Children are not small adults. Because this statement seems so obvious, we often spend little time and energy thinking about their differences. The unique properties of the pediatric musculoskeletal system result in distinct patterns of injury and establish an alternative natural history. These variations are fundamental to the differing orthopedic care and treatment options for children and adolescences as compared to adults.
At birth the body has about 270 bones, many of which are unossified cartilage. During skeletal maturation, which includes the process of ossification and bone modeling, some of the 270 bones fuse together, resulting in an adult skeleton with 206 distinct bones. Although adults are said to be skeletally mature, this should not be presumed to indicate a static state. The skeleton is a dynamic tissue, continuously renewing itself throughout our lifetime. As the remodeling process transpires, “older” bone is removed and replaced with “newer” bone, preserving structural integrity of the skeleton and contributing calcium and phosphate to the mineral balance of the body.
During the growing years (childhood through adolescence), there is a greater rate of new bone formation as compared to removal of the older bone. The imbalance, favoring bone formation, allows for the growing skeleton to gain bone density and strength. Peak bone mass is usually achieved by a person’s mid-20s, establishing the best-case scenario for lifetime bone health. Beyond genetic determinants, there are numerous environmental influences on bone mass attainment. In addition to a healthy diet, regular weight-bearing and resistance exercises are also beneficial to building and maintaining a strong skeleton.
While bone remodeling is an ongoing maintenance process, bone growth and modeling is exclusive to children and adolescents. In addition to accrual of bone mass, the bones of the immature skeleton experience growth in both their diameter (or width) and length. Appositional bone growth is responsible for the increasing diameter of bones. This process continues in adults and is related to bone remodeling. In contrast, bone length does not continue to increase beyond adolescence, concluding when the physis, also known as the growth plate, closes. In other words, children’s bones have physis while adult bones do not.
The physis is a specialized cartilage structure within the bone, housing the cellular machinery responsible for longitudinal growth. Physiologic physeal closure marks the termination of longitudinal bone growth and is an expected event in skeletal maturation. The timing of growth plate closure varies by the specific bone and is influenced by genetic and environmental factors. On average, bone length does not increase after the mid-teens in females and late teens in males.
The presence of an active (or open) growth plate warrants special consideration when children sustain fractures. Because of ongoing bone growth, children’s fractures heal quickly and do not require perfect alignment at the time of healing. Instead, the presence of an open growth plate imparts the potential for the bone to straighten, or return to its expected shape, during the process of natural growth and modeling. This means that fractures requiring surgery in an adult do not necessarily require as intense of a treatment in a child.
While this is a distinct advantage for children, the presence of an open growth plate is also a liability. In some cases, injuries in skeletally immature patients produce a fracture involving the growth plate, known as a physeal fracture. This injury may disrupt the complex physeal structure and result in irreversible damage. In such circumstances—usually associated with more severe injuries accompanied by greater displacement and/or higher energy transfer to the injured area—the bone may grow abnormally, leading to complete cessation of growth from the injured growth plate. This situation, called premature physeal closure, can potentially cause a limb length difference.
Because an angular deformity of the limb can develop when there is continued but abnormal growth, it is critical that physeal injuries are recognized and respected. Ongoing surveillance imaging is often necessary to monitor for evidence of a developing deformity. Prompt identification allows for the full spectrum of available treatment options to be considered, whereas delayed identification may limit the potential interventions. Treatment of deformity after physeal injuries is a complex undertaking and depends on a multitude of factors, including the percentage of the growth plate that is damaged and the patient’s remaining skeletal growth.
