Understanding how axial loads on the spine influence segmental biomechanics for idiopathic scoliosis patients: A magnetic resonance imaging study

BACKGROUND

Segmental biomechanics of the scoliotic spine are important since the overall spinal deformity is comprised of the cumulative coronal and axial rotations of individual joints. This study investigates the coronal plane segmental biomechanics for adolescent idiopathic scoliosis patients in response to physiologically relevant axial compression.

METHODS

Individual spinal joint compliance in the coronal plane was measured for a series of 15 idiopathic scoliosis patients using axially loaded magnetic resonance imaging. Each patient was first imaged in the supine position with no axial load, and then again following application of an axial compressive load. Coronal plane disc wedge angles in the unloaded and loaded configurations were measured. Joint moments exerted by the axial compressive load were used to derive estimates of individual joint compliance.

FINDINGS

The mean standing major Cobb angle for this patient series was 46°. Mean intra-observer measurement error for endplate inclination was 1.6°. Following loading, initially highly wedged discs demonstrated a smaller change in wedge angle, than less wedged discs for certain spinal levels (+2,+1,-2 relative to the apex, (p<0.05)). Highly wedged discs were observed near the apex of the curve, which corresponded to lower joint compliance in the apical region.

INTERPRETATION

While individual patients exhibit substantial variability in disc wedge angles and joint compliance, overall there is a pattern of increased disc wedging near the curve apex, and reduced joint compliance in this region. Approaches such as this can provide valuable biomechanical data on in vivo spinal biomechanics of the scoliotic spine, for analysis of deformity progression and surgical planning.

A biomechanical investigation of dual growing rods used for fusionless scoliosis correction

BACKGROUND

The use of dual growing rods is a fusionless surgical approach to the treatment of early onset scoliosis which aims to harness potential growth and correct spinal deformity. The purpose of this study was to compare the in-vitro biomechanical response of two different dual rod designs under axial rotation loading.

METHODS

Six porcine spines were dissected into seven level thoracolumbar multi-segment units. Each specimen was mounted and tested in a biaxial Instron machine, undergoing nondestructive left and right axial rotation to peak moments of 4 Nm at a constant rotation rate of 8 deg. s(-1). A motion tracking system (Optotrak) measured 3D displacements of individual vertebrae. Each spine was tested in an un-instrumented state first and then with appropriately sized semi-constrained and 'rigid' growing rods in alternating sequence. The range of motion, neutral zone size and stiffness were calculated from the moment-rotation curves and intervertebral range of motion was calculated from Optotrak data.

FINDINGS

Irrespective of test sequence, rigid rods showed a significant reduction of total rotation across all instrumented levels (with increased stiffness) whilst semi-constrained rods exhibited similar rotational behavior to the un-instrumented spines (P<0.05). An 11.1% and 8.0% increase in stiffness for left and right axial rotation respectively and 14.9% reduction in total range of motion were recorded with dual rigid rods compared with semi-constrained rods.

INTERPRETATION

Based on these findings, the Semi-constrained growing rods were shown to not increase axial rotation stiffness compared with un-instrumented spines. This is thought to provide a more physiological environment for the growing spine compared to dual rigid rod constructs.

A three-dimensional mathematical model of the thoracolumbar fascia and an estimate of its biomechanical effect

The thoracolumbar fascia (TLF) provides a means of attachment to the lumbar spine for several muscles including the transverse abdominis, and parts of the latissimus dorsi and internal oblique muscles. Previous biomechanical models of the lumbar spine either tend to omit the TLF on the assumption that its contribution would be negligible or incorporate only part of the TLF. Here, a three-dimensional model of the posterior and middle layers of the TLF is presented to enable its action to be included in future three-dimensional models of the spine. It is used illustratively to estimate the biomechanical influence of this structure on the lumbar spine. The formulation of the model allows the lines of action of the fibres comprising the fascia to be calculated for any posture whilst ensuring that anatomical constraints are satisfied. Application of the model suggests that the TLF produces moments primarily in flexion and extension. The simulated results demonstrate that the abdominal muscles, acting via the TLF, are capable of contributing extension moments comparable to those produced by other smaller muscles associated with the lumbar spine.

A new approach for assigning bone material properties from CT images into finite element models

Generation of subject-specific finite element (FE) models from computed tomography (CT) datasets is of significance for application of the FE analysis to bone structures. A great challenge that remains is the automatic assignment of bone material properties from CT Hounsfield Units into finite element models. This paper proposes a new assignment approach, in which material properties are directly assigned to each integration point. Instead of modifying the dataset of FE models, the proposed approach divides the assignment procedure into two steps: generating the data file of the image intensity of a bone in a MATLAB program and reading the file into ABAQUS via user subroutines. Its accuracy has been validated by assigning the density of a bone phantom into a FE model. The proposed approach has been applied to the FE model of a sheep tibia and its applicability tested on a variety of element types. The proposed assignment approach is simple and illustrative. It can be easily modified to fit users' situations.

