A multibody modelling approach to determine load sharing between passive elements of the lumbar spine

The human spinal segment is an inherently complex structure, a combination of flexible and semi-rigid articulating elements stabilised by seven principal ligaments. An understanding of how mechanical loading is shared among these passive elements of the segment is required to estimate tissue failure stresses. A 3D rigid body model of the complete lumbar spine has been developed to facilitate the prediction of load sharing across the passive elements. In contrast to previous multibody models, this model includes a non-linear, six degrees of freedom intervertebral disc, facet bony articulations and all spinal ligaments. Predictions of segmental kinematics and facet joint forces, in response to pure moment loading (flexion–extension), were compared to published in vitro data. On inclusion of detailed representation of the disc and facets, the multibody model fully captures the non-linear flexibility response of the spinal segment, i.e. coupled motions and a mobile instantaneous centre of rotation. Predicted facet joint forces corresponded well with reported values. For the loading case considered, the model predicted that the ligaments are the main stabilising elements within the physiological motion range; however, the disc resists a greater proportion of the applied load as the spine is fully flexed. In extension, the facets and capsular ligaments provide the principal resistance. Overall patterns of load distribution to the spinal ligaments are in agreement with previous predictions; however, the current model highlights the important role of the intraspinous ligament in flexion and the potentially high risk of failure. Several important refinements to the multibody modelling of the passive elements of the spine have been described, and such an enhanced passive model can be easily integrated into a full musculoskeletal model for the prediction of spinal loading for a variety of daily activities.     

Load-sharing in the lumbosacral spine in neutral standing & flexed postures – A combined finite element and inverse static study

Understanding load-sharing in the spine during in-vivo conditions is critical for better spinal implant design and testing. Previous studies of load-sharing that considered actual spinal geometry applied compressive follower load, with or without moment, to simulate muscle forces. Other studies used musculoskeletal models, which include muscle forces, but model the discs by simple beams or spherical joints and ignore the articular facet joints.This study investigated load-sharing in neutral standing and flexed postures using a detailed Finite Element (FE) model of the ligamentous lumbosacral spine, where muscle forces, gravity loads and intra-abdominal pressure, as predicted by a musculoskeletal model of the upper body, are input into the FE model. Flexion was simulated by applying vertebral rotations following spine rhythm measured in a previous in-vivo study, to the musculoskeletal model. The FE model predicted intradiscal pressure (IDP), strains in the annular fibers, contact forces in the facet joints, and forces in the ligaments. The disc forces and moments were determined using equilibrium equations, which considered the applied loads, including muscle forces and IDP, as well as forces in the ligaments and facet joints predicted by the FE model. Load-sharing was calculated as the portion of the total spinal load carried along the spine by each individual spinal structure. The results revealed that spinal loads which increased substantially from the upright to the flexed posture were mainly supported by the discs in the upright posture, whereas the ligaments’ contribution in resisting shear, compression, and moment was more significant in the flexed posture.     

Investigation of impact loading rate effects on the ligamentous cervical spinal load-partitioning using finite element model of functional spinal unit C2–C3

The cervical spine functions as a complex mechanism that responds to sudden loading in a unique manner, due to intricate structural features and kinematics. The spinal load-sharing under pure compression and sagittal flexion/extension at two different impact rates were compared using a bio-fidelic finite element (FE) model of the ligamentous cervical functional spinal unit (FSU) C2–C3. This model was developed using a comprehensive and realistic geometry of spinal components and material laws that include strain rate dependency, bone fracture, and ligament failure. The range of motion, contact pressure in facet joints, failure forces in ligaments were compared to experimental findings. The model demonstrated that resistance of spinal components to impact load is dependent on loading rate and direction. For the loads applied, stress increased with loading rate in all spinal components, and was concentrated in the outer intervertebral disc (IVD), regions of ligaments to bone attachment, and in the cancellous bone of the facet joints. The highest stress in ligaments was found in capsular ligament (CL) in all cases. Intradiscal pressure (IDP) in the nucleus was affected by loading rate change. It increased under compression/flexion but decreased under extension. Contact pressure in the facet joints showed less variation under compression, but increased significantly under flexion/extension particularly under extension. Cancellous bone of the facet joints region was the only component fractured and fracture occurred under extension at both rates. The cervical ligaments were the primary load-bearing component followed by the IVD, endplates and cancellous bone; however, the latter was the most vulnerable to extension as it fractured at low energy impact.     

