Imaging description

When visible on radiographs, transient osteoporosis and osteonecrosis are quite distinct. Transient osteoporosis of the hip demonstrates radiographically marked osteoporosis. The osteoporosis can be so profound that the subchondral cortex of the head becomes nearly invisible. In contradistinction, when osteonecrosis is visible radiographically, it will have patchy areas of sclerosis or increased density (Figure 48.1). As the osteonecrosis progresses, areas of subchondral collapse and eventual osteoarthritis will develop.

In the earlier course of both of these diseases, the radiographs can be negative. However, MRI will be abnormal. Transient osteoporosis (or transient bone marrow edema of the hip, if osteoporosis never develops) demonstrates diffuse hypointense signal on T1-weighted images and diffuse high signal intensity on water-sensitive sequences (Figure 48.2). MRI of osteonecrosis of the hip may also demonstrate diffusely abnormal bone marrow signal that appears edematous; however, there should be a superimposed focus of infarct. This area of infarct will be seen as a large area of high T1-weighted signal surrounded by a serpentine border of low signal intensity (Figure 48.3). This border may also exhibit the double-line sign of adjacent marked high and low signal on T2-weighted images.