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. The long bones include the femurs, tibias, and fibulas of the lower limb, the humeri, radii, and ulnas of the upper limb (arm + forearm), and the phalanges of the fingers and toes. The long bones of the leg comprise nearly half of adult height. The other primary skeletal component of height is the spine and skull.
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. As growth nears conclusion and attainment of adult height, bones begin to approach the size and shape of adult bones. The remaining cartilaginous portions of the epiphyses become thinner. As these cartilaginous zones become obliterated, the epiphyses are said to be "closed" and no further lengthening of the bones will occur. A small amount of spinal growth concludes an adolescent's growth.
Pediatric endocrinologists are the physicians who most commonly order and interpret bone age x-rays and evaluate children for advanced or delayed growth and physical development.
A more complex method also based on hand x-rays is the "TW2" method An atlas based on knee maturation has also been compiled.
The hands of infants do not change much in the first year of life and if precise bone age assessment is desired, an x-ray of approximately half of the skeleton (a "hemiskeleton" view) may be obtained to assess some of the areas such as shoulders and pelvis which change more in infancy.
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 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. Bone age may be significantly advanced in genetic overgrowth syndromes, such as Sotos syndrome, Beckwith-Wiedemann syndrome and Marshall-Smith syndrome.