Effects of geometric individualisation of a human spine model on load sharing: neuro-musculoskeletal simulation reveals significant differences in ligament and muscle contribution

In spine research, two possibilities to generate models exist: generic (population-based) models representing the average human and subject-specific representations of individuals. Despite the increasing interest in subject specificity, individualisation of spine models remains challenging. Neuro-musculoskeletal (NMS) models enable the analysis and prediction of dynamic motions by incorporating active muscles attaching to bones that are connected using articulating joints under the assumption of rigid body dynamics. In this study, we used forward-dynamic simulations to compare a generic NMS multibody model of the thoracolumbar spine including fully articulated vertebrae, detailed musculature, passive ligaments and linear intervertebral disc (IVD) models with an individualised model to assess the contribution of individual biological structures. Individualisation was achieved by integrating skeletal geometry from computed tomography and custom-selected muscle and ligament paths. Both models underwent a gravitational settling process and a forward flexion-to-extension movement. The model-specific load distribution in an equilibrated upright position and local stiffness in the L4/5 functional spinal unit (FSU) is compared. Load sharing between occurring internal forces generated by individual biological structures and their contribution to the FSU stiffness was computed. The main finding of our simulations is an apparent shift in load sharing with individualisation from an equally distributed element contribution of IVD, ligaments and muscles in the generic spine model to a predominant muscle contribution in the individualised model depending on the analysed spine level.

     

Cervical spine morphology and ligament property variations: A finite element study of their influence on sagittal bending characteristics

Cervical spine finite element models reported in biomechanical literature usually represent a static morphology. Not considering morphology as a model parameter limits the predictive capabilities for applications in personalized medicine, a growing trend in modern clinical practice. The objective of the study was to investigate the influence of variations in spinal morphology on the flexion-extension responses, utilizing mesh-morphing-based parametrization and metamodel-based sensitivity analysis. A C5-C6 segment was used as the baseline model. Variations of intervertebral disc height, facet joint slope, facet joint articular processes height, vertebral body anterior-posterior depth, and segment size were parametrized. In addition, material property variations of ligaments were considered for sensitivity analysis. The influence of these variations on vertebral rotation and forces in the ligaments were analyzed. The disc height, segmental size, and body depth were found to be the most influential (in the cited order) morphology variations; while among the ligament material property variations, capsular ligament and ligamentum flavum influenced vertebral rotation the most. Changes in disc height influenced forces in the posterior ligaments, indicating that changes in the anterior load-bearing column of the spine could have consequences on the posterior column. A method to identify influential morphology variations is presented in this work, which will help automation efforts in modeling to focus on variations that matter. This study underscores the importance of incorporating influential morphology parameters, easily obtained through computed tomography/magnetic resonance images, to better predict subject-specific biomechanical responses for applications in personalized medicine.     

Load-bearing role of facets in a lumbar segment under sagittal plane loadings

In the present work, the load-bearing role of the facet joints in a lumbar I2-3 segment is quantitatively determined by means of a three dimensional nonlinear finite element program. The analysis accounts for both material and geometric nonlinearities and treats the facet articulation as a nonlinear moving contact problem. The disc nucleus is considered as an inviscid incompressible fluid and the annulus as a composite of collagenous fibres embedded in a matrix of ground substance. The spinal ligaments are modelled as a collection of nonlinear axial elements. The loadings consist of axial compression and sagittal plane shears and bending moments, acting alone or combined. The results show that in pure compression, the external axial force is transmitted primarily by the intervertebral disc. The facet joints carry only a small percentage of the force. However, the facet joints carry large forces in extension, whereas in small flexion they carry none. Addition of compression tends to increase these contact forces in extension while it has no effect on them in flexion. In extension, the forces on the facet joints are transmitted by both the articular surfaces and the capsular ligaments. Although in small flexion the facets carry no load, large contact forces are predicted to develop as the segment is flexed beyond 7-8 degrees. These forces are of the same magnitude as those computed under large extension rotation and are oriented nearly in the horizontal plane with negligible component in the axial direction. The horizontal components of the contact forces generated during articulation are often larger than the axial components which directly resist the applied compressive force. The axial components of the contact forces, therefore, grossly underestimate the total forces acting on the facets. The transfer of forces from one facet to the adjacent one occurs through distinct areas in flexion and in extension loadings. That is, on the superior articular surface, the contact area shifts from the upper tip in large flexion to the lower margin in extension. On the inferior articular surface, the contact area shifts from the upper and central regions in large flexion to the lower tip in extension.     

