Fabio Galbusera

Response analysis of the lumbar spine during regular daily activities—A finite element analysis

A non-linear poroelastic finite element model of the lumbar spine was developed to investigate spinal response during daily dynamic physiological activities. Swelling was simulated by imposing a boundary pore pressure of 0.25 MPa at all external surfaces. Partial saturation of the disc was introduced to circumvent the negative pressures otherwise computed upon unloading. The loading conditions represented a pre-conditioning full day followed by another day of loading: 8h rest under a constant compressive load of 350 N, followed by 16 h loading phase under constant or cyclic compressive load varying in between 1000 and 1600 N. In addition, the effect of one or two short resting periods in the latter loading phase was studied. The model yielded fairly good agreement with in-vivo and in-vitro measurements. Taking the partial saturation of the disc into account, no negative pore pressures were generated during unloading and recovery phase. Recovery phase was faster than the loading period with equilibrium reached in only approximately 3h. With time and during the day, the axial displacement, fluid loss, axial stress and disc radial strain increased whereas the pore pressure and disc collagen fiber strains decreased. The fluid pressurization and collagen fiber stiffening were noticeable early in the morning, which gave way to greater compression stresses and radial strains in the annulus bulk as time went by. The rest periods dampened foregoing differences between the early morning and late in the afternoon periods. The forgoing diurnal variations have profound effects on lumbar spine biomechanics and risk of injury.

Biomechanics of the lumbar spine after dynamic stabilization.

Target of the study was to predict the biomechanics of the instrumented and adjacent levels due to the insertion of the DIAM spinal stabilization system (Medtronic Ltd). For this purpose, a 3-dimensional finite element model of the intact L3/S1 segment was developed and subjected to different loading conditions (flexion, extension, lateral bending, axial rotation). The model was then instrumented at the L4/L5 level and the same loading conditions were reapplied. Within the assumptions of our model, the simulation results suggested that the implant caused a reduction in range of motion of the instrumented level by 17% in flexion and by 43% in extension, whereas at the adjacent levels, no significant changes were predicted. Numerical results in terms of intradiscal pressure, relative to the intact condition, predicted that the intervertebral disc at the instrumented level was unloaded by 27% in flexion, by 51% in extension, and by 6% in axial rotation, while no variations in pressure were caused by the device in lateral bending. At the adjacent levels, a change of relative intradiscal pressure was predicted in extension, both at the L3/L4 level, which resulted unloaded by 26% and at the L5/S1 level, unloaded by 8%. Furthermore, a reduction in terms of principal compressive stress in the annulus fibrosus of the L4/L5 instrumented level was predicted, as compared with the intact condition. These numerical predictions have to be regarded as a theoretical representation of the behavior of the spine, because any finite element model represents only a simplification of the real structure.

Design concepts in lumbar total disc arthroplasty

The implantation of lumbar disc prostheses based on different design concepts is widely accepted. This paper reviews currently available literature studies on the biomechanics of TDA in the lumbar spine, and is targeted at the evaluation of possible relationships between the aims of TDA and the geometrical, mechanical and material properties of the various available disc prostheses. Both theoretical and experimental studies were analyzed, by a PUBMED search (performed in February 2007, revised in January 2008), focusing on single level TDA. Both semi-constrained and unconstrained lumbar discs seem to be able to restore nearly physiological IAR locations and ROM values. However, both increased and decreased ROM was stated in some papers, unrelated to the clinical outcome. Segmental lordosis alterations after TDA were reported in most cases, for both constrained and unconstrained disc prostheses. An increase in the load through the facet joints was documented, for both semi-constrained and unconstrained artificial discs, but with some contrasting results. Semi-constrained devices may be able to share a greater part of the load, thus protecting the surrounding biological structure from overloading and possible early degeneration, but may be more susceptible to wear. The next level of development will be the biomechanical integration of compression across the motion segment. All these findings need to be supported by long-term clinical outcome studies.

