What is the diagnostic approach for a patient with medial left knee pain 1 month after a low-energy fall?

Diagnostic Approach to Medial Left Knee Pain 1 Month After Low-Energy Fall

Begin with plain radiographs (anteroposterior and lateral views) as the initial imaging study, followed by MRI without contrast if radiographs are negative and symptoms persist, as this represents a subacute injury where soft-tissue pathology (meniscal tear, medial collateral ligament injury, or bone contusion) is most likely. 1, 2

Initial Clinical Assessment

Evaluate for specific clinical findings that guide imaging decisions:

• Assess ability to bear weight - inability to bear weight for 4 steps or persistent difficulty after 1 month suggests significant internal derangement requiring advanced imaging 1, 2
• Palpate for focal bony tenderness - specifically at the medial femoral condyle, medial tibial plateau, and patella to identify potential occult fractures 1
• Evaluate for joint effusion - presence of significant effusion at 1 month post-injury indicates ongoing pathology requiring further investigation 2, 3
• Test joint stability - apply valgus stress at 0° and 30° of flexion to assess medial collateral ligament integrity 2, 4
• Check for mechanical symptoms - locking, catching, or giving way suggests meniscal or ligamentous injury 1, 2

Initial Imaging: Plain Radiographs

Obtain radiographs even though the injury occurred 1 month ago, as they remain essential to rule out fractures and establish a baseline. 1, 3

Minimum views required:

• Anteroposterior view 2
• Lateral view with knee at 25-30° flexion 2
• Consider patellofemoral view if anterior knee pain is present 2

Next Step: MRI Without Contrast

If radiographs are negative, proceed directly to MRI without contrast, as 93.5% of acute knee injuries involve soft-tissue rather than osseous pathology. 1

MRI is indicated at this 1-month timepoint if any of the following are present:

• Persistent inability to fully bear weight after the initial injury period 2, 3
• Significant joint effusion on examination or radiographs 1, 2
• Mechanical symptoms suggesting meniscal injury (locking, catching) 2, 3
• Joint instability on valgus stress testing suggesting ligamentous injury 2, 3
• Persistent medial pain despite conservative management 5, 6

MRI is superior to CT for detecting bone marrow contusions, meniscal tears, and ligamentous injuries, which are the most common causes of persistent medial knee pain after low-energy trauma. 1

Specific Medial Knee Pathology to Consider

At 1 month post-injury with persistent medial pain, evaluate for:

• Medial meniscus tear - most common cause of persistent medial knee pain after trauma 1, 7
• Medial collateral ligament injury - particularly deep MCL injury, which can cause chronic pain even after grade I/II sprains 5, 4
• Bone marrow contusion - may persist for weeks and is best visualized on MRI 1
• Medial plica syndrome - synovial fold irritation that can mimic meniscal pathology 8
• Medial tibial crest friction syndrome - rare cause of refractory medial pain deep to the MCL 6

Alternative Imaging: CT

CT may be considered instead of MRI only if there is high suspicion for an occult fracture (persistent focal bony tenderness) and MRI is unavailable. 1

However, CT has significant limitations:

• Low sensitivity for meniscal injuries requiring subsequent MRI for preoperative planning 1
• Inferior to MRI for bone marrow abnormalities and soft-tissue evaluation 1
• Not appropriate as the sole advanced imaging study in this clinical scenario 1

Critical Pitfalls to Avoid

• Do not skip radiographs even at 1 month post-injury - they remain essential to rule out fractures and chronic changes 2, 3
• Do not delay MRI beyond 5-7 days if significant effusion, instability, or mechanical symptoms are present 2, 3
• Do not assume negative radiographs exclude significant pathology - soft-tissue injuries are far more common than fractures in this scenario 1
• Do not overlook deep MCL injury - this structure can cause persistent medial pain even after minor sprains and may require targeted injection therapy 5, 6
• Consider arthrocentesis if large effusion persists at 1 month to rule out infection or crystal arthropathy, though less likely given the trauma history 3