Computational investigations of mechanical failures of internal plate fixation

This paper investigated the biomechanics of two clinical cases of bone fracture treatments. Both fractures were treated with the same locking compression plate but with different numbers of screws as well as different plate materials. The fracture treated with 12 screws (rigid fixation) failed at 7 weeks with the plate breaking; the fracture with six screws (flexible fixation) endured the entire healing process. It was hypothesized that the plate failure in the unsuccessful case was due to the material fatigue induced by stress concentration in the plate. As the two clinical cases had different fracture locations and different plate materials, finite element simulations were undertaken for each fractured bone fixed by both a rigid and a flexible method. This enabled comparisons to be made between the rigid and flexible fixation methods. The fatigue life was assessed for each fixation method. The results showed that the stress in the rigid fixation methods could be significantly higher than that in flexible fixation methods. The fatigue analyses showed that, with the stress level in flexible fixation (i.e. with fewer screws), the plate was able to endure 2000 days, and that the plate in rigid fixation could fail by fatigue fracture in 20 days. The paper concludes that the rigid fixation method resulted in serious stress concentrations in the plate, which induced fatigue failure. The flexible fixation gave sufficient stability and was better for fracture healing.

The mechanical response of the ovine lumbar anulus fibrosus to uniaxial, biaxial and shear loads

Analytical and computational models of the intervertebral disc (IVD) are commonly employed to enhance understanding of the biomechanics of the human spine and spinal motion segments. The accuracy of these models in predicting physiological behaviour of the spine is intrinsically reliant on the accuracy of the material constitutive representations employed to represent the spinal tissues. There is a paucity of detailed mechanical data describing the material response of the reinforced-ground matrix in the anulus fibrosus of the IVD. In the present study, the 'reinforced-ground matrix' was defined as the matrix with the collagen fibres embedded but not actively bearing axial load, thus incorporating the contribution of the fibre-fibre and fibre-matrix interactions. To determine mechanical parameters for the anulus ground matrix, mechanical tests were carried out on specimens of ovine anulus, under unconfined uniaxial compression, simple shear and biaxial compression. Test specimens of ovine anulus fibrosus were obtained with an adjacent layer of vertebral bone/cartilage on the superior and inferior specimen surface. Specimen geometry was such that there were no continuous collagen fibres coupling the two endplates. Samples were subdivided according to disc region - anterior, lateral and posterior - to determine the regional inhomogeneity in the anulus mechanical response. Specimens were loaded at a strain rate sufficient to avoid fluid outflow from the tissue and typical stress-strain responses under the initial load application and under repeated loading were determined for each of the three loading types. The response of the anulus tissue to the initial and repeated load cycles was significantly different for all load types, except biaxial compression in the anterior anulus. Since the maximum applied strain exceeded the damage strain for the tissue, experimental results for repeated loading reflected the mechanical ability of the tissue to carry load, subsequent to the initiation of damage. To our knowledge, this is the first study to provide experimental data describing the response of the 'reinforced-ground matrix' to biaxial compression. Additionally, it is novel in defining a study objective to determine the regionally inhomogeneous response of the 'reinforced-ground matrix' under an extensive range of loading conditions suitable for mechanical characterisation of the tissue. The results presented facilitate the development of more detailed and comprehensive constitutive descriptions for the large strain nonlinear elastic or hyperelastic response of the anulus ground matrix.

A naturally shaped silicone ventricle evaluated in a mock circulation loop: a preliminary study

Mock circulation loops are used to evaluate the performance of cardiac assist devices prior to animal and clinical testing. A compressible, translucent silicone ventricle chamber that mimics the exact size, shape and motion of a failing heart is desired to assist in flow visualization studies around inflow cannulae during VAD support. The aim of this study was therefore to design and construct a naturally shaped flexible left ventricle and evaluate its performance in a mock circulation loop. The ventricle shape was constructed by the use of CT images taken from a patient experiencing cardiomyopathic heart failure and used to create a 3D image and subsequent mould to produce a silicone ventricle. Different cardiac conditions were successfully simulated to validate the ventricle performance, including rest, left heart failure and VAD support.

Simulation of the nutrient supply in fracture healing

The healing process for bone fractures is sensitive to mechanical stability and blood supply at the fracture site. Most currently available mechanobiological algorithms of bone healing are based solely on mechanical stimuli, while the explicit analysis of revascularization and its influences on the healing process have not been thoroughly investigated in the literature. In this paper, revascularization was described by two separate processes: angiogenesis and nutrition supply. The mathematical models for angiogenesis and nutrition supply have been proposed and integrated into an existing fuzzy algorithm of fracture healing. The computational algorithm of fracture healing, consisting of stress analysis, analyses of angiogenesis and nutrient supply, and tissue differentiation, has been tested on and compared with animal experimental results published previously. The simulation results showed that, for a small and medium-sized fracture gap, the nutrient supply is sufficient for bone healing, for a large fracture gap, non-union may be induced either by deficient nutrient supply or inadequate mechanical conditions. The comparisons with experimental results demonstrated that the improved computational algorithm is able to simulate a broad spectrum of fracture healing cases and to predict and explain delayed unions and non-union induced by large gap sizes and different mechanical conditions. The new algorithm will allow the simulation of more realistic clinical fracture healing cases with various fracture gaps and geometries and may be helpful to optimise implants and methods for fracture fixation.

