Bone fluid flow transports nutrients to, and carries waste from, the bone cells embedded in the bony matrix. In long bones, it is driven by the blood pressure differentials between the medullary cavity and the periosteal surface and it is enhanced by mechanical loading. Loading of bone tissue deforms the bone matrix and changes the volume of the medullary cavity. Both mechanisms alter the interstitial fluid flow in the bone cortex. The former changes the volume of the fluid cavities in the cortex, while the latter modifies the intramedullary pressure (IMP). This study aims to investigate and compare, for the first time, the effects of these two mechanisms combined on the interstitial fluid flow in the bone cortex.

A hydraulic-fluid method is proposed to investigate the enhancement of IMP induced by the external loading. An intact sheep tibia is represented by a hollow cylinder, with the bone marrow being completely constrained in the cavity and assumed to behave as an icompressible liquid. The cortex is supposed to be a purely elastic material, and its permeability is ignored at this stage. The numerical results show that an axial compressive load of 500 N increases the IMP from 4000 Pa to 48900 Pa.

The influence of the enhanced IMP on the interstitial fluid flow is examined in a subsequent poroelastic analysis. At this stage, the cortex is assumed to be a biphasic material that permits fluid perfusion. The poroelastic analyses were conducted for both initial and enhanced IMPs. The results of the simulations demonstrate that the external load induces very high interstitial pressure. The highest pressure could be 25 times higher than the initial marrow pressure, but its magnitude decreases quickly. Furthermore, the influence of the IMP on the interstitial pressure is limited to the inner half of the cortical wall adjacent to the endosteal surface. However, the influence becomes more significant with decreasing load-induced interstitial pressure.

In conclusion, these simulations suggest that the increase in IMP during mechanical loading further enhances interstitial fluid movements in cortical bone, which highlights the importance of mechanical loading for the maintenance of healthy bones.

Introduction: Vertebral rotation is an important aspect of spinal deformity in scoliosis, associated with ribcage deformity (rib hump). Although both lateral curvature and axial rotation appear to increase together in progressive scoliosis, the mechanisms driving vertebral rotation are not clearly established and it is not known whether lateral curvature precedes rotation, or vice versa. This study investigates the hypothesis that intravertebral (within the bone) rotation in idiopathic scoliosis is caused by growth in the presence of gravity-induced torsions, the twisting moments generated by gravitational forces acting on the scoliotic spine.

Methods: The twisting moment Tp acting at an arbitrary point P on a three-dimensional spinal curve is given by Tp=Mp·â, where Mp=r¥F is the total moment due to gravity force F acting at (vector) distance r, and â is the tangent to the spinal curve at P (Figure One). Standing radiographs for five idiopathic scoliosis patients were used to define three-dimensional curves representing the approximate axes of rotation of each spine, running along the anterior edge of the neural canal from T1 to S1. The equilibrium equations above were then solved to calculate gravity-induced torsions exerted by head and torso weight about the spinal axes for each patient. Intravertebral rotations were measured for the same patients using Aaro & Dahlborn’s technique with reformatted computed tomography images in the plane of superior and inferior endplates of each vertebra. The gravity-induced torsion curves were compared with intravertebral rotation measurements to see whether gravity-induced torsion is a likely contributor to intravertebral rotation.

Results: Gravity-induced torques as high as 4 Nm act on the spines of idiopathic scoliosis patients due to static body weight in the standing position. Maximum intravertebral rotations (for a single vertebra) were approximately 78. There appears to be general agreement between the measured intravertebral rotations and profiles of gravity-induced torsion along the length of the spine (Figure 2). Rotation measurements confirm the finding of previous authors that maximum intravertebral rotations occur at the ends of a scoliotic curve (with little relative rotation at the apex), and this finding is consistent with the gravity-induced torsion profiles calculated.

Conclusion: Gravity-induced torsion is a potential cause of vertebral rotation in idiopathic scoliosis. Since the spine must be curved in three-dimensions (out of plane) to produce such torques, vertebral rotation would be expected to occur subsequent to an initial lateral deviation, suggesting that coronal curvature ‘drives’ axial rotation during scoliosis progression.

