Arachidonic Acid Metabolism
Arachidonic acid metabolism exerts complex control over many bodily systems, mainly involved in inflammation and immunity. The enzymes cyclooxygenase (COX)-1 and COX-2 metabolize arachidonic acid to prostaglandin G2 and prostaglandin H2
, which in turn may be converted to various prostaglandins. The classic COX inhibitors are the NSAIDs, which prevent the production of prostaglandin products. They are not selective and inhibit all phases of the inflammatory process, including fracture healing. Because COX-2 is specific to inflamed tissue, there is much less gastric irritation associated with COX-2 inhibitors. This selectivity of COX-2 does not seem to negate other fracture healing side effects of NSAIDs and should be avoided in patients at risk for delayed fracture healing.11
Wnt signaling pathways are a group of signal transduction pathways made of proteins that pass signals from outside of a cell through cell surface receptors to the inside of the cell. These pathways are activated by the binding of a Wnt protein ligand to a Frizzled cell surface receptor. This leads to regulation of gene transcription and conversion of undifferentiated MSCs into an osteoblastic lineage.12
is produced by the osteocyte and binds to a co-receptor group inhibiting the Wnt signaling pathway,13
which leads to decreased bone formation. Antibodies against sclerostin have demonstrated promising results in promoting bone formation and increased callus size.14
An antibody for sclerostin (romosozumab) is available and in clinical use (antisclerostin antibody). Its use has demonstrated increased bone growth and increased bone mineral density of the hip and spine,15
but its effect on fresh fractures and nonunions is not known.
FIGURE 2 A, Image depicts mature callous phase and shows removal of calcified cartilage with remodeling of early calcified cartilage to secondary woven bone. B, Histologic image (magnification ×50) shows islands of young cellular cartilage (black arrows) have become mineralized with interlocking trabeculae of new woven bone rimmed by osteoblasts (white arrows). This intramembranous bone formation has intertrabecular stroma composed of cellular fibrocollagenous tissue (white arrows) C, Image depicts remodeling phase and shows formation of the woven bone with remodeling to form mature lamellar bone with cortical end plates and cortical structure. D, Histologic image (magnification x50) shows remodeling phase (mature callous). Osseous trabeculae reveal numerous lacunar spaces with prominent osteocyte characteristics of late woven bone in which the lining osteoblast population is prominent (clear arrows). The woven bone is being converted to mature lamellar bone with remodeling into cortical end plates and restoration of the marrow component.
Early Callus Phase
The process of chondrogenesis begins as the early callus phase of healing approximately 7 to 10 days after injury. The migration and proliferation of chondroprogenitor MSCs and differentiation into chondrocytes with the expression of cartilage-specific matrix occur. The formation of a cartilage callus provides immediate mechanical stability to the fracture. Within 2 weeks, protein synthesis is complete and hypertrophic chondrocytes release calcium into the extracellular matrix to precipitate with phosphate ions17
and calcification begins. Once enough cartilaginous callus is formed, mineralization occurs with the removal of the proteoglycan inhibitors,18
which prevent mineralization (Figure 1
Platelet concentrate contains alpha granules that contain more than 30 bioactive proteins, many of which have a fundamental role in hemostasis and/or tissue healing. Of these, platelet-derived growth factor (PDGF) and transforming growth factor beta (TGF-β) appear to have the most potent effect on the soft callus stage of healing.19
TGF-β activates fibroblasts to induce collagen formation, endothelial cells for angiogenesis, chondroprogenitor cells for cartilage, and MSCs in an effort to increase the population of factors all crucial to the propagation of the soft callus phase of healing. PDGF also activates macrophages, resulting in débridement of the traumatic site, which then triggers a second source of growth factors released from the host tissues.19
Mature Callus Phase
Calcification of the cartilaginous callus matrix occurs as hypertrophic chondrocytes develop budding of their membrane structures to form vesicularized bodies. These matrix vesicles migrate to the extracellular matrix. The chondrocytes then undergo apoptosis and mineral is laid down on the callus surfaces.18
Neovascularization brings in perivascular cells and osteoblast progenitors, which infiltrate the calcified matrix surfaces.21
Chondroclasts invade along with bone-forming osteoblasts to begin the remodeling of the primary spongiosa (early calcified cartilage) to secondary spongiosa (woven bone), resulting in fracture union by approximately 4 to 5 weeks (Figure 2
As the formation of woven bone takes place, it remodels to mature lamellar bone and restores the original cortical end plates and cortical structure.22
Osteoblastic cells secrete factors that induce fully differentiated osteoblasts to express ligands that regulate the activity of osteoclasts. Receptor activator of nuclear factor kappa B ligand (RANKL) is essential for the development of osteoclast precursors22
because osteoclasts and osteoblasts play a vital role in normal bone remodeling. RANKL is strongly induced by fracture to increase its activity22
and demonstrates a peak in expression just before calcified cartilage removal begins (remodeling phase) (Figure 2
Estrogen inhibits the formation and activation of the bone-resorbing osteoclasts via suppression of RANKL signaling within the osteoclast. Selective estrogen receptor modulators are ligands for estrogen receptors that induce these receptors to affect intracellular transcriptional modulators. Raloxifene is such a class of selective estrogen receptor modulator, mimics estrogen’s suppression of RANKL, and thus can also inhibit osteoclast formation. Denosumab is the first RANKL inhibitor to be approved by the FDA. Denosumab inhibits this maturation of osteoclasts by binding to and inhibiting RANKL, thus preventing the binding to RANK.