Development of a biaxial compression device for biological samples: preliminary experimental results for a closed cell foam

Biological tissues are subjected to complex loading states in vivo and in order to define constitutive equations that effectively simulate their mechanical behaviour under these loads, it is necessary to obtain data on the tissue's response to multiaxial loading. Single axis and shear testing of biological tissues is often carried out, but biaxial testing is less common. We sought to design and commission a biaxial compression testing device, capable of obtaining repeatable data for biological samples. The apparatus comprised a sealed stainless steel pressure vessel specifically designed such that a state of hydrostatic compression could be created on the test specimen while simultaneously unloading the sample along one axis with an equilibrating tensile pressure. Thus a state of equibiaxial compression was created perpendicular to the long axis of a rectangular sample. For the purpose of calibration and commissioning of the vessel, rectangular samples of closed cell ethylene vinyl acetate (EVA) foam were tested. Each sample was subjected to repeated loading, and nine separate biaxial experiments were carried out to a maximum pressure of 204 kPa (30 psi), with a relaxation time of two hours between them. Calibration testing demonstrated the force applied to the samples had a maximum error of 0.026 N (0.423% of maximum applied force). Under repeated loading, the foam sample demonstrated lower stiffness during the first load cycle. Following this cycle, an increased stiffness, repeatable response was observed with successive loading. While the experimental protocol was developed for EVA foam, preliminary results on this material suggest that this device may be capable of providing test data for biological tissue samples. The load response of the foam was characteristic of closed cell foams, with consolidation during the early loading cycles, then a repeatable load-displacement response upon repeated loading. The repeatability of the test results demonstrated the ability of the test device to provide reproducible test data and the low experimental error in the force demonstrated the reliability of the test data.

Parametric equations to represent the profile of the human intervertebral disc in the transverse plane

Computational and finite element models of the spine are used to investigate spine and disc mechanics. Subject specific data for the transverse profile of the disc could improve the geometric accuracy of these models. The current study aimed to develop a mathematical algorithm to describe the profile of the disc components, using subject-specific data points. Using data points measured from pictures of human intervertebral discs sectioned in the transverse plane, parametric formulae were derived that mapped the outer profile of the anulus and nucleus. The computed anulus and nucleus profile were a similar shape to the discs from which they were derived. The computed total disc area was similar to the experimental data. The nucleus:disc area ratios were sensitive to the data points defined for each disc. The developed formulae can be easily implemented to provide patient specific data for the disc profile in computational models of the spine.

Re-design of the Exeter V40 long-stem femoral component for ease of removal

Clinical experience shows that removal of the Exeter long-stem femoral component (220 mm, 240 mm, 260 mm) of total hip arthroplasty is extremely difficult, often requiring splitting of the femur. To identify the reason for this, measurements of stem geometry and force required to pull the stems out of the cement mantle were conducted on three original Exeter long-stem and one standard femoral components. All implants required an initial force of approximately 4 kN for release from the cement. The long-stem components then required much larger forces and hence much higher expenditure of energy to pull them clear of the cement. This was attributed to the reverse taper seen on the nominally cylindrical distal section of the long-stem components. Following re-design of the manufacturing process to ensure the taper continued to the implant's distal tip, four further implants were tested. These demonstrated the requirement for initial cement release but then required no further energy expenditure similar to the standard stem. This study clearly demonstrated that the original difficulty in removing these long stems was owing to the manufacturing process resulting in a reverse taper on the distal stem. The adoption of recommended manufacturing changes to ensure the taper continues to the distal tip removed this difficulty.

Nonlinear finite element analysis of anular lesions in the L4/5 intervertebral disc

Degenerate intervertebral discs exhibit both material and structural changes. Structural defects (lesions) develop in the anulus fibrosus with age. While degeneration has been simulated in numerous previous studies, the effects of structural lesions on disc mechanics are not well known. In this study, a finite element model (FEM) of the L4/5 intervertebral disc was developed in order to study the effects of anular lesions and loss of hydrostatic pressure in the nucleus pulposus on the disc mechanics. Models were developed to simulate both healthy and degenerate discs. Degeneration was simulated with either rim, radial or circumferential anular lesions and by equating nucleus pressure to zero. The anulus fibrosus ground substance was represented as a nonlinear incompressible material using a second-order polynomial, hyperelastic strain energy equation. Hyperelastic material parameters were derived from experimentation on sheep discs. Endplates were assumed to be rigid, and annulus lamellae were assumed to be vertical in the unloaded state. Loading conditions corresponding to physiological ranges of rotational motion were applied to the models and peak rotation moments compared between models. Loss of 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 alone (prior to degeneration of the nucleus) has minimal effect on disc mechanics, but that disc stiffness is significantly reduced by the loss of hydrostatic pressure in the nucleus. With the degeneration of the nucleus, the outer innervated anulus or surrounding osteo-ligamentous anatomy may therefore experience increased strains.

Nonlinear finite element analysis of anular lesions in the L4/5 intervertebral disc

Degenerate intervertebral discs exhibit both material and structural changes. Structural defects (lesions) develop in the anulus fibrosus with age. While degeneration has been simulated in numerous previous studies, the effects of structural lesions on disc mechanics are not well known. In this study, a finite element model (FEM) of the L4/5 intervertebral disc was developed in order to study the effects of anular lesions and loss of hydrostatic pressure in the nucleus pulposus on the disc mechanics. Models were developed to simulate both healthy and degenerate discs. Degeneration was simulated with either rim, radial or circumferential anular lesions and by equating nucleus pressure to zero. The anulus fibrosus ground substance was represented as a nonlinear incompressible material using a second-order polynomial, hyperelastic strain energy equation. Hyperelastic material parameters were derived from experimentation on sheep discs. Endplates were assumed to be rigid, and annulus lamellae were assumed to be vertical in the unloaded state. Loading conditions corresponding to physiological ranges of rotational motion were applied to the models and peak rotation moments compared between models. Loss of 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 alone (prior to degeneration of the nucleus) has minimal effect on disc mechanics, but that disc stiffness is significantly reduced by the loss of hydrostatic pressure in the nucleus. With the degeneration of the nucleus, the outer innervated anulus or surrounding osteo-ligamentous anatomy may therefore experience increased strains.

