The onset of ossification in the human calcaneus.

A (silver) radiographic and microscopic study of the onset of ossification in the calcaneus of 177 human fetuses between 49 and 150 mm C.-R. length has revealed the presence of two independent and developmentally different ossific sites. A lateral locus, intramembranous (parachondral) in origin and precocious in appearance, was observed in slightly over 16% of the fetuses examined between 93 mm (the first appearance of this bone) and 150 mm C.-R. It occupied the vascular connective tissue within the anterior portion of a distinct groove on the inferolateral wall of the cartilaginous calcaneus between the retrotrochlear eminence anterosuperiorly, and the lateral process of the tuber posteroinferiorly. A centrally situated, primary ossific centre, endochondral in origin, was detected in only 11% of the fetuses between 118 mm (the initial appearance of this centre) and 150 mm C.-R. It was situated in the centre of the anterior third of the cartilaginous calcaneus in relation to the sustenaculum tali medially and to a distinct cartilaginous prominence on its lateral surface. Only four fetuses possessed both ossific sites (lateral and central): at 122, 143, 145, and 150 mm C.-R., and in only one of these was continuity established between them. One fetus (122 mm) possessed two independent endochondral centres (superior and inferior).