Radiology · 1946 · 17 citations · 0 references
Osseous involvement occurs in only a small proportion of cases of echinococcus infestation, probably bout 1 per cent. The six-hooked embryo of the Taenia echinococcus reaches the bone by way of the arterial circulation, establishing itself in the interstices of the spongy tissue, usually in the most highly ascularized areas, as in the epiphyseal ends of the long bones. Microvesicles replace the medullary tissue, molding themselves to the contours of the bony spaces and diffusely infiltrating the bone. Since there is no intermixture of connective-tissue elements no adventitious membrane is formed, as in other parts of the body. I-Iydatid disease of the bone thus differs in two respects from hydatid disease elsewhere, namely (a) exogenous vesiculation and (b) absence of an adventitious membrane. “Hydatid cyst” of the bone is, therefore, a misnomer, not in accord with the facts, and should be replaced by the term osteohydatidosis. In the lungs and other viscera, the characteristic feature of hydatid disease is a large cyst containing the hydatid fluid and the germinal layer, surrounded by a fibrous capsule representing the reaction of the adjacent tissues. When the parasite lodges In bone, however, it is unable, because of the resistance of the tissue, to follow its usual mode of development and therefore assumes an exogenous type of vesiculation. The microvesicles arising in the walls of the primary vesicle advance eccentrically, taking on an independent existence and producing “granddaughter vesicles,” which invade the bony tissue. This process, so far as the bone is concerned, is in the nature of a mechanical effect and may continue silently for years without exciting any reaction such as an osteitis. Only later is there an associated necrosis of toxic or ischemic origin. As the bone is infiltrated, destroyed, and replaced by the exogenous hydatids (Fig. 1), the integrity of the cortex is not at first interrupted and the general configuration of the bone is unchanged. Eventually, however, with progressive invasion, the cortex also gives way, and extra-osseous signs of hydatidosis become evident. Among the most significant of these is the ossifluent abscess of hydatid origin. Once the periosteum is penetrated, the vesicles are no longer imprisoned within the confines of the bone, and growth assumes a more rapid pace. The adjacent tissues react in an attempt at encystment, and a common adventitious membrane is formed enclosing numerous vesicles of various size The abscess thus formed has no germinal layer, which should always suggest its bony origin. Growth is slow and may continue over a period of years. Just as there is an absence of clinical signs of osteohydatidosis over a long period, so there is an absence of radiologic manifestations of the bony changes. In general, the development may be said to comprise two stages.