Parathyroid hormone (PTH) is a polypeptide involved in the regulation of calcium and phosphate metabolism. Bone resorption is caused by osteoclasts, which are indirectly stimulated by PTH. Because osteoclasts do not have a receptor for PTH, PTH binds to the osteoblasts, stimulating them to increase their expression of RANKL. PTH inhibits their expression of osteoprotegerin, which binds to RANKL and blocks it from interacting with RANK. The binding of RANKL to RANK stimulates these osteoclast precursors to fuse, forming new osteoclasts.23
Although continual exposure to PTH leads to an increase in osteoclast activity and density, intermittent exposure stimulates osteoblasts more than osteoclasts and results in increased bone formation.
Teriparatide is a recombinant form of PTH identical to a portion of human PTH. It is an effective anabolic (ie, bone growing) agent24
used in the treatment of some forms of osteoporosis. Clinically, recombinant PTH has been approved by the FDA for its use in the treatment of osteoporosis and several recent clinical trials have demonstrated that daily systemic treatment with PTH increases bone mineral density and reduces fracture risk in patients with osteoporosis. It is also used off-label to speed fracture healing and treat nonunions and has demonstrated increased fracture callus volume, enhanced mechanical properties, as well as increased bone mineral density, bone mineral content, and total osseous tissue volume.25
Diphosphonates are widely used for the treatment of osteoporosis and the prevention of fragility fractures and have two phosphonate [PO(OH)2
] groups. Diphosphonates inhibit the digestion of bone by encouraging osteoclasts to (1) undergo cell death or (2) inhibit the ruffled border with subsequent dysfunction of resorption. Both mechanisms thereby slow bone loss.30
There are concerns that long-term diphosphonate use can result in oversuppression of bone turnover. It is hypothesized that microfractures in the bone are unable to heal and eventually unite and propagate, resulting in atypical fractures. This phenomenon has been seen most prominently in the proximal femur, often in patients using biphosphates for prolonged periods of time.33
In cases where there is concern of such fractures, teriparatide is potentially an alternative because of reducing damage caused by suppression of bone turnover.35
In cases of atypical femur fracture where teriparatide has been shown to be effective in combination with surgical management, it is proposed that PTH increases bone remodeling resulting in the removal of more densely mineralized bone replaced with new, less-compact normal mineralized bone.36
Morphology for Acute Fractures and the Development of Nonunion
Many factors that can affect fracture healing are associated with the characteristics of the injury, including the pattern of bony injury, location of the fracture, status of the soft-tissue envelope, extent of bone loss, and the degree of stability (strain) afforded to the fracture site.38
Motion at the fracture site will allow callus to form and intermittent shear stresses are thought to encourage ossification.39
The larger the stress, the greater the amount of callus formed up to an undefined limit of strain. Interfragmentary micromotion and low amounts of strain can stimulate both intramembranous and endochondral ossification.40
If, however, the motion is excessive, a high-strain environment will result in fibrous tissue proliferation and damage the neovascularity, resulting in a deficient inflammatory phase of healing. The ideal degree of mechanical stability has yet to be determined.41
Studies have shown that excessively rigid
fixation may paradoxically impair fracture healing42
via the inhibition of external callus formation, maintenance of a fracture gap aggravated by bone-end resorption, and excessive protection of the healing bone from normal stresses (stress shielding), all leading to adverse remodeling and nonunion. This has been clinically shown with the initial widespread use of locking plates and the overuse of locking screws, minimizing the micromotion and localizing the strain directed at the fracture site. Radiographs documented minimal callus formation and no cortical bridging. All these factors contributed to the high rate of plate failure and nonunion when this fixation montage was used.
Highly comminuted fractures with wide fracture gaps with increased strain fill these gaps with fibrous connective tissue, intervening areas of necrotic bone, and giant cells. This prevents neovascularization response and inhibits the chemotaxis and migration of MSCs. A wide diastasis makes any cellular interaction with circulating inductive factors very difficult (Figure 3
Atrophic nonunion develops commonly under these circumstances and these types of nonunion have fracture gaps interspersed with dense avascular fibrous tissue, secondary to bone loss or infection. These nonunion types typically occur in concert with mechanical instability and thus both the biologic potential and mechanical stability must be restored. A stimulus to initiate the inflammatory phase of fracture healing and revascularization is necessary along with providing mechanical stability. Biologic adjuvants are then required and are delivered either through the direct application of viable osteoblasts (bone graft) or by providing inductive factors to initiate the chemotaxis and proliferation of MSCs.
FIGURE 3 AP radiograph of the lower leg shows nonmodifiable factors of a fracture that went on to nonunion. Significant open soft-tissue injury, comminution, bone loss, and wide fragment diastasis are seen.
Basic principles of fracture fixation assert that comminuted fractures respond well to bridge plating and relative stability,43
whereas simple fractures should be anatomically reduced, compressed, and rigidly stabilized.41
Hypertrophic nonunion occurs when the biologic potential is intact and fracture callus is highly proliferative, but because of mechanical instability, large amounts of strain and excessive motion result in fibrous tissue at the fracture site. The treatment of this nonunion requires the augmentation of stability to allow fracture bridging to occur. No additional biologic stimulus is necessary.