An experimental and finite element poroelastic creep response analysis of an intervertebral hydrogel disc model in axial compression

A hydrogel intervertebral disc (IVD) model consisting of an inner nucleus core and an outer anulus ring was manufactured from 30 and 35% by weight Poly(vinyl alcohol) hydrogel (PVA-H) concentrations and subjected to axial compression in between saturated porous endplates at 200 N for 11 h, 30 min. Repeat experiments (n=4) on different samples (N=2) show good reproducibility of fluid loss and axial deformation. An axisymmetric nonlinear poroelastic finite element model with variable permeability was developed using commercial finite element software to compare axial deformation and predicted fluid loss with experimental data. The FE predictions indicate differential fluid loss similar to that of biological IVDs, with the nucleus losing more water than the anulus, and there is overall good agreement between experimental and finite element predicted fluid loss. The stress distribution pattern indicates important similarities with the biological IVD that includes stress transference from the nucleus to the anulus upon sustained loading and renders it suitable as a model that can be used in future studies to better understand the role of fluid and stress in biological IVDs.

Impact data for the investigation of injuries in inflatable rescue boats (IRBS)

Inflatable Rescue Boats (IRBs) are arguably the most important rescue tools utilised by Australian Surf Lifesavers. The crews in the IRB are continuously battling the fierce element that is the ocean. This force of nature takes its toll on man and machine. Initial impact data for this unique situation has been gathered as part of a biomechanical study investigating the increasing frequency of injuries to surf lifesavers whilst using an IRB. This paper outlines the scope of the research topic and concentrates on the data gathering equipment and an analysis of this unique data set. This initial testing has revealed impact acceleration peaks up to and exceeding 400 m/s2 (40 g) for a period of about 20 ms. These values were a result of an impact with waves of moderate size (approximately 1 m). It was therefore concluded that the impact is of a significant nature and further work should be performed to determine more concise ride characteristics for the IRB. From that it is hoped that methods will be discovered to lessen the impact on the crew with the aim of decreasing the injury rate.

Disc lesions and the mechanics of the intervertebral joint complex

STUDY DESIGN

Correlations between tears in the disc and the mechanics of both the intervertebral joint and vertebral body bone were analyzed.

OBJECTIVES

To examine the effect of disc degeneration on the mechanics of spinal motion segments.

SUMMARY OF BACKGROUND DATA

Degeneration of the intervertebral disc results in changes to the mechanics of the spine. The actual effect of tear type and size on the mechanics of the intervertebral joint is unknown.

METHODS

Thirty spinal specimens (median age, 68 years) were divided into T12-L1, L2-L3, and L4-L5 motion segments. Mechanical tests recorded stiffness in flexion, extension, and torsion. Disc morphology was ascertained by taking three transverse sections of the disc and mapping and measuring the concentric tears, radial tears, and rim lesions. The severity of each tear type within each disc then was quantified. Bone cubes from the adjacent vertebral bodies were tested in compression to determine the elastic moduli and tested to failure in the longitudinal direction.

RESULTS

Groups with tears were older and had reduced bone elastic moduli than groups without tears. Extension stiffness for the intact joint tended to increase with increasing tear severity. A decrease in torsional stiffness was present with increased severity of rim lesions at both L2-L3 and L4-L5.

CONCLUSIONS

Tears in the intervertebral disc are reflected in a reduction in vertebral bone elastic modulus and in changes in the mechanics of the intervertebral joints in flexion, extension, and torsion.

The skeletal response to matt and polished cemented femoral stems

We studied the effect of the surface finish of the stem on the transfer of load in the proximal femur in a sheep model of cemented hip arthroplasty. Strain-gauge analysis and corresponding finite-element (FE) analysis were performed to assess the effect of friction and creep at the cement-stem interface. No difference was seen between the matt and polished stems. FE analysis showed that the effects of cement creep and friction at the stem-cement interface on femoral strain were small compared with the effect of inserting a cemented stem.

The skeletal response to matt and polished cemented femoral stems

We studied the effect of the surface finish of the stem on the transfer of load in the proximal femur in a sheep model of cemented hip arthroplasty. Strain-gauge analysis and corresponding finite-element (FE) analysis were performed to assess the effect of friction and creep at the cement-stem interface.

No difference was seen between the matt and polished stems. FE analysis showed that the effects of cement creep and friction at the stem-cement interface on femoral strain were small compared with the effect of inserting a cemented stem.

Three-dimensional clinical measurement of bilateral hip and knee rotations

This paper reports a technique of using an electromagnetic system to measure three-dimensional bilateral hip and knee joint rotations during walking and of using peak cross-correlation between rotation angles to describe the pattern of rotations. Three-dimensional rotations of thigh and shank during gait were recorded using five receivers of the electromagnetic system synchronised with four foot-switches. Thirteen normal subjects were tested on two separate occasions to examine repeatability of the measurements. The relationship between the rotations was represented as lags at peak cross-correlation. Twelve parameters of the lags at peak cross-correlation were calculated. The analysis showed that the joint rotation could be measured reliably and the cross-correlation analysis provided parameters that were generally suitable for defining characteristics of hip and knee joint rotations during gait.

