What is the best approach to diagnose and manage a patient with unilateral knee pain that is tender to touch?

Diagnostic and Management Approach for Unilateral Knee Pain with Tenderness

Begin with plain radiographs (AP, lateral, tunnel, and tangential patellar views) as the initial imaging study when focal tenderness is present, followed by MRI without contrast if radiographs are normal but symptoms persist. 1

Initial Clinical Assessment

Key clinical features to identify:

• Mechanism of injury: Determine if trauma occurred (fall, twisting, motor vehicle accident, or significant impact) versus insidious onset 1
• Focal tenderness location: Patellar tenderness, joint line tenderness, or fibular head tenderness guide differential diagnosis 1, 2
• Mechanical symptoms: Locking, catching, or giving way suggest meniscal tear or loose body 3, 4
• Effusion presence: Joint swelling indicates intra-articular pathology 1, 2
• Weight-bearing ability: Inability to bear weight for 4 steps or flex knee to 90 degrees increases fracture probability 2
• Age consideration: Patients >45 years with activity-related pain and <30 minutes morning stiffness suggest osteoarthritis (95% sensitivity, 69% specificity) 5

Imaging Algorithm

Step 1: Plain Radiographs (Initial Study)

Radiographs are the mandatory first imaging study when focal tenderness is present, regardless of trauma mechanism. 1

• Ottawa knee rules validate this approach: Order radiographs if age >55 years, focal patellar or fibular head tenderness, inability to bear weight for 4 steps, or inability to flex to 90 degrees 2
• Standard views required: AP, lateral, tunnel, and tangential patellar projections 3, 6
• Radiographs identify: Fractures, degenerative changes, osteochondral defects, effusions, and loose bodies 6

Step 2: MRI Without Contrast (If Radiographs Normal)

When radiographs are normal or non-diagnostic but symptoms persist, MRI without IV contrast is the next appropriate study. 1, 3

• MRI sensitivity/specificity: 88% sensitivity and 90% specificity for meniscal tears 3
• MRI detects: Occult fractures, meniscal tears, ligamentous injuries, articular cartilage damage, bone marrow lesions, and popliteal cysts 1, 3, 7
• MRI is superior to physical examination: More sensitive for ligamentous and meniscal damage, though slightly less specific 2

Differential Diagnosis by Clinical Pattern

Traumatic Presentation (Acute Injury)

• Meniscal tear indicators: Joint line tenderness (75% sensitive, 27% specific) and McMurray test (52% sensitive, 97% specific) 2, 5
• Ligamentous injury indicators: Lachman test for ACL (74% sensitive, 95% specific) is superior to drawer sign 2
• Fracture risk: Apply Ottawa rules; CT may be needed after radiographs to better delineate complex fractures 1, 6

Non-Traumatic/Chronic Presentation

• Osteoarthritis: Age >50 years, morning stiffness <30 minutes, crepitus, bony enlargement (89% sensitive, 88% specific) 2, 5
• Patellofemoral pain: Typically age <40 years, anterior knee pain during squatting (91% sensitive, 50% specific) 5
• Degenerative meniscal tear: Common in patients >40 years with OA; may be asymptomatic incidental finding 3, 5

Critical Pitfalls to Avoid

Do not skip radiographs before ordering MRI. Approximately 20% of patients with chronic knee pain undergo MRI without recent radiographs, leading to unnecessary costs and potential misdiagnosis 7

Consider referred pain sources before attributing all symptoms to knee pathology:

• Hip pathology: Obtain ipsilateral hip radiographs if knee radiographs unremarkable and clinical suspicion exists 1, 7
• Lumbar spine pathology: Evaluate for radicular symptoms if knee imaging is normal 7

Recognize age-related asymptomatic findings: The majority of people >70 years have asymptomatic meniscal tears; mechanical symptoms (locking, catching) distinguish symptomatic from incidental tears 3, 5

Avoid premature surgical referral for degenerative meniscal tears: Exercise therapy is first-line treatment even with mechanical symptoms; surgery is indicated only for severe traumatic bucket-handle tears with displaced tissue 5

Management Framework

Conservative Management (First-Line)

• Osteoarthritis: Exercise therapy, weight loss if overweight, education, and self-management programs 5
• Patellofemoral pain: Hip and knee strengthening exercises combined with foot orthoses or patellar taping 5
• Meniscal tears: Exercise therapy for 4-6 weeks; surgery only for severe traumatic tears with displacement 5

Advanced Interventions (When Conservative Fails)

• Intra-articular corticosteroid injections: Effective for OA when conservative measures insufficient 8
• Radiofrequency ablation of genicular nerves: Conventional and cooled RF shown effective for refractory OA pain 8
• Surgical referral: Consider for end-stage OA (minimal joint space, inability to cope with pain) after exhausting conservative options 5

Urgent Evaluation Required

Significant trauma (motor vehicle accident, knee dislocation) requires immediate radiographs and potentially CTA of lower extremity to evaluate for vascular injury, which occurs in 30% of posterior knee dislocations. 1