C4–C5 segment finite element model development,validation, and load-sharing investigation

Detailed cervical spine models are necessary to better understand cervical spine response to loading, improve our understanding of injury mechanisms, and specifically for predicting occupant response and injury in auto crash scenarios. The focus of this study was to develop a C4–C5 finite element model with accurate representations of each tissue within the segment. This model incorporates more than double the number of elements of existing models, required for accurate prediction of response. The most advanced material data available were then incorporated using appropriate nonlinear constitutive models to provide accurate predictions of response at physiological levels of loading. This tissue-scale segment model was validated against a wide variety of experimental data including different modes of loading (axial rotation, flexion, extension, lateral bending, and translation), and different load levels. In general, the predicted response of the model was within the single standard deviation response corridors for both low and high load levels. Importantly, this model demonstrates that appropriate refinement of the finite element mesh, representation at the tissue level, and sufficiently detailed material properties and constitutive models provide excellent response predictions without calibration of the model to experimental data. Load sharing between the disc, ligaments, and facet joints was investigated for various modes of loading, and the dominant load-bearing structure was found to correlate with typical anatomical injury sites for these modes of loading. The C4–C5 model forms the basis for the development of a full cervical spine model. Future studies will focus on tissue-level injury prediction and dynamic response.     

Diurnal variations in intervertebral disc height affect spine flexibility,intradiscal pressure and contact compressive forces in the facet joints

Diurnal changes of intervertebral disc height are caused by high compressive loading during the day, which expulses fluid from the disc, and by osmotic pressure, which imbibes fluid into the disc at low loading. The aim of the present study was to determine the magnitude of diurnal changes in spine flexibility, intradiscal pressures and contact forces in the facet joints. A validated osseoligamentous finite element model of the lumbar spine was used to determine these quantities for morning and evening situations. Disc height varied by 10% for these two situations. Spine flexibility and facet joint forces were markedly higher in the evening than in the morning. Intradiscal pressures were higher in the morning than in the evening. The different spine flexibilities in the morning and evening should be taken into account during kinematical measurements. Predicted facet joint forces may be used for the designing and pre-clinical testing of artificial facet joint replacements.     

Realistic loading conditions for upper body bending

Different modes of load applications are used to simulate flexion and extension of the upper body. It is not clear which loading modes deliver realistic results and allow the comparison of different studies.In a numerical study, a validated finite element model of the lumbar spine, ranging from the vertebra L1 to the disc L5–S1 was employed. Each of six different loading modes was studied for simulating flexion and extension, including pure moments, an eccentric axial force, using a wedged fixture, and applying upper body weight plus follower load plus muscle forces. Intersegmental rotations, intradiscal pressures and facet joint contact forces were calculated. Where possible, results were compared to data measured in vivo.The results of the loading modes studied show a large variance for some values. Outcome measures such as flexion angle and intradiscal pressure differed at a segment by up to 44% and 88%, respectively, related to their maximum values. Intradiscal pressure is mainly determined by the magnitude of the applied compressive force. For flexion maximum contact forces between 0 and 69 N are predicted in each facet joint for different loading modes. For both flexion and extension, applying upper body weight plus follower load plus muscle forces as well as a follower load together with a bending moment delivers results which agreed well with in vivo data from the literature.Choosing an adequate loading mode is important in spine biomechanics when realistic results are required for intersegmental rotations, intradiscal pressure and facet joint contact forces. Only then will results of different studies be comparable.     

A combined passive and active musculoskeletal model study to estimate L4-L5 load sharing

A number of geometrically-detailed passive finite element (FE) models of the lumbar spine have been developed and validated under in vitro loading conditions. These models are devoid of muscles and thus cannot be directly used to simulate in vivo loading conditions acting on the lumbar joint structures or spinal implants. Gravity loads and muscle forces estimated by a trunk musculoskeletal (MS) model under twelve static activities were applied to a passive FE model of the L4-L5 segment to estimate load sharing among the joint structures (disc, ligaments, and facets) under simulated in vivo loading conditions. An equivalent follower (FL), that generates IDP equal to that generated by muscle forces, was computed in each task. Results indicated that under in vivo loading conditions, the passive FE model predicted intradiscal pressures (IDPs) that closely matched those measured under the simulated tasks (R² = 0.98 and root-mean-squared-error, RMSE = 0.18 MPa). The calculated equivalent FL compared well with the resultant force of all muscle forces and gravity loads acting on the L4-L5 segment (R² = 0.99 and RMSE = 58 N). Therefore, as an alternative approach to represent in vivo loading conditions in passive FE model studies, this FL can be estimated by available in-house or commercial MS models. In clinical applications and design of implants, commonly considered in vitro loading conditions on the passive FE models do not adequately represent the in vivo loading conditions under muscle exertions. Therefore, more realistic in vivo loading conditions should instead be used.     

