What does a hypointense marrow signal in the right proximal femur on MRI indicate?

Hypointense Marrow Signal in Right Proximal Femur: Diagnostic Implications

A hypointense marrow signal in the right proximal femur on MRI most commonly indicates bone marrow edema, which may represent stress fracture, osteonecrosis, infection, or infiltrative bone marrow process requiring further evaluation to determine the underlying cause. 1

MRI Findings and Their Significance

MRI is highly sensitive for detecting bone marrow abnormalities due to its excellent soft tissue contrast. On MRI, normal bone marrow in adults appears hyperintense on T1-weighted images due to its fat content. When marrow signal becomes hypointense on T1-weighted images, this represents replacement of normal fatty marrow with:

• Fluid (edema)
• Cellular infiltration
• Fibrosis
• Blood products

Common Causes of Hypointense Marrow Signal

1. Stress Fracture/Stress Reaction

   • MRI shows linear T1 hypointense signal (fracture line) with surrounding bone marrow edema
   • Often seen in athletes, military recruits, or patients with osteoporosis
   • May present with groin or thigh pain that worsens with activity 1

2. Osteonecrosis (Avascular Necrosis)

   • Typically appears as a focal area of hypointense signal on T1-weighted images
   • Often has a characteristic "double-line sign" on T2-weighted images
   • Common in patients on corticosteroids, with alcohol abuse, or sickle cell disease 2

3. Infection (Osteomyelitis)

   • Presents as hypointense marrow signal on T1 with surrounding edema
   • May show enhancement with contrast
   • Associated soft tissue abnormalities are often present 1

4. Bone Marrow Infiltration

   • Seen in hematologic malignancies (leukemia, lymphoma, multiple myeloma)
   • Presents as diffuse or multifocal hypointense signal replacing normal marrow
   • May be an incidental finding requiring further workup 3, 4

5. Traumatic Injury

   • Bone contusion or occult fracture
   • Usually associated with a history of trauma

6. Charcot Neuroarthropathy

   • In diabetic patients with neuropathy
   • Shows bone marrow edema on fluid-sensitive sequences 1

Diagnostic Approach

Initial Evaluation

1. Correlate with Clinical History:

   • Presence of pain (acute vs chronic)
   • History of trauma
   • Underlying medical conditions (diabetes, cancer, osteoporosis)
   • Medication use (especially corticosteroids)

2. Review Plain Radiographs:

   • May be normal in early stress fractures or osteonecrosis
   • Sensitivity only 15-35% for occult femoral fractures 5

Additional Imaging Based on Clinical Suspicion

1. For Suspected Stress Fracture:

   • MRI without contrast is the gold standard (nearly 100% sensitivity) 1, 5
   • CT scan if MRI contraindicated (sensitivity 69-87%) 5

2. For Suspected Infection:

   • MRI with contrast to evaluate for abscess or synovitis
   • Consider nuclear medicine studies if hardware present 1

3. For Suspected Malignancy:

   • Complete MRI protocol with and without contrast
   • Consider whole-body imaging depending on findings 3

Management Considerations

Management depends on the underlying cause:

1. Stress Fracture:

   • Non-weight bearing for compression-type fractures
   • Surgical fixation may be needed for tension-type femoral neck fractures due to high risk of displacement 1, 5

2. Osteonecrosis:

   • Early diagnosis is crucial to prevent femoral head collapse
   • Treatment ranges from conservative management to core decompression or arthroplasty depending on stage 2

3. Malignancy:

   • Further workup including laboratory studies (CBC, SPEP)
   • Bone marrow biopsy may be indicated
   • Approximately 6% of patients with incidental abnormal bone marrow signal are diagnosed with malignancy 3

Common Pitfalls

1. Misinterpreting normal red marrow reconversion as pathologic (especially in young adults or patients with anemia)

2. Overlooking bilateral findings - up to 30% of atypical femoral fractures are bilateral 1

3. Failing to correlate with clinical symptoms - not all marrow signal abnormalities are symptomatic or clinically significant

4. Inadequate follow-up of incidental findings - abnormal marrow signal should prompt appropriate clinical evaluation 3

Key Points

• Hypointense marrow signal on T1-weighted MRI is abnormal in adults and warrants further investigation
• MRI is the most sensitive imaging modality for detecting bone marrow abnormalities
• Clinical correlation is essential for determining the significance of imaging findings
• Systematic evaluation including laboratory studies may be necessary to establish the diagnosis