Quantitative phenotyping of bone fracture repair: a review

Abstract

Fracture repair is a complex process that involves the interaction of numerous molecular factors, cell lineages and tissue types. These biological processes allow for an impressive feat of engineering: an elastic soft callus is progressively replaced by a more rigid and mineralized callus. During this reparative phase, the healing bone is exposed to a risk of re-fracture. Bone volume and bone quality are the two major factors determining the strength of the callus. Although both factors are important, often only bone volume is analyzed and reported in preclinical studies. Recent developments in techniques for examining bone quality in the callus will enable the rapid and detailed analysis of its material properties and its microstructure. This review aims to give an overview of the methods available for quantitatively phenotyping the bone callus in preclinical studies such as Raman spectroscopy, nanoindentation, scanning acoustic microscopy, in vivo micro-computed tomography (micro-CT) and high-resolution micro-CT. Consolidated and emerging experimental methods are described with a focus on their applicability, and with examples of their utilization.

Figure 1

A sequential schematic of four classical stages of fracture healing. (a) After inflammation a hematoma is generated. (b) In the first stage of the reparative phase, the initial fibrin is gradually replaced by cartilaginous tissue and woven bone starts to form. (c) In a later stage of the reparative phase, the cartilaginous tissue mineralizes, more bone is formed and the volume of granulation tissue substantially decreases. (d) Eventually, once the bone is bridged, remodeling restores the original cortex.

Figure 2

Qualitative scheme of the tissue evolution during bone fracture repair. Granulation tissue is the only tissue present in the fracture gap shortly after fracture. After a few days, the fibroblasts in the granulation tissue make way for chondrocytes producing cartilaginous tissue. Bone formed by intramembranous ossification will appear within the first few days after fracture. About 2 weeks later, the cartilage formed in the fracture gap will start mineralizing to be converted into lamellar bone by endochondral ossification.

Figure 3

Techniques for the assessment of bone quality and bone competence. Mechanical tests (dark gray bars), imaging techniques to study geometry and microarchitecture (medium gray bars) and techniques to analyze tissue composition (light gray bars) are indicated in logarithmic scale, according to their spatial resolution. The techniques include dual energy X-ray absorptiometry (DXA), energy-dispersive X-ray spectroscopy (EDX), Fourier transform infrared imaging (FTIR), high-resolution micro-computed tomography (HR micro-CT), high-resolution peripheral quantitative computed tomography (HR-pQCT), in vivo micro-computed tomography (in vivo micro-CT), micro-magnetic resonance imaging (micro-MRI), phase contrast-based computed tomography (pc imaging), quantitative backscattered electron imaging (qBEI), scanning acoustic microscopy (SAM) and small-angle X-ray scattering (SAXS). Adapted from Donnelly with kind permission.