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A current outlook on bone metabolism and fracture healing

Issue: January 2016

ByGazi Huri, MD

ByMehmet Kaymakoglu, MD

ByAltug Yücekul, MD

ByEgemen Turhan, MD

ByMahmut Nedim Doral, MD

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Editor’s note: In this second part of the FORTE article by members of the Orthopaedics and Traumatology Department at Hacettepe University in Ankara, Turkey, they discuss principles of fracture healing and bone metabolism. Read part 1 of the article here and look for more articles from FORTE members in future issues of Orthopaedics Today Europe.

Bone is the only tissue in our body that repairs itself without scar tissue, which is a unique process because only some species are capable of regenerating specific tissue. As knowledge of this process expands with findings from new studies, the bone healing process appears to be both astonishing and complex.

Fractures heal either via direct or indirect healing. Open reduction and internal fixation with precise anatomical reduction is all that is required for direct healing to occur. When those conditions exist, direct fracture healing occurs via remodeling of lamellar bone and Haversian systems (intramembranous healing), and there is no callus formation during that process. The fracture unites through contact healing, in which osteoclasts form cutting cones that, in turn, form tunnels and cross the fracture line. Osteoblasts then fill in the tunnels with newly formed bone. Indirect healing is more common than other types of healing, and consists of both an endochondral and intramembranous pathway. Non-stable conditions are then followed by an indirect healing process, which produces callus formation.

The phases of fracture healing are inflammation, soft callus, hard callus and remodeling. During inflammation, hundreds of cytokines, growth factors and molecules work together to perform one of the rare regeneration processes of body. Should this process fail to create a new composition of tissue, it is called a nonunion. However, there are molecular aspects of bone healing that are important and about which new information is now being learned.

Stages of fracture healing

When a fracture occurs, blood vessels on both sides of fracture line are damaged, causing a hematoma at the fracture site and termination of the blood supply at the edges of fractured bone. During the inflammation stage, these edges develop necrosis, which is followed by an immediate cytokine flow. A long-standing cytokine flow has diverse effects on bone healing, whereas the fast, regulated secretion of pro-inflammatory molecules is critical to create an environment for a regenerative process. During this phase, macrophages, fibroblasts, multipotent MSCs, neutrophils, cytokines, such as tumor necrosis factor-alpha (TNF-α), interleukin (IL)-1, IL-6, IL-11 and growth factors, such as transforming growth factor-beta (TGF-β), vascular endothelial growth factor (VEGF), platelet-derived growth factor (PDGF), fibroblast growth factor (FGF), bone morphogenetic protein (BMP) and insulin-like growth factor (IGF)-1, are released in the hematoma. TGF-β and IGF-1 seem to be factors that organize this molecule-rich environment for callus formation.

TNF-α, IL-1 and IL-6 are responsible for chemotaxis, osteoblast differentiation and starting of angiogenesis. TNF-α is thought to be the most important factor in the acute inflammation phase and has a leading role in promoting progenitor cell differentiation. Studies into local administration of recombinant human TNF-α within the first 24 hours of a fracture show this speeds up the fracture healing process. Chemokine ligand or CCL-2, which was originally called monocyte chemoattractant protein-1 or MCP-1, neutrophil recruitment and the TNA-α signaling pathway are the main milestones for the bone healing process, especially during the acute phase of inflammation. It has also been shown that IL-6 is needed to start angiogenesis by promoting VEGF signaling in the acute inflammation phase.

Soft and hard callus formation

After recruitment of all molecules and cells, endochondral formation begins with the precipitation of TGF-β and intramembranous ossification is induced by BMP at periosteal sites. The structure of a soft callus consists of type II collagen and proteoglycan molecules, and accumulation of mesenchymal cells and angiogenesis are both important events during the callus formation phases. Soft callus formation is typically followed by hard callus formation so that bone regeneration progresses. As chondrocytes proliferate, they start to mineralize the extracellular matrix with the initiation of the RANKL/RANK/OPG pathway and macrophage colony-stimulating factor (M-CSF). Cartilaginous matrix is resorbed with the help of these two pathways to establish the infrastructure of calcified bone matrix.

Angiogenesis is essential for successful bone healing. VEGF is highly expressed by osteoblasts and hypertrophic chondrocytes at the fracture site. Periosteal vascularization of bone is also important. Bones and bone marrow receive about 10% of cardiac output, and the bone’s vascular supply enables it to undergo rapid growth and remodeling that is not possible with cartilage. Nutrient arteries within long bones are the leading source of the blood supply used in bone metabolism. Another source of blood supply to bone comes from periosteal arteries that can provide the whole blood requirement if damage occurs to the nutrition artery. This anatomical structure of vasculature shows the importance of soft tissue around the bone and its role in bone healing.