Introduction It is important to understand the mechanics of the lumbar spine, as it has been shown that much low back pain is attributable to mechanical factors. One important aspect of spinal mechanics is the neutral zone, defined as a region of little or no resistance to motion on either side of the neutral position for a motion segment. If the neutral zone is a significant feature of intervertebral joint mechanics then the spinal joints will have little intrinsic stability and rely on muscles to control their movement around the neutral position. This has significant implications for our understanding of how degenerative changes to the spinal joints might destabilise the spine. This study was performed to characterise the size of the neutral zone and the effect of axial preload for different spinal motions.

Methods Using a 6 degree-of-freedom (DOF) ABB industrial robot incorporating a 6-DOF JR3 force sensor, six isolated ovine lumbar joint segments were subjected to 5 repetitive movements in 3 directions (6° extension / 15° flexion, +/− 7° lateral bend, +/− 3° axial twist) with 4 different preloads (0, 150, 300, 450N) under 2 conditions (facet joints intact and facets removed). For each direction, the fixed axis about which the joint would rotate with a minimal motion-opposing moment was determined in advance. In accordance with a previous study by this group, the neutral zone was defined as the region where absolute rotational stiffness is less than 0.05 Nm/°.

Results When moving from 6° of extension to −15° (flexion) a neutral zone was usually observed starting around 0° and continuing as far as −8 or −9°. The neutral zone was in the same region when moving in the opposite direction, except when the specimen showed a considerable amount of hysteresis, in which case the neutral zone could start as early as −11° or −12° and usually continued to −2°or −3°. Increasing preload usually made the joint stiffer in the regions outside the neutral zone, but did not affect the neutral zone itself. If present without preload, hysteresis usually increased with increasing preload. In lateral bend and axial twist no neutral zone was generally observed. In lateral bend the stiffness gradually increased with rotation, whereas in axial twist the stiffness was usually constant over the range of movement. For all movements, the only effect of facet removal was a constant reduction in stiffness over the whole movement. For lateral bend this meant that the stiffness around 0° usually would drop below the threshold of 0.05Nm/°, hence creating a neutral zone extending over a couple of degrees.

Discussion Ovine spinal joints have a region where there is little to no resistance to flexion/extension. This region can be in excess of 10°. This means in their neutral position, the individual spinal joints have virtually no stability and the spine depends on other measures such as muscle activation to maintain stability in the sagittal plane. For lateral bend there is a region of little resistance as well, but it is not nearly as profound as in flexion/extension.

Introduction Biomechanical modelling of the human body requires measurement of the relative positions of skeletal elements. This information provides data on joint kinematics directly affects muscle attachment site locations and hence determines muscle moment calculations. Spinal orientation is particularly difficult to measure due to small joint movements and relative inaccessibility of the bones to direct measurement. This study presents a novel method of accurately determining relative bone position in vivo using magnetic resonance imaging (MRI).

Methods A process incorporating both positional and conventional MRI was used to determine the skeletal positions of the lumbar spine and pelvis. The method uses higher quality conventional MRI to determine bone geometries and then registers these with lower resolution, positional MRI images of various postures.

Using the positional scanner four postures were investigated: Neutral Standing, Neutral Sitting, Flexed Sitting and Extended Sitting. These scans comprised simultaneous sagittal and coronal non-contiguous slices to facilitate three-dimensional registration and reduce acquisition time. Conventional MRI was then used to scan the subject at higher resolution contiguous slices. After segmentation and surface extraction of all bones from all scans, each bone geometry was registered with each of the positional scans to produce high quality in vivo skeletal position data.

For 2 subjects, each of the 5 lumbar vertebrae and the pelvis were registered 5 times in the 4 postures to investigate intra-tester reliability. This resulted in 48 sets of 5 registrations. Each bone surface was represented by surface points and a local coordinate system. Angular and translational differences between coordinate axes were examined for each set of five registrations.

Results The results indicate good intra-measurer reliability with a maximum rotational difference for all vertebral registrations of less than 1 degree and a maximum origin translation of less than 3mm. The pelvic registrations demonstrated larger discrepancies. Flexion/extension, lateral bend and axial twist rotations were measured for each joint. While there did not appear to be patterns between the two subjects, there were obvious trends within each subject and in particular trends of lateral bending throughout sagittal plane motion were identified.

