bone age

Bone age is the body's biological age as read from the skeleton rather than from a birth certificate. Two children born on the same day can have skeletons that look more or less mature, and that maturity is often a better guide to growth than calendar age.

It is usually estimated from a single X-ray of the left hand and wrist. As a child matures, the cartilage growth centers gradually turn to bone in a predictable order, so a radiologist compares the pattern to standard reference atlases and assigns the bone age that the image most resembles.

Bone age is so useful because it tracks the hormones that drive maturation. Sex steroids and thyroid hormone advance it, while growth hormone deficiency and hypothyroidism delay it. Comparing bone age to chronological age, and to remaining open growth plates, lets clinicians estimate how much growth is left and predict adult height.

A practical caveat is that bone-age reading carries some subjectivity and inter-reader variation, and the classic reference atlases were built from specific historical populations, so results are an informed estimate rather than an exact number. Still, an advanced or delayed bone age is a powerful clue when sorting out short stature, tall stature, or abnormal puberty.