Bone Fracture Acute Phase Response-A Unifying Theory of Fracture Repair: Clinical and Scientific Implications

Abstract

Bone fractures create five problems that must be resolved: bleeding, risk of infection, hypoxia, disproportionate strain, and inability to bear weight. There have been enormous advancements in our understanding of the molecular mechanisms that resolve these problems after fractures, and in best clinical practices of repairing fractures. We put forth a modern, comprehensive model of fracture repair that synthesizes the literature on the biology and biomechanics of fracture repair to address the primary problems of fractures. This updated model is a framework for both fracture management and future studies aimed at understanding and treating this complex process. This model is based upon the fracture acute phase response (APR), which encompasses the molecular mechanisms that respond to injury. The APR is divided into sequential stages of "survival" and "repair." Early in convalescence, during "survival," bleeding and infection are resolved by collaborative efforts of the hemostatic and inflammatory pathways. Later, in "repair," avascular and biomechanically insufficient bone is replaced by a variable combination of intramembranous and endochondral ossification. Progression to repair cannot occur until survival has been ensured. A disproportionate APR-either insufficient or exuberant-leads to complications of survival (hemorrhage, thrombosis, systemic inflammatory response syndrome, infection, death) and/or repair (delayed- or non-union). The type of ossification utilized for fracture repair is dependent on the relative amounts of strain and vascularity in the fracture microenvironment, but any failure along this process can disrupt or delay fracture healing and result in a similar non-union. Therefore, incomplete understanding of the principles herein can result in mismanagement of fracture care or application of hardware that interferes with fracture repair. This unifying model of fracture repair not only informs clinicians how their interventions fit within the framework of normal biological healing but also instructs investigators about the critical variables and outputs to assess during a study of fracture repair.

Keywords: Acute phase response; Endochondral ossification; Fracture repair; Fracture vascularity; Non-union; Strain.

Fig. 1

The body’s response to fracture injury: the acute phase response (APR). a Following a fracture, the body must resolve 5 primary problems: bleeding, susceptibility to infection, disproportionate strain, bone hypoxia, and inability to bear weight. The APR is the body’s hormonal response system to injury. The APR first resolves lethal problems such as bleeding and susceptibility to infection in the “survival phase,” then transitions to the “repair phase” where strain is reduced by cellular and acellular factors allowing new vasculature to extend across the fracture site, reducing bone hypoxia, and leading to vascular union. b If the APR is insufficient or c inappropriately exuberant, complications, or “villains,” arise, such as hemorrhage, infection/sepsis, deep vein thrombosis (DVT), SIRS, and, in severe cases, death. Complications that prolong the survival phase will delay the initiation of repair

Fig. 2

Fibrin must be removed following fracture injury for revascularization and subsequent ossification to occur. Wild-type mice exhibit robust vascularity at the fracture site 14 days post fracture. In plasminogen knockout mice, fibrin cannot be removed from fracture site, and angiogenesis is significantly inhibited. When fibrinogen was depleted in plasminogen knockout mice, angiogenesis is largely restored. Fibrin acts as a barrier for bone revascularization, preventing VEGF produced by the hypertrophic chondrocytes from effectively reaching the VEGF receptor on endothelial cells. This demonstrates the importance of completely resolving one phase of repair before a next can begin

Fig. 3

Matrix evolution at the fracture site. The evolution of matrices is shown in a murine model of a transverse femur fracture with fixation. By 14 days post fracture, the fluorescently labeled fibrin matrix (red) has begun to be cleared and type 2 collagen and type 10 collagen (not pictured fluorescently) begin to form at the fracture site. By 21 days post fracture, the presence of type 1 collagen (green) is pronounced at the site of fracture healing in the form of hard callus. As remodeling of the hard callus occurs through 42 days post fracture, there is a decrease in observable type 1 collagen signal

Fig. 4

Cells in the fracture callus. Following a fracture, pre-hypertrophic chondrocytes function to resolve strain by producing a biomechanical matrix composed of cellular and acellular materials, primarily collagen 2. Once the pre-hypertrophic chondrocytes have sufficiently minimized strain, they hypertrophy, becoming hypertrophic chondrocytes that will provide vascular endothelial growth factor (VEGF) to attract endothelial cells, and promote ossification in conjunction with osteoblasts by producing bone morphogenic protein (BMP) and nanohydroxyapatite

Fig. 5

Intramembranous ossification/primary bone healing. In fractures with (a) intact vascularity, little avascular necrosis, (b) low strain, good oxygenation, and healthy periosteum and endosteum, intramembranous ossification or primary bone healing is possible. Progenitor cells invade the fracture site (c) and are followed by (d) endothelial cells and subsequently transition directly into osteoblasts. The osteoblasts (e) gradually achieve bony union that includes (f) union of cortical bone, intramedullary vascularity, periosteum, and endosteum