Discussion The results showed that the technique was able to register the surfaces reliably. The intervertebral movements between postures were within normal ranges of motion and demonstrated kinematic trends within an individual. At present, the greatest disadvantage of the method described lies in its large data processing times. The data collected are three dimensional and represent the anatomy and movement of a specific individual. The method can be used to examine joint mechanics and centres of rotation in three dimensions, validate the predictions of finite element models and investigate the effects of medical interventions.

Introduction The NIH estimates that 30–50% of women and 20–30% of men will develop a vertebral fracture in their lifetime. 700,000 vertebral fractures occur each year in the United States alone, 85% of which are associated with osteoporosis. Osteoporosis leads to reduced stiffness of vertebral cancellous bone and eventual loss of cortical wall thickness. This study aims to investigate the effects of cortical wall thickness and cancellous bone elastic modulus on vertebral strength and fracture patterns using synthetic vertebrae made from bone analogue materials.

Methods Synthetic vertebrae were created using rapid prototyping for the cortical shell and expanding polyurethane foam filler for the cancellous core. Dimensions were based on human L1 vertebra as specified in Panjabi et al. (1992). Silicone mouldings were used as intervertebral disk phantoms. The synthetic vertebrae were subjected to uniaxial compression at constant strain rate (5mm/min) using a Hounsfield testing machine. Force and displacement were logged until ultimate specimen failure, as well as video to record gross fracture patterns.

Results Post-failure examination indicated that successful filling of the synthetic shell by the expanding foam was achieved. Pilot results demonstrate the repeatability of the technique, with < 4% variation between specimens compared to mean initial fracture load and < 2.5% variation from mean ultimate load. Initial fracture occurred at approximately 67% of ultimate failure load. Initial fracture occurred consistently at the vertebral endplates which is similar to reported in vitro behaviour with cadaveric specimens. Investigation of the effects of cancellous foam elastic modulus is currently underway.

Discussion A synthetic L1 vertebra has been successfully developed, providing a highly repeatable analogue for investigation of the biomechanics of osteoporotic vertebral compression fractures. While the magnitude of the force obtained from the synthetic vertebrae differs from real human vertebrae due to differing material properties, comparative biomechanics between the synthetic and real vertebrae appear consistent, and fracture patterns are similar to those observed in cadaveric studies.

Introduction Endoscopic single rod anterior fusion surgery for the treatment of adolescent idiopathic scoliosis (AIS) offers the advantages of improved cosmetic results, the fusion of fewer segments and faster patient rehabilitation. The development of a patient-specific finite element model of the spine to be used to predict post-operative biomechanical outcomes of anterior AIS surgery will improve the pre-operative planning and performance of scoliosis instrumentation. This study aims to develop a methodology for validating the finite element modeling approach to scoliosis surgical planning by producing biomechanical data for movements of ovine lumbar spines both with and without anterior rod scoliosis instrumentation.

Methods Ovine lumbar spine specimens were CT scanned, dissected and instrumented across four levels (L2–L5) with a generic anterior single rod and screw implant for scoliosis correction. A displacement controlled 6 degree-of-freedom robotic facility was used to perform biomechanical testing on the spine segments for rotations of ±4 degrees in flexion/extension and lateral bending, and ±3 degrees in axial rotation. The tests were repeated with the rod removed. Resistive force and moment data was recorded using a force transducer and strain gauges on the surface of the rod yielded torsion and bending moment strain data, recorded on a data logger. All data was synchronized with the robot position data and filtered using moving average methods. The stiffness of the spines for each movement was calculated in units of Nm/degree of rotation.

Results As expected the results reflect the variability found in biological materials. The similarities of behaviour profiles however, support the use of this method for FE model validation. The addition of the rod caused an increase in stiffness for each movement. This increase was 17±7% and 23±10% for left and right axial rotation, 93±35% and 73±50% for left and right lateral bending, and 78±46% and 67±35% for flexion and extension respectively. Recorded strains on the rod surface did not exceed 400με.

Discussion The outcomes of this study have provided an experimental method for validating behaviour predicted by finite element models of the spine fitted with anterior scoliosis instrumentation. Using the CT scans of the ovine spines along with documentation of the experimental positioning of the specimens, the testing conditions can be simulated in a finite element model and the experimental and predicted biomechanical outcomes compared. The study also offers comparative information about the relative stiffness of the spine with and without scoliosis instrumentation.

