Realistic tool-tissue interaction models for surgical simulation and planning

Surgical simulators present a safe and potentially effective method for surgical training, and can also be used in pre- and intra-operative surgical planning. Realistic modeling of medical interventions involving tool-tissue interactions has been considered to be a key requirement in the development of high-fidelity simulators and planners. The soft-tissue constitutive laws, organ geometry and boundary conditions imposed by the connective tissues surrounding the organ, and the shape of the surgical tool interacting with the organ are some of the factors that govern the accuracy of medical intervention planning.\ud \ud This thesis is divided into three parts. First, we compare the accuracy of linear and nonlinear constitutive laws for tissue. An important consequence of nonlinear models is the Poynting effect, in which shearing of tissue results in normal force; this effect is not seen in a linear elastic model. The magnitude of the normal force for myocardial tissue is shown to be larger than the human contact force discrimination threshold. Further, in order to investigate and quantify the role of the Poynting effect on material discrimination, we perform a multidimensional scaling study. Second, we consider the effects of organ geometry and boundary constraints in needle path planning. Using medical images and tissue mechanical properties, we develop a model of the prostate and surrounding organs. We show that, for needle procedures such as biopsy or brachytherapy, organ geometry and boundary constraints have more impact on target motion than tissue material parameters. Finally, we investigate the effects surgical tool shape on the accuracy of medical intervention planning. We consider the specific case of robotic needle steering, in which asymmetry of a bevel-tip needle results in the needle naturally bending when it is inserted into soft tissue. We present an analytical and finite element (FE) model for the loads developed at the bevel tip during needle-tissue interaction. The analytical model explains trends observed in the experiments. We incorporated physical parameters (rupture toughness and nonlinear material elasticity) into the FE model that included both contact and cohesive zone models to simulate tissue cleavage. The model shows that the tip forces are sensitive to the rupture toughness. In order to model the mechanics of deflection of the needle, we use an energy-based formulation that incorporates tissue-specific parameters such as rupture toughness, nonlinear material elasticity, and interaction stiffness, and needle geometric and material properties. Simulation results follow similar trends (deflection and radius of curvature) to those observed in macroscopic experimental studies of a robot-driven needle interacting with gels

The development of a soft tissue mimicking hydrogel: Mechanical characterisation and 3D printing

Accurate tissue phantoms are difficult to design due to the complex hyperelastic, viscoelastic and biphasic properties of real soft tissues. The aim of this work is to demonstrate the tissue mimicking ability of a composite hydrogel (CH), constituting of poly(vinyl alcohol) (PVA) and phytagel (PHY), as a soft tissue phantom over a range mechanical properties, for a variety of biomedical and tissue engineering applications. Its compressive stress-strain behaviour, relaxation response, tensile impact stresses and surgical needle-tissue interactions were mapped and characterised with respect to its constituent hydrogel formulation. The mechanical characterisation of biological tissues was also investigated and the results were used as the ground truth for mimicking. The best mimicking hydrogel compositions were determined by combining the most relevant mechanical properties for each desired application. This thesis demonstrates the use of the tissue mimicking composite hydrogel formulations as tissue phantoms for various surgical procedures, including convection enhanced drug delivery, and traumatic brain injury studies. To expand the applications of the CH, a preliminary biological evaluation of the hydrogel was performed using human dermal fibroblasts. Cell seeded on the collagen-coated composite hydrogel showed good attachment and viability. Finally, a novel fabrication method with the aim of creating samples that replicate the anisotropic properties of biological tissues was developed. A cryogenic 3D printing method utilising the liquid to solid phase change of the composite hydrogel ink was achieved by rapidly cooling the ink solution below its freezing point. The setup was able to successfully create complex 3D brain mimicking material. The method was validated by showing that the mechanical and microstructural properties of the 3D printed material was well matched to its cast-moulded equivalent. This greatly widens the applications of the CH as a mechanically accurate tool for in-vitro testing and also demonstrates promise for future mechanobiology and tissue engineering studies.Open Acces

Experimental and Analytical Investigation of the Cavity Expansion Method for Mechanical Characterization of Soft Materials

