A novel tool to quantify in vivo lumbar spine kinematics and 3D intervertebral disc strains using clinical MRI

Medical imaging modalities that calculate tissue morphology alone cannot provide direct information regarding the mechanical behaviour of load-bearing musculoskeletal organs. Accurate in vivo measurement of spine kinematics and intervertebral disc (IVD) strains can provide important information regarding the mechanical behaviour of the spine, help to investigate the effects of injuries on the mechanics of the spine, and assess the effectiveness of treatments. Additionally, strains can serve as a functional biomechanical marker for detecting normal and pathologic tissues. We hypothesised that combining digital volume correlation (DVC) with 3T clinical MRI can provide direct information regarding the mechanics of the spine. Here, we have developed a novel non-invasive tool for in vivo displacement and strain measurement within the human lumbar spine and we used this tool to calculate lumbar kinematics and IVD strains in six healthy subjects during lumbar extension. The proposed tool enabled spine kinematics and IVD strains to be measured with errors that did not exceed 0.17 mm and 0.5%, respectively. The findings of the kinematics study identified that during extension the lumbar spine of healthy subjects experiences total 3D translations ranging from 1 mm to 4.5 mm for different vertebral levels. The findings of strain analysis identified that the average of the maximum tensile, compressive, and shear strains for different lumbar levels during extension ranged from 3.5% to 7.2%. This tool can provide base-line data that can be used to describe the mechanical environment of healthy lumbar spine, which can help clinicians manage preventative treatments, define patient-specific treatments, and to monitor the effectiveness of surgical and non-surgical interventions.

Effect of radiation-induced damage of trabecular bone tissue evaluated using indentation and digital volume correlation

Exposure to X-ray radiation for an extended amount of time can cause damage to the bone tissue and therefore affect its mechanical properties. Specifically, high-resolution X-ray Computed Tomography (XCT), in both synchrotron and lab-based systems, has been employed extensively for evaluating bone micro-to-nano architecture. However, to date, it is still unclear how long exposures to X-ray radiation affect the mechanical properties of trabecular bone, particularly in relation to lab-XCT systems. Indentation has been widely used to identify local mechanical properties such as hardness and elastic modulus of bone and other biological tissues. The purpose of this study is therefore, to use indentation and XCT-based investigative tools such as digital volume correlation (DVC) to assess the microdamage induced by long exposure of trabecular bone tissue to X-ray radiation and how this affects its local mechanical properties. Trabecular bone specimens were indented before and after X-ray exposures of 33 and 66 h, where variation of elastic modulus was evaluated at every stage. The resulting elastic modulus was decreased, and micro-cracks appeared in the specimens after the first long X-ray exposure and crack formation increased after the second exposure. High strain concentration around the damaged tissue exceeding 1% was also observed from DVC analysis. The outcomes of this study show the importance of designing appropriate XCT-based experiments in lab systems to avoid degradation of the bone tissue mechanical properties due to radiation and these results will help to inform future studies that require long X-ray exposure for in situ experiments or generation of reliable subject-specific computational models.

3D strain analysis of trabecular bone within the osteoarthritic humeral head subjected to stepwise compressive loads

Understanding the local mechanical properties of trabecular bone at the humeral head-neck junction is essential for the safe design of stemless humeral head implants. Recent advancements in mechanical testing coupled with volumetric imaging have allowed for the ability to quantify full-field strain distributions throughout trabecular bone. Within this study, digital volume correlation (DVC) was applied to micro-computed tomography images to investigate the local load carrying characteristics of trabecular bone within osteoarthritic (OA) humeral heads subjected to stepwise loading. A multi-pegged indenter was used to transfer loads from a custom-fabricated loading apparatus to trabecular bone on the resection surface of OA humeral head osteotomies retrieved from patients undergoing total shoulder arthroplasty (TSA). In regions of trabecular bone that eventually fractured, third principal strains were significantly higher (95^th percentile third principal strain = -12,558 μstrain, p < 0.001) compared to regions that did not fracture (95^th percentile third principal strain = -7,806 μstrain). As well, bone volume fraction (p = 0.012), trabecular separation (p = 0.014), and trabecular number (p = 0.007) were found to influence the likelihood of trabecular bone fracture. Collectively, this work has led to a deeper understanding of the local load carrying characteristics of trabecular bone specific to patients receiving TSA for osteoarthritis.