Introduction: Low back pain (LBP) is an ailment affecting a large portion of the population and may result from degeneration of the intervertebral discs. Degeneration of the discs may be characterized by a loss of hydration, a more granular texture in the disc components and the presence of anular lesions which are tears in the anulus fibrosus. Research to date has been lacking in defining a relationship between LBP and anular lesions. In this study a materially and geometrically accurate finite element model (FEM) of an L4/5 intervertebral disc was developed in order to study the effects of anular lesions on the disc mechanics.

Methods: An anatomically accurate transverse profile for the disc FEM was derived from transversely sectioned human cadaveric discs. The anulus fibrosus ground substance was represented as an incompressible material using an Ogden hyperelastic strain energy equation. Material parameters were derived from experimentation on sheep discs. In order to separately assess the effects of degeneration of the nucleus and of the entire disc, four models were analysed. A healthy disc was modelled as reference and the three degenerate models comprised a degenerate nucleus (no hydrostatic nucleus pressure) with either a healthy anulus, or with a radial or rim anular lesion. Loading conditions to simulate the extreme range of physiological motions about the 6 axes of rotation were applied to the models and the peak rotation moments compared.

Results: The reduction in peak moment between the Healthy Disc FEM and the Healthy Anulus FEM ranged from 24% under flexion to 86% under right lateral bending. When the lesions were simulated, the rim and radial lesion resulted in variations in peak moment from the Healthy Anulus FEM of 1–10% and 0–4%, respectively.

Conclusions: The analysis suggested that loss of the nucleus pulposus pressure had a much greater effect on the disc mechanics than the presence of anular lesions. This indicated that the development of anular lesions prior to the degeneration of the nucleus would have minimal effect on the disc mechanics. But the response of an entirely degenerate disc would show significantly different mechanics compared to a healthy disc. With the degeneration of the nucleus, the disc stiffness will reduce and the outer innervated anulus may become overloaded and painful.

Introduction Vertebral rotation is an important aspect of spinal deformity in idiopathic scoliosis, associated with ribcage asymmetry. Although both lateral curvature and rotation appear to increase together in progressive scoliosis, the mechanisms driving vertebral rotation are not clearly established and it is not known whether lateral curvature precedes rotation, or vice versa. This paper investigates the hypothesis that intravertebral (within the bone) rotation in idiopathic scoliosis is caused by growth in the presence of gravity-induced torsions, the twisting moments generated by body weight forces acting on the scoliotic spine.

Methods Three-dimensional spinal curvature was measured for a small group of idiopathic scoliosis patients using standing radiographs and equations of static equilibrium were used to calculate gravity-induced torsion profiles along the length of each spine due to head, neck and torso weight. Intravertebral rotations were then measured for the same patients using Aaro & Dahlborn’s technique (Aaro S, et al; Spine 1982) with reformatted computed tomography images. The gravity-induced torsion curves were compared with rotation measurements to see whether gravity-induced torsion is a likely contributor to intravertebral rotation in scoliosis.

Results Gravity-induced torques as high as 7.5Nm act on the spines of idiopathic scoliosis patients due to static body weight in the standing position, and maximum intravertebral rotations (for a single vertebra) are approximately 7°. There appears to be general agreement between the measured intravertebral rotations and profiles of gravity-induced torsion along the length of the spine.

Discussion Gravity-induced torsion is a potential cause of vertebral rotation in idiopathic scoliosis. Since the spine must be curved in three-dimensions (out of plane) to produce such torques, vertebral rotation would be expected to occur subsequent to an initial lateral deviation, suggesting that lateral curvature precedes vertebral rotation in idiopathic scoliosis.

Introduction A very specific group within the 80 percent of the population that suffers from low back pain at some stage in life are young cricket fast bowlers. Amongst them a high occurrence of unilateral L4 pars interarticularis fractures exists, which shows a strong statistical correlation to the presence of a contralateral volumetric increase in the Quadratus Lumborum (QL) muscle. However, there is no clear physical link between these two phenomena. To investigate this relationship, we have combined a mathematical model of the lumbar spine muscles with a finite element model of the fourth lumbar vertebra and analysed the stresses occurring in the L4 vertebra throughout the bowling motion.