Finite element modeling of soft tissues: Material models, tissue interaction and challenges

BACKGROUND Musculoskeletal soft tissues, such as articular cartilage, ligaments, knee meniscus and intervertebral disk, have a complex structure, which provides elasticity and capability to support and distribute the body loads. Soft tissues describe an inhomogeneous and multiphasic structure, and exhibit a nonlinear, time-dependent behavior. Their mechanical response is governed by a substance composed of protein fiber-rich and proteoglycan-rich extracellular matrix and interstitial fluid. Protein fibers (e.g. collagen) give the tissue direction dependent stiffness and strength. To investigate these complex biological systems, the use of mathematical tools is well established, alone or in combination with experimental in vitro and in vivo tests. However, the development of these models poses many challenges due to the complex structure and mechanical response of soft tissues. METHODS Non-systematic literature review. FINDINGS This paper provides a summary of different modeling strategies with associated material properties, contact interactions between articulating tissues, validation and sensitivity of soft tissues with special focus on knee joint soft tissues and intervertebral disk. Furthermore, it reviews and discusses some salient clinical findings of reported finite element simulations. INTERPRETATION Model studies extensively contributed to the understanding of functional biomechanics of soft tissues. Models can be effectively used to elucidate clinically relevant questions. However, users should be aware of the complexity of such tissues and of the capabilities and limitations of these approaches to adequately simulate a specific in vivo or in vitro phenomenon.

Comparison of four methods to simulate swelling in poroelastic finite element models of intervertebral discs

Osmotic phenomena influence the intervertebral disc biomechanics. Their simulation is challenging and can be undertaken at different levels of complexity. Four distinct approaches to simulate the osmotic behaviour of the intervertebral disc (a fixed boundary pore pressure model, a fixed osmotic pressure gradient model in the whole disc or only in the nucleus pulposus, and a swelling model with strain-dependent osmotic pressure) were analysed. Predictions were compared using a 3D poroelastic finite element model of a L4-L5 spinal unit under three different loading conditions: free swelling for 8 h and two daily loading cycles: (i) 200 N compression for 8 h followed by 500 N compression for 16 h; (ii) 500 N for 8 h followed by 1000 N for 16 h. Overall, all swelling models calculated comparable results, with differences decreasing under greater loads. Results predicted with the fixed boundary pore pressure and the fixed osmotic pressure in the whole disc models were nearly identical. The boundary pore pressure model, however, cannot simulate differential osmotic pressures in disc regions. The swelling model offered the best potential to provide more accurate results, conditional upon availability of reliable values for the required coefficients and material properties. Possible fields of application include mechanobiology investigations and crack opening and propagation. However, the other approaches are a good compromise between the ease of implementation and the reliability of results, especially when considering higher loads or when the focus is on global results such as spinal kinematics.

Computational modeling of combined cell population dynamics and oxygen transport in engineered tissue subject to interstitial perfusion

This work presents a computational model of tissue growth under interstitial perfusion inside a tissue engineering bioreactor. The model accounts both for the cell population dynamics, using a model based on cellular automata, and for the hydrodynamic microenvironment imposed by the bioreactor, using a model based on the Lattice–Boltzmann equation and the convection-diffusion equation. The conditions of static culture versus perfused culture were compared, by including the population dynamics along with oxygen diffusion, convective transport and consumption. The model is able to deal with arbitrary complex geometries of the spatial domain; in the present work, the domain modeled was the void space of a porous scaffold for tissue-engineered cartilage. The cell population dynamics algorithm provided results which qualitatively resembled population dynamics patterns observed in experimental studies, and these results were in good quantitative agreement with previous computational studies. Simulation of oxygen transport and consumption showed the fundamental contribution of convective transport in maintaining a high level of oxygen concentration in the whole spatial domain of the scaffold. The model was designed with the aim to be computationally efficient and easily expandable, i.e. to allow straightforward implementability of further models of complex biological phenomena of increasing scientific interest in tissue engineering, such as chemotaxis, extracellular matrix deposition and effect of mechanical stimulation.

What have we learned from finite element model studies of lumbar intervertebral discs in the past four decades

Finite element analysis is a powerful tool routinely used to study complex biological systems. For the last four decades, the lumbar intervertebral disc has been the focus of many such investigations. To understand the disc functional biomechanics, a precise knowledge of the disc mechanical, structural and biochemical environments at the microscopic and macroscopic levels is essential. In response to this need, finite element model studies have proven themselves as reliable and robust tools when combined with in vitro and in vivo measurements. This paper aims to review and discuss some salient findings of reported finite element simulations of lumbar intervertebral discs with special focus on their relevance and implications in disc functional biomechanics. Towards this goal, the earlier investigations are presented, discussed and summarized separately in three distinct groups of elastic, multi-phasic transient and transport model studies. The disc overall response as well as the relative role of its constituents are markedly influenced by loading rate, magnitude, combinations/preloads and posture. The nucleus fluid content and pressurizing capacity affect the disc compliance, annulus strains and failure sites/modes. Biodynamics of the disc is affected by not only the excitation characteristics but also preloads, existing mass and nucleus condition. The role of fluid pressurization and collagen fiber stiffening diminish with time during diurnal loading. The endplates permeability influences the time-dependent response of the disc in both loaded and unloaded recovery phases. The transport of solutes is substantially influenced by the disc size, tissue diffusivity and endplates permeability.

