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