Methods A mathematical model of the lumbar spine muscles has been developed previously at QUT. It contains 170 fascicles representing all major muscles in the lumbar region and allows for analysis of the forces and moments on the intervertebral joints caused by these muscles in any given posture. A Finite Element Model (FEM) of an L4 vertebra and intervertebral disc (IVD) was developed based on one created by Theo Smit and obtainable from the Internet through the BEL Repository of the Istituti Ortopedici Rizzoli, Bologna, Italy. Material properties were obtained from literature, while muscle forces, directions and attachment locations in the different postures came from the mathematical model. Six postures occurring in right-handed fast bowling were modelled to determine the differences in stresses between having symmetric and asymmetric QL muscles. The asymmetric condition consisted of a 30% increase in Physiological Cross-Sectional Area (PCSA) on the right side. In all cases it was assumed the left facet joints were ‘locked up’, to create a presumed worst-case scenario for the stress build-up in the pars.

Results It was found that when using muscle activation levels from literature an enlarged right-side QL did not increase the stresses in the left pars noticeably, in fact in some cases it even slightly reduced those stresses. When only the right-side QL muscle was activated, while all other muscles only provided passive muscle force, a 30% PCSA increase of this muscle produced an increase in maximum Von Mises and principal stresses in the left-side pars from typically 30 MPa to 40 MPa but only in the postures close to upright stance. In more extreme postures where the maximum stresses in the pars are higher, the increased PCSA of the right QL only led to small stress increases from typically 125 to 129 MPa.

Discussion Even in the worst-case scenario where only the right-side QL is active and the left-side facet joint is locked up, a PCSA increase of that muscle does not cause a large increase in stresses in postures where the stresses are high. Hence, this study has not demonstrated a clear physical link between asymmetric hypertrophy of QL and pars fractures. It may even suggest the hypertrophy is a response to postural overload attempting to reduce stresses in the pars. To clarify this, an improved FEM of the L3 and L4 vertebrae and IVDs, including all ligaments, is currently being developed. We believe that in the future this combination of models can be used for many more purposes where the influence of posture and musculature on the lumbar spine biomechanics needs investigation.

Introduction: Contemporary surgical interventions for adolescent idiopathic scoliosis (AIS) include both anterior and posterior rod systems, in which a single or double rod construct provides curve correction and stability. This paper presents a methodology for development of patient-specific finite element methods to predict the biomechanical outcomes of scoliosis surgery pre-operatively, with the aim of optimising the performance of instrumentation constructs for anterior single rod AIS surgery.

Methods: Geometry for each patient-specific finite element model is obtained from pre-operative thoracolumbar CT scans taken in the supine position using a low dose multi-slice imaging protocol. The finite element model incorporates vertebrae, intervertebral discs, and posterior processes with associated ligaments and zygapophysial joints. A custom pre-processor generates the entire model according to user-specified meshing parameters, providing rapid model generation once the geometric parameters have been extracted from each CT dataset. Material properties are currently based on published values. Simulated movements about axes corresponding to flexion/extension, left/right lateral bending, and trunk rotation are solved using the ABAQUS/Standard software, allowing assessment of predicted loads and stresses before and after addition of instrumentation.

Results: The total time per patient required for model generation is currently about six hours, with manual measurement of spine geometry from the CT stack accounting for most of this time. Actual solution time for each finite element model is expected to be around four hours, making patient-specific pre-operative planning for endoscopic scoliosis surgery a feasible option at least in terms of processing time per patient.

Discussion: A finite element methodology has been developed for patient-specific simulation of endoscopic scoliosis surgery. Issues to be addressed in future include prescription of patient-specific material properties, analysis of errors associated with geometry measurement from CT scans, and validation of the methodology by comparison of predicted and actual outcomes for scoliosis patients. Patient-specific simulation of scoliosis surgery has the potential to optimize surgical outcomes and reduce biomechanical complications associated with the use of endoscopic scoliosis instrumentation systems.

Introduction: An estimated 80% of all adults will experience back pain at sometime during their life. To aid in the understanding of how the spine functions as a mechanical system and assist clinicians in their diagnosis this study produced 3D models of the muscles in the lumbar spine region. The models show selected muscles at rest and during controlled activities.

