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THE NORMAL GROWTH PLATE

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The growth plate is an important site of involvement with many of the metabolic bone diseases. The physis, or growth plate, is composed of chondrocytes that are arranged in an orderly fashion. There are 3 histological and functional zones of the physis. The reserve, or germinal cell, zone is adjacent to the epiphysis and contains chondrocyte precursors. This is an important site of regulation of the cellular components of the physis. The proliferative and hypertrophic zones are characterized by the presence of chondrocytes that are organized into columns. The hypertrophic zone is subdivided into zones of maturation (columns of cartilage cells), degeneration, and provisional calcification. The transition between cartilage and bone occurs at the zone of provisional calcification and adjacent portion of the metaphysis. Metaphyseal vessels invade chondrocytes in the portion of the hypertrophic zone closest to the metaphysis. Mineralization of the longitudinal cartilaginous septa of the hypertrophic zone with hydroxyapatite crystal forms a dense but fragile layer, the zone of provisional calcification.

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The zones of primary and secondary spongiosa are in the metaphysis immediately adjacent to the zone of provisional calcification of the growth plate. In these layers, osteoblasts ensheath the calcified cartilage columns and produce layers of osteoid. Bone formation occurs by enchondral ossification. There is also resorption of calcified cartilage in the metaphyseal spongiosa. Appropriate osteoclastic activity in the metaphysis is essential for bone modeling. The perichondral region of the metaphysis, which is located at the margin of the physis, is composed of osteoblasts and chondrocytes; this provides circumferential growth of the physis during skeletal maturation.

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The growth plate and adjacent portion of the metaphysis are involved in various forms of skeletal system pathology in children. Metaphyseal infractions in abused children occur through the primary spongiosum of the metaphysis. Growth plate injuries, such as slipped capital femoral epiphysis, usually occur within the nonmineralized layer of the hypertrophic zone of the growth plate, adjacent to the zone of provisional calcification. The manifestations of rickets in the growth plate include disorganized cartilage cell growth in the hypertrophic zone, as well as deficient mineralization in the zone of provisional calcification and the metaphyseal spongiosa. Poisoning of osteoclasts in patients with lead intoxication causes failure of normal resorption of mineralized cartilage in the primary spongiosa.

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On radiographs, the zone of provisional calcification appears as a thin calcified band along the surface of the metaphysis (Figure 59-1). In young infants, this zone of provisional calcification is of equal or slightly greater density than adjacent spongy bone in the metaphysis. This layer is wider and denser in older children who are experiencing rapid growth. The remainder of the physis is radiolucent. The medullary portion of the metaphysis usually has similar radiodensity as that of the diaphysis. It has the lacy character of overlapping trabeculae.

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Figure 59–1

Normal growth plate.

A–C. Wrist radiographs of 3 different children, aged 1 month ...

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