The difference between a person’s chronological age and their skeletal maturity, or bone age, reveals important information about physical development. While chronological age is a fixed number based on the date of birth, bone age is a biological measure that varies significantly among individuals. This measurement is a valuable tool in pediatric medicine for assessing a child’s overall growth trajectory.
Chronological Age Versus Skeletal Maturity
Chronological age is the simple passage of time an individual has lived since their birth, calculated in years and months. This measure provides a universal standard of time but does not account for individual biological differences in growth and maturation. Two children celebrating their twelfth birthday, for instance, are chronologically the same age, but their bodies may be developing at very different rates.
Skeletal maturity, or bone age, is a biological indicator measuring the degree of development and ossification of a person’s bones. It is expressed as the age in years corresponding to the average skeletal development of a healthy peer group. The bones of a growing child contain specialized areas of cartilage called growth plates, or physes, which are responsible for all bone lengthening.
The appearance and eventual closure of these growth plates reflect a child’s physiological progress toward adulthood. Bone age is a more reliable index of biological maturity than chronological age because growth spurts and the timing of puberty are closely correlated with skeletal development. A bone age may be younger or older than the chronological age, suggesting a delay or acceleration in physical maturation.
How Bone Age is Determined
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 bones (radius, ulna, carpals, and phalanges) that display a wide range of ossification centers maturing sequentially. This variety provides a comprehensive snapshot of the skeletal development process.
A radiologist or pediatric specialist compares the image of the patient’s hand and wrist to a set of standardized reference images. The most commonly used resource in the United States is the Greulich and Pyle Atlas, which contains X-ray images of the hands of healthy children at various ages. The patient’s bone age is assigned by identifying the reference image that most closely matches the appearance of their bones.
Alternatively, the Tanner-Whitehouse (TW) method involves assigning a numerical score to the individual bones in the hand and wrist based on their specific developmental stage. The total score is then correlated with a corresponding bone age. Both atlas-based and scoring methods rely on evaluating the size, shape, and degree of fusion of the growth plates to determine the level of skeletal maturity.
Underlying Causes of Advanced or Delayed Bone Age
A difference between chronological age and bone age is often the first sign of an underlying medical or biological factor influencing a child’s growth. An advanced bone age, where the skeleton is maturing faster than expected, is frequently linked to conditions that cause an early or excessive presence of sex hormones. Precocious puberty, for example, causes a premature surge in estrogen or testosterone, which accelerates the growth plate fusion process.
Other endocrine conditions, such as congenital adrenal hyperplasia or hyperthyroidism, can also lead to an advanced bone age due to an excess of hormones affecting skeletal maturation. Children with obesity may sometimes exhibit a moderately advanced bone age, possibly due to hormonal factors produced by adipose tissue. An advanced bone age indicates that the child’s period of growth is likely to conclude sooner than their peers.
Conversely, a delayed bone age, where skeletal development lags behind chronological age, is seen in several different conditions. One of the most common is constitutional delay of growth and puberty, often referred to as being a “late bloomer,” which is a variation of normal development where the growth spurt and puberty occur later. This delay is usually benign and inherited.
Medical conditions that affect hormone production or nutrient absorption can also cause significant delays in bone age. Untreated hypothyroidism, involving insufficient thyroid hormone, severely impairs normal bone maturation. Likewise, a deficiency in growth hormone can result in delayed skeletal development and a corresponding lag in bone age.
Other Causes of Delayed Bone Age
Chronic illnesses, such as inflammatory bowel disease or malnutrition, can also slow down the body’s overall biological clock, including bone growth.
Why Bone Age Measurement Matters
Assessing a child’s skeletal maturity is a routine procedure in pediatric endocrinology because the result offers valuable prognostic and diagnostic information. The measurement allows physicians to predict a child’s final adult height with greater accuracy than relying on chronological age alone. This prediction is made by combining the current height, the bone age, and established growth charts.
The bone age assessment is also instrumental in diagnosing and monitoring various growth disorders. A significant discrepancy between the two ages often prompts a deeper investigation into potential hormonal imbalances, such as growth hormone deficiency or precocious puberty. Once a diagnosis is made, bone age is used to monitor the effectiveness of treatments, such as growth hormone therapy or medications designed to delay puberty.
In a child receiving growth hormone, a desired outcome is the acceleration of growth velocity without an excessive advance in bone age. The measurement also has applications in other fields, including orthodontics, where the timing of jaw and facial growth is important for treatment planning, and in forensic science, where it helps estimate the age of unidentified individuals. The bone age provides a functional timeline of the body’s physical progress, guiding medical decisions that affect a child’s future development.