A finite element model technique to determine the mechanical response of a lumbar spine segment under complex loads

This study presents a CT-based finite element model of the lumbar spine taking into account all function-related boundary conditions, such as anisotropy of mechanical properties, ligaments, contact elements, mesh size, etc. Through advanced mesh generation and employment of compound elements, the developed model is capable of assessing the mechanical response of the examined spine segment for complex loading conditions, thus providing valuable insight on stress development within the model and allowing the prediction of critical loading scenarios. The model was validated through a comparison of the calculated force-induced inclination/deformation and a correlation of these data to experimental values. The mechanical response of the examined functional spine segment was evaluated, and the effect of the loading scenario determined for both vertebral bodies as well as the connecting intervertebral disc.     

Three-dimensional static modeling of the lumbar spine

This paper presents three-dimensional static modeling of the human lumbar spine to be used in the formation of anatomically-correct movement patterns for a fully cable-actuated robotic lumbar spine which can mimic in vivo human lumbar spine movements to provide better hands-on training for medical students. The mathematical model incorporates five lumbar vertebrae between the first lumbar vertebra and the sacrum, with dimensions of an average adult human spine. The vertebrae are connected to each other by elastic elements, torsional springs and a spherical joint located at the inferoposterior corner in the mid-sagittal plane of the vertebral body. Elastic elements represent the ligaments that surround the facet joints and the torsional springs represent the collective effect of intervertebral disc which plays a major role in balancing torsional load during upper body motion and the remaining ligaments that support the spinal column. The elastic elements and torsional springs are considered to be nonlinear. The nonlinear stiffness constants for six motion types were solved using a multiobjective optimization technique. The quantitative comparison between the angles of rotations predicted by the proposed model and in the experimental data confirmed that the model yields angles of rotation close to the experimental data. The main contribution is that the new model can be used for all motions while the experimental data was only obtained at discrete measurement points.     

Biomechanical implications of lumbar spinal ligament transection

Many lumbar spine surgeries either intentionally or inadvertently damage or transect spinal ligaments. The purpose of this work was to quantify the previously unknown biomechanical consequences of isolated spinal ligament transection on the remaining spinal ligaments (stress transfer), vertebrae (bone remodelling stimulus) and intervertebral discs (disc pressure) of the lumbar spine. A finite element model of the full lumbar spine was developed and validated against experimental data and tested in the primary modes of spinal motion in the intact condition. Once a ligament was removed, stress increased in the remaining spinal ligaments and changes occurred in vertebral strain energy, but disc pressure remained similar. All major biomechanical changes occurred at the same spinal level as the transected ligament, with minor changes at adjacent levels. This work demonstrates that iatrogenic damage to spinal ligaments disturbs the load sharing within the spinal ligament network and may induce significant clinically relevant changes in the spinal motion segment.     

Effect of a pedicle-screw-based motion preservation system on lumbar spine biomechanics: A probabilistic finite element study with subsequent sensitivity analysis

Pedicle-screw-based motion preservation systems are often used to support a slightly degenerated disc. Such implants are intended to reduce intradiscal pressure and facet joints forces, while having a minimal effect on the motion patterns.In a probabilistic finite element study with subsequent sensitivity analysis, the effects of 10 input parameters, such as elastic modulus and diameter of the elastic rod, distraction of the segment, level of bridged segments, etc. on the output parameters intervertebral rotations, intradiscal pressures, and facet joint forces were determined. A validated finite element model of the lumbar spine was employed. Probabilistic studies were performed for seven loading cases: upright standing, flexion, extension, left and right lateral bending and left and right axial rotation.The simulations show that intervertebral rotation angles, intradiscal pressures and facet joint forces are in most cases reduced by a motion preservation system. The influence on intradiscal pressure is small, except in extension. For many input parameter combinations, the values for intervertebral rotations and facet joint forces are very low, which indicates that the implant is too stiff in these cases. The output parameters are affected most by the following input parameters: loading case, elastic modulus and diameter of the elastic rod, distraction of the segment, and angular rigidity of the connection between screws and rod.The designated functions of a motion preservation system can best be achieved when the longitudinal rod has a low stiffness, and when the connection between rod and pedicle screws is rigid.     

