Bone regeneration: new findings and potential clinical applications.

Bone is a biologically privileged tissue in that it has the capacity to undergo regeneration as part of a repair process. Fracture healing is the most common and recognizable form of bone regeneration, but several other examples of bone regeneration have been observed in humans, suggesting that the ability to regulate bone regeneration as a therapeutic tool should be possible. Historically, efforts at limb lengthening have led to procedures for regenerating bone, such as the method of Ilizarov. This procedure, known as distraction osteogenesis, has applications in a variety of skeletal conditions, including the restoration of large skeletal defects, the transport of bone in cases of severe trauma with bone loss, and the correction of skeletal deformities. Fibrodysplasia ossificans progressiva is an example of how an abnormal metabolic condition can be viewed as evidence for the capacity of humans to regenerate large amounts of bone if the cellular and molecular signaling events are altered. Elucidation of the cellular and molecular basis for bone regeneration in humans - particularly the role of the human genome in relation to the expression of various growth factors and cytokines, such as the bone morphogenetic proteins - offers great potential for the treatment of orthopaedic conditions. Development of specific bone morphogenetic proteins as therapeutic substances to induce bone regeneration in patients is well under way. As methods for enhancing fracture healing, distraction osteogenesis, and other procedures are refined, the development of protein- and gene-based therapies for regulating bone formation should lead to a new era of orthopaedic practice.