Dominique Saletti

Links between mechanical behavior of cancellous bone and its microstructural properties under dynamic loading

Previous studies show that in vivo assessment of fracture risk can be achieved by identifying the relationships between microarchitecture description from clinical imaging and mechanical properties. This study demonstrates that results obtained at low strain rates can be extrapolated to loadings with an order of magnitude similar to trauma such as car crashes. Cancellous bovine bone specimens were compressed under dynamic loadings (with and without confinement) and the mechanical response properties were identified, such as Young׳s modulus, ultimate stress, ultimate strain, and ultimate strain energy. Specimens were previously scanned with pQCT, and architectural and structural microstructure properties were identified, such as parameters of geometry, topology, connectivity and anisotropy. The usefulness of micro-architecture description studied was in agreement with statistics laws. Finally, the differences between dynamic confined and non-confined tests were assessed by the bone marrow influence and the cancellous bone response to different boundary conditions. Results indicate that architectural parameters, such as the bone volume fraction (BV/TV), are as strong determinants of mechanical response parameters as ultimate stress at high strain rates (p-value<0.001). This study reveals that cancellous bone response at high strain rates, under different boundary conditions, can be predicted from the architectural parameters, and that these relations with mechanical properties can be used to make fracture risk prediction at a determined magnitude.

Increased intra-cortical porosity reduces bone stiffness and strength in pediatric patients with osteogenesis imperfecta

Osteogenesis imperfecta (OI) is a heritable disease occurring in one out of every 20,000 births. Although it is known that Type I collagen mutation in OI leads to increased bone fragility, the mechanism of this increased susceptibility to fracture is not clear. The aim of this study was to assess the microstructure of cortical bone fragments from patients with osteogenesis imperfecta (OI) using polarized light microscopy, and to correlate microstructural observations with the results of previously performed mechanical compression tests on bone from the same source. Specimens of cortical bone were harvested from the lower limbs of three (3) OI patients at the time of surgery, and were divided into two groups. Group 1 had been subjected to previous micro-mechanical compression testing, while Group 2 had not been subjected to any prior testing. Polarized light microscopy revealed disorganized bone collagen architecture as has been previously observed, as well as a large increase in the areal porosity of the bone compared to typical values for healthy cortical bone, with large (several hundred micron sized), asymmetrical pores. Importantly, the areal porosity of the OI bone samples in Group 1 appears to correlate strongly with their previously measured apparent Youngs modulus and compressive strength. Taken together with prior nanoindentation studies on OI bone tissue, the results of this study suggest that increased intra-cortical porosity is responsible for the reduction in macroscopic mechanical properties of OI cortical bone, and therefore that in vivo imaging modalities with resolutions of ~100 μm or less could potentially be used to non-invasively assess bone strength in OI patients. Although the number of subjects in this study is small, these results highlight the importance of further studies in OI bone by groups with access to human OI tissue in order to clarify the relationship between increased porosity and reduced macroscopic mechanical integrity.

Kinematic and dynamic responses of the scrum.

In a rugby game, the scrum allows to restart a game to restart after a minor infringement. It is an important sequence for physical and psychological domination of the opponent (Quarrie and Wilson 2000). Milburn (1990) has showed that during a scrum, the front row produces 38% of the total pushing force, the locks 42% and the looseforwards 20%. In this study, it was also recommended for optimal force production to have a low body position with aligned trunk–head–neck and a large angle at the hip. However, the correlation between force production and ankle, knee and hip angle observed by Quarrie and Wilson (2000) was low. In a recent study, Preatoni et al. (2013) have also investigated the forces developed during scrummaging, but as a factor of playing level. Nevertheless, no information is available on the variation of body angles, especially for the head and trunk, and on force history during a pushing phase. The aim of this study was to obtain a description of force during scrummaging with the kinematic angles of players, in order to develop a model to understand the scrum.

The behavior of cancellous bone from quasi-static to dynamic strain rates with emphasis on the intermediate regime

Previous studies, conducted using quasi-static and dynamic compression tests, have shown that the mechanical strength of cancellous bone is strain rate dependent. However, these studies have not included the intermediate strain rate (ISR) regime (1/s to 100/s), which is important since it is representative of the loading rates at which non-fatal injuries typically occur. In this study, 127 bovine bone specimens were compressed in 3 regimes spanning 8 distinct strain rates, from 0.001/s to 600/s, using three different devices: a conventional quasi-static testing machine, a wedge-bar (WB) apparatus and a conventional split Hopkinson pressure bar (SHPB) implemented with a cone-in-tube (CiT) striker and a tandem momentum trap. Due to the large sample size, a new robust automated algorithm was developed with which the material properties, such as the apparent Young׳s modulus and the yield and ultimate values of stress and strain, were identified for each individual specimen. A statistical summary of the data is presented. Finally, this study demonstrates that results obtained at intermediate strain rates are essential for a fuller understanding of cancellous bone behavior by providing new data describing the transition between the quasi-static and dynamic regimes.

Fracture characterization in cancellous bone specimens via surface difference evaluation of 3D registered pre- and post-compression micro-CT scans.

To cite this article: M. Prot, G. Dubois, T. J. Cloete, D. Saletti & S. Laporte (2015) Fracture characterization in cancellous bone specimens via surface difference evaluation of 3D registered preand post-compression micro-CT scans, Computer Methods in Biomechanics and Biomedical Engineering, 18:sup1, 2030-2031, DOI: 10.1080/10255842.2015.1069608 To link to this article: http://dx.doi.org/10.1080/10255842.2015.1069608

Links between microstructural properties of cancellous bone and its mechanical response to different strain rates

Automobile accidents and sporting injuries may lead to osseous fractures. To reduce the number of road accidents and their societal costs, governments have partnered with car manufacturers to develop an overall road safety. To achieve this, researchers are working on improving the design of automotive structures. However, researchers must first quantify the risk of injury incurred during an impact. Indeed, in France, osteoporosis is responsible of approximately 150,000 fractures per year. A better understanding of this fracture mechanism will aid in the design of protective features that will guard against fracture under these loading conditions. Bone is generally divided into two micro-structural types: cortical bone and cancellous bone. Cortical bone is a compact bone, denser than cancellous bone, and accounts for 80% of the skeletal mass in the human body. Cancellous bone, also called trabecular or spongy bone, has a porous structure that protects the bone marrow, acts as a core material to support the shape of thin layers of cortical bone and assists in transferring joint forces to the thick load bearing cortical bone layers. Several studies have been able to make great progress on characterising and modelling the behaviour of cortical bone. Regarding the cancellous bone, studies are mainly focused on quasi-static loading cases (Follet 2002). In order to analyse and understand the mechanism of cancellous bone for speed ranges above the quasi-static regime, experimental work using Split Hopkinson Bar Technique (SHPB) has been undertaken. However, no modelling, including the different parameters of cancellous bone, has yet been developed to analyse and understand the mechanism of rupture of the cancellous bone. This issue raises a keen interest in the scientific community, and this comes through in the work presented here. Indeed, the aim of this study was to characterise the mechanical properties of cancellous bovine bone for compression loading under different strain rates and to identify links with the microstructural description.

Sled acceleration control for low-speed impact testing and transient response studies

Whiplash Associated Disorder is the most common soft-tissue injury arising from low-speed car crashes (Siskind et al. 2013). To better understand whiplash injury mechanisms in the head-neck system, a sled was acquired. The sled was previously controlled in open loop mode, without any feedback of the resulting motion. The aim of this project is to safely control the motion and acceleration of the sled in order to be able to generate reproducible acceleration profiles.