What is the initial management and differential diagnosis for a patient presenting with acute knee pain?

Acute Knee Pain: Differential Diagnosis and Initial Management

Immediate Assessment for Urgent Conditions

First, rapidly exclude life-threatening or limb-threatening emergencies that require immediate intervention, including septic arthritis (fever, severe pain, refusal to bear weight, joint effusion), acute compartment syndrome (severe pain out of proportion, neurovascular compromise), and popliteal artery injury (diminished pulses, cool extremity). 1, 2, 3

Red Flags Requiring Urgent Evaluation:

• Septic arthritis indicators: Fever, severe joint swelling, erythema, warmth, inability to move the joint, and recent bacteremia 1, 2, 4
  • In children <5 years with CRP >2.0 mg/dL, probability of septic arthritis exceeds 90% 2
  • Requires urgent ultrasound-guided joint aspiration 2
• Vascular injury: Diminished pulses, cool extremity, severe pain after trauma 3
• Acute compartment syndrome: Severe pain disproportionate to injury, neurovascular compromise 3

Differential Diagnosis by Mechanism

Traumatic Causes:

• Fractures: Patella, tibial plateau, femoral condyle, fibular head 5
• Ligamentous injuries: ACL, PCL, MCL, LCL tears 6, 7
• Meniscal tears: Medial or lateral meniscus 6, 7
• Patellar dislocation/subluxation 5, 6
• Osteochondral defects 7

Atraumatic Causes:

• Septic arthritis (emergency) 1, 2
• Inflammatory arthropathy: Gout, pseudogout, rheumatoid arthritis 4
• Degenerative osteoarthritis 7
• Bursitis: Prepatellar, pes anserine 8, 4
• Tendinopathy: Patellar, quadriceps 8, 4

Initial Imaging Algorithm

When to Order Radiographs (Ottawa Knee Rule):

Obtain plain radiographs (minimum AP and lateral views) if ANY of the following Ottawa criteria are present: 5, 1

• Age ≥55 years 5
• Isolated patellar tenderness (no other bony tenderness) 5
• Tenderness at the fibular head 5
• Inability to flex knee to 90 degrees 5
• Inability to bear weight immediately after injury 5
• Inability to take 4 weight-bearing steps in the emergency department 5

Override Ottawa rules and obtain radiographs immediately if: 5, 1

• Gross deformity present 5, 1
• Palpable mass 5, 1
• Penetrating injury 5, 1
• Prosthetic hardware 5, 1
• Altered mental status (head injury, intoxication, dementia) 5, 1
• Neuropathy (paraplegia, diabetes) 5, 1

Standard Radiographic Views:

• Minimum: AP and lateral (at 25-30 degrees flexion) 5, 1
• Additional views when indicated: Cross-table lateral with horizontal beam (for lipohemarthrosis), patellofemoral view (for patellar fracture/dislocation), internal/external oblique views 5, 1

Advanced Imaging Indications

When to Order CT:

CT is appropriate when radiographs are normal but occult fracture is suspected, particularly for tibial plateau fractures where CT demonstrates 100% sensitivity compared to 83% for radiographs. 5, 1, 2

When to Order MRI:

MRI should NOT be used as initial imaging for acute trauma but becomes appropriate when: 5, 1, 7

• Radiographs are normal but pain persists beyond conservative management 1
• Surgery is being contemplated 1
• Evaluation of meniscal, ligamentous, or cartilaginous structures is needed 7
• Persistent effusion, inability to fully bear weight, mechanical symptoms, or joint instability present at 5-7 days post-injury 2

Physical Examination Essentials

Key Components to Document:

• Focal bony tenderness: Patella, fibular head, tibial plateau, femoral condyles 5, 1, 2
• Joint effusion: Presence and size 5, 1
• Weight-bearing ability: Can patient take 4 steps? 5, 2
• Range of motion: Specifically flexion to 90 degrees 5, 2
• Ligamentous stability: Lachman, anterior/posterior drawer, valgus/varus stress tests 4, 6
• Meniscal signs: McMurray test, joint line tenderness 4, 6
• Neurovascular status: Pulses, sensation, motor function 3, 4

Common Pitfalls to Avoid

• Never order MRI without obtaining recent radiographs first—this occurs in approximately 20% of chronic knee pain cases and represents inappropriate utilization 1
• Remember that meniscal tears are often incidental findings in older patients and may not be the source of pain 1
• Clinical judgment should supersede clinical guidelines in specific cases where physician suspicion is high 5
• Do not miss septic arthritis in children—use age and CRP criteria to guide urgent aspiration 2
• Beware of vascular injury after seemingly minor trauma, especially in bicycle falls or posterior knee trauma 3

Initial Management Based on Findings

If Fracture Identified:

• Orthopedic consultation for definitive management 2
• Immobilization and non-weight-bearing as appropriate 2

If No Fracture but Traumatic Mechanism:

• Conservative management with RICE (rest, ice, compression, elevation) 2, 6
• Most MCL tears, meniscal tears, and patellar dislocations can be managed non-operatively 6
• Short period of knee bracing in extension with progression to weight-bearing as tolerated for patellar dislocation 6
• Close follow-up at 5-7 days; consider MRI if persistent symptoms 2

If Atraumatic Effusion:

• Consider laboratory tests (cell count, crystal analysis, culture) if infection or inflammatory arthropathy suspected 4
• Urgent aspiration if septic arthritis cannot be excluded 2