Mechanical loading of the intervertebral disc: From the macroscopic to the cellular level

PurposeMechanical loading represents an integral part of intervertebral disc (IVD) homeostasis. This review aims to summarise recent knowledge on the effects of mechanical loads on the IVD and the disc cells, taking into consideration the changes that IVDs undergo during ageing and degeneration, from the macroscopic to the cellular and subcellular level.MethodsNon-systematic literature review.ResultsSeveral scientific papers investigated the external loads that act on the spine and the resulting stresses inside the IVD, which contribute to estimate the mechanical stimuli that influence the cells that are embedded within the disc matrix. As disc cell responses are also influenced by their biochemical environment, recent papers addressed the role that degradation pathways play in the regulation of (1) cell viability, proliferation and differentiation and (2) matrix production and turnover. Special emphasis was put on the intracellular-signalling pathways, as mechanotransduction pathways play an important role in the maintenance of normal disc metabolism and in disc degenerative pathways.ConclusionsDisc cells are exposed to a wide range of mechanical loads, and the biochemical environment influences their responses. Degeneration-associated alterations of the disc matrix change the biochemical environment of disc cells and also the mechanical properties of the disc matrix. Recent studies indicate that these factors interact and regulate disc matrix turnover.

Ageing and degenerative changes of the intervertebral disc and their impact on spinal flexibility

PurposeDegeneration of the intervertebral disc is associated with various morphological changes of the disc itself and of the adjacent structures, such as reduction of the water content, collapse of the intervertebral space, disruption and tears, and osteophytes. These morphological changes of the disc are linked to alterations of the spine flexibility. This paper aims to review the literature about the ageing and degenerative changes of the intervertebral disc and their link with alterations in spinal biomechanics, with emphasis on flexibility.MethodsNarrative literature review.ResultsClinical instability of the motion segment, usually related to increased flexibility and hypothesized to be connected to early, mild disc degeneration and believed to be responsible for low back pain, was tested in numerous in vitro studies. Despite some disagreement in the findings, a trend toward spinal stiffening with the increasing degeneration was observed in most studies. Tests about tears and fissures showed inconsistent results, as well as for disc collapse and dehydration. Vertebral osteophytes were found to be effective in stabilizing the spine in bending motions.ConclusionsThe literature suggests that the degenerative changes of the intervertebral disc and surrounding structures lead to subtle alteration of the mechanical properties of the functional spinal unit. A trend toward spinal stiffening with the increasing degeneration has been observed in most studies.

Cervical spine biomechanics following implantation of a disc prosthesis

This study presents a finite element model of the C4-C7 segment in healthy conditions and after implantation of a disc prosthesis at a single level, in order to investigate of the influence of disc arthroplasty on the biomechanics of the cervical spine. A nonlinear finite element model of the C4-C7 segment in intact conditions was developed and run in flexion and extension. A detailed model of the Bryan disc prosthesis, including contacts between the different components of the device, was built and positioned at C5-C6. The calculated segmental motion resulted preserved after disc arthroplasty, with respect to the model of the intact spine, in both flexion and extension. A general preservation of the forces transmitted through the facet joints was obtained; a minor force increase at the implanted level was detected. The analysis of the instantaneous centers of rotation (ICR) in flexion-extension showed the preservation of a physiological kinematics. The mechanical behaviour showed an asymmetry between flexion and extension, probably due to the removal of the anterior longitudinal ligament and the anterior part of the annulus fibrosus, and the preservation of the posterior structures. In general, the disc prosthesis showed to be able to reproduce a nearly physiological motion. However, other important mechanical aspects, such as the possible micromotion at the bone-implant interface and the possible degenerative conditions of the spine, need to be evaluated before drawing a conclusion about total disc arthroplasty from an engineering point of view.