Presence of intervertebral discs alters observed stiffness and failure mechanisms in the vertebra

Abstract

Ex vivo mechanical testing is an essential tool for study of vertebral mechanics. However, the common method of testing vertebral bodies in the absence of adjacent intervertebral discs (IVDs) may limit the physiological relevance of the results. The goal of this study was to determine the influence of IVDs on vertebral mechanical properties and failure mechanisms. Rabbit thoracic vertebral bodies were tested with and without IVDs in a stepwise fashion that incorporated a micro-computed tomography scan at each loading step. The image sequences were analyzed using digital volume correlation to quantify deformations throughout the vertebral body. The observed deformation patterns differed substantially between the groups. Specimens tested with IVDs exhibited a slow increase in strain in the inferior and posterior regions, followed by a sudden increase in strain in the anterior cortex right at the yield point. In contrast, the highest strains in the isolated vertebral bodies were in the posterior regions throughout the test. Specimens tested with IVDs had lower stiffness (507.49 ± 184.73 N/mm vs. 845.61 ± 296.09 N/mm; p = 0.044), higher ultimate displacement (2.00 ± 0.68 mm vs. 1.17 ± 0.54 mm; p = 0.043), and higher maximum shear strains (e.g. top 25th percentile: 0.19 ± 0.11 vs. 0.06 ± 0.07mm/mm; p < 0.0458), and tended to have lower ultimate force (690.28 ± 160.25 N vs. 873.81 ± 131.48 N; p = 0.056). Similar work to failure (648.15 ± 317.86 N-mm vs. 603.49 ± 437.95 N-mm; p = 0.844) was observed between the two groups. These results indicate that testing vertebral bodies in the absence of IVDs can elicit artifactual failure mechanisms. These artifacts may be more prominent than the effects on vertebral strength and toughness.

Bone mechanical properties and changes with osteoporosis

“…Vertebral bodies with higher density posteriorly than anteriorly would be expected to exhibit higher strength under this type of load distribution, as has been shown [52]. In addition, a prevailing hypothesis has emerged that degeneration of the intervertebral disc results in transfer of more of the applied load to the outer regions of the vertebral body, thus causing resorption in the central and mid-transverse regions [54]. Vertebrae that have undergone this adaptation may thus be less likely to fracture [53].…”

Digital Volume Correlation: Review of Progress and Challenges

“…This is especially true in biomechanics where the specimen shape is not chosen but given [106]. For instance, the failure mechanisms may change when intervertebral disks are present or not.…”

“…Other applications followed in the field of biomechanics (e.g., see Refs. [194,11,106,241,17,193,44,40,43,109]). At the beginning of the current decade, DVC was clearly identified as one technique very suited to biomechanical applications [239].…”

Microdamage assessment of bone-cement interfaces under monotonic and cyclic compression

“…With the recent and rapid progress of micro-focus computed tomography (mCT) in conjunction with in situ mechanical testing (Nazarian & Muller, 2004;Youssef et al, 2005;Buffière et al, 2010), digital volume correlation (DVC) has become a powerful tool to examine full-field deformations in trabecular bone (Liu & Morgan, 2007;Gillard et al, 2014;Dall'Ara et al, 2014), cortical bone (Dall'Ara et al, 2014), whole bones (Hussein et al, 2012(Hussein et al, , 2013 and cellular scaffolds (Madi et al, 2013). However, to date DVC has been mainly used in the analysis of solids and cellular materials, but little is known on the volumetric strain distribution in composite materials (Mortazavi et al, 2014) and there are no data with regard to bone-cement composites.…”