Multi-scale mechanical and morphological characterisation of sintered porous magnesium-based scaffolds for bone regeneration in critical-sized defects

Magnesium (Mg) and its alloys are very promising degradable, osteoconductive and osteopromotive materials to be used as regenerative treatment for critical-sized bone defects. Under load-bearing conditions, Mg alloys must display sufficient morphological and mechanical resemblance to the native bone they are meant to replace to provide adequate support and enable initial bone bridging. In this study, unique highly open-porous Mg-based scaffolds were mechanically and morphologically characterised at different scales. In situ X-ray computed tomography (XCT) mechanics, digital volume correlation (DVC), electron microscopy and nanoindentation were combined to assess the influence of material properties on the apparent (macro) mechanics of the scaffold. The results showed that Mg exhibited a higher connected structure (38.4mm^-3 and 6.2mm^-3 for Mg and trabecular bone (Tb), respectively) and smaller spacing (245µm and 629µm for Mg and Tb, respectively) while keeping an overall appropriate porosity of 55% in the range of trabecular bone (30-80%). This fully connected and highly porous structure promoted lower local strain compared to the trabecular bone structure at material level (i.e. -22067 ± 8409µε and -40120 ± 18364µε at 6% compression for Mg and trabecular bone, respectively) and highly ductile mechanical behaviour at apparent level preventing premature scaffold failure. Furthermore, the Mg scaffolds exceeded the physiological strain of bone tissue generated in daily activities such as walking or running (500-2000µε) by one order of magnitude. The yield stress was also found to be close to trabecular bone (2.06MPa and 6.67MPa for Mg and Tb, respectively). Based on this evidence, the study highlights the overall biomechanical suitability of an innovative Mg-based scaffold design to be used as a treatment for bone critical-sized defects. STATEMENT OF SIGNIFICANCE: Bone regeneration remains a challenging field of research where different materials and solutions are investigated. Among the variety of treatments, biodegradable magnesium-based implants represent a very promising possibility. The novelty of this study is based on the characterisation of innovative magnesium-based implants whose structure and manufacturing have been optimised to enable the preservation of mechanical integrity and resemble bone microarchitecture. It is also based on a multi-scale approach by coupling high-resolution X-ray computed tomography (XCT), with in situ mechanics, digital volume correlation (DVC) as well as nano-indentation and electron-based microscopy imaging to define how degradable porous Mg-based implants fulfil morphological and mechanical requirements to be used as critical bone defects regeneration treatment.

Micromechanical evaluation of cortical bone using in situ XCT indentation and digital volume correlation

The overall mechanical behaviour of cortical bone is strongly dependant on its microstructure. X-ray computed tomography (XCT) has been widely used to identify the microstructural morphology of cortical tissue (i.e. pore network, Haversian and Volkmann's canals). However, the connection between microstructure and mechanics of cortical bone during plastic deformation is unclear. Hence, the purpose of this study is to provide an in-depth evaluation of the interplay of plastic strain building up in relation to changes in the canal network for cortical bone tissue. In situ step-wise XCT indentation was used to introduce a localised load on the surface of the tissue and digital volume correlation (DVC) was employed to assess the three-dimensional (3D) full-field plastic strain distribution in proximity of the indent. It was observed that regions adjacent to the imprint were under tensile strain, whereas the volume underneath experienced compressive strain. Canal loss and disruption was detected in regions of higher compressive strains exceeding -20000 με and crack formation occurred in specimens where Haversian canals were running parallel to the indentation tip. The results of this study outline the relationship between the micromechanical and structural behaviour of cortical bone during plastic deformation, providing information on cortical tissue fracture pathways.

Variability in strain distribution in the mice tibia loading model: A preliminary study using digital volume correlation

It is well known that bone has an enormous adaptive capacity to mechanical loadings, and to this extent, several in vivo studies on mouse tibia use established cyclic compressive loading protocols to investigate the effects of mechanical stimuli. In these experiments, the applied axial load is well controlled but the positioning of the hind-limb between the loading endcaps may dramatically affect the strain distribution induced on the tibia. In this study, the full field strain distribution induced by a typical in vivo setup on mouse tibiae was investigated through a combination of in situ compressive testing, µCT scanning and a global digital volume correlation (DVC) approach. The precision of the DVC method and the effect of repositioning on the strain distributions were evaluated. Acceptable uncertainties of the DVC approach for the analysis of loaded tibiae (411 ± 58µɛ) were found for nodal spacing of approximately 50 voxels (520 µm). When pairs of in situ preloaded and loaded images were registered, low variability of the strain distributions within the tibia were seen (range of mean differences in principal strains: 585-1800µɛ). On contrary, larger differences were seen after repositioning (range of mean differences in principal strains: 2500-5500µɛ). To conclude, these preliminary results on thee specimens showed that the DVC approach applied to the mouse tibia can be precise enough to evaluate local strain distributions under loads, and that repositioning of the hind-limb within the testing machine can induce large differences in the strain distributions that should be accounted for when modelling this system.