Bone/Osteoid Producing Lesions

Abstract

There are many lesions that are associated with reactive new bone formation; this chapter predominantly covers those in which deposition of osteoid/bone matrix represents the primary pathological process. The key in recognizing these lesions is the identification of osteoid or woven bone (vs. lamellar bone) on the frozen section slide. Osteoid is the organic nonmineralized matrix of bone and, being predominantly composed of type I collagen fibers, appears homogeneously eosinophilic and almost keloid-like in nature. This matrix is almost always associated with osteoblasts within clear spaces or halos. Bone matrix is further classified as lamellar or woven depending upon the predominant fiber arrangement of its collagen. In lamellar bone, the bone collagen fibers are arranged in tightly packed stacks that are parallel to one another but run at slightly different angles so that the bone appears to be layered. Moreover, the osteoblasts/osteocytes within lamellar bone also run parallel to the collagen fibers. After about 3 years of age, normal compact (cortical) and cancellous (trabecular, spongy, and medullary) bone exclusively consist of lamellar bone. In contrast, woven bone is found in the fetal skeleton, in the growing parts of the skeleton in infants and adolescents, and in processes in which there is very rapid bone production secondary to neoplastic or nonneoplastic conditions. Accordingly, identification of lesional woven bone and its distinction from adjacent lamellar bone is crucial during frozen section evaluation. This is based on the fact that, in contrast to lamellar bone, woven bone is characterized by the random distribution of its collagen fibers and the irregular distribution of osteoblasts within it. Although the distinction between lamellar and woven bone can, for the most part, be made using regular bright-field microscopy, the process can be facilitated with the use of polarized light.