Bone development and function: Genetic and environmental mechanisms

Bone manipulation is fundamental to orthodontics and dentofacial orthopedics. This clinically oriented overview of bone biology is related to the fundamental genetic and environmental mechanisms of bone development. The genome codes for growth factors, ischemic agents, vascular induction/invasion mechanisms, and mechanically induced inflammation. These biological mechanisms interact with the physical factors of diffusion limitation and mechanical loading to produce bone morphology. The patterning of facial ossification centers, the nasal cartilage, and the secondary facial cartilages is genetically determined. Growth against pressure gradients is accomplished by secondary cartilages: condylar cartilage, angular synchondrosis, symphyseal synchondrosis, nasal septum, and posterior palatal synchondrosis. Once a functional occlusion is established (∼12-18 months in humans) the symphyseal synchondrosis fuses, and the palatal synchondrosis reverts to a midpalatal suture. Thereafter, facial growth is dictated by oropharyngeal function and manifest at secondary growth sites: mandibular condyle, facial sutures, and subperiosteal bone modeling. The basic genetic mechanisms for bone adaptation are vascular invasion, inflammatory response, and localized cellular control agents (cytokines, growth factors). Approximately 90 genes are known to be involved in craniofacial development.