RELATIONSHIP BETWEEN THE OSSIFICATION CENTER AND CARTILAGINOUS ANLAGEIN THE NORMAL HINDFOOT IN CHILDREN - STUDY WITH MR-IMAGING

OBJECTIVE. Although many reports have documented when ossification cen ters can first be visualized on radiographs, few studies have evaluate d the position of the ossification center within its cartilaginous anl age. In the skeletally immature child, the ossification centers of the tarsals are used to evaluate the positions of the tarsals and their i nterrelationships. It is convenient to assume that tarsal ossification begins in the center of its cartilaginous anlage and proceeds in a ra dial fashion; however, this may not be the case. Accordingly, we used MR imaging to evaluate the location of the ossification centers of the tarsals within their cartilaginous anlage in the mid and hindfoot in children. MATERIALS AND METHODS. MR studies of 69 feet in 40 children, 3 months to 7 years old (mean, 2.5 years), were reviewed retrospectiv ely. The location of the ossification center within its cartilaginous anlage and the percentage of ossification of the cartilaginous anlagen of the talus, calcaneus, cuboid bone, and navicular bone were determi ned from coronal and sagittal images. In the talus, the difference bet ween the orientation of the long axis of its ossification center and t he long axis of its cartilaginous anlage was measured on coronal and s agittal images. RESULTS. Early talar ossification was centered on the neck of the talus; the proximal aspect of the bone ossified last. The long axis of the talar ossification center and the long axis of its ca rtilaginous anlage differed in orientation. Early calcaneal ossificati on was centered on the distal two thirds of the cartilaginous anlage o f the calcaneus; the proximal aspect and the area of the subtalar join t ossified last. Early navicular ossification was centered on the cent ral or lateral third of the navicular cartilaginous anlage; the medial aspect ossified last. The ossification center of the cuboid bone was in the middle of the cuboidal cartilaginous anlage. CONCLUSION. Our re sults show that early ossification in the talus, calcaneus, and navicu lar bones does not begin in the center of the bones cartilaginous anla gen. The orientations of the long axis of the talar ossification cente r and the long mid axis of its cartilaginous anlage are different. The refore, part of the changes in the alignment of the tarsals seen on ra diographs with growth is due to ossification beginning and proceeding eccentrically within the cartilaginous anlage and not to a true change in the alignment of the tarsals. These data provide new information a bout the normal development of the child's hindfoot and midfoot.