Bone age

Bone age is distinct from an individual's biological or chronological age, which is the amount of time that has elapsed since birth. 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, a bone age that is older than a person's chronological 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 assisting 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 an x-ray of the patient's 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 paediatric radiologist specially trained in estimating bone age assesses the patient's x-ray for growth, shape, size, and other bone 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, including x-ray comparisons of the bones of the knee or elbow to a reference atlas and magnetic resonance imaging approaches.

Measurement techniques

Estimating the bone age of a living child is typically performed by comparing images of their bones to images of models of the average skeleton for a given age and sex acquired from healthy children and compiled in an atlas. Features of bone development assessed in determining bone age include the presence of bones (have certain bones ossified yet), the size and shape of bones, the amount of mineralization (also called ossification), and the degree of fusion between the epiphyses and metaphyses. The first atlas published in 1898 by John Poland consisted of x-ray images of the left hand and wrist. Since then, updated atlases of the left hand and wrist have appeared, along with atlases of the foot and ankle, knee, and elbow. An alternative approach to the atlas method just described is the so-called "single-bone method" where maturity scales are assigned to individual bones. Here, a selection of bones are given a score based on their perceived development, a sum is totaled based on the individual bone scores, and the sum is correlated to a final bone age.

Evaluation of the bones of the hand and wrist

The two most common techniques for estimating bone age are based on a posterior-anterior x-ray of a patient's left hand, fingers, and wrist. The reason for imaging only the left hand and wrist are that a hand is easily x-rayed with minimal radiation and shows many bones in a single view. Further, most people are right-hand dominant and the left hand is therefore less likely to be deformed due to trauma. Finally, only the wrist and hand are imaged out of a desire to minimize the amount of potentially harmful ionizing radiation delivered to a child.

Greulich and Pyle atlas

In the United States, bone age is usually determined by comparing an x-ray of the patient's left hand and wrist to a set of reference images contained in the Greulich and Pyle atlas. Drs. William Walter Greulich and Sarah Idell Pyle published the first edition of their standard reference atlas of x-ray images of the left hands and wrists of boys and girls in 1950. The Greulich and Pyle atlas contains x-ray images of the left hands and wrists of different children deemed to be good models of the average appearance of the bones of the hand at a given age. The atlas has a set of images arranged in chronological order by age for males ranging from 3 months to 19 years and for females ranging from 3 months to 18 years in varying intervals of 3 months to 1 year.

Images in the Greulich and Pyle atlas came from healthy white boys and girls enrolled in the Brush Foundation Study for Human Growth and Development between the years 1931 and 1942.

To assign a bone age to the patient under review, a radiologist compares the patient's hand and wrist x-ray to images in the Greulich and Pyle atlas. Assessment of the carpals, metacarpals, and phalanges are used to find the closest match in the atlas; the chronological age of the patient in the atlas becomes the bone age assigned to the patient under review. If a patient's x-ray is found to be very close in appearance to two contiguous images in the atlas, then an average of the chronological ages in the atlas may be used as the patient's bone age, although some evaluators choose to interpolate the closest age while others report a range of possible bone ages.

A drawback associated with the Greulich and Pyle method of assessing bone age is that it relies on x-ray imaging and therefore requires exposing the patient to ionizing radiation. Further, there can be moderate levels of variability in the bone ages assigned to the same patient by different assessors. Other downsides are that the atlas has not been updated since 1959 and the images in the atlas were acquired from healthy white children living in Cleveland, Ohio in the 1930s and 1940s and therefore may not yield accurate bone age assignments when applied to non-white patients or unhealthy children.

Tanner-Whitehouse method

The Tanner-Whitehouse (TW) technique of estimating bone is a "single-bone method" based on an x-ray image of a patient's left hand and wrist. There have been two updates since the first publication of the TW method in 1962: the TW2 method in 1975 and the TW3 method in 2001. The TW methods consist of evaluating individual bones and assigning a letter grade to each bone based on its degree of maturation. Next, the scores for all evaluated bones are compiled into a sum, and that sum is correlated to bone age through a lookup table for males or females depending on the sex of the patient.

The bones considered in the TW3 method include the distal radius and ulna, the metacarpals and phalanges of the 1st, 3rd, and 5th fingers, and all of the carpal bones except the pisiform.

Knee maturation

An atlas based on knee maturation has also been compiled.

Hemiskeleton method

The bones in the hand a wrist in a newborn do not change much in the first year of life. However, most pediatric radiologists still use the Greulich and Pyle technique for estimating bone age in infancy. Alternative techniques for estimating bone age in infancy include tallying the number of ossification centers present in the left half of the infant's body requiring a hemiskeleton x-ray. One common method based on x-rays of the hemiskeleton is the Sontag method. This technique was created to avoid errors in estimating bone age thought to arise from focusing on only one area of the body. The Sontag method uses x-rays of all the bones and joints of the upper and lower limbs on the left side of the body. Then, a radiologist counts the number of ossification centers present and uses a chart to convert the sum of ossification centers to a bone age. There is a chart for males and another for females with possible bone ages ranging from 1 month to 5 years. Since most of the ossification centers counted using this technique appear early in life, this method is only valid for measuring bone age up to around 5 years of age.

Evaluation of cervical vertebrae

Lamparski (1972) used 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 the assessment of skeletal age for both males and females. This method has the advantage of eliminating the need for additional radiographic exposure in cases where the vertebrae have already been recorded on a lateral cephalometric radiographic. This method is called the Cervical vertebral maturation method.

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.