Difficulties in estimating muscle forces from muscle cross-sectional area. An example using the psoas major muscle

Most biomechanical models use muscle cross-sectional area (CSA) as an indicator of maximum isometric muscle force. In general, there are multiple estimates of CSA for the same muscle. For example, numerous studies have estimated the CSA of the psoas major muscle using different subject populations and positions. However, few studies have combined the available information to obtain an overall estimate of CSA or investigated the effect different subject characteristics may have on CSA. In the present update, nine studies that reported psoas major CSA or physiologic CSA were compared with respect to subject characteristics, methodology, and results. Corrections to cadaveric data were made to adjust physiologic CSA to CSA. Comparison of reported values for living subjects indicated that females have smaller mean CSA than males for the psoas major muscle and that body size does not significantly influence muscle CSA in males. Areas derived from cadaveric data were smaller than similar studies on living subjects, possibly because of subject age, removal of tendinous and fatty components of fascicles, and lack of detailed data for fascicle angles in the supine position. Results indicate that researchers who use muscle CSA in biomechanical models should carefully assess the appropriateness of the data used, particularly in relation to potential sex differences and the influence of postural changes on CSA.

Kinematics and movement sequencing during flexion of the lumbar spine

OBJECTIVES

This study investigated the sequence of intervertebral joint movements and range of motion during three tasks involving lumbar flexion.

DESIGN

Position sensors were used to measure position and rotation of lumbar vertebrae during unconstrained flexion.

BACKGROUND

In the development of mathematical models, numerous assumptions need to be made. Few studies have attempted to assess the validity of the assumptions regarding kinematics in models of the lumbar spine.

METHODS

Position sensors were attached to the skin overlying the lumbar vertebrae of 14 volunteers. Volunteers performed three flexion tasks; unconstrained flexion from upright standing, with and without a mass of 5 kg held close to the body, and the transition from upright standing to a seated position.

RESULTS

Four definitive movement sequences were identified for those subjects with consistency between replicates; 'top down' motion (where the top of the lumbar spine starts to move first and the bottom moves last), 'bottom up' (where the bottom of the lumbar spine moves first and the top moves last), 'all together' (where all segments commence movement together), and 'middle last' (where the middle segments of the lumbar spine are last to commence movement). Subjects not fitting one of these sequences were categorised into a miscellaneous group. Only two subjects exhibited the same sequence for each of the three tasks, while other subjects exhibited two or three different sequences for the three tasks, or showed a lack of consistency for one of the tasks.

CONCLUSIONS

The results from this study indicate that there is no single movement sequence exhibited by the sample population.

RELEVANCE

Incorrect assumptions which are incorporated into mathematical models have the potential to influence model output. Given that output from spinal models is often used to assess ergonomic issues such as safe lifting loads, validation of the assumptions is essential.

Radiographic and histologic analysis of cemented double tapered femoral stems

The macroscopic, radiographic, and histologic features of the prosthesis-cement and cementbone interfaces and adjacent bone were studied in 21 cemented hemiarthroplasties in sheep that had lived until sacrifice at 9 months. The features were compared with those immediately after implantation of the stem in the contralateral femur. The femoral stem was a double taper that was either polished collarless, matte collarless, or matte collared. There was no prosthesis to cement debonding or cement to bone radiolucent line immediately after implantation, and there was excellent interdigitation at the cement-bone interface. After 9 months there was no evidence of prosthesis to cement debonding and no stem with definite loosening. At 9 months after implantation there was evidence of bone remodeling with new bone filling what were presumed to be gaps at the cement-bone interface from immediately after implantation. Radiolucent lines at the cement-bone interface were found to represent trabeculation of the cortical bone rather than the presence of a complete fibrous interface, which was not seen. There was no difference between stem types. Sheep have been shown to be useful in a model of cemented hip arthroplasty and, although no differences were seen between stem types at 9 months after implantation, long term differences cannot be excluded.

Power spectrum analysis of human femoral rotations during gait

Femoral rotational waveforms sampled at 15 Hz for a duration 20 s satisfy the sampling rate and frequency resolution requirements of such waveforms during walking. Spectral analysis provides a unique signature of the frequency composition of such signals. This identity may prove useful in characterising human gait and could be of value in future studies of walking in health and disease.

IN VITRO HUMAN MONOCYTE RESPONSE TO WEAR PARTICLES OF TITANIUM ALLOY CONTAINING VANADIUM OR NIOBIUM

Our aim was to determine whether in vitro studies would detect differences in the cellular response to wear particles of two titanium alloys commonly used in the manufacture of joint replacement prostheses. Particles were of the order of 1 μm in diameter representative of those found adjacent to failed prostheses.

Exposure of human monocytes to titanium 6-aluminium 4- vanadium (TiAlV) at concentrations of 4 x 107 particles/ml produced a mean prostaglandin E2 release of 2627.6 pM; this was significantly higher than the 317.4 pM induced by titanium 6-aluminium 7-niobium alloy (TiAlNb) particles (p = 0.006). Commercially-pure titanium particles induced a release of 347.8 pM. In addition, TiAlV stimulated significantly more release of the other cell mediators, interleukin-1, tumour necrosis factor and interleukin-6. At lower concentrations of particles there was less mediator release and less obvious differences between materials. None of the materials caused significant toxicity.

The levels of inflammatory mediators released by phagocytic cells in response to wear particles may influence the amount of periprosthetic bone loss. Our findings have shown that in vitro studies can detect differences in cellular response induced by particles of similar titanium alloys in common clinical use, although in vivo studies have shown little difference. While in vitro studies should not be used as the only form of assessment, they must be considered when assessing the relative biocompatibility of different implant materials.