Biomechanical comparison between fusion of two vertebrae and implantation of an artificial intervertebral disc

Surgical treatments for lower back pain can be distributed into two main groups: fusion (arthrodesis) and disc replacement (arthroplasty). The objective of this study was to compare, under severe loading conditions, the biomechanics of the lumbar spine treated either by fusion or total disc replacement (TDR). A three-dimensional model of a two-level ligamentous lumbar segment was created and simulated through static analyses with the finite-element method (FEM) software ABAQUS. The model was validated by comparing mobility, pressure on the facets, force in the ligaments, maximum stresses, disc bulge, and endplate deflection with measured data given in the literature. The FEM analysis predicted that the mobility of the model after arthrodesis on the upper level was reduced in all rotational degrees of freedom by an average of approximately 44%, relative to healthy normal discs. Conversely, the mobility of the model after TDR on the upper level was increased in all rotational degrees of freedom by an average of approximately 52%. The level implanted with the artificial disc showed excessive ligament tensions (greater than 500 N), high facet pressures (greater than 3 MPa), and a high risk of instability. The mobility and the stresses in the level adjacent to the arthroplasty were also increased. In conclusion, the model for an implanted movable artificial disc illustrated complications common to spinal arthroplasty and showed greater risk of instability and further degeneration than predicted for the fused model. This modeling technique provides an accurate means for assessing potential biomechanical risks and can be used to improve the design of future artificial intervertebral discs.     

Search for critical loading condition of the spine--a meta analysis of a nonlinear viscoelastic finite element model

The relative vulnerability of spinal motion segments to different loading combinations remains unknown. The meta-analysis described here using the results of a validated L2-L3 nonlinear viscoelastic finite element model was designed to investigate the critical loading and its effect on the internal mechanics of the human lumbar spine. A Box-Behnken experimental design was used to design the magnitude of seven independent variables associated with loads, rotations and velocity of motion. Subsequently, an optimization method was used to find the primary and secondary variables that influence spine mechanical output related to facet forces, disc pressure, ligament forces, annulus matrix compressive/shear stresses and anulus fibers strain. The mechanical responses with respect to the two most-relevant variables were then regressed linearly using the response surface quadratic model. Axial force and sagittal rotation were identified as the most-relevant variables for mechanical responses. The procedure developed can be used to find the critical loading for finite element models with multi input variables. The derived meta-models can be used to predict the risk associated with various loading parameters and in setting safer load limits.     

A finite element study of a lumbar motion segment subjected to pure sagittal plane moments

A nonlinear finite element program has been developed and applied to the analysis of a three-dimensional model of the lumbar L2-3 motion segment subjected to sagittal plane moments. The analysis accounts for both material and geometric nonlinearities and is based on the Updated Lagrangian approach. The disc nucleus has been considered as an incompressible inviscid fluid and the annulus as a composite of collagenous fibres embedded in a matrix of ground substance. Articulation at the facet joints has been treated as a general moving contact problem and the spinal ligaments have been modelled as a collection of nonlinear axial elements. Effects of the loss of intradiscal pressure in flexion and of facetectomy in extension have been analyzed. Comparison of the predicted gross response characteristics with available measurements indicates satisfactory agreement. In flexion relatively large intradiscal pressures are generated, while in extension negative pressures (i.e. suction) of low magnitude are predicted. The stress distribution results indicate that the load transfer path through the posterior elements of the joint in flexion is different from that in extension. In flexion the ligaments are the means of load transfer, while in extension the load is transmitted through the pedicles, laminae and articular processes. In flexion, the inner annulus fibres at the posterolateral location are subject to maximum tensile strain. It is suggested that large flexion moment in combination with other loads is a likely cause of disc prolapse commonly found at this location of the annulus.     

