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Bone age is the degree of a person's skeletal development. In children, bone age serves as a measure of physiological maturity and is used to aid in the diagnosis of growth abnormalities, endocrine disorders, and other medical conditions. As a person grows from fetal life through childhood, puberty, and finishes growth as a young adult, the bones of the skeleton change in size and shape. These changes can be seen by x-ray techniques. A comparison between the appearance of a patient's bones to a standard set of bone images known to be representative of the average bone shape and size for a given age can be used to assign a "bone age" to the patient under review.

Bone age is distinct from an individual's biological or chronological age, which is the amount of time that has elapsed since childbirth. Discrepancies between bone age and biological age can be seen in people with stunted growth, where bone age may be less than biological age; similarly, an advanced bone age may be detected in a child growing faster than normal. A delay or advance in bone age is most commonly associated with normal variability in growth, but significant deviations between bone age and biological age may indicate an underlying medical condition that requires treatment. A child's current height and bone age can be used to predict adult height. Other uses of bone age measurements include aiding in the diagnosis of medical conditions affecting children, such as constitutional growth delay, precocious puberty, thyroid dysfunction, growth hormone deficiency, and other causes of abnormally short or tall stature.

In the United States, the most common technique for estimating a person’s bone age is to compare the patient’s x-ray of their left hand and wrist to a reference atlas containing x-ray images of the left hands of children considered to be representative of how the skeletal structure of the hand appears for the average person at a given age. A radiologist specially trained in estimating bone age assesses the patient’s hand and wrist bones for their amount of growth, shape, size, and other features. The image in the reference atlas that most closely resembles the patient’s x-ray is then used to assign a bone age to the patient. Other techniques for estimating bone age exist and include x-ray comparisons of the bones of the knee or elbow to a reference atlas and magnetic resonance imaging approaches.

Evaluation of the bones of the hand a wrist
The most commonly used method is based on a single x-ray of the left hand, fingers, and wrist. A hand is easily x-rayed with minimal radiation and shows many bones in a single view. The bones in the x-ray are compared to the bones of a standard atlas, usually "Greulich and Pyle".

Tanner-Whitehouse Method
A more complex method also based on hand x-rays is the "TW2" or the "TW3" method (TW = Tanner Whitehouse) 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.

Lamparski (1972) utilized the cervical vertebrae and found them to be as reliable and valid as the hand-wrist area for assessing skeletal age. He developed a series of standards for assessment of skeletal age for both males and females. This method has the advantage of eliminating the need for an additional radiographic exposure since the vertebrae are already recorded on the lateral cephalometric radiographic.

Hassel & Farman (1995) developed an index based on the second, third, and fourth cervical vertebrae (C2, C3, C4) and proved that atlas maturation was highly correlated with skeletal maturation of the hand-wrist. Several smartphone applications have been developed to facilitate the use of vertebral methods such as Easy Age.

Height prediction
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. These tables, the Bayley-Pinneau tables, are included as an appendix in the Greulich and Pyle atlas.

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.

Clinical application of bone age readings
For the average person with average puberty, the bone age would match the person's chronological age. In terms of height growth and height growth related to bone age, average females stop growing taller two years earlier than average males. Peak height velocity (PHV) occurs at the average age of 11 years for girls and at the average age of 13 years for boys. A girl has reached 99% of her adult height at a bone age of 15 years and has a small amount of height growth left from this point on. A boy has reached 99% of his adult height at a bone age of 17 years and has a small amount of height growth left from this point on. When the bone age reaches 16 years in females and 18 years in males, growth in height is over and they have reached their full adult height.

There are exceptions with people who have an advanced bone age (bone age is older than chronological age) due to being an early bloomer (someone starting puberty and hitting PHV earlier than average), being an early bloomer with precocious puberty, or having another condition. There are also exceptions with people who have a delayed bone age (bone age is younger than chronological age) due to being a late bloomer (someone starting puberty and hitting PHV later than average), being a late bloomer with delayed puberty, or having another condition.

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. Those with an advanced bone age typically hit a growth spurt early on but stop growing at an earlier age. Consequently, when a naturally short child has an advanced bone age, it stunts their growth at an early age leaving them even shorter than they would have been. Because of this, those who are short with an advanced bone age, need medical attention before their bones fully fuse.

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. Bone age may be significantly advanced in genetic overgrowth syndromes, such as Sotos syndrome, Beckwith-Wiedemann syndrome and Marshall-Smith syndrome.

Bone maturation is delayed with the variation of normal development termed Constitutional delay of growth and puberty, but delay also accompanies growth failure due to growth hormone deficiency and hypothyroidism.

Recent studies show that organs like liver can also be used to estimate age and sex, because of the unique feature of liver. Liver weight increases with age and is different between males and females. Thus, liver can be employed in special medico-legal cases of skeletal deformities or mutilation.