Bone remodeling takes place within small cavities formed by osteoclasts and it is induced by RANKL and M-CSF. Factors, such as IGF-1, TGF-β, BMPs, FGF and PDGF, are released during the bone remodeling phase of fracture healing. We know about the general effects of these factors from research in which specific changes were seen in animal models that were gradually genetically altered. Although some bone modeling research focuses on a specific gene and its overall effect, all the connections between cells and the interactions of molecules must be evaluated to fully understand this process. Researchers have noticed osteocytes play a key role in this network of connections. It has been thought the surface of an osteocyte is 400-fold larger than all the Haversian and Volkmann systems. Its biomechanical signaling mechanism works with Wnt-type/β-catenin signaling. Using its signaling system, bone, as a mechanosensitive organ, appears to decide where it will build itself and how much of it will be built and resorbed. This theory, called Wolff’s law, states that bones adapt to mechanical forces in healthy humans and animals. In the century since this theory was developed, we found new clues to how bones may actualize this remodeling mechanism. The well-accepted theory is the mechanotransduction theory, which states that remodeling depends on the canalicular signaling mechanism of osteocytes.

The Wnt-type/β-catenin signaling mechanism works through the low-density lipoprotein receptor-related protein-5 (LRP5) receptor, and is important for embryological development and the sensory mechanism. In LRP5 gene-deleted mice, bone regeneration is negatively affected. The down-regulation of Wnt-signaling is observed after osteoblastic cells are exposed to 15 minutes of stretching, which suggests this sensory mechanism may have a fast tolerance rate. This demonstrates that bones need recurrent stimuli for an adaptation process to occur and a simple non-recurrent force is unable to activate this mechanosensory mechanism and change the structure of bone. The Wnt-signaling pathway also is needed for osteogenic cell differentiation and is widely accepted as a factor that causes osteoinduction, just as BMPs do.

BMP molecules

BMPs are a subfamily of the TGF-β superfamily. Twenty subtypes of human BMP molecules have been described to date, and three subgroups of BMPs are osteogenic. The other subgroups of BMPs perform different tasks in tissues. In bones, BMPs are produced by multiple cells in bone tissue. Chondrogenesis, osteogenesis, angiogenesis and bone matrix are induced by this growth factor. Recombinant human BMPs (rhBMPs) are used clinically as osteoinducers. Both rhBMP-2 and rhBMP-7 are FDA-approved for use in spinal fusion surgeries. Govender and colleagues and Geesink and colleagues studied BMP use in a fracture site and found it accelerates the healing process and increases rates of successful healing. In addition to being a novel osteoinducer factor, BMP also can have adverse effects that include heterotopic ossification and oncogenesis. Furthermore, BMPs are expensive to use.

Sclerostin, another important molecule for bone metabolism and remodeling, is a glycoprotein produced by osteocytes. It binds to Wnt receptors and inhibits osteogenic differentiation and decreases bone production. Pre-clinical studies have been conducted into its monoclonal antibody, and results of clinical studies in osteoporotic patients have been encouraging. Sclerostin-based therapies may eventually be used in patients with osteoporosis and fractures.

Discussion

As more has been learned about bone metabolism and fracture healing, it is has become more difficult to describe and chart these processes. Bone undergoes several interactions with the rest of the body. Our understanding of the molecular basis of bone should consider the body as a whole, since numerous factors and molecule interactions may cause simple treatment models to fail during fracture management. We should be aware of biological treatment models that take advantage of the body’s supernatural capabilities to manage mechanisms that are ultimately complex. We now know it is important for orthopaedic surgeons to consider the soft tissue around the bone in their treatment of patients and be aware of new research in this area that will advance orthopaedic outcomes.

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• For more information:
• Mahmut Nedim Doral, MD, can be reached at email: mndoral@gmail.com.
• Gazi Huri, MD, can be reached at email: gazihuri@yahoo.com.
• Mehmet Kaymakoglu, MD, can be reached at email: kaymakoglumehmet@gmail.com.
• Egemen Turhan, MD, can be reached at email: dregementurhan@yahoo.com.
• Altug Yücekul, MD, can be reached at email: ayucekul@hotmail.com.
• Doral, Huri, Kaymakoglu, Turhan and Yücekul are at the Orthopaedics and Traumatology Department, Hacettepe University, 06230 Ankara, Turkey.

Disclosures: Doral, Huri, Kaymakoglu, Turhan and Yücekul report no relevant financial disclosures.