Methods: The images were acquired on a Siemens Sonata 1.5T System using breathhold FISP sequences. Twenty slices of thickness 5mm and zero separation were acquired using an in-plane resolution of .68mm and Fast-Fourier-Transformed to 512 x 512. Single acquisitions were acquired per slice. Imaging time per posture (rest, extension, left rotation and right rotation) was approximately 17–20 seconds. All image series conformed to the DICOM Standard.

The code developed for this study was written in Interactive Data Language (IDL) Version 5.5 from Research Systems Inc (RSI).

Each slice from an image series was displayed to an Operator, who roughly selected the muscle(s) boundary. The user-selected points were then compared with the 24-neighbouring pixels, and the vertices moved to the minimum value in the 5x5 area, which corresponds to the muscle boundary. The adjusted region of interest was then displayed to the user for verification. Once the Operator had completed selection of the regions of interest in all slices, spatial smoothing was performed on the data, and 3D models of the muscles constructed.

Results: This analysis produces 3D images of the muscles in the lower back. The visualisation of the data enables different combinations of muscle and posture to be displayed. Typically, a muscle at rest is overlaid with one of the three controlled activities – extension, left or right extension. The 3D models can be displayed as either a meshed or solid object.

The 3D model is displayed in a window that enables an operator using a mouse to rotate, scale and/or translate the model.

To aid visualisation, the volume of each muscle of interest is calculated using the number of pixels within the region of interest, pixel spacing and slice thickness. The result, in mm³, is displayed alongside the 3D model.

Discussion: The refinement of MR Imaging techniques for subjects in a variety of postures, and the development of post processing techniques provides a useful tool for all in the understanding of the mechanics of the lumbar spine. It is envisaged that this tool with further analysis will assist in determining if there is a link between muscle volume during movement and lower back pain.

INTRODUCTION: A computer model of the L4/5 human intervertebral disc is currently under development. An integral aspect of this model is the material properties assigned to its components. Detailed data on the material properties of the anulus fibrosus ground matrix are not available in the existing literature. To determine these properties, mechanical tests were carried out on specimens of anulus fibrosus harvested from sheep spines. The tests included unconfined uniaxial compression, simple shear and biaxial compression. Data on the strain required to cause permanent damage in the anulus ground matrix and data on the mechanical response of the anulus to repeated loading were obtained.

METHODS: Intervertebral discs were isolated from the lumbar spines of recently sacrificed sheep. These discs were sectioned into test specimens ensuring there were no continuous collagen fibres bearing load. The edge dimensions of the cubic specimens were 3 ± 0.2 mm. To ascertain the strain to initiate tissue damage, the specimens underwent successive loadings, which were carried out one hour apart to allow recovery. The maximum strain in each test was increased incrementally by 5% until a reduction in stiffness was observed in the following test. Separate tests were carried out to quantify and characterise the response of the anulus ground matrix in the three modes of loading and to strains greater than that which initiates damage.

RESULTS: The strains at which permanent tissue damage occurred were between 20 and 27% in uniaxial compression and between 25 and 35% in simple shear. Testing the specimen beyond these strains showed an obvious reduction in stiffness. The biaxial compression tests showed similar changes but did not result in such pronounced losses in stiffness. The material characteristics were reproducible up to 20% strain. Following deformation to higher strains the altered mechanics were also shown to be reproducible indicating that the matrix had been deranged but not failed.

DISCUSSION: Average physiological strains in the L4/5 intervertebral disc are in the order of 10–50% based on maximum deformations observed in vivo. The current results demonstrate that this strain will cause some permanent damage to the anulus ground matrix, however, the matrix will still be capable of carrying stress upon repeated loading. Thompson et. al1 found that people over the age of 35 all exhibited signs of disc degeneration. We hypothesise that the regenerative ability of the anulus ceases to function effectively as we age and the continual damage caused to the anulus tissue by daily activities may lead to the degenerative changes seen in the anulus.

Knowledge of the material characteristics up to 20% strain and following exposure to higher strains will enable a more realistic model of the intervertebral disc and the effects of degeneration to be studied.

INTRODUCTION: The complexity of the spine has made a complete understanding of its mechanical function difficult. As a consequence, biomechanical models have been used to describe the behaviour of the spine and its various components. A comprehensive mathematical model of the muscles of the lumbar spine and trunk is presented to enable computation of the forces and moments experienced by the lumbar intervertebral joints during physiological activities.

