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The goal of this section is to introduce the basic building blocks for understanding the processes of bone healing. This includes an understanding of the importance of vascularity, the role of inflammatory cells and the immune complex within the healing environment, the role of progenitor cells that are released within or migrate to the area, the importance of the extracellular matrix and biochemical factors that influence healing.

Bone follows the same basic healing processes as other tissues. Immediately following the insult/bone failure, the immune system responds not only to remove damaged tissue but also to signal a number of cellular processes. The immune system is integrated with the osteoresponsive cascade as immune‐based cells, namely macrophages and monocytes, not only stimulate vascular, osteoblastic and osteoclastic responses, but can also form into osteoclastic cells [15].

At a cellular level, the biological and mechanical environments are interconnected. The process in which the mechanical environment influences cell processes and development is termed developmental mechanics [5]. Relatively unstable environments induce cellular mechanisms that can delay or inhibit healing and vice versa. There is an emerging field of study around the inflammatory and immune regulation of the fracture environment, particularly the role of osteal macrophages (osteomacs) and their influence on vascularity and both anabolic and catabolic processes in bone healing [15]. Cells become activated in the deranged environment caused by tissue damage and subsequently signal other cellular cascades to mobilize for healing.

Fractures result in immediate disruption of vascularity and, if complete, compromise stability. In cortical bone, the periosteum is also damaged resulting in further vascular disruption. In trabecular bone, vascularity and marrow homeostasis in the area are compromised. Failure of subchondral compacta can result in an articular fracture. All stimulate immediate cellular and cell signalling responses resulting in pain and inflammation, which influence subsequent healing [15].

In the haematoma phase of healing, bleeding occurs at the site, stimulating the coagulation cascade and formation of fibrin (Figure 6.1a). This usually occurs within the first 24 hours, and the fibrin becomes the initial matrix upon which inflammatory and progenitor cells will have an effect. Within 24 hours, inflammatory cells invade the area, first neutrophils and then monocytes and macrophages [15]. At this point, the environment can be altered markedly by case management and specifically by reduction, repair and reconstruction techniques. If a degree of instability remains, vascular integrity and hence oxygen tension are reduced, creating an environment conducive to a chondrocytic cell population [15]. If fibrocartilaginous matrix produced by chondrocytes improves the local strain environment, then vascular ingrowth principally from peripheral tissues follows [16] (Figure 6.1b). As the fracture gap fills with chondrocytes and fibrocartilage matrix, mechanical stability increases. Chondrocytes undergo hypertrophic differentiation that further increases stabilization, decreases strain and enhances neovascularization. The cartilage matrix is remodelled, local bone morphogenic proteins (BMPs) are stimulated and osteoblasts are recruited to form woven bone [17]. During this cascade, stability progressively increases, strains are reduced and the healing environment begins to enter the hard callus phase (Figure 6.1c). Woven bone is initially produced at the periphery and ultimately replaces the entire cartilaginous callus. As osseous callus matures, further improvement in local strains allows secondary remodelling that ultimately leads to a functional and usually more anatomically correct shape to the bone. In long bones, all phases are considered complete once the medullary cavity is reformed (Figure 6.1d). In the presence of continued instability, there is usually persistently compromised vascularity that allows formation of granulation and fibrous tissue only and results in a non‐union [18].

Vascularity strongly influences healing capability. Initially, the damaged environment becomes avascular and as chondrocytes thrive within a hypoxic environment this leads to their initial proliferation. As healing progresses, neovascularization brings nutrients essential to cellular optimization and minerals needed for hard callus formation. Oxygenation is key for conversion of soft to hard callus. Thus, the mechanical environment and degree of soft tissue damage influence vascularity, neovascularity and hence stability.

Immediately after fracture, inflammatory cells are released into the environment. This is stimulated by multiple factors, including neurovascular components (especially if the periosteum is involved) and local immune cells and factors that are embedded within the bone tissue. These cells can then stimulate a systemic response for migration of inflammatory cells to the site. The term ‘osteomacs’ has been introduced to describe the importance of the immune cells in regulating bone healing [19]. Polymorphonuclear cells remove debris, macrophages and monocytes stimulate osteoclastic and progenitor cells, and based on the integrity of the vascularity, chondrocytes and osteoblasts are activated.

