Bone age is the degree of maturation of a child’s bones. As a person grows from fetal life through childhood, puberty, and syndromes in children of early age growth as a young adult, the bones of the skeleton change in size and shape. At birth, only the metaphyses of the “long bones” are present. The long bones are those that grow primarily by elongation at an epiphysis at one end of the growing bone.
As a child grows the epiphyses become calcified and appear on the x-rays, as do the carpal and tarsal bones of the hands and feet, separated on the x-rays by a layer of invisible cartilage where most of the growth is occurring. As sex steroid levels rise during puberty, bone maturation accelerates. Pediatric endocrinologists frequently order bone age x-rays to evaluate children for advanced or delayed growth and physical development. These are interpreted by pediatric radiologists, physicians who are experts in using medical imaging for pediatric diagnosis and therapy.
X-ray of a hand, with automatic calculation of bone age by a computer software. The most commonly used method is based on a single x-ray of the left hand, fingers, and wrist. The bones in the x-ray are compared to the bones of a standard atlas, usually “Greulich and Pyle”. An atlas based on knee maturation has also been compiled. Bone age rating is a procedure well-suited for automation by computer. The main advantage is the elimination of the variability of rating between different human raters. Statistics have been compiled to indicate the percentage of height growth remaining at a given bone age.
By simple arithmetic, a predicted adult height can be computed from a child’s height and bone age. Separate tables are used for boys and girls because of the sex difference in timing of puberty, and slightly different percentages are used for children with unusually advanced or delayed bone maturation. In a number of conditions involving atypical growth, bone age height predictions are less accurate. For example, in children born small for gestational age who remain short after birth, the bone age is a poor predictor of adult height.
An advanced or delayed bone age does not always indicate disease or “pathologic” growth. Conversely, the bone age may be normal in some conditions of abnormal growth. Children do not mature at exactly the same time. Just as there is wide variation among the normal population in age of losing teeth or experiencing the first menstrual period, the bone age of a healthy child may be a year or two advanced or delayed. An advanced bone age is common when a child has had prolonged elevation of sex steroid levels, as in precocious puberty or congenital adrenal hyperplasia. The bone age is often marginally advanced with premature adrenarche, when a child is overweight from a young age or when a child has lipodystrophy. Those with an advanced bone age typically hit a growth spurt early on but stop growing at an earlier age.