In biomedical engineering, the mechanical properties of biological tissues are commonly determined by using conventional methods such as tensile stretching, confined and unconfined compression, indentation and elastography. With the exception of elastography, most techniques are implemented on ex-vivo soft tissue samples. This study evaluated a newly developed technique that has the potential to measure the mechanical properties of soft tissues in their in-vivo condition. This technique is based on the mechanics of internal spherical cavity expansion inside soft materials. Experimental, mathematical and numerical investigations were conducted. Experimentally, the pressure-cavity volume relationship was measured using two types of polyvinyl alcohol (PVA) hydrogels of different stiffnesses, namely Sample1 and Sample 2. In addition, unconfined compression tests were conducted to measure the stress-strain relationship of the two gels. Based on the cavity expansion test results, the measured pressure-volume data was translated into the stress-strain relationship using a mathematical model. The stiffness of the two gels was then compared to that determined by the unconfined compression technique. The resulting stiffness of the two techniques was then compared at overlapping range of strains, with the average percentage of difference being 8.46% for Sample1 and 5.36% for Sample 2. A numerical model was developed to investigate the analytical solution of the new technique. This investigation was based on verifying the displacement predicted by the analytical solution. The promising outcome of the technique encouraged extending this study to include bovine liver tissues. A tissue sample was extracted from a bovine liver and subjected to tensile loading to evaluate its stiffness. The result was a stiffness of 76.92 kPa. A second sample of the same bovine liver was evaluated using the spherical expansion technique which resulted in a stiffness of 87.94 kPa

Three-dimensional modeling and simulation of muscle tissue puncture process

Needle biopsy is an essential part of modern clinical medicine. The puncture accuracy and sampling success rate of puncture surgery can be effectively improved through virtual surgery. There are few three-dimensional puncture (3D) models, which have little significance for surgical guidance under complicated conditions and restrict the development of virtual surgery. In this paper, a 3D simulation of the muscle tissue puncture process is studied. Firstly, the mechanical properties of muscle tissue are measured. The Mooney-Rivlin (M-R) model is selected by considering the fitting accuracy and calculation speed. Subsequently, an accurate 3D dynamic puncture model is established. The failure criterion is used to define the breaking characteristics of the muscle, and the bilinear cohesion model defines the breaking process. Experiments with different puncture speeds are carried out through the built in vitro puncture platform. The experimental results are compared with the simulation results. The experimental and simulated reaction force curves are highly consistent, which verifies the accuracy of the model. Finally, the model under different parameters is studied. The simulation results of varying puncture depths and puncture speeds are analyzed. The 3D puncture model can provide more accurate model support for virtual surgery and help improve the success rate of puncture surgery

3D cohesive finite element minimum invasive surgery simulation based on Kelvin-Voigt model

Minimally invasive surgery is an important technique used for cytopathological examination. Recently, multiple studies have been conducted on a three-dimensional (3D) puncture simulation model as it can reveal the internal deformation state of the tissue at the micro level. In this study, a viscoelastic constitutive equation suitable for muscle tissue was derived. Additionally, a method was developed to define the fracture characteristics of muscle tissue material during the simulation process. The fracture of the muscle tissue in contact with the puncture needle was simulated using the cohesive zone model and a 3D puncture finite element model was established to analyze the deformation of the muscle tissue. The stress nephogram and reaction force under different parameters were compared and analyzed to study the deformation of the biological soft tissue and guide the actual operation process and reduce pain

Ductile sliding between mineral crystals followed by rupture of collagen crosslinks: Experimentally supported micromechanical explanation of bone strength

International audienceThere is an ongoing discussion on how bone strength could be explained from its internal structure and composition. Reviewing recent experimental and molecular dynamics studies, we here propose a new vision on bone material failure: mutual ductile sliding of hydroxyapatite mineral crystals along layered water films is followed by rupture of collagen crosslinks. In order to cast this vision into a mathematical form, a multiscale continuum micromechanics theory for upscaling of elastoplastic properties is developed, based on the concept of concentration and influence tensors for eigenstressed microheterogeneous materials. The model reflects bone's hierarchical organization, in terms of representative volume elements for cortical bone, for extravascular and extracellular bone material, for mineralized fibrils and the extrafibrillar space, and for wet collagen. In order to get access to the stress states at the interfaces between crystals, the extrafibrillar mineral is resolved into an infinite amount of cylindrical material phases oriented in all directions in space. The multiscale micromechanics model is shown to be able to satisfactorily predict the strength characteristics of different bones from different species, on the basis of their mineral/collagen content, their intercrystalline, intermolecular, lacunar, and vascular porosities, and the elastic and strength properties of hydroxyapatite and (molecular) collagen

Recent Advances in Soft Biological Tissue Manipulating Technologies

Biological soft tissues manipulation, including conventional (mechanical) and nonconventional (laser, waterjet and ultrasonic) processes, is critically required in most surgical innervations. However, the soft tissues, with their nature of anisotropic and viscoelastic mechanical properties, and high biological and heat sensitivities, are difficult to manipulated. Moreover, the mechanical and thermal induced damage on the surface and surrounding tissue during the surgery can impair the proliferative phase of healing. Thus, understanding the manipulation mechanism and the resulted surface damage is of importance to the community. In recent years, more and more scholars carried out researches on soft biological tissue cutting in order to improve the cutting performance of surgical instruments and reduce the surgery induced tissue damage. However, there is a lack of compressive review that focused on the recent advances in soft biological tissue manipulating technologies. Hence, this review paper attempts to provide an informative literature survey of the state-of-the-art of soft tissue manipulation processes in surgery. This is achieved by exploring and recollecting the different soft tissue manipulation techniques currently used, including mechanical, laser, waterjet and ultrasonic cutting and advanced anastomosis and reconstruction processes, with highlighting their governing removal mechanisms as well as the surface and subsurface damages

