Femoral head AVN represents ischemic injury of femoral head. By convention, the term avascular (ischemic) necrosis generally is applied to areas of epiphyseal or subarticular involvement, whereas "bone infarct" usually is reserved for metaphyseal and diaphyseal involvement. AVN has many causes (see below). The hip is the most common site. This process can be identified by conventional radiograps, CT, nuclear medicine study and MRI. A classic appearance of AVN on MRI is "double line" sign seen in up to 80% of cases.
AP
Lateral
Cor T1
Cor STIR
Sag T2 Fat Sat
59 yo with R hip pain.
Femoral head AVN represents ischemic injury of femoral head. By convention, the term avascular (ischemic) necrosis generally is applied to areas of epiphyseal or subarticular involvement, whereas "bone infarct" usually is reserved for metaphyseal and diaphyseal involvement. AVN has many causes including hemoglobinopathies, Cushing syndrome, and exogenous steroid use, alcoholism, pancreatitis, human immunodeficiency virus, Gaucher disease, Caisson disease etc. but trauma is the most common. It may also be idiopathic. The hip is the most common site, but the knee, shoulder, and carpal and tarsal bones can be affected. This process can be identified by conventional radiograps, CT, nuclear medicine study and MRI. MRI is more sensitive than computed tomography, scintigraphy, or conventional radiography.
The findings on conventional radiographs lags several months behind the time of injury. These findings include areas of radiolucency, sclerosis, bone collapse, joint space narrowing, and, in the femoral head, a characteristic subchondral radiolucent crescent sign. Some of these findings indicate early some late, irreversible stage of disease. Ficat staging system is commonly used to classify the radiographic appearance of AVN of the hip:
Stage 0: No clinical or radiographic findings.
Stage 1: Clinical symptoms of AVN are present but no radiographic findings.
Stage 2: Radiographicaly osteopenia, cystic areas, and sclerosis.
Stage 3: Radiographicaly flattening of the femoral head; a subchondral crescentic area of radiolucency also may be seen.
Stage 4: Secondary osteoarthritis of the hip with associated joint space narrowing.
MR imaging is reported to have a sensitivity of 97% and specificity of 98% in the diagnosis of AVN of the hip. It detects the bone marrow edema associated with early AVN, which is seen as decreased signal intensity with poorly defined margins on T1-weighted images. A classic appearance of AVN, the "double line" sign, occurs later in the disease process, after the start of osseous repair. This describes a focal area of high or intermediate signal intensity (white) that is surrounded by a rim of low signal intensity (black) on both T1 and T2 images. A high-signal-intensity line may represent hypervascular granulation and low-signal intensity line correlate with the reactive zone at the outer margin of a necrotic lesion. Double line sign is seen in up to 80% of cases.
The MR imaging findings of AVN of the hip may be classified according to a system proposed by Mitchell:
Class A lesion: Signal intensity characteristics analogous to those of fat that is, high signal intensity on T1-weighted images and intermediate signal intensity on T2-weighted images.
Class B lesion: Signal intensity characteristics that are similar to those of blood, which has high signal intensity on both T1- and T2-weighted images.
Class C lesion: Signal intensity properties that are similar to those of fluid that is, low signal intensity on T1-weighted images and high signal intensity on T2-weighted images.
Class D lesion: Signal is similar to that of fibrous tissue, which has low signal intensity on both T1- and T2-weighted images.
Class A signal intensity tends to reflect early disease, and class D signal intensity tends to reflect late disease.
Suggested reading:
Zurlo JV. The Double-Line Sign. Radiology. 1999; 212: 541-542.
Mitchell DG, Kressel HY, Rao VM, et al. The unique MRI appearance of the reactive interface in avascular necrosis: the double-line sign. Magn Reson Imaging 1987; 5(suppl 1): 41.
Mitchell DG, Rao VM, Dalinka MK, et al. Femoral head avascular necrosis: correlation of MR imaging, radiographic staging, radionuclide imaging, and clinical findings. Radiology 1987; 162: 709-715.
Print
Email