In vitro human monocyte response to wear particles of titanium alloy containing vanadium or niobium

Our aim was to determine whether in vitro studies would detect differences in the cellular response to wear particles of two titanium alloys commonly used in the manufacture of joint replacement prostheses. Particles were of the order of 1 microm in diameter representative of those found adjacent to failed prostheses. Exposure of human monocytes to titanium 6-aluminium 4- vanadium (TiAlV) at concentrations of 4 x 10(7) particles/ml produced a mean prostaglandin E2 release of 2627.6 pM; this was significantly higher than the 317.4 pM induced by titanium 6-aluminium 7-niobium alloy (TiAlNb) particles (p = 0.006). Commercially-pure titanium particles induced a release of 347.8 pM. In addition, TiAlV stimulated significantly more release of the other cell mediators, interleukin-1, tumour necrosis factor and interleukin-6. At lower concentrations of particles there was less mediator release and less obvious differences between materials. None of the materials caused significant toxicity. The levels of inflammatory mediators released by phagocytic cells in response to wear particles may influence the amount of periprosthetic bone loss. Our findings have shown that in vitro studies can detect differences in cellular response induced by particles of similar titanium alloys in common clinical use, although in vivo studies have shown little difference. While in vitro studies should not be used as the only form of assessment, they must be considered when assessing the relative biocompatibility of different implant materials.

The role of polyethylene wear in joint replacement failure

Aseptic loosening is the major cause of failure of joint replacement prostheses. Polyethylene implants removed at revision surgery regularly show wear. It is proposed that the polyethylene particles released into the tissues as a consequence of this wear induce a tissue response that precedes aseptic loosening. This paper presents the results of recent in vivo and in vitro studies of the biological response to polyethylene wear particles undertaken in the authors' laboratories. A clinical perspective is provided by the inclusion of the authors' recent observations of retrieval analyses of joint replacement implant wear and the tissue response to polyethylene in humans.

CLINICAL IMPLICATIONS OF STIFFNESS AND STRENGTH CHANGES IN FRACTURE HEALING

In the assessment of fracture healing by monitoring stiffness with vibrational analysis or instrumented external fixators, it has been assumed that there is a workable correlation between stiffness and strength. We used four-point bending tests to study time-related changes in stiffness and strength in healing tibial fractures in sheep. We aimed to test the validity of the measurement of stiffness to assess fracture strength.

At each duration of healing examined, we found marked variations in stiffness and strength. Stiffness was shown to be load-dependent: measurements at higher loads reflected ultimate strength more accurately. There was a biphasic relationship between stiffness and strength: at first there was a strong correlation regardless of loading conditions, but in the second phase, which included the period of ‘clinical healing’, stiffness and strength were not significantly correlated.

We conclude that the monitoring of stiffness is useful primarily in assessing progress towards union but is inherently limited as an assessment of strength at the time of clinical union. Any interpretation of stiffness must take into account the load conditions.

Clinical implications of stiffness and strength changes in fracture healing

In the assessment of fracture healing by monitoring stiffness with vibrational analysis or instrumented external fixators, it has been assumed that there is a workable correlation between stiffness and strength. We used four-point bending tests to study time-related changes in stiffness and strength in healing tibial fractures in sheep. We aimed to test the validity of the measurement of stiffness to assess fracture strength. At each duration of healing examined, we found marked variations in stiffness and strength. Stiffness was shown to be load-dependent: measurements at higher loads reflected ultimate strength more accurately. There was a biphasic relationship between stiffness and strength: at first there was a strong correlation regardless of loading conditions, but in the second phase, which included the period of 'clinical healing', stiffness and strength were not significantly correlated. We conclude that the monitoring of stiffness is useful primarily in assessing progress towards union but is inherently limited as an assessment of strength at the time of clinical union. Any interpretation of stiffness must take into account the load conditions.

The reliability of postural sway measures using the 3SPACE Tracker

OBJECTIVE: To investigate the reliability of three-dimensional postural control sway measures, in a normal population, using an electromagnetic device. DESIGN: A repeated measures design was used within and between sessions. BACKGROUND: Electromagnetic measurement has been proposed as a method of measuring postural sway; however, reliability within a normal population has not been shown. METHOD: An electromagnetic device, the 3SPACE Tracker, measured postural sway at the pelvis using sway path length and displacements in the anterior-posterior, medial-lateral and vertical direction. Ten subjects performed a postural control task involving two trials for six different sensory conditions, on two separate occasions, two weeks apart. RESULTS: The reliability of the technique was demonstrated by the repeatability of results for individual subjects within and between sessions. Individual subjects and sensory conditions were discriminated. Sway path length was the most consistent of the measures used. CONCLUSIONS: Three dimensional measures of postural sway are reliable and discriminatory in a normal population.

Bilateral femoral rotations measured during walking: a new parameter to summarize and describe individual gait

The aim of this study was to develop and assess a method of analysis of femoral rotations to describe and summarize an individual's gait. Twelve normal subjects underwent walking trials at both natural and slow speeds, controlled by a metronome. The three-dimensional rotations of each femur were measured, simultaneously, during walking using a 3SPACE Tracker. The phase lags between the three rotations for each leg, obtained by cross-correlations, provided the primary data. The device was found to be reliable and gave significant test--retest repeatability. The analyses provided summary parameters describing mathematically the patterns of rotations and showed there was repeatability within and between sessions. A strong systematic subject effect in all analyses indicated that these parameters provide a highly specific description of how an individual walks.

