Bone is one of the few tissues that can regenerate following an injury. Most others heal with scar. Although most fractures heal uneventfully following traditional therapy, systemic condition, poor fixation, and other local factors can lead to a delayed union or nonunion. Once the bone heals, it establishes its normal anatomic, structural, and mechanical properties. The outer layer of bone is surrounded by a thick nonbony connective tissue layer, the periosteum, which is itself surrounded in many locations by muscle. The outer cortex of bone is a well-organized structure on the gross and microscopic levels, providing much of the torsion and bending strength of the whole bone. There are relatively few cells in the cortex, primarily vascular and bone-forming cells. The inner region of the bone is the medullary canal, which is much more cellular, in which hematopoietic progenitor cells and their progeny also reside. The medullary bone provides much of the compressive strength of bone. Here we will review the basic biology of fracture repair, some of the methods to potentially improve repair based on recent preclinical work, and discuss the early phase clinical data that are available for some of these methods.

Fracture healing is a complex regenerative process initiated in response to injury, in which bone can heal by primary or secondary mechanisms. In primary healing, new cortical bone is laid down without any intermediate. This type of healing occurs when a fracture is rigidly fixed usually through certain types of surgery. In the more common secondary healing, immature and disorganized bone forms between the fragments, which is termed the callus.¹ Osseous repair progresses through three closely integrated and overlapping phases, during which multiple cell types and growth factors are activated in a coordinated manner.

In the initial phase of fracture repair, bleeding from the damaged tissues causes a hematoma at the fracture site, stopping blood loss and liberating growth factors and cytokines. Endothelial cells respond and increase their vascular permeability, allowing leukocytes, monocytes, and macrophages to reach the fracture site.² The blood supply is temporarily disrupted for a few millimeters on the bone, on either side of the fracture, producing local necrosis and hypoxia. It is likely that necrosis also results in the release of sequestered growth factors (eg, bone morphogenetic proteins, BMPs), which promotes differentiation of the surrounding mesenchymal cells into bone-forming cells.³ In the proliferative phase, undifferentiated mesenchymal cells aggregate at the site of injury, proliferate, and differentiate, presumably in response to growth factors produced by the response to injured tissues.⁴ This process involves
both intramembranous and endochondral ossification. Intramembranous ossification involves the formation of bone directly from committed osteoprogenitor cells and undifferentiated mesenchymal cells that reside in the periosteum, resulting in hard callus formation.⁵ During endochondral ossification, mesenchymal cells differentiate into chondrocytes, producing cartilaginous matrix, which then undergoes calcification and eventually is replaced by bone. The formation of primary bone is followed by extensive remodeling until the damaged skeletal element regains original shape and size^1,5,6,7 (Figure 1).

Secondary healing shares many factors with endochondral ossification during development, but differs in that injury and activation of inflammatory cells initiate the healing response. Damage to the extracellular matrix, factors released by platelets, proteins in the initial blood clot at the injury site, and activation of hemopoietic cells initiate the repair process in fracture healing. Increasing evidence suggests that an appropriate inflammation response is critical to successful fracture repair. Macrophage cells are particularly important in the early stages of the repair process as without these cells, fractures will not heal.^8,9 Many cells can produce new osteoblasts, including periosteal cells, muscle derived cells, blood vessel cells (in particular pericytes), and undifferentiated mesenchymal cells in the bone marrow (MSCs). These cells can initially differentiate into chondrocytes, or can differentiate directly into osteoblasts. Cartilage precedes bone formation in the initial or soft callus which bridges the bone gaps but lacks the degree of strength in a hard callous that is composed of bone. Blood vessels are critical to the repair process, and without appropriate vascularity, fractures also cannot heal. Osteoclasts are required for the remodeling phase, and these cells interact with bone-forming cells much like in physiologic bone homeostasis. Several growth factors and signaling proteins are activated in the early phases of repair, which stimulate new blood vessel formation, mesenchymal cell differentiation, and osteoblast formation. The mechanical environment also alters cell differentiation, with too little or too much mechanical stimulation of cells at the fracture site preventing bone healing. Many of these factors have the possibility to be modulated to improve poor bone headlining, but as the process is a coordinated response involving many factors and cells, no one protein or cell is completely responsible for the healing response.

When bone heals by secondary healing, a callus forms at the fracture site that is larger in diameter than the uninjured bone. The bone in the callus is initially weaker than the mature bone until it undergoes remodeling, a process that takes many months to complete. The larger diameter of the callus allows the bone as a unit to achieve strength similar to the uninjured bone. This is because having weaker material further from the center of the bone allows the bone as a unit to exhibit greater strength. This is a function of the moment of inertia, which is a measure of the distance of a material from the center of the bone, and predicts the torsional and bending strength of the bone.

When fracture healing is impaired, osteoblastic differentiation is inhibited, and undifferentiated mesenchymal tissue remains at the fracture site. In patients, this outcome results in delayed union, or nonunion, usually requiring additional surgery for successful fracture healing. Several risk factors have been associated with a delayed union. Patient-dependent risk factors include older age, diabetes, smoking, nutritional deficiencies, and the use of anti-inflammatory agents.^10,11 Other patient factors such as local infection and immune disorders can be deleterious to fracture healing. Local factors associated with a delayed union include the extent of soft-tissue injury, compartment syndrome, and certain anatomic locations, such as the tibia. Interestingly, bones with less muscle coverage are more apt to delayed union.¹² Because up to 5% of fractures go on to a delayed union, the development of therapies to reduce the rate of nonunion and to better treat nonunions is an area of intense investigation.

The mechanical environment will alter the ability of fractures to heal. A gap in bone held rigidly, or too much motion at a fracture site will result in a delayed union. How the local mechanical environment influences cells to repair a fracture varies with different stages of healing. Loading during the early stages of repair may impede stabilization of the injury site, whereas loading during matrix deposition and remodeling are ongoing may enhance stabilization.¹³ One method to mechanically stimulate cells during the repair process is through ultrasound. Low-intensity pulsed ultrasound will increase in cell proliferation, protein synthesis, collagen synthesis, membrane permeability, integrin expression, and increased cytosolic Ca(2+) levels as well as other increased indicators of bone repair in response to low-intensity pulsed ultrasound exposure.^14,15 Ultrasound also enhances angiogenesis mechanisms during bone healing.¹⁶ These are all changes that should enhance fracture repair. Despite these preclinical findings, clinical studies have shown variable results. A recent large randomized trial showed no difference in acute fracture repair by adding ultrasound,¹⁷ but a meta-analysis for nonunion showed a mild improved healing effect.¹⁸ Despite the disparate reported clinical findings, preclinical data suggest that ultrasound may be effective in inducing bone healing when combined with other therapies.¹⁹