Feasibility of extracting tissue material properties via cohesive elements: a finite element approach to probe insertion procedures in non-invasive spine surgeries.

Modeling the mechanical behavior of soft tissue probe insertion remains a challenging endeavor due to involved interdependent phenomena comprising tissue nonlinear deformation, contact between the probe and the tissue, crack propagation, and viscoelastic effects. To that matter, cohesive elements allow simulating crack formation and propagation, which provides a promising path to modeling the mechanical behavior of probe insertion in soft tissues. As such, the aim of the present study was to investigate the feasibility of devising and integrating an algorithm in a finite element (FE) case study in efforts of reverse engineering the material properties of non-homogeneous soft tissues. A layered nonlinear tissue model with a cohesive zone was created in the commercial software ABAQUS. Material properties were iteratively modified via a hybrid gradient descent optimization algorithm: minimizing the resultant error to first find optimum Ogden's hyperelastic parameters, followed by obtaining the damage parameters. Perceived material properties were then compared to those obtained via experimental human cadaver testing. Under the investigated four-layered muscle model, numerical results overlapped, to a great extent, with six different force-insertion experimental profiles with an average error of [Formula: see text] 15%. The best profile fit was realized when the highest sudden force drop was less than 60% of the peak force. Lastly, the FE analysis revealed an increase in stiffness as the probe advanced inside the tissue. The optimization algorithm demonstrated its capability to reverse engineer the material parameters required for the FE analysis of real, non-homogeneous, soft tissues. The significance of this procedure lies within its ability to extract tissue material parameters, in real time, with little to no intervention or invasive experimental tests. This could potentially further serve as a database for different muscle layers and force-insertion profiles, used for surgeon and physician clinical training purposes.

Developing a Quantifying Device for Soft Tissue Material Prop-Erties around Lumbar Spines.

Knowing the material properties of the musculoskeletal soft tissue could be important to develop rehabilitation therapy and surgical procedures. However, there is a lack of devices and information on the viscoelastic properties of soft tissues around the lumbar spine. The goal of this study was to develop a portable quantifying device for providing strain and stress curves of muscles and ligaments around the lumbar spine at various stretching speeds. Each sample was conditioned and applied for 20 repeatable cyclic 5 mm stretch-and-relax trials in the direction and perpendicular direction of the fiber at 2, 3 and 5 mm/s. Our device successfully provided the stress and strain curve of the samples and our results showed that there were significant effects of speed on the young's modulus of the samples (p < 0.05). Compared to the expensive commercial device, our lower-cost device provided comparable stress and strain curves of the sample. Based on our device and findings, various sizes of samples can be measured and viscoelastic properties of the soft tissues can be obtained. Our portable device and approach can help to investigate young's modulus of musculoskeletal soft tissues conveniently, and can be a basis for developing a material testing device in a surgical room or various lab environments.

Computational modeling of human bone fracture healing affected by different conditions of initial healing stage.

BACKGROUND: Bone healing process includes four phases: inflammatory response, soft callus formation, hard callus development, and remodeling. Mechanobiological models have been used to investigate the role of various mechanical and biological factors on bone healing. However, the effects of initial healing phase, which includes the inflammatory stage, the granulation tissue formation, and the initial callus formation during the first few days post-fracture, are generally neglected in such studies. METHODS: In this study, we developed a finite-element-based model to simulate different levels of diffusion coefficient for mesenchymal stem cell (MSC) migration, Young's modulus of granulation tissue, callus thickness and interfragmentary gap size to understand the modulatory effects of these initial phase parameters on bone healing. RESULTS: The results quantified how faster MSC migration, stiffer granulation tissue, thicker callus, and smaller interfragmentary gap enhanced healing to some extent. However, after a certain threshold, a state of saturation was reached for MSC migration rate, granulation tissue stiffness, and callus thickness. Therefore, a parametric study was performed to verify that the callus formed at the initial phase, in agreement with experimental observations, has an ideal range of geometry and material properties to have the most efficient healing time. CONCLUSIONS: Findings from this paper quantified the effects of the initial healing phase on healing outcome to better understand the biological and mechanobiological mechanisms and their utilization in the design and optimization of treatment strategies. It is also demonstrated through a simulation that for fractures, where bone segments are in close proximity, callus development is not required. This finding is consistent with the concepts of primary and secondary bone healing.

Tissue rheology in embryonic organization.

Tissue morphogenesis in multicellular organisms is brought about by spatiotemporal coordination of mechanical and chemical signals. Extensive work on how mechanical forces together with the well-established morphogen signalling pathways can actively shape living tissues has revealed evolutionary conserved mechanochemical features of embryonic development. More recently, attention has been drawn to the description of tissue material properties and how they can influence certain morphogenetic processes. Interestingly, besides the role of tissue material properties in determining how much tissues deform in response to force application, there is increasing theoretical and experimental evidence, suggesting that tissue material properties can abruptly and drastically change in development. These changes resemble phase transitions, pointing at the intriguing possibility that important morphogenetic processes in development, such as symmetry breaking and self-organization, might be mediated by tissue phase transitions. In this review, we summarize recent findings on the regulation and role of tissue material properties in the context of the developing embryo. We posit that abrupt changes of tissue rheological properties may have important implications in maintaining the balance between robustness and adaptability during embryonic development.

Intermittent Parathyroid Hormone After Prolonged Alendronate Treatment Induces Substantial New Bone Formation and Increases Bone Tissue Heterogeneity in Ovariectomized Rats.

Postmenopausal osteoporosis is often treated with bisphosphonates (eg, alendronate, [ALN]), but oversuppression of bone turnover by long-term bisphosphonate treatment may decrease bone tissue heterogeneity. Thus, alternate treatment strategies after long-term bisphosphonates are of great clinical interest. The objective of the current study was to determine the effect of intermittent parathyroid hormone (PTH) following 12 weeks of ALN (a bisphosphonate) treatment in 6-month-old, ovariectomized (OVX) rats on bone microarchitecture, bone remodeling dynamics, and bone mechanical properties at multiple length scales. By using in vivo µCT and 3D in vivo dynamic bone histomorphometry techniques, we demonstrated the efficacy of PTH following ALN therapy for stimulating new bone formation, and increasing trabecular thickness and bone volume fraction. In healthy bone, resorption and formation are coupled and balanced to sustain bone mass. OVX results in resorption outpacing formation, and subsequent bone loss and reduction in bone tissue modulus and tissue heterogeneity. We showed that ALN treatment effectively reduced bone resorption activity and regained the balance with bone formation, preventing additional bone loss. However, ALN treatment also resulted in significant reductions in the heterogeneity of bone tissue mineral density and tissue modulus. On the other hand, PTH treatment was able to shift the bone remodeling balance in favor of formation, with or without a prior treatment with ALN. Moreover, by altering the tissue mineralization, PTH alleviated the reduction in heterogeneity of tissue material properties induced by prolonged ALN treatment. Furthermore, switching to PTH treatment from ALN improved bone's postyield mechanical properties at both the whole bone and apparent level compared to ALN alone. The current findings suggest that intermittent PTH treatment should be considered as a viable treatment option for patients with prior treatment with bisphosphonates. © 2017 American Society for Bone and Mineral Research.