The morphologic and pathogenetic bases of heterotopic bone formation in the extremities.

Heterotopic bone formations, involving the insertional portion of a tendon, fascia or ligament, were observed in the extremities. Histopathological examinations suggest that the connective tissue is transformed by direct metaplasia into bone tissue. The development of ossification is favored by the special structure of the connective tissue in these areas. Whereas in a small number of these cases a trauma could be identified as the competent cause, in the majority of the cases excessive or repeated minor stresses probably accounted for the ossification; in other cases with no obvious stresses a constitutional overreaction of the connective tissue had to be assumed as an explanation for the occurrence of ossification.