The appearance of a miniature vertebral body that conforms in its contours to its larger host is no mere radiological curiosity, but a sign of significant alteration in bone growth. The same importance may be attached to such an appearance in the innominate bones and the round bones of the carpus and tarsus.1
Usually, the "bone within a bone" appearance in the spine results from a previous severe systemic illness or intermittent chronic disease (Figs 1 and 2). The analogous finding in the faster-growing long bones (ie, the transverse metaphyseal lines of Park and Harris) are much more commonly observed but less important. Nonetheless, the pathological findings are identical in the spine, long bones, and innominate bones. Osteoblasts form a thin, transversely oriented bony template on the underside of the zone of proliferative cartilage that has stopped growing during the period of stress or illness. With recovery, chondroblastic and
Frager DH, Subbarao K. The 'Bone Within a Bone'. JAMA. 1983;249(1):77–79. doi:10.1001/jama.1983.03330250057032
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