Skeletal age, often called “bone age,” provides an estimate of a person’s physical maturity by examining the development of their bones. It is a measure of how far a child’s skeleton has progressed toward its adult form. This assessment reflects the biological timeline of growth, which is regulated by hormones and genetics. Determining skeletal age gives medical professionals insights into a child’s overall rate of development.
Understanding the Difference Between Skeletal and Chronological Age
Skeletal age is distinct from chronological age, which is simply the number of years and months passed since birth. Skeletal age reflects biological maturation through bone ossification. In many healthy children, these two ages align closely, meaning their bones are developing at a rate consistent with their actual age.
It is common for a child’s skeletal age to vary from their chronological age by a year or more without underlying medical concern. The difference tells doctors whether a child is maturing faster or slower than the average population. Skeletal maturation is determined by the appearance and eventual fusion of the growth plates.
The Process of Determining Skeletal Age
The standard procedure for assessing skeletal maturity involves taking a single X-ray, typically of the non-dominant hand and wrist. This area is chosen because it contains numerous small bones that undergo predictable changes in shape and structure throughout childhood and adolescence. The X-ray image allows doctors to visualize the growth plates, which are areas of developing cartilage located near the ends of the long bones.
On an X-ray, these growth plates, also known as epiphyseal plates, appear as dark lines because they are less dense than the surrounding mineralized bone. As a child grows, the appearance of these plates changes, becoming progressively thinner until they completely fuse with the main part of the bone. This process, called ossification, marks the end of bone lengthening. To assign a skeletal age, a doctor or radiologist compares the child’s X-ray to a standardized set of images in an atlas.
The most widely used reference is the Greulich and Pyle Atlas, which contains X-rays representing the average skeletal development for children of various ages. Another method, the Tanner-Whitehouse method, provides a score based on the maturity of individual bones in the hand and wrist. By matching the child’s X-ray features—such as the size, shape, and degree of fusion of the wrist and finger bones—to the atlas images, a doctor assigns a numerical “bone age” in years.
Why Doctors Measure Skeletal Age
Skeletal age assessment is a valuable diagnostic tool used to investigate and manage conditions related to growth and development. A primary application is predicting a child’s potential final adult height. By combining current height with skeletal age, doctors use specialized formulas to estimate how much growth remains before the growth plates close permanently.
The measurement is used extensively in pediatric endocrinology to evaluate the timing of puberty. A skeletal age that is significantly advanced can indicate precocious (early) puberty, while a delayed bone age may suggest delayed puberty. This information helps clinicians determine whether intervention is necessary to regulate the pace of development. Doctors also use bone age to diagnose and monitor various endocrine disorders, such as growth hormone deficiency or hypothyroidism.
For children undergoing treatment for growth disorders, serial bone age X-rays allow physicians to monitor the effectiveness of therapies, such as growth hormone administration. Orthopedic specialists also use this information to time surgical procedures that must coincide with specific phases of skeletal development. The assessment provides a biological roadmap that is often more informative for clinical decisions than chronological age alone.
Causes of Advanced or Delayed Bone Maturation
The rate at which bone maturation occurs is influenced by a complex interplay of hormones and other biological factors. An advanced skeletal age, meaning bone development is ahead of chronological age, can be caused by conditions that lead to an excess of growth-promoting hormones. This includes certain types of precocious puberty, where excessive sex hormones like estrogen cause growth plates to mature and close prematurely. Obesity is also a common non-endocrine factor associated with a mildly advanced bone age.
Conversely, a delayed bone age, where skeletal maturity lags behind chronological age, is often seen in conditions that involve hormone deficiencies. These can include untreated growth hormone deficiency or hypothyroidism, as both hormones are necessary for normal bone development. Chronic illnesses, such as inflammatory bowel disease or kidney disease, and prolonged malnutrition can also slow the rate of skeletal maturation. A significant delay of two years or more may indicate a need for further evaluation, even though a mild delay can be a normal variation known as constitutional delay of growth and puberty.