Inflammation has a major impact on bone healing, and dysregulation of the inflammatory cascade can lead to increased resorption and decreased formation [15]. Studies in osteoimmunology have identified critical links between the immune system and bone healing [20]. The fact that haematopoietic stem cells and mesenchymal stem cells both reside within the bone marrow and share similar signalling factors is evidence of their integrated and coordinated function. Haematopoietic stem cells function along the monocytic–macrophagic–osteoclastic line, while mesenchymal stem cells are necessary for osteoblast formation. Cross‐talk between inflammatory and bone formation cells is necessary for healing, and the two lines interact through cell signalling to optimize repair [21]. As in other tissues, following an adverse event, macrophages and monocytes regulate a sequence of events to mitigate the insult and optimize the healing cascade. The cells remove damaged tissue, stimulate neovascularity and trigger healing by release of signalling factors.

Neutrophils are the first inflammatory cells to occupy a fracture site. They function to recruit monocytes and macrophages and to regulate the signalling factors that are necessary for the healing cascade to occur. Although neutrophils are necessary for healing, with severe trauma, ongoing inflammation can slow healing [22]. Overproduction of cytokines continually damages tissue and impairs vascularity. However, if inflammation is suppressed, particularly along the monocytic and macrophagic cell lines, then a decrease in healing signalling can occur [23]. Osteomacs are macrophages located within periosteum and endosteum that work in the local environment. Macrophages remove and remodel the fibrin matrix, while osteoclasts, differentiated from monocytes, remove bone fragments. In the presence of tissue damage, signalling factors from osteomacs recruit other inflammatory cells, increase vascularity and regulate mesenchymal stem cell migration and differentiation. Osteomacs also influence and regulate the remodelling cascade. In this manner, inflammatory cells and the immune system have essential anabolic effects on bone, and suppression of the inflammatory phase can negatively influence bone healing [24].

Progenitor cells can either be local in origin or can be produced systemically and migrate to the area. Trabecular bone has a vast marrow network with osteoprogenitor cells that can act locally. Cortical bone also contains progenitor cells that can be released and act locally. Osteoprogenitor cell migration and differentiation is regulated by the osteomacs. Osteoclasts are stimulated and released systemically, while osteoblasts can be triggered and recruited locally or systemically (Figure 6.2).

The extracellular matrix is essential for cells to have an influence on tissue. Damaged and non‐viable tissues must be debrided by the inflammatory cells to optimize the environment for repair. At the same time, clotting factors in cells induce fibrin formation that provides the initial framework on which cells and osteogenic factors can act. Matrix components, both mineralized and non‐mineralized, within bone allow cellular and biochemical functions necessary for fracture healing and also provide the foundation for which various repair techniques can be used. The characteristics of the extracellular matrix can positively or negatively influence cellular and biochemical factors within the environment. First, fibrin is formed at the site followed by type III collagen, proteoglycans and glycoproteins. Type III collagen can induce capillary proliferation and osteoprogenitor cell migration to the site. These influence mechanical integrity and cell signalling [10]. Type II collagen matrix is common with chondrogenic cell proliferation typical of endochondral ossification; although type I collagen is present in limited amounts during all phases of healing, it significantly increases with bone formation.

In the acute stages, inflammatory cytokines such as interleukins, tumour necrosis factor and prostaglandins are released from platelets and inflammatory cells, stimulating progenitor cells to migrate to the area of damage [15]. Local cells and the forming matrix then release stimulatory factors such as fibroblast growth factor (FGF), platelet derived growth factor, transforming growth factor beta, BMPs and Wnt glycoproteins [15]. These have all been shown to have a stimulatory effect on bone healing, at both cellular and macroscopic levels. Vascularity at the site is also stimulated by release of vascular endothelial growth factor (VEGF) and prostaglandin E2 (PGE2) [25].

In contrast, chronic inflammation (which is most commonly due to infection) results in persistence of inflammatory cytokines, consequential degradation of healing tissues, pain and continuation of an immature matrix that cannot stimulate an osteogenic response. At the cellular level, the controlled release of inflammatory mediators is overwhelmed, leading to dysregulation of the osteoimmunologic response. Osteoblast proliferation is reduced, thus decreasing the immunosuppressive balance they induce. In the presence of infection, bacterial factors such as endotoxic lipopolysaccharide (LPS) produce soluble inflammatory factors leading to osteolysis [26].

Figure 6.2 Relationship of inflammatory and osteogenic cells.

Source: Florence Loi et al. [15]. Reproduced with permission of Elsevier.