Search for critical loading condition of the spine–A meta analysis of a nonlinear viscoelastic finite element model

The relative vulnerability of spinal motion segments to different loading combinations remains unknown. The meta-analysis described here using the results of a validated L2–L3 nonlinear viscoelastic finite element model was designed to investigate the critical loading and its effect on the internal mechanics of the human lumbar spine. A Box-Behnken experimental design was used to design the magnitude of seven independent variables associated with loads, rotations and velocity of motion. Subsequently, an optimization method was used to find the primary and secondary variables that influence spine mechanical output related to facet forces, disc pressure, ligament forces, annulus matrix compressive/shear stresses and anulus fibers strain. The mechanical responses with respect to the two most-relevant variables were then regressed linearly using the response surface quadratic model. Axial force and sagittal rotation were identified as the most-relevant variables for mechanical responses. The procedure developed can be used to find the critical loading for finite element models with multi input variables. The derived meta-models can be used to predict the risk associated with various loading parameters and in setting safer load limits.     

Effects of lumbo-pelvic rhythm on trunk muscle forces and disc loads during forward flexion: A combined musculoskeletal and finite element simulation study

Previous in-vivo studies suggest that the ratio of total lumbar rotation over pelvic rotation (lumbo-pelvic rhythm) during trunk sagittal movement is essential to evaluate spinal loads and discriminate between low back pain and asymptomatic population. Similarly, there is also evidence that the lumbo-pelvic rhythm is key for evaluation of realistic muscle and joint reaction forces and moments predicted by various computational musculoskeletal models. This study investigated the effects of three lumbo-pelvic rhythms defined based on in-vivo measurements on the spinal response during moderate forward flexion (60°) using a combined approach of musculoskeletal modeling of the upper body and finite element model of the lumbosacral spine. The muscle forces and joint loads predicted by the musculoskeletal model, together with the gravitational forces, were applied to the finite element model to compute the disc force and moment, intradiscal pressure, annular fibers strain, and load-sharing. The results revealed that a rhythm with high pelvic rotation and low lumbar flexion involves more global muscles and increases the role of the disc in resisting spinal loads, while its counterpart, with low pelvic rotation, recruits more local muscles and engages the ligaments to lower the disc loads. On the other hand, a normal rhythm that has balanced pelvic and lumbar rotations yields almost equal disc and ligament load-sharing and results in more balanced synergy between global and local muscles. The lumbo-pelvic rhythm has less effect on the intradiscal pressure and annular fibers strain. This work demonstrated that the spinal response during forward flexion is highly dependent on the lumbo-pelvic rhythm. It is therefore, essential to adapt this parameter instead of using the default values in musculoskeletal models for accurate prediction of muscle forces and joint reaction forces and moments. The findings provided by this work are expected to improve knowledge of spinal response during forward flexion, and are clinically relevant towards low back pain treatment and disc injury prevention.     

Spinal facet joint biomechanics and mechanotransduction in normal, injury and degenerative conditions

The facet joint is a crucial anatomic region of the spine owing to its biomechanical role in facilitating articulation of the vertebrae of the spinal column. It is a diarthrodial joint with opposing articular cartilage surfaces that provide a low friction environment and a ligamentous capsule that encloses the joint space. Together with the disc, the bilateral facet joints transfer loads and guide and constrain motions in the spine due to their geometry and mechanical function. Although a great deal of research has focused on defining the biomechanics of the spine and the form and function of the disc, the facet joint has only recently become the focus of experimental, computational and clinical studies. This mechanical behavior ensures the normal health and function of the spine during physiologic loading but can also lead to its dysfunction when the tissues of the facet joint are altered either by injury, degeneration or as a result of surgical modification of the spine. The anatomical, biomechanical and physiological characteristics of the facet joints in the cervical and lumbar spines have become the focus of increased attention recently with the advent of surgical procedures of the spine, such as disc repair and replacement, which may impact facet responses. Accordingly, this review summarizes the relevant anatomy and biomechanics of the facet joint and the individual tissues that comprise it. In order to better understand the physiological implications of tissue loading in all conditions, a review of mechanotransduction pathways in the cartilage, ligament and bone is also presented ranging from the tissue-level scale to cellular modifications. With this context, experimental studies are summarized as they relate to the most common modifications that alter the biomechanics and health of the spine-injury and degeneration. In addition, many computational and finite element models have been developed that enable more-detailed and specific investigations of the facet joint and its tissues than are provided by experimental approaches and also that expand their utility for the field of biomechanics. These are also reviewed to provide a more complete summary of the current knowledge of facet joint mechanics. Overall, the goal of this review is to present a comprehensive review of the breadth and depth of knowledge regarding the mechanical and adaptive responses of the facet joint and its tissues across a variety of relevant size scales.