Asterisk Sign

The quest for an early radiologic imaging sign of avascular necrosis continued despite Alex Norman’s and Peter Bullough’s 1963 report ¹ of the crescent sign on plain radiographs and the development of radioisotope bone scanning (see Crescent sign). Wolfgang Dihlmann’s contribution ² to the quest was the description of an early computerized tomography (CT) sign—the asterisk sign—in 1982. He studied 71 patients and four gross specimens at the Institute of Roentgenology in Hamburg. He showed that the normal arrangement of the trabeculae in the femoral head was in the form of a small star or asterisk (Figure 1). ‘The image of the asterisk of the femoral head in CT is formed by physiologically thickened load-transmitting bone trabeculae.’ ² The appearance of this asterisk was altered in patients with avascular necrosis before they had any changes evident on plain radiographs (Figure 2). ‘The reduced mechanical loadability of dead bone trabeculae … leads to (micro-) fractures which disturb the arrangement of the trabeculae and alter the form of the asterisk.’ ² This CT sign is useful, however, magnetic resonance imaging (MRI) has supplanted CT scanning for the early diagnosis of avascular necrosis and this sign is not used during MRI interpretation. MRI allows direct visualization of the changes within the marrow and does not rely on changes in trabecular structure. Figure 1 Normal ‘asterisk’ pattern in femoral head. Reprinted from Dihlmann<xref ref-type="bibr" rid="ref2_2"> ² </xref>. CT analysis of the upper end of the femur. <italic>Skeletal Radiol.,</italic> 8, 251–8, with permission of Springer Verlag https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781003076568/34c29fd5-5662-4e8f-b58c-badd7fa5d724/content/fig2_1_B.jpg"/> Figure 2 Early stage of avascular necrosis. ‘Hyperdense spots and hyperdense streets running from the centre of the asterisk to the cortical bone, indicate the presence of [avascular necrosis]. The diagnosis … was first established by CT in this case. Reprinted from Dihlmann<xref ref-type="bibr" rid="ref2_2"> ² </xref>. CT analysis of the upper end of the femur. <italic>Skeletal Radiol.,</italic> 1982, 8, 251–8, with permission of Springer-Verlag https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781003076568/34c29fd5-5662-4e8f-b58c-badd7fa5d724/content/fig2_2_B.jpg"/>