METHODS: The model includes the nine major muscles crossing the region and concentrates on improving the estimated line of action for the muscles. The muscles are considered to consist of numerous fascicles, each with its own force producing potential based on size and line of action. The model respects the physical constraints imposed by the skeletal structure by ensuring that muscles maintain their anatomical position in various spinal postures. Validation was performed by comparing model predictions of maximum moments to published data from maximum isometric exertions in male volunteers. To highlight the potential novel uses of the model, three examples of muscle injury caused by surgical procedures were investigated; posterior lumbar surgery, impairment of abdominal muscles from anterior surgery and removal of the psoas major unilaterally during total hip replacement.

RESULTS: The validation indicated that the model predicted forces similar to those measured in normal volunteers. The biomechanical changes resulting from the muscle injuries during the surgical procedures share several common features: decreased spinal compression and production of asymmetric moments during symmetric tasks.

DISCUSSION: The results suggest that interference with muscles crossing or attaching to the lumbar spine can have a significant impact on its function.

INTRODUCTION: An estimated 80% of all adults will experience back pain at some time during their life. To aid in the understanding of how the spine functions as a mechanical system and assist clinicians in their diagnosis this study produced 3D models of the muscles in the lumbar spine region. The models show selected muscles at rest and during controlled activities.

METHODS: The images were acquired on a Siemens Sonata 1.5T System using breathhold FISP sequences. Twenty slices of thickness 5 mm and zero separation were acquired using an in-plane resolution of .68 mm and Fast-Fourier-Transformed to 512 x 512. Single acquisitions were acquired per slice. Imaging time per posture (rest, extension, left rotation and right rotation) was approximately 17–20 seconds. All image series conformed to the DICOM Standard.

The code developed for this study was written in Interactive Data Language (IDL) Version 5.5 from Research Systems Inc (RSI).

Each slice from an image series was displayed to an Operator, who roughly selected the muscle(s) boundary. The user-selected points were then compared with the 24-neighbouring pixels, and the vertices moved to the minimum value in the 5x5 area, which corresponds to the muscle boundary. The adjusted region of interest was then displayed to the user for verification. Once the Operator had completed selection of the regions of interest in all slices, spatial smoothing was performed on the data, and 3D models of the muscles constructed. RESULTS: This analysis produces 3D images of the muscles in the lower back. The visualisation of the data enables different combinations of muscle and posture to be displayed. Typically, a muscle at rest is overlaid with one of the three controlled activities – extension, left or right extension. The 3D models can be displayed as either a meshed or solid object.

The 3D model is displayed in a window that enables an operator using a mouse to rotate, scale and/or translate the model.

To aid visualisation, the volume of each muscle of interest is calculated using the number of pixels within the region of interest, pixel spacing and slice thickness. The result, in mm³, is displayed alongside the 3D model.

DISCUSSION: The refinement of MR Imaging techniques for subjects in a variety of postures, and the development of post processing techniques provides a useful tool for all in the understanding of the mechanics of the lumbar spine. It is envisaged that this tool with further analysis will assist in determining if there is a link between muscle volume during movement and lower back pain.

INTRODUCTION: Contrary to the prevailing conviction that lumbar segments affected by lytic spondylolisthesis are unstable, multiple studies have failed to find evidence of increased or abnormal motion at these segments. Affected segments do not exhibit excessive anterior translation: the so-called slip. Previous studies, however, did not use techniques that might reveal abnormalities in the quality of motion, as opposed to its magnitude.

METHODS: To determine if features of instability could be detected in the radiographs of patients with spondylolisthesis, a retrospective, cohort study was conducted of the kinematics of the lumbar spine of patients with spondylolisthesis compared with asymptomatic normal subjects. The flexion-extension radiographs of 15 patients with spondylolytic spondylolisthesis were analysed to determine the location of their instantaneous centres of rotation, and their magnitudes of translation and sagittal rotation. Normative data were obtained by applying the same techniques to the radiographs of 20 asymptomatic subjects.

