Bone infarct is ischemic death of the cellular elements of the bone and marrow in metaphysis and diaphysis. Lesions in the epiphysis are called avascular necrosis (AVN). Presently the term osteonecrosis is accepted and used widely.
Causes: Idiopathic, Trauma, Idiopathic causes such as Legg-Calvé-Perthes disease, Renal transplantation, Increase in endogenous steroid levels, as in patients with Cushing syndrome, Collagen vascular disorders such as systemic lupus erythematosus (SLE), rheumatoid arthritis, and scleroderma, Hemoglobinopathies such as sickle cell disease and thalassemia, Hemophilia, Gaucher disease, Fabry disease, Infection, Pancreatitis, Pregnancy, Gout and hyperuricemia, Diabetes, Use of immunosuppressants and other drugs such as exogenous steroids, indomethacin, and phenylbutazone, Alcohol use, Dysbaric osteonecrosis, Radiation therapy, Arteritis.
Radiograph: Findings are characteristic in established case. Early stages radiograph is not of much use. In the epiphysis region, an arc like, subchondral, lucent lesion may be associated with areas of patchy loss of bone opacity intermingled with sclerotic areas and bone collapse. In the diametaphyseal region, a sheet like lucency of varying size is usually surrounded by shell-like sclerosis and/or calcification and periostitis. In flat or complex bones, patchy lucencies and sclerosis are often associated with bone collapse or fractures.
Steinberg has classified the radiologic appearance into 6 stages, as follows12 :
- Stage 0 - Normal findings are demonstrated.
- Stage I - The appearance may vary from normal to subtle trabecular mottling, but an isotopic bone scan or MRI shows abnormal bone.
- Stage II
- Stage IIa - Focal radiopacity is associated with osteopenia.
- Stage IIb - Radiopacity is associated with osteoporosis and an early crescent sign.
- Stage III
- Stage IIIa - An established crescent sign is associated with cyst formation.
- Stage IIIb - Mild alteration in the configuration of the femoral head is caused by a subchondral fracture, but the joint space is maintained.
- Stage IV - Marked collapse of the femoral head is demonstrated with an associated acetabular abnormality.
- Stage V - Joint space narrowing is demonstrated with changes of secondary osteoarthrosis.
CT scans: Central or peripheral areas of reduced attenuation. Reformatted sagittal and coronal images show subchondral fractures and collapse of the articular surface. May show subtle trabecular irregularity associated with bone necrosis in early stages.
MRI: Ischemic bone changes become apparent in hematopoietic tissues on MRIs within 6-12 hours. MRI characteristics of bone infarction are patchy areas of low signal intensity on T1-weighted spin-echo images. Diffuse abnormal signal intensity may be present in osteonecrosis of the femoral head; these changes are reflected on both T1- and T2-weighted images.
The most characteristic appearance is the double-line sign, which consists of a hyperintense inner ring and a hypointense outer ring, on T2-weighted MRIs. This finding reflects the reactive interface between ischemic and non ischemic bone.