Direct measurement of hoop strains in the intact and torn human medial meniscus

OBJECTIVE: To measure the circumferential or hoop strains generated in the medial meniscus during loading of the knee joint and to examine the effect of longitudinal and radial tears in the meniscus on these strain values. DESIGN: An in vitro investigation measuring the circumferential strains in the medial menisci of cadaveric human knees as they were loaded in a materials testing machine. BACKGROUND: The menisci transmit approximately 50% of the load through the knee, the rest being transmitted by direct contact of the articular cartilage. Damage to the menisci will alter the pattern of load transmission as will meniscectomy. This study examined the changes in the mechanics of the meniscus in situ as a result of simulated tears to assess the effect of its load carrying capacity and the implications of surgery to remove part or all of a damaged meniscus. METHODS: Nineteen human cadaveric knees were tested. Windows were made in the joint capsule and strain gauges inserted into the anterior, middle and posterior sections of the medial meniscus. The knees were then loaded to three times body weight at speeds of 50 and 500 mm/min, with the knee joint at 0 degrees and 30 degrees of flexion. The tests were repeated following the creation of a longitudinal or a radial tear in the meniscus. RESULTS: The intact menisci showed significantly less strain in the posterior section compared to the anterior and middle sections (P < 0.003, with strains of 1.54%, 2.86% and 2.65% respectively). With a longitudinal tear this pattern changed with strains decreasing anteriorly and increasing posteriorly. There were also significant differences at different angles of knee joint flexion not seen in the intact meniscus. 50% radial tears reduced the strains anteriorly whilst a complete radial tear completely defunctioned the meniscus. CONCLUSIONS: This study has shown that there are significantly different hoop strains produced in different sections of the medial meniscus under load and the patterns of strain distribution are disturbed by meniscal tears. RELEVANCE: These results provide important data for mathematical models which must include non-uniform behaviour. They also have implications for the surgical management of torn menisci. Undamaged portions should be preserved and the integrity of the circumferential fibres maintained to ensure the menisci retain a load bearing capability.

Three-dimensional analysis of active cervical motion: the effect of age and gender

OBJECTIVE: To describe the effects of age and gender on three-dimensional (3D) active cervical spine motion. DESIGN: This was a descriptive study. BACKGROUND: This study expanded on previous investigations of age and gender effects on single plane motion of the cervical spine. METHODS: Sixty female and 60 male asymptomatic, normal volunteers, aged between 20 and 59 years, were examined in a standardized seated position. The 3 SPACE Isotrak system was used to measure simultaneous 3D motion of the cervical spine. RESULTS: The mean range of all the primary movements decreased significantly with age. For flexion/extension the greatest decrease occurred between the 20- and 30-year-olds, whereas for both lateral flexion and rotation, significant differences were demonstrated in subjects aged two decades apart. The coupling of motion associated with rotation was significantly related to age. CONCLUSION: Age had a significant effect on all of the primary movements. Age had less effect on the range of the coupled movements, in that the only movements to be affected were lateral flexion and extension occurring during cervical rotation. Gender had no marked effect on the primary or the coupled movements. RELEVANCE: This study provides normative data for the effects of age and gender on three-dimensional analysis of active cervical spine motion, which can be used for comparison with specific patient populations. The high level of intra-subject test-retest reliability renders the3 SPACE system of value for clinical measurement of movement pre- and post-treatment intervention for cervical spine disorders.

Drug inhibition of the macrophage response to metal wear particles in vitro

The wear particles of cobalt chrome alloy and titanium alloy have been implicated as a cause of aseptic loosening of prostheses. It is thought that their ability to induce either cell death or the release of mediators that induce bone resorption contributes to this loosening. This study was designed to test the hypothesis that these adverse biologic effects are due to wear particle corrosion at low pH after they have been phagocytosed by macrophages. Cobalt chrome alloy and titanium alloy particles of similar size and concentration to those found in the tissues surrounding failed prostheses were added to cultured rodent peritoneal macrophages. Treatment of macrophages with drugs that prevent a drop in pH within phagosomes significantly reduced the toxicity of phagocytosed cobalt chrome alloy particles. The same drugs also reduced the levels of prostaglandin E2 and interleukin-6 release induced by phagocytosed titanium alloy particles. When both types of particles were incubated at a low pH, similar to that encountered by phagocytosed particles, soluble products were released that induced the same effects as the particles themselves. These results show that enhanced corrosion of wear particles by phagocytic cells may contribute significantly to the adverse biologic effects of wear particles and identify drug therapies that may be investigated further.