RESULTS: All but one of the 15 patients exhibited at least one segment with abnormal motion. Only one patient had excessive translation at the lytic segment. Four had minor abnormalities affecting either the lytic segment or ones above. Nine patients exhibited major abnormalities. Seven had paradoxical motion at the lytic segment, in which the centre of rotation was located above L5, instead of below, and in which L5 translated backwards, instead of forwards, during flexion. Two patients exhibited axial dropping of L4, instead of horizontal translation, during extension.

DISCUSSION: Not all patients with spondylolisthesis show features of instability. However, a proportion of patients exhibit highly abnormal movements that are consistent with instability. The abnormalities involve movements within normal range but in abnormal directions. Visual inspection of radiographs will not reveal these abnormalities but they can be detected by plotting the instantaneous axes of rotation.

INTRODUCTION: Structural changes to the intervertebral disc (IVD) in the form of anular lesions are a feature of IVD degeneration. Degeneration has been related to changes in the mechanical function of the IVD. This study determined the mechanical effect of individual concentric tears, radial tears and rim lesions of the anulus in an in vitro experiment.

METHODS: The lumbar spines from five sheep were taken post mortem and divided into three motion segments. The disc body units were tested on a robotic testing facility, using position control, in flexion/extension, lateral bending and axial rotation. Concentric tears, radial tears and rim lesions were experimentally introduced and the motions repeated after the introduction of each lesion. The mechanical response after the lesion creation was compared to the undamaged response to assess the mechanical effect of each lesion.

RESULTS: It was found that an anterior rim lesion reduced the peak moment resisted by the disc in extension, lateral bending and axial rotation. Concentric tears and radial tears did not affect the peak moment resisted, however, radial tears reduced the hysteresis of response in flexion/extension and lateral bending. The neutral zone was not affected by the presence of IVD lesions.

DISCUSSION: These results show that rim lesions reduce the disc’s ability to resist motion. Radial tears change the hysteresis of response indicating an altered stress distribution in the disc. These changes may lead to overloading of the spinal ligaments, muscles and zygapophysial joints, possibly damaging these structures. This suggests a mechanism for a cycle of degeneration that is instigated by small changes in the mechanical integrity of the IVD.

Introduction: the neutral zone is defined as a region of no or little resistance to motion in the middle of an intervertebral joint’s range of movement. Previous studies have used quasistatic loading regimes that do not model physiological activity¹. The aim of the present study was to assess experimentally the existence of the neutral zone of intervertebral joints during spinal motion in flexion/extension, lateral bending and axial rotation during physiological movements simulated using a robotic testing facility. Sheep intervertebral joints were used as they have been shown to exhibit similar mechanical behaviour to human joints².

Methods: five spines from mature sheep were used. Three specimens were tested from each spine to simulate human l1/2, l3/4 and l4/5 intervertebral joints. The robotic facility enabled the testing regime to be defined for each individual joint based on its geometry. The joints were tested by cycling through the full range of physiological movement in flexion/extension, lateral bending and axial rotation.

Results: a neutral zone was found to exist during dynamic movements only in flexion/extension. The results were equivocal for lateral bending and suggested that a neutral zone does not exist in axial rotation. The zygapophysial joints were shown to be significant in determining the mechanics of the intervertebral joints as their removal increased the neutral zone in all cases. A criterion for defining the size of the neutral zone was proposed.

Conclusions: a neutral zone exists in flexion/extension during dynamic movements of intervertebral joints. This has important implications for the muscular control of the spine consisting of several intrinsically lax joints stacked on one another.

Assessment of bony union after anterior fusion of the lumbar spine has previously relied on the skilled interpretation of plain radiograph. A biplanar radiographic technique was used to measure small movements between vertebrae and to give a quantitative measure of bony union in 11 patients who had undergone interbody fusion with autogenous bone chips at one level in the lumbar spine. The investigation gave three types of results: bony union, where the fused level showed marked restriction of movement relative to the rest of the lumbar spine; paradoxical movement, where the fused joint showed marked reverse movement (when the patient flexed, the fused level of the lumbar spine extended) which was thought to be due to an anterior bony bar which caused an altered pattern of movement; and non-union, where the level of fusion showed no restriction of movement. The intervertebral joint above the level of fusion was shown to move more than the other joints in the lumbar spine. The study showed that bony union is possible with the use of autogenous cancellous bone chips, and that biplanar radiographic technique can determine the extent of union.