Bone age refers to the degree of physical maturity of a child’s skeleton, distinct from their chronological age (the actual time elapsed since birth). This measure of physiological maturity reflects how much growth a child has completed and how much remains. A delayed bone age means the bones appear younger on an X-ray than the child’s chronological age would suggest. It can indicate either a normal variation in development or an underlying medical condition affecting the rate of skeletal maturation.
Assessing Skeletal Maturity
Physicians determine a child’s bone age using a standardized process centered on a single X-ray image. Standard practice involves taking a radiograph of the non-dominant hand and wrist, an area containing numerous bones whose developmental stages are easily observed. The X-ray reveals the appearance and extent of the growth plates (epiphyseal plates), which are layers of cartilage where new bone is formed to lengthen the skeleton.
The resulting image is compared to established reference atlases containing X-rays of children whose skeletal maturity is known for specific ages. The two most common methods are the Greulich and Pyle atlas and the Tanner-Whitehouse method. The Greulich and Pyle method involves visually matching the patient’s X-ray to the closest standard image in the atlas to assign a skeletal age. In contrast, the Tanner-Whitehouse method assigns a score to multiple individual bones, such as the radius, ulna, and small hand bones, and then calculates the bone age from the total score.
The degree of fusion of the growth plates indicates skeletal maturity, signaling the approaching end of linear growth. Once the growth plates fully fuse, bone lengthening stops, and the child reaches their final adult height. The bone age assessment thus provides a proxy for the total growth time remaining, useful for evaluating growth abnormalities.
Common Causes for Delayed Bone Maturation
Delayed bone maturation frequently results from a normal developmental pattern known as Constitutional Delay of Growth and Puberty (CDGP). This common, non-pathological cause often has a genetic component, seen when parents were “late bloomers” in their own development. Children with CDGP typically have a bone age that lags their chronological age by two years or more, but they maintain a normal growth rate and are otherwise healthy.
A delayed bone age can also be a sign of an endocrine disorder, where hormone levels necessary for growth are insufficient. Hypothyroidism, caused by an underactive thyroid gland, is a condition where low levels of thyroid hormone impair normal bone and physical development. Similarly, Growth Hormone Deficiency (GHD), where the pituitary gland does not produce enough growth hormone, is strongly associated with a significant bone age delay, often exceeding two years.
Chronic systemic illnesses or nutritional deficiencies can also slow down skeletal maturation. Conditions causing malabsorption, such as celiac disease or inflammatory bowel disease, limit the uptake of necessary nutrients for bone development. Severe chronic diseases affecting the heart, kidneys, or liver can also interfere with the hormonal and metabolic pathways that govern bone growth.
Implications for Puberty and Final Height
The primary implication of a delayed bone age relates to the timing of puberty and the duration of the growth period. Skeletal maturity, rather than chronological age, is a better predictor of when a child will enter puberty and when their growth plates will ultimately close. A delayed bone age therefore suggests a corresponding delay in the onset of pubertal development.
For a child diagnosed with CDGP, the delayed bone age indicates a longer period available for growth. While these children may be shorter than their peers throughout childhood, they continue to grow after their peers have finished their growth spurts. This extended growth phase usually allows the individual to reach an adult height consistent with their genetic potential, calculated based on parental heights.
However, when the bone age delay is caused by a hormonal issue like GHD, the prognosis is different, and the child will not simply “catch up” without intervention. In these cases, the predicted adult height may be compromised if the underlying deficiency is not treated. The use of current height and bone age allows doctors to predict final adult height with relative accuracy, helping to distinguish between a benign delay and a condition requiring treatment.
Treatment Approaches
The management strategy for a delayed bone age is entirely dependent on the underlying cause identified during the medical evaluation. For children diagnosed with CDGP, the approach is often watchful waiting, coupled with careful monitoring of growth velocity and bone age progression. In these cases, the body is expected to initiate puberty and complete growth on its own natural, albeit delayed, timeline.
When a significant delay in the onset of puberty causes severe psychosocial distress, a short course of hormone therapy may be considered for boys. Low-dose testosterone injections administered over a period of months can safely initiate pubertal development and accelerate the growth rate. This intervention is primarily aimed at addressing the child’s emotional well-being and is not typically intended to increase their final adult height.
If the delayed bone age is due to a treatable deficiency, such as hypothyroidism or GHD, the focus shifts to correcting the hormonal imbalance. Administering the missing hormone, such as thyroid hormone replacement or growth hormone therapy, generally restores the child’s growth rate to a normal pace. Effective treatment of the underlying condition is often successful in normalizing the rate of bone maturation and allowing the child to maximize their height potential.