Mechanical properties of the human anterior cruciate ligament

The aim was to measure the stiffness and strength of the femur-anterior cruciate ligament-tibia complex tested in a physiological manner with a force exerted anteriorly on the tibia, at knee joint flexion angles of 0 degrees, 10 degrees and 30 degrees and at speeds of 50 and 500 mm/min. Ligaments were preconditioned by cycling five times, with data from the fifth cycle used to determine the stiffness of the ligament in a low-load range. The ligaments were then tested to failure with the knee at 30 degrees flexion. The specimens were divided into two groups, middle-aged (40-60) and old (>60). For each group no statistical difference was observed between stiffness of the ligament at different joint flexion angles or speeds. Seven of the 21 specimens in the older age group failed by avulsion at the bone-ligament interface. All the other specimens failed by tears in the substance of the ligament. Ultimate failure load was found to have a significant correlation with bodyweight. It was 1.6 and 1.3 times bodyweight for the middle-aged and older age groups respectively. This study has highlighted the importance of identifying different modes of failure, of making corrections for bodyweight and testing in a physiological manner. The results allow a better understanding of the mechanical behaviour of the anterior cruciate ligament and provide design data for anterior cruciate ligament grafts and prostheses. RELEVANCE:--Our clinical experience indicates that the anterior cruciate ligament is frequently ruptured during uncoordinated contraction of the quadriceps mechanism. The results of this study, in which the mechanical properties of the anterior cruciate ligament have been measured with force exerted anteriorly on the tibia, allow a more complete understanding of the mechanical behaviour of the anterior cruciate ligament and provide design data for anterior cruciate ligament grafts and prostheses.

Failure strengths of different meniscal suturing techniques

The aim of this study was to measure the failure strengths of three arthroscopic meniscal suturing methods. The techniques investigated were a single horizontal loop, a double vertical loop, and a single vertical loop. Eleven human, lateral menisci obtained at autopsy were cut with a scalpel to simulate peripheral longitudinal tears. The menisci were then repaired with one suture at a time, using each of the three methods in turn. The two parts of the meniscus were then pulled apart using a computer-controlled materials testing machine until failure occurred, either by the suture itself failing or by the suture pulling out because of tearing of the meniscus. The horizontal sutures had a mean failure strength of 29.3 N, whereas the double vertical loop failed at 63.2 N and the single vertical loop at 67.3 N. The horizontal loop sutures and the double vertical loop sutures all failed by pulling out of the meniscus. The single vertical loop sutures failed by rupture of the suture itself. This study showed the superior mechanical characteristics of the single vertical loop suturing system over the other techniques tested. Owing to the more consistent failure strength, decreased costs, and shorter surgical time over the double vertical loop system, the single vertical loop technique is recommended for arthroscopic meniscal repair.

Mechanical consequences of annular tears and subsequent intervertebral disc degeneration

The relationship between degeneration of the intervertebral disc and changes to its mechanics is unclear. The aim of this study was to examine, in a sheep model, the effect of creating a lesion in the outer, anterior annulus on the mechanics of the intervertebral joint complex and the disc. Forty-one 2-year-old Merino wethers were allocated randomly into a control group or an annular lesion group and additionally to non-survivors which were sacrificed immediately or survivors sacrificed 6 months later. The annular lesion group had incisions made in two non-adjacent intervertebral discs and a plate was secured across the vertebrae at one level. Mechanical tests were performed on specimens consisting of the two vertebrae, the intervening disc and associated ligaments. Stiffness of the specimens was measured in flexion, extension, and in pure torsion. The tests were conducted first on the intact intervertebral joints and then after removal of the zygapophyseal joints and the interspinous and supraspinous ligaments. The results showed that the creation of an annular lesion caused immediate changes to the mechanics of the disc. In torsion, where no axis of rotation was imposed on the joints, there was a clear reduction in stiffness compared with controls. After 6 months the discs in the lesion groups approached the stiffness of the controls. The plates had a marked effect on the stiffness of the joints in flexion and extension, but after 6 months this difference was not apparent. The mechanics of the intact joints were not affected immediately by the lesion but after 6 months they were less stiff than the controls. There was clear evidence of a progressive degenerative response in the nucleus in all discs with a lesion. The addition of a plate to limit movement did not markedly affect this biological response to the injury but there was some evidence that after 6 months there were fewer degenerative changes to the zygapophyseal joints in the plated specimens. Recovery of the mechanical integrity of the disc was more marked in the joints that were plated, supporting the concept that limiting motion of an injured intervertebral disc facilitates a healing response in the annulus.

A method for production and characterization of metal prosthesis wear particles

The wear of joint prostheses generates wear particles that produce an inflammatory response in the surrounding tissues and may contribute to bone resorption resulting in prosthetic loosening. Although the effects of particles produced from prosthetic materials have been studied extensively in vitro and in vivo, little attention has been paid to the standardisation of methods for the generation and characterization of these particles. This paper describes a reproducible method for generation of metal particles by the abrasive shaking of joint replacement components. Particular attention was given to the production of metal particles that closely resembled particles found around solid and loose human prostheses. To achieve this, particle size, size distribution, chemical composition, and shape were characterized. Particles that were 0.5-3.0 microns in diameter were isolated by differential sedimentation, and the distribution of particle sizes was determined with use of a Coulter Multisizer. Chemical composition was measured by atomic absorption spectrophotometry, and transmission electron microscopy was used to characterize particle shape. The techniques were shown to be reproducible, since there was little variation between batches over a lengthy time period. These or similar methods of particle production and characterization should be an essential part of future in vitro and in vivo studies of wear particles.

The response to particulate debris

The most common cause of failure of total hip and other arthroplasties is aseptic loosening of the prosthesis. Substantial evidence suggests that the adverse tissue response to prosthesis wear particles is an important contributor to bone loss around implants. The results may be an increased risk of loosening and severe bone loss, which makes revision surgery more difficult. Studies in humans have demonstrated that the appearances of the periprosthetic tissues are related to the type, number, and size of wear particles. The appearance and tissue response around any given prosthesis is related to the balance among the rate of production of wear particles, the ability of the tissues to deal with the particles, