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

Differential Diagnosis for Knee Pain

The differential diagnosis for knee pain is primarily determined by patient age, pain location, mechanism of onset, and associated symptoms, with osteoarthritis being the most common cause in patients ≥45 years, patellofemoral pain in active patients <40 years, and meniscal tears affecting approximately 12% of adults across all ages. 1, 2

Age-Stratified Differential Diagnosis

Patients ≥45 Years Old

• Osteoarthritis is the most likely diagnosis when patients present with activity-related knee joint pain and less than 30 minutes of morning stiffness (95% sensitivity, 69% specificity) 2
• Degenerative meniscal tears commonly coexist with osteoarthritis in this age group and may not be the primary pain source 2
• Subchondral insufficiency fractures most commonly affect the medial femoral condyle in middle-aged to elderly females and are often radiographically occult initially 3
• Chondrocalcinosis and crystal-induced arthropathy become more prevalent 1, 4

Patients <40 Years Old

• Patellofemoral pain syndrome has a lifetime prevalence of approximately 25% in physically active individuals, with anterior knee pain during squatting being 91% sensitive and 50% specific 2
• Acute traumatic meniscal tears occur following twisting injuries in younger patients 2
• Patellar tracking disorders including patellar subluxation are more common in teenage girls and young women 4
• Knee extensor mechanism problems such as Osgood-Schlatter disease and patellar tendonitis are more common in teenage boys and young men 4

Location-Based Differential Diagnosis

Anterior Knee Pain

• Patellofemoral pain syndrome presents with anterior pain during squatting (91% sensitivity) 2
• Patellar tendinopathy can be identified on MRI 1
• Fat pad impingement syndromes and Hoffa's disease require contrast-enhanced MRI for optimal visualization 1
• Deep infrapatellar bursitis is best evaluated with contrast-enhanced MRI 1
• Osgood-Schlatter or Sinding-Larsen-Johansson syndrome in adolescents 1

Medial or Lateral Knee Pain

• Meniscal tears with joint line tenderness (83% sensitivity, 83% specificity) and positive McMurray test (61% sensitivity, 84% specificity) 2
• Pes anserine bursitis in active patients 4
• Medial plica syndrome can be evaluated with ultrasound or MRI 1, 4
• Iliotibial band syndrome causing lateral pain 1

Diffuse Knee Pain

• Osteoarthritis with diffuse joint involvement 1
• Inflammatory arthropathy including rheumatoid arthritis 1
• Septic arthritis presenting with fever, swelling, erythema, and limited range of motion requiring urgent evaluation 5

Intra-articular Pathology

Structural Lesions

• Osteochondritis dissecans (OCD) requires MRI for characterization, though hyperintense rim or cysts at fragment periphery are less specific for instability in children compared to adults 1
• Loose bodies can be localized with ultrasound, particularly within popliteal cysts, lateral recess, or suprapatellar recess 1
• Articular cartilage damage can be present despite normal radiographs, particularly in patients 45-55 years with elevated T2 mapping values on 3T MRI 3

Soft Tissue Pathology

• Bone marrow lesions (BMLs) appear as increased edema-like signal on MRI and are strongly associated with knee pain, especially in males or those with family history of osteoarthritis 3
• Synovitis with enhancing tissue thicker than 2mm in Hoffa's fat correlates with peripatellar pain 1
• Popliteal (Baker's) cyst with or without rupture 1, 3
• Pigmented villonodular synovitis requires contrast-enhanced MRI 1

Critical Referred Pain Sources

Hip Pathology

• Hip joint pathology must be considered when knee radiographs are unremarkable, especially with clinical evidence of limited hip range of motion, groin pain, or positive impingement signs 1, 3
• Slipped capital femoral epiphysis in adolescents can present as knee pain 4
• Hip radiographs should be obtained if clinical suspicion exists before proceeding to knee MRI 3

Lumbar Spine Pathology

• Lumbar radiculopathy or neurogenic claudication patterns can refer pain to the knee 1, 3
• Lumbar spine radiographs should be obtained if clinically indicated 3

Urgent Diagnoses Requiring Immediate Evaluation

• Septic arthritis with fever, swelling, erythema, and limited range of motion 5
• Acute fracture with severe pain, swelling, instability, or inability to bear weight following trauma 5
• Acute ligamentous rupture causing hemarthrosis and joint instability 4
• Bucket-handle meniscal tears with displaced meniscal tissue requiring surgical intervention 2

Common Pitfalls in Diagnosis

Overinterpretation of Imaging

• In patients >70 years, the majority have asymptomatic meniscal tears, making MRI findings potentially misleading 3
• In the 45-55 age group, meniscal tears occur with similar frequency in both painful and asymptomatic knees 3
• The presence of a meniscal tear on MRI does not automatically establish it as the pain source 3

Premature or Inappropriate Imaging

• Approximately 20% of patients undergo MRI without recent radiographs (within past year), which is inappropriate 1, 3
• Radiographic imaging should be reserved for chronic knee pain (>6 weeks) or acute traumatic pain meeting evidence-based criteria 5
• MRI should generally be considered only when surgery is being contemplated or pain persists despite adequate conservative treatment 5