Development of Nanofiber Scaffolds with Controllable Structure and Mineral Content for Tendon-to-Bone Repair

Rotator cuff tears are common and lead to significant pain and disability. Effective repair of torn rotator cuff tendons requires healing of tendon to bone. Unfortunately, healing does not reproduce the structural and compositional features of the natural tendon-to-bone bone attachment that are necessary for effective load transfer, and surgical repairs often rupture. Recent efforts for improving tendon-to-bone healing have focused on tissue engineering approaches. Scaffolds, cells, and/or growth factors are implanted at the repair site to guide the healing process and improve outcomes. To that end, a polymer-mineral tissue engineered scaffold was developed for this thesis which mimics two of the primary features of the tendon-to-bone insertion: aligned nanofibers and hydroxyapatite mineral crystals. The nanofibrous component was created by electrospinning poly lactic-co-glycolic acid to create non-woven mats. The bone-like mineral was then deposited onto the nanofibers using mineralizing solutions. The structure (alignment and crimp microstructure) and composition (mineral content and morphology) of the scaffolds were modulated to understand their influence on scaffold mechanics. Experimental and modeling results demonstrated that: (1) the orientation distribution of the nanofibers was a major determinant of modulus, strength, and anisotropy, (2) crimp microstructure was a major determinant of low strain non-linear mechanical behavior, (3) mineral content positively correlated with modulus and strength and negatively correlated with toughness, (4) mineral morphology was a significant determinant of its stiffening effect, and (5) scaffold-level stiffening by mineral was due to mineral cross-bridges between nanofibers, not due to stiffening of individual nanofibers. Scaffolds were tested in a rotator cuff tendon-to-bone animal model in an effort to improve healing, but were found to be ineffective; the scar-mediated wound healing response dominated over any effects from the scaffold. In summary, a number of mechanisms driving nanofiber mechanics were defined, but further study is needed to effectively apply these scaffolds in the setting of tendon-to-bone repair

Ihh Signaling and Muscle Forces are Required for Enthesis Development

Tendon-to-bone repair is clinically challenging and plagued by high failure rates. The attachment of relatively stiff bone (~20GPa) to more compliant tendon (~200MPa) represents a fundamental engineering challenge. In the native tendon-bone attachment, termed the enthesis, transitional tissue contains gradients of structure and composition that effectively reduce stress concentrations at the boundary between hard and soft tissue. This transitional tissue is replaced by scar after injury and repair, resulting in a mechanically inferior attachment. The goal of this thesis is to study biological and mechanical cues that are critical to the development of the structure and function of the native tendon, which could inspire novel repair strategies to improve tendon-to-bone healing. To accomplish this, we characterized mineralization patterns in the murine supraspinatus enthesis throughout postnatal development on the micro-scale using Raman spectroscopy and at the nano-scale using transmission electron microscopy - electron energy loss spectroscopy. Mineralization of this tissue occured postnatally via endochondral ossification. We observed a constant and approximately linear increase in the mineral-to-collagen ratio at the mineralizing front within the enthesis at all developmental stages. Using a multi-scale linear elastic model of the tendon enthesis, we demonstrated that the mineral gradient amplifies stresses near mineralizing cells early in development while reducing stress concentrations at the mature tendon-bone interface. Next, we investigated tendon enthesis development in the absence of muscle forces. This localized paralysis model resulted in joint level deformities and mineralization defects. We observed a dramatic decrease in the enthesis biomechanical properties accompanied by structural and compositional changes. Collagen fiber alignment was reduced and mineralization defects were observed using Raman spectroscopy and X-ray diffraction. In order to probe the biological mechanisms that might influence development of this tissue, we hypothesized that factors critical to endochondral bone formation will also influence enthesis mineralization. Using a murine reporter of active Indian hedgehog (Ihh) signaling, we identified a population of cells present early in development that populate the mature enthesis. Lineage tracing analyses indicated that this cell population remained at the mature enthesis while down-regulating Ihh signaling in mineralized regions. In the case of reduced muscle forces, Ihh signaling was slightly elevated in this model compared to controls. Eliminating Ihh signaling throughout development using a conditional Smoothened (Smo) knockout mouse model specific to tendon lineage cells resulted in dramatic mineralization defects in the enthesis and reduced biomechanical behavior of the attachment. Taken together, this thesis demonstrates that Ihh signaling and muscle loading are necessary for mineralization and maturation of a mechanically robust tendon-to-bone attachment