Bone Development

Skeletal maturity is a measure of development incorporating the size, shape and degree of mineralization of bone to define its proximity to full maturity. The assessment of skeletal maturity involves a rigorous examination of multiple factors and a fundamental knowledge of the various processes by which bone develops.

Longitudinal growth in the long bones of the extremities occurs through the process of endochondral ossification. In contrast, the width of the bones increases by development of skeletal tissue directly from fibrous membrane. The latter is the mechanism by which ossification of the calvari-um, the flat bones of the pelvis, the scapulae, and the body of the mandible occurs. Initial calcification begins near the center of the shaft of long bones in a region called the primary ossification center [1].

Although many flat bones, including the carpal bones, ossify entirely from this primary center, all of the long bones develop secondary centers that appear in the cartilage of the extremities of the bone. Maturation in these centers proceeds in a manner identical to that in the primary centers

Fig. 1. Schematic representation of endochondral bone formation. Skeletal maturity is mainly assessed by the degree of development and ossification of the secondary ossification centers in the epiphysis

with ossification of cartilage and invasion of osteoclasts and osteoblasts. The bone ossified from the primary center is the diaphysis, while the bone ossified from the secondary center is the epiphysis. As the secondary center is progressively ossified, the cartilage is replaced by bone until only a thin layer of cartilage, the epiphyseal plate, separates the diaphyseal bone from the epiphysis. The part of the diaphysis that abuts on the epiphysis is referred to as the metaphysis and represents the growing end of the bone. As long as the epiphyseal cartilage plate persists, both the diaphysis and epiphysis continue to grow, but, eventually, the osteoblasts cease to multiply and the epiphyseal plate is ossified. At that time, the osseous structures of the diaphysis and epiphysis are fused and growth ceases [1].

In the fetal phase of life, the principle interest in skeletal growth is associated with the diagnosis of prematurity. The end of the embryonic period and the beginning of the fetus is marked by the event of calcification, which begins at 8 or 9 weeks. By the 13th fetal week, most primary centers of the tubular bones are well-developed into diaphyses, and, at birth, all diaphyses are completely ossified, while most of the epiphyses are still cartilaginous. Ossification of the distal femoral epiphysis begins during the last two months of gestation, and this secondary center is present in most full term babies. Similarly, the ossification center for the proximal epiphysis of the humerus usually appears about the 40th week of gestation. On the other hand, the centers for the proximal epiphyses of the femur and tibia may not be present in full term infants, but appear in the first few months of life [2, 3].

After birth, the epiphyses gradually ossify in a largely predictable order, and, at skeletal maturity, fuse with the main body of the bone. Comparing the degree of maturation of the epiphyses to normal age-related standards forms the basis for the assessment of skeletal maturity, the measure of which is commonly called "bone age" or "skeletal age". It is not clear which factors determine a normal maturational pattern, but it is certain that genetics, environmental factors, and hormones, such as thyroxine, growth hormone, and sex steroids, play important roles. Studies in patients with mutations of the gene for the estrogen receptor or for aromatase enzyme have demonstrated that it is estrogen that is primarily responsible for ultimate epiphyseal fusion, although it seems unlikely that estrogen alone is responsible for all skeletal maturation [4].

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