What is the significance of a bone marrow infarct seen on a routine x-ray of the knee?

Significance of Bone Marrow Infarct on Routine Knee X-ray

A bone marrow infarct seen on routine knee X-ray is a marker for systemic avascular necrosis and requires further investigation for underlying risk factors and additional sites of involvement, despite generally having a good prognosis on its own. 1

Understanding Bone Marrow Infarcts

Bone marrow infarcts are areas of avascular necrosis occurring in the metaphyses or diaphyses of long bones, while the term osteonecrosis is typically used when these lesions occur in the epiphyses 1. They most commonly affect:

• Distal femur
• Proximal tibia
• Distal tibia

Radiographic Appearance

• On routine X-rays, bone infarcts initially appear as subtle, mottled, ill-defined radiolucencies in the diametaphyseal region 2
• As they mature, they develop typical high-density lesions in the center of the marrow cavity 1
• A periosteal reaction may be the first and only radiographic change 1
• Early infarcts can be easily missed or misinterpreted on plain radiographs 2

Clinical Significance

Symptoms and Prevalence

• Although often considered silent, bone infarcts cause symptoms in approximately 50% of cases 1
• The true prevalence is unknown 1
• Bone infarcts are multifocal in over 50% of cases 1
• When multifocal, they are usually accompanied by multiple foci of epiphyseal avascular necrosis 1

Associated Conditions

Bone marrow infarcts are frequently associated with underlying systemic diseases:

• Sickle cell disease
• Systemic lupus erythematosus
• Leukemia and lymphoma
• Post-renal transplantation
• Gaucher's disease
• High-dose systemic steroid use 3

Some cases may be idiopathic, but this is less common 2.

Diagnostic Approach

Imaging Recommendations

1. MRI is the gold standard for evaluation of bone marrow infarcts:

   • Shows typical features that obviate the need for other investigations 1
   • Demonstrates a central area with high or intermediate signal surrounded by a serpentine, thin, low-signal border 2
   • Has near 100% sensitivity for fracture detection and excellent capability for detecting bone marrow contusions 4

2. CT scanning may be considered when MRI is contraindicated:

   • Superior to knee radiographs in detection and classification of fractures 4
   • Provides excellent visualization of bony details 4
   • Less sensitive than MRI for soft tissue injuries 4

Differential Diagnosis

When evaluating bone marrow lesions, consider:

• Traumatic contusion and fractures
• Cyst formation and erosions
• Hematopoietic and infiltrated marrow
• Developmental chondroses
• Disuse and overuse
• Transient bone marrow edema syndrome
• Subchondral insufficiency fractures
• True osteonecrosis 5

Management Implications

Follow-up Recommendations

When a bone marrow infarct is identified on routine X-ray:

1. Perform a comprehensive evaluation for underlying risk factors 1
2. Consider MRI to better characterize the lesion 2
3. Consider whole-body MRI to screen for multifocal involvement, as bone infarcts are often markers for systemic avascular necrosis 3

Treatment Considerations

• Bone infarcts generally have a good prognosis and often resolve without intervention 6
• Conservative therapy is typically the first step:
  • Limited weight-bearing for 3-6 weeks on the affected side
  • Anti-inflammatory medications or analgesics for symptom management 6
• In non-responding cases or more advanced stages, minimally invasive procedures like subchondroplasty or core decompression may be considered 6

Important Caveats

1. Malignant transformation risk: While extremely low, there is a small risk of malignant transformation in bone infarcts 1

2. Misdiagnosis risk: Plain films of immature bone infarcts can be misinterpreted as aggressive lesions (malignancy or infection), potentially leading to unnecessary biopsies 2

3. Systemic implications: The presence of a bone marrow infarct should prompt investigation for other sites of avascular necrosis, which may have function-threatening effects 1

4. Evolution monitoring: Bone marrow infarcts evolve over time, from subtle signal abnormalities to the classic serpiginous appearance on MRI 3