What is the differential diagnosis for an [AGE]-year-old patient with inferior knee pain after a fall and a normal X-ray?

Differential Diagnosis for Inferior Knee Pain After Fall with Normal X-Ray

The most likely diagnoses are soft tissue injuries including meniscal tears, ligamentous injuries (ACL/PCL/collateral ligaments), patellar tendon pathology, or occult fractures not visible on plain radiographs, and MRI without IV contrast is the appropriate next imaging study. 1

Primary Differential Considerations

Soft Tissue Injuries (Most Common with Normal X-Ray)

• Meniscal tears are highly likely, particularly if there is joint line tenderness (83% sensitivity, 83% specificity) or a positive McMurray test (61% sensitivity, 84% specificity) 2
• Ligamentous injuries including ACL, PCL, or collateral ligament tears should be suspected with mechanism of fall and instability 1
• Patellar tendinopathy or quadriceps tendon injury, especially if pain is localized to the inferior pole of the patella or tibial tubercle 3
• Bone marrow contusions or occult fractures that are radiographically invisible but detectable on MRI 1, 4

Age-Specific Considerations

• In adolescents: Consider Osgood-Schlatter disease (tibial tubercle apophysitis), Sinding-Larsen-Johansson syndrome (inferior pole patella apophysitis), or osteochondritis dissecans 3
• In adults ≥45 years: Degenerative meniscal tears associated with early osteoarthritis are common, even with minimal trauma 2

Recommended Diagnostic Algorithm

Immediate Next Step

• Order MRI knee without IV contrast as this is the ACR-recommended next imaging study after normal radiographs when pain persists, to evaluate for occult fractures, meniscal tears, ligament injuries, and bone marrow lesions 1, 5

Physical Examination Priorities

• Assess for joint effusion (presence indicates intra-articular pathology) 1, 4
• Test joint line tenderness (medial or lateral) to localize meniscal pathology 2
• Perform McMurray test (knee rotation with extension) for meniscal tears 2
• Evaluate ligamentous stability with Lachman, anterior/posterior drawer, and varus/valgus stress tests 6
• Check for mechanical symptoms such as locking or catching (suggests meniscal tear) 2

Critical Pitfalls to Avoid

• Do not assume normal X-rays exclude significant injury: Meniscal tears, ligament injuries, and bone marrow contusions are invisible on radiographs but cause substantial morbidity 1, 4
• Do not skip MRI if symptoms persist: The ACR explicitly recommends MRI without contrast as the next appropriate study after negative radiographs in patients with ongoing pain 1
• Do not order MRI with contrast: IV contrast adds no diagnostic value for traumatic knee injuries and increases cost and potential adverse effects 5
• In geriatric patients: Even "minor" falls can cause occult fractures or significant soft tissue injuries that require higher clinical suspicion 1

Management Considerations Based on Diagnosis

If MRI Confirms Soft Tissue Injury

• Initiate physical therapy emphasizing quadriceps and hip strengthening, neuromuscular education, and movement retraining 3
• Use NSAIDs or acetaminophen (≤4g/day) for pain control, though NSAIDs are more effective than acetaminophen for knee pain 3, 7
• Avoid glucosamine, chondroitin, and acupuncture as they lack evidence of effectiveness 3

Orthopedic Referral Indications

• Osteochondritis dissecans identified on imaging 3
• Significant ligamentous injury (complete ACL/PCL tear) 3
• Bucket-handle meniscal tears with displaced tissue requiring surgical intervention 2
• Recurrent patellar dislocations 3

Conservative Management for Most Cases

• Most meniscal tears respond to exercise therapy for 4-6 weeks, even with mechanical symptoms 2
• Surgery is not indicated for degenerative meniscal tears even with locking or catching 2