What is the differential diagnosis and initial management for a 39-year-old female presenting with left knee pain?

Differential Diagnosis for Left Knee Pain in a 39-Year-Old Female

In a 39-year-old woman with left knee pain, the most likely diagnoses are patellofemoral pain syndrome, meniscal pathology (either degenerative or traumatic), early osteoarthritis, and overuse injuries such as pes anserine bursitis or iliotibial band syndrome. 1, 2, 3

Key Historical Features to Elicit

The critical history elements that distinguish between diagnoses include:

• Onset and mechanism: Acute twisting injury suggests meniscal tear or ligamentous injury, while insidious onset points toward patellofemoral pain syndrome or early osteoarthritis 1, 3
• Pain location: Anterior knee pain indicates patellofemoral syndrome (especially if worse with squatting, which has 91% sensitivity), while medial/lateral joint line pain suggests meniscal pathology 2, 3
• Duration: Pain >6 weeks is considered chronic and warrants different imaging considerations than acute presentations 1
• Mechanical symptoms: True locking or catching may indicate meniscal tear (though these symptoms alone don't mandate surgery for degenerative tears) 3
• Morning stiffness: Less than 30 minutes of morning stiffness with activity-related pain in patients ≥45 years has 95% sensitivity for knee OA 3

Physical Examination Findings

Critical examination maneuvers include:

• Joint line tenderness: 83% sensitive and 83% specific for meniscal tears 3
• McMurray test: Knee rotation with extension has 61% sensitivity and 84% specificity for meniscal pathology 3
• Anterior knee pain during squat: 91% sensitive and 50% specific for patellofemoral pain 3
• Effusion assessment: Presence of effusion may indicate internal derangement, inflammatory arthropathy, or infection 4, 5
• Range of motion: Restricted motion may suggest osteoarthritis or adhesive capsulitis 4

Complete Differential Diagnosis by Category

Patellofemoral Disorders (Most Common in This Age Group)

• Patellofemoral pain syndrome: Lifetime prevalence ~25% in physically active individuals <40 years; presents with anterior knee pain worse with stairs, squatting, or prolonged sitting 2, 3
• Patellar maltracking/subluxation: More common in teenage girls and young women 2
• Patellar stress fracture: Rare but can occur with high-impact activities; may be bilateral 6

Meniscal Pathology

• Traumatic meniscal tear: Occurs with acute twisting injury in patients <40 years 3
• Degenerative meniscal tear: Can occur in this age group, especially if early OA present; affects ~12% of adult population 3

Early Degenerative Changes

• Early osteoarthritis: While more common ≥45 years, can begin earlier with risk factors including obesity, prior trauma, genetic predisposition, or certain occupations 4, 3
• Chondrocalcinosis: Crystal deposition disease 4

Overuse/Inflammatory Conditions

• Pes anserine bursitis: Common in active patients 2
• Iliotibial band syndrome: Lateral knee pain in active individuals 4, 2
• Medial plica syndrome: Can cause chronic medial knee pain 4, 2
• Hoffa's disease: Inflammation of infrapatellar fat pad 4

Ligamentous Injuries

• ACL/PCL/MCL/LCL sprains or tears: Usually associated with acute trauma and instability 1, 2

Other Considerations

• Referred pain from hip: Must be excluded, especially if knee radiographs are normal 4, 5
• Referred pain from lumbar spine: Consider if knee imaging unremarkable 4, 5
• Septic arthritis: Can occur at any age; presents with fever, swelling, erythema, and limited ROM requiring urgent evaluation 1, 2
• Popliteal (Baker) cyst: Can cause posterior knee pain and swelling 4
• Osteonecrosis: Less common but important not to miss 4

Initial Management Algorithm

Step 1: Determine Urgency

Urgent referral is required if:

• Severe pain, swelling, and instability with inability to bear weight after acute trauma 1
• Signs of infection: fever, swelling, erythema, severely limited ROM 1

Step 2: Initial Imaging Decision

Obtain plain radiographs (minimum AP and lateral views) if: 4, 5

• Chronic pain (>6 weeks duration) 1
• Acute traumatic pain meeting Ottawa knee rules criteria (age ≥55, isolated patellar tenderness, fibular head tenderness, inability to flex to 90°, or inability to bear weight) 4
• Focal tenderness or effusion with fall/twisting mechanism 4

Common pitfall: Approximately 20% of patients inappropriately receive MRI without recent radiographs within the prior year—always obtain plain films first 4, 5

Step 3: Further Imaging Based on Radiograph Results

If radiographs are normal or show only effusion with persistent pain:

• MRI without IV contrast is the next appropriate study to evaluate for meniscal tears, ligamentous injury, cartilage damage, bone marrow edema, osteonecrosis, and soft tissue pathology 4, 5

If radiographs show osteoarthritis:

• MRI is not usually indicated unless symptoms are unexplained by radiographic findings (e.g., to evaluate for stress fractures or osteonecrosis) 4

Ultrasound may be appropriate for:

• Confirming suspected effusion and guiding aspiration 4, 5
• Evaluating popliteal cysts 4
• Assessing medial plicae or iliotibial band syndrome 4

Step 4: Conservative Management (First-Line for Most Conditions)

For suspected patellofemoral pain:

• Hip and knee strengthening exercises combined with foot orthoses or patellar taping 3
• No indication for surgery 3

For suspected meniscal tears:

• Exercise therapy for 4-6 weeks is first-line treatment 3
• Surgery only indicated for severe traumatic bucket-handle tears with displaced tissue 3
• Degenerative tears: exercise therapy is first-line; surgery not indicated even with mechanical symptoms 3

For early OA:

• Exercise therapy, weight loss if overweight, education, and self-management programs 3

Step 5: Consider Aspiration if Effusion Present

• Ultrasound or fluoroscopy-guided aspiration can evaluate for crystals or low-grade infection 4, 5
• Use ICD-10 codes M25.462 (effusion, left knee) and M25.562 (pain, left knee) to justify the procedure 5

Critical Pitfalls to Avoid

• Do not order MRI without recent radiographs first (within past year) unless there are contraindications to radiography 4, 5
• Do not assume knee pathology without excluding referred pain from hip or lumbar spine, particularly if knee radiographs are unremarkable 4, 5, 7
• Do not rush to surgery for degenerative meniscal tears—conservative management with exercise is first-line even with mechanical symptoms 3
• Do not obtain CT scan if both radiographs and MRI are normal—CT provides no additional diagnostic value in this scenario 7