bearing as tolerated
- Acetaminophen ± short-course NSAIDs; avoid opioids if possible [2]
- Aspiration of tense hemarthrosis may provide symptomatic relief
Definitive management — individualized shared decision-making: [24]
Guidelines vary: the AAOS recommends early ACL reconstruction (within 6 weeks to 3 months) to reduce future meniscal tears, while British and Dutch guidelines recommend structured rehabilitation first with surgery reserved for persistent instability. [24]
- Surgical reconstruction favored for: Young/active patients, pivoting/cutting sports, concomitant meniscal or multiligamentous injury, persistent instability after rehabilitation [1-2]
- Nonoperative management reasonable for: Less active patients, straight-ahead activities only, patients willing to modify activity; involves 3+ months of supervised physiotherapy with ROM, strengthening, and neuromuscular training [1-2]
- Key point: Bracing alone does not prevent subluxation and is ineffective as sole treatment [1-2]
- Reevaluation: At 6–12 weeks to assess rehabilitation effectiveness and need for delayed reconstruction [1]
17. Disposition
- Discharge from ED (majority of cases): Isolated ACL tear with stable neurovascular exam, no fracture requiring operative fixation
- Orthopedic consultation in ED: Knee dislocation (even if spontaneously reduced), multiligamentous injury, tibial spine avulsion fracture, locked knee
- Emergent vascular surgery consultation: Absent or diminished pulses, ABI <0.9, signs of limb ischemia [7][9]
- Observation/admission: Knee dislocation requiring serial vascular exams, compartment syndrome concern, associated fractures requiring operative management
18. Follow Up / Return Precautions
Follow-up timing
- Orthopedic/sports medicine referral within 1–2 weeks of injury for definitive evaluation and MRI if not obtained in ED
- Reevaluation at 6–12 weeks if nonoperative pathway chosen, to assess functional recovery and need for surgery [1]
- If surgical reconstruction performed, return to sport is criterion-based (not solely time-based), typically 9–12 months [1]
Return precautions — instruct patients to return immediately for:
- Increasing swelling, pain, or inability to bear weight
- Numbness, tingling, or weakness in the foot/leg
- Knee locking or inability to straighten the knee
- Recurrent giving way or buckling episodes
- Signs of DVT: calf swelling, warmth, tenderness (post-surgical)
Patient counseling
- ACL injury carries a long-term risk of knee osteoarthritis regardless of treatment strategy [1-2]
- Neuromuscular prevention programs (hamstring/gluteal strengthening, landing mechanics) can reduce ACL injury risk by up to 50% and should be discussed for return-to-sport and contralateral injury prevention [11][25]
References
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2. Initial Assessment and Management of Select Musculoskeletal Injuries: A Team Physician Consensus Statement. — Herring SA, Kibler WB, Putukian M, et al. Medicine and Science in Sports and Exercise. 2024.
3. Diagnostic Values of History Taking, Physical Examination and KT-1000 Arthrometer for Suspect Anterior Cruciate Ligament Injuries in Children and Adolescents: A Prospective Diagnostic Study. — Dietvorst M, van der Steen MCM, Reijman M, Janssen RPA. BMC Musculoskeletal Disorders. 2022.
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