Figure 10b. Posttraumatic osteonecrosis of the talus in a 42-year-old man who subsequently underwent below-the-knee amputation. (a) Sagittal T1-weighted MR image through the ankle demonstrates a serpiginous low-signal-intensity line in the talar dome (arrows). (b) Corresponding sagittal fat-saturated T2-weighted MR image shows a high-signal-intensity line in the talar dome (arrow) that outlines an avascular segment. (c) Low-power photomicrograph (original magnification, x25; WHO stain [hematoxylin, phloxine, saffron, alcian green]) helps confirm AVN of the talus. The cartilage is thinned and is stained green by the alcian green dye, which helps identify mucopolysaccharides. Note the paler than expected green staining of the articular cartilage of the talar dome (white arrow). Near the ankle joint, a curvilinear band of fibrous tissue (black arrows) appears as a pale pink band outlining the area of necrosis; trabeculae are preserved in this area. On the external aspect of the arch of fibrosis, there are coarser, thicker trabeculae, indicating that this area of the bone is viable and is capable of reaction. (d) High-power photomicrograph (original magnification, x250; WHO stain) of a decalcified section shows viable tibia (Ti) on one side of the joint and necrotic talus (Ta) on the other. The bone trabeculae on the viable (tibial) side show smooth endosteal contours (white arrows); the adipose tissue of the bone marrow shows a fine filiform pattern (white arrowheads). These findings contrast with the necrotic (talar) side of the joint: Here, the cartilage shows a more homogeneous pattern with loss of all cellular detail, the subjacent bone contains irregular and partly fragmented trabeculae (black arrows), and the bone marrow shows patchy eosinophilia (black arrowheads), which indicate that the fat has undergone degenerative change with saponification.