When to Order Knee MRI After a Fall
Order knee MRI without IV contrast when radiographs are normal but the patient has persistent pain, focal tenderness, joint effusion, inability to bear weight, or suspected internal derangement—MRI is the next appropriate imaging study after negative X-rays to detect occult fractures, meniscal tears, ligament injuries, and bone marrow contusions that are invisible on radiographs. 1, 2
Initial Imaging: Start with Radiographs
Always obtain knee radiographs first in patients ≥5 years old presenting with acute knee pain after a fall 1. The American College of Radiology explicitly states that MRI is not routinely used as the initial imaging study for acute knee trauma 1.
When Radiographs Are Indicated (Ottawa and Pittsburgh Rules):
Apply clinical decision rules to determine if radiographs are needed 1:
Ottawa Knee Rule criteria (patients ≥18 years):
- Age ≥55 years
- Palpable fibular head tenderness
- Isolated patellar tenderness
- Cannot flex knee to 90°
- Cannot bear weight immediately after injury
- Cannot walk 4 steps in the emergency room 1
Pittsburgh Rule criteria:
- Age <12 years or >50 years
- Cannot take 4 weight-bearing steps 1
Common pitfall: Do not apply clinical decision rules if the patient has gross deformity, palpable mass, penetrating injury, prosthetic hardware, altered mental status, neuropathy, or history suggesting increased fracture risk—obtain radiographs regardless 1.
When to Proceed to MRI After Radiographs
Clear Indications for MRI:
Order MRI without IV contrast when radiographs show no fracture but any of the following exist 1, 2:
- Persistent pain with focal tenderness or effusion despite normal X-rays 1, 2
- Suspected occult fracture (especially in older patients with osteoporosis) 1, 2
- Suspected internal derangement (meniscal tears, ligament injuries) 1, 2
- Inability to bear weight with normal radiographs 1
- Mechanical symptoms (locking, catching, instability) 2
Age-specific consideration: In patients >50 years with minimal osteoarthritis (Kellgren-Lawrence grade 0-1), MRI altered management in 70% of cases, but provided no benefit in end-stage disease (KL grade 4) 3.
Special Populations:
Patients with history of osteoporosis or previous knee injuries: MRI is particularly valuable as bone marrow contusions and occult fractures are common but radiographically invisible 2, 4. Subchondral fractures were more prevalent in older patients (mean age 72.4 years) and those with ≥10 pack-year smoking history 3.
Skeletally immature children: MRI without IV contrast is appropriate after normal radiographs to evaluate suspected occult fractures or internal derangement 1.
What Knee MRI Detects That X-Rays Miss
Soft Tissue Injuries:
- Meniscal tears: Medial meniscus tears occur in 47% of all knee MRIs, increasing to 100% in severe osteoarthritis 5, 6
- Ligament injuries: ACL, PCL, and collateral ligament tears are invisible on radiographs 2, 6
- Tendon pathology: Patellar tendinopathy, quadriceps tendon injury 2
Osseous Pathology:
- Bone marrow contusions: Cause substantial morbidity but are radiographically occult 2, 4
- Occult fractures: Including subchondral insufficiency fractures in older patients 1, 3
- Osteochondritis dissecans: Particularly in adolescents 7
Other Findings:
- Articular cartilage abnormalities 7, 6
- Popliteal cysts (Baker's cysts) and their rupture 8, 6
- Joint effusion extent and synovitis 8, 6
Critical Pitfalls to Avoid
Never assume normal X-rays exclude significant injury—meniscal tears, ligament injuries, and bone marrow contusions are invisible on radiographs but cause substantial morbidity 2. The American College of Radiology explicitly recommends not skipping MRI if symptoms persist after negative radiographs 2.
Do not order MRI without obtaining radiographs first—62.2% of knee MRIs in one study were performed without recent radiographs, representing inappropriate utilization 5. Radiographs must always be the initial study 1.
Consider referred pain from hip or lumbar spine before attributing all symptoms to knee pathology, especially if radiographs are unremarkable 8. A thorough clinical examination including lumbar spine and hip assessment should be completed 8.
Not all MRI findings are symptomatic—particularly meniscal tears in patients >45 years may be degenerative and asymptomatic 8. MRI findings must be correlated with clinical presentation 5.