Differential Diagnosis for Knee Pain and Swelling
The diagnosis depends critically on whether the presentation is acute or chronic, with acute swelling suggesting trauma, infection, or inflammatory arthropathy, while chronic presentations most commonly indicate osteoarthritis, patellofemoral pain syndrome, or degenerative meniscal pathology. 1, 2
Acute Swelling (< 2 weeks)
Traumatic Causes
- Ligamentous injury (ACL/PCL tear): Immediate swelling within 2 hours, audible "pop," hemarthrosis on aspiration, and giving-way sensation strongly suggest acute ligament rupture 3
- Meniscal tear (traumatic): Occurs with twisting injury in patients under 40 years; McMurray test has 61% sensitivity and 84% specificity, while joint line tenderness has 83% sensitivity and 83% specificity 2
- Fracture: Requires radiographic evaluation if trauma history is present 4
Non-Traumatic Acute Causes
- Septic arthritis: Can develop at any age and requires urgent aspiration for diagnosis 1, 5
- Crystal-induced arthropathy (gout/pseudogout): More common in adults, diagnosed via joint aspiration 5
- Osteochondritis dissecans: Presents with pain, swelling, locking, catching, popping, or giving way; physical exam reveals tenderness, effusion, loss of motion, or crepitus 6
Chronic Swelling (> 2 weeks)
Most Common Diagnoses by Age
Patients ≥ 45 years:
- Osteoarthritis: Most likely diagnosis with activity-related joint pain and less than 30 minutes of morning stiffness (95% sensitivity, 69% specificity) 2
- Degenerative meniscal tear: Affects 12% of adults; often asymptomatic (majority of people over 70 have asymptomatic tears) 6, 2
Patients < 40 years:
- Patellofemoral pain syndrome: Lifetime prevalence ~25%; anterior knee pain during squatting is 91% sensitive and 50% specific 2
- Patellar tracking problems: More common in teenage girls and young women 5
- Knee extensor mechanism problems (Osgood-Schlatter, patellar tendonitis): More common in teenage boys and young men 5
Other Chronic Causes
- Popliteal (Baker's) cyst: Causes posterior knee swelling; often communicates with joint and may indicate underlying intra-articular pathology 6, 7
- Pes anserine bursitis: Overuse injury in active patients 5
- Medial plica syndrome: Overuse injury in active patients 5
Critical Pitfall: Rule Out Referred Pain
Before attributing all symptoms to knee pathology, you must evaluate for referred pain from the hip or lumbar spine, particularly when knee radiographs are unremarkable. 6, 8, 7
- Hip pathology: Commonly refers pain to the knee; examine hip range of motion and perform hip provocation tests 8
- Lumbar spine pathology: Can present with knee symptoms alone; perform straight leg raise and lumbar spine examination 6, 8
Diagnostic Algorithm
Initial Evaluation
- History specifics: Mechanism of injury, timing of swelling (immediate vs. delayed), mechanical symptoms (locking, catching), age, activity level 6, 1, 9
- Physical examination: Location of tenderness, effusion presence, range of motion, ligamentous stability testing, McMurray test, patellar tracking 6, 2
Imaging Strategy
- Initial radiographs (AP, lateral, sunrise/Merchant, tunnel views): Indicated for chronic knee pain, acute trauma, or suspected fracture/degenerative changes 6, 4
- Avoid premature MRI: Approximately 20% of chronic knee pain patients undergo MRI without recent radiographs 8, 7
- MRI indications: Only when surgery is being considered, pain persists despite adequate conservative treatment, or initial radiographs are normal but symptoms persist 4, 8
- Ultrasound: Useful for detecting synovial pathology and popliteal cysts 4, 7
Joint Aspiration
- Perform aspiration when acute swelling is present to aid diagnosis (rule out infection, crystal disease, hemarthrosis) and relieve pain 1
Age-Specific Diagnostic Considerations
Patients over 70 years: Bilateral structural abnormalities can be present with primarily unilateral symptoms; not all meniscal tears or degenerative changes seen on imaging are symptomatic 6, 7, 2
Patients 45-55 years: The likelihood of meniscal tear being present in either a painful or asymptomatic knee is not significantly different 6