Bone age

Even among children who are twelve years old, some are well into puberty while others are just beginning. Height alone does not readily reveal these differences in maturity. Bone age involves looking directly at the bones and expressing which age group their appearance most closely resembles.

Within bones, areas of hard bone tissue called ossification centers appear one by one and gradually enlarge, and growth ends as the growth plates at the ends close. The timing varies somewhat from person to person, so children of the same chronological age may have different bone ages.

Usually, a single X-ray of the left hand and wrist is taken. The hand contains many small bones, and the way they appear and fuse with age is distinct, making it useful for assessing maturity. Comparing the image with standard reference material is a widely used approach. For example, it may be matched against an atlas containing representative images for each age, like a picture book (the Greulich–Pyle method), or each bone may be assigned a maturity score and the scores added together (the Tanner–Whitehouse method). Readings can vary slightly between assessors, so a single result is viewed as an indicator of an approximate position rather than an absolute number.

The resulting bone age serves as a reference for estimating how much more the bones may grow. If the ends of the bones are already largely fused, there is little growth potential left; if they are still open, more time for growth is considered to remain.

Genetics, nutritional status, hormones such as growth hormone and sex hormones, and the timing of puberty all play a role. When puberty begins earlier than in peers, bone age tends to be ahead of chronological age; when it begins later, bone age tends to lag behind. For this reason, bone age is generally interpreted alongside other information, such as the height growth curve, pubertal development, and parental height, rather than on its own.

Even among children who are 13 years old, some are nearly finished growing, while others have not yet begun their growth spurt. Skeletal maturity is the concept used to describe this difference. Developmental stage is assessed by how many ossification centers have appeared and how far the growth plates have closed. If the growth plates are still open and ossification is less advanced, there is considerable room for further growth, as well as scope for various growth-related processes to continue. Bone age expresses skeletal maturity in terms of age, while the methods below assess the same process on a different scale: stages.

Risser sign. This indicator divides the progression of ossification from front to back along the upper rim of the pelvic ilium (the iliac crest) into six stages, from 0 to 5. If no bone is visible, it is stage 0. Ossification progresses through one-quarter, one-half, three-quarters, and the full length, and stage 5 is assigned when the iliac crest apophysis has completely fused with the main body of the ilium. Because it can conveniently be assessed on the same X-ray as the spine, it has long been used as a standard in scoliosis care. However, the period of fastest height growth falls within stage 0, making fine distinctions within this interval difficult. The European and American grading systems also differ, so care is needed when comparing publications.

Sanders skeletal maturity staging. This method uses hand and wrist X-rays to classify maturity into 8 stages based on the width, shape, and degree of fusion of the ends of the finger bones (the epiphyses). It divides the growth-spurt interval grouped within Risser stage 0 into several stages and is reported to distinguish the period around peak growth velocity more clearly.

Supplementary indicators. The timing of the first menstrual period in adolescent girls, the point when height is increasing most rapidly (peak height velocity), and the stage of secondary sexual development (Tanner stage) are also considered. These do not necessarily align with skeletal indicators, so they are generally assessed together rather than relying on just one.

This concept comes up repeatedly in discussions of adolescent idiopathic scoliosis. This is because the usual clinical approach is to judge the likelihood of further spinal curve progression from the current curve size and the amount of growth remaining. A curve in a child with substantial growth remaining is more likely to progress, and the rate of progression slows markedly once skeletal maturity is reached. The same reasoning explains why brace treatment (bracing for scoliosis) is discussed specifically during the growth period. The landmark BrAIST study (Weinstein et al., 2013) also enrolled adolescents whose skeletal maturity stages indicated that growth remained.

This also relates to various bone-related processes during growth. Ongoing skeletal maturation means that the growth plates and epiphyses are still open. During a growth spurt, when height increases rapidly, the lengthening of bones may outpace adaptation by muscles and tendons. Osgood-Schlatter disease and discomfort related to sports overuse during growth are reported relatively frequently during this period. Once skeletal maturity is reached and the growth plates close, the setting for these processes also disappears.

"Doesn't an advanced bone age mean a child will grow taller?" It is closer to the opposite. If bone age is ahead of chronological age, it means that much less growth time remains, which often works against a taller predicted final height. Conversely, a delayed bone age means more time remains for growth. However, final height results from the combined effects of genetics, nutrition, and hormones, so bone age alone does not determine it.

"Can you tell from appearance?" It is difficult to tell from height, body shape, or flexibility alone, and imaging is needed to check the state of the growth plates.

• It is more helpful to view a child's growth through their height growth curve over time than through a number at a single point. Measuring and recording height regularly provides useful information for medical appointments.
• Bone age is simply information about the 'room left to grow.' It does not change the fact that eating well, sleeping well, and staying regularly active form the foundation for growth.
• If pain repeatedly occurs in the same place during exercise while a child is growing, consider it in relation to the timing of growth (overuse injuries in youth sports).