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Development of the skeletal system during childhood and adolescence

This is an excerpt from Life Span Motor Development 8th Edition With HKPropel Access by Kathleen M. Haywood & Nancy Getchell.

Postnatal growth in bone length occurs at secondary ossification centers at the ends of bone shafts. A secondary center can also be called the epiphyseal plate, growth plate, or pressure epiphysis (figure 9.4). The epiphyseal plate has many cellular layers (figure 9.5) where cartilage cells form, grow, align, and finally erode to leave new bone in place. Bone is thus laid down at the epiphyseal plates to increase the length of the bone. The process of laying down new bone depends on an adequate blood supply. Any injury that disturbs this blood supply threatens the bone’s normal growth in length. In contrast to the long bones, small round bones such as those in the wrist and ankle simply ossify from the center outward.

Growth at the ossification centers ceases at different times in different bones. At the epiphyseal plates, the cartilage zone eventually disappears and the shaft, or diaphysis, of the bone fuses with the epiphysis. Once the epiphyseal plates of a long bone fuse, the length of the bone is fixed. Almost all epiphyseal plates are closed by age 18 or 19.

Recall that girls as a group mature faster than boys. It is no surprise, then, that the various ossification centers appear at younger chronological ages in girls than in boys. Likewise, the epiphyseal plates close at younger chronological ages in girls than in boys. For example, on average, the epiphysis at the head of the humerus closes in girls between 12 and 16 years but in boys between 14 and 19 years (Crowder and Austin, 2005; Hansman, 1962). Individuals, of course, have their own unique timing, and a group of children at the same chronological age can easily vary in skeletal age by 3 years or more. This demonstrates how variable maturity is in comparison to chronological age during the growth period.

While the long bones are growing in length they also increase in girth, a process called appo­sitional bone growth. Girth is increased by the addition of new tissue layers under the periosteum, a very thin outer covering of the bone, much like a tree adds to its girth under its bark.

There are also epiphyses at the sites where the muscles’ tendons attach to bones. They are called traction epiphyses. You might have heard of a familiar condition that occurs during the growth period in some youths—Osgood-Schlatter disease. This is an irritation of the traction epiphysis where the patellar tendon attaches to the shin bone below the knee. Pediatricians usually have youths with this condition refrain from vigorous (in particular, weight-bearing and jumping) activities for a time to prevent further irritation of the site. Overuse injuries to traction epiphyses during the growth period can threaten pain-free movement at a joint in later life. For example, a traction epiphysis near the elbow can be injured by repeatedly and forcefully pronating the forearm, as in throwing.

Ay et al. (2011) found that infants in the Generation R Study with low birth weight as well as those in the lowest weight groups at age 6 months tended to have a low bone mineral density measure at age 6 months. A positive relationship existed between postnatal growth in weight and bone mineral density as well as bone mineral content. Even infants who showed catch-up growth in weight during the first 6 weeks after birth were less likely to have low bone mineral density. As the Generation R Study continues, it will be interesting to observe the relationship between early levels of bone mineral density and content and later levels so we might learn whether adults are at risk for bone fractures based on their early growth patterns.

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