Fig. 6

Secondary bone healing. In the majority of fractures, the structural integrity of the bone and the vascular supply to the fracture site are disrupted, leading to (a) hypoxia and interfragmentary motion. Under these conditions, (b) progenitor cells are drawn to the fracture site and, depending on the conditions of strain and oxygen tension, either (c) intramembranous or endochondral ossification will ensue. (d) At the periphery of the fracture (relatively preserved oxygen supply and low strain), progenitor cells in close association with the bone’s intact blood supply differentiate into osteoblasts and begin the process of intramembranous ossification. Within the center of the fracture site (high strain and low oxygen tension) (e), the progenitor cells develop into pre-hypertrophic chondrocytes, proliferate in response to strain, and resolve strain by forming a biomechanical extracellular matrix. When strain is sufficiently resolved, (f) these chondrocytes undergo hypertrophy and become hypertrophic chondrocytes that direct angiogenesis and osteogenesis. (g) Hypertrophic chondrocytes promote vascular invasion and osteogenesis by releasing BMP, VEGF, and hydroxyapatite. (h) Vascular union always precedes bony union at the fracture site, as the endothelial cells are necessary for ossification. (i) With bony union of the fracture callus, the fracture is stabilized, and the remaining chondrocytes become hypertrophic. (j) The fracture is now healed and remodeling proceeds

Fig. 7

Temporo-spatial fracture repair and angiogenesis. In a murine diaphyseal fracture model, the production and resolution of soft tissue callus, hard tissue callus, and vascularity are highly correlated with one another. Safranin-O staining, radiographs, and angiograms of fractured femurs demonstrate the temporal and spatial development of the fracture callus and associated vasculature. Seven days post fracture (7-DPF), the diaphyseal intramedullary vasculature remains disrupted by regional hematoma, resulting in an avascular femoral segment flanked proximally and distally by intact intramedullary vasculature and shunting blood to the periosteum. Radiographic and histopathologic examination shows formation of a cartilaginous soft tissue callus without evidence of osteoid formation within this avascular zone. The soft tissue callus rapidly enlarges to its maximal size by 10-DPF. Simultaneously, hard tissue callus is initially formed via intramembranous ossification at the extreme proximal and distal aspects of the fracture site, where the periosteum inserts on unaffected adjacent cortical bone. This process occurs in conjunction with the formation of small highly branching extramedullary vessels recruited by cells in the periosteum expressing VEGF-A (10-DPF). As hard tissue callus replaces soft tissue callus (14-DPF), it is accompanied by an expansion of newly formed vasculature. The regions of vascular expansion begin at the proximal and distal aspects of the fracture site and migrate centrally toward the soft tissue callus, directed by the ordered release of VEGF by hypertrophic chondrocytes. Vascular ingrowth continues until anastomoses are developed, coinciding with complete dissolution of soft tissue callus and formation of bridging hard tissue callus (21-DPF). Following a vascular anastomosis and bridging of hard callus across the fracture site, the fracture callus remodels back to within the original cortices coinciding with the vasculature returning to larger vessels with reduced branching (28–42-DPF)

Fig. 8

Radiographs as “angiograms.” These radiographs depict a healing femur fracture in a young adult treated with open reduction and internal fixation with plate and screws. The hardware provides greater stabilization on the ipsilateral side of the fracture and more chondroid soft tissue callus is required on the contralateral fracture side for equivalent stabilization. The lack of motion on the side of the plate coupled with the compression of the fracture prevents callus formation. The hazy soft tissue callus become radiopaque as it is replaced by hard tissue, which is definitive evidence of vascular ingress to the area

Fig. 9

Fixation methods. (a) Intramedullary nailing disrupts the medullary vasculature yet leaves periosteal vessels intact. It also allows for limited motion, promoting chondrocyte proliferation. This combination enables robust callus formation that is highly vascular. (b) Plate and screw fixation of fractures provides rigid fixation, limiting callus formation and causing little disruption of the intramedullary vasculature. However, the compression of the plate against bone disrupts extramedullary vasculature and can cause hypoxia under the plate. (c) Limited contact plating aims to provide rigid fixation, leaving remaining intramedullary vasculature intact while also causing as little disruption of extramedullary vasculature as possible. This theoretically enables improved fracture healing with limited callus formation compared to full contact plating. Periosteum disrupted by these two plating methods is marked in green. (d) When fixation is inadequate, as is often the case of flexible nailing of an adult fracture, chondrocytes proliferate to try to reduce the strain that has not been adequately treated. However, the strain and motion may be too great for the chondrocytes to stabilize, preventing chondrocyte hypertrophy and bony union, leading to pseudarthrosis