MRI Knee Without Contrast
Order an MRI of the knee without contrast to evaluate the soft-tissue swelling and rule out occult fracture, ligamentous injury, meniscal tear, and complications related to anticoagulation such as hemarthrosis or pseudoaneurysm. 1
Rationale for MRI as Next Step
With negative plain radiographs in the setting of acute knee trauma, MRI is the definitive next imaging modality according to ACR Appropriateness Criteria. 1 The guideline explicitly states that MRI should be the next study to evaluate for radiographically occult fractures and internal derangements of the acutely injured knee, with its superior contrast resolution and multiplanar capability proven highly accurate for detecting:
- Bone marrow contusions and occult fractures that X-rays miss
- Meniscal injuries (superior to CT)
- Ligamentous injuries including ACL, PCL, and collateral ligaments
- Soft-tissue pathology including hemarthrosis
Why Not CT?
While CT can detect some occult fractures and has been used after negative radiographs, MRI is explicitly superior to CT for bone marrow abnormalities, meniscal injuries, and ligamentous injuries. 1 CT without IV contrast also cannot evaluate for vascular complications. The guidelines are clear that MRI should be performed as clinically indicated after CT if CT is done first—so proceeding directly to MRI is more efficient.
Critical Consideration: Anticoagulation Complication
The anticoagulation status raises concern for hemarthrosis or even pseudoaneurysm, both of which require MRI for diagnosis. 2 A case report documents bilateral hemarthrosis from pseudoaneurysms in an anticoagulated patient that was diagnosed by MRI after failing conservative management—ultimately requiring transarterial embolization. The warmth, redness, and persistent swelling two weeks post-injury in an anticoagulated patient makes this a real concern that plain films and ultrasound cannot adequately assess.
What MRI Will Show
The MRI will definitively characterize:
- Extent of hemarthrosis (blood in joint)
- Bone marrow edema/contusions from the traumatic impact 3
- Occult fractures not visible on X-ray
- Meniscal or ligamentous tears that could explain ongoing symptoms
- Vascular abnormalities such as pseudoaneurysm (though MRA would be more specific if high suspicion)
- Soft-tissue masses or collections in the lower leg
Common Pitfalls to Avoid
- Don't assume extensive bruising is "just" from anticoagulation—the warmth and erythema around the patella could indicate underlying structural injury requiring intervention
- Don't delay imaging waiting for symptoms to resolve—occult fractures and ligamentous injuries need timely diagnosis to guide appropriate weight-bearing restrictions and treatment
- Don't order CT first unless MRI is contraindicated (pacemaker, severe claustrophobia, metallic foreign body)—you'll likely need MRI anyway for soft-tissue evaluation
Infection Consideration
While the warmth and erythema might raise concern for septic arthritis, the unremarkable labs from last night (presumably including inflammatory markers) and the clear trauma history make infection less likely. However, if the MRI shows a large effusion and clinical suspicion remains high, arthrocentesis could be performed for definitive exclusion.
The ACR guidelines rate MRI knee without IV contrast as "usually appropriate" (rating 8-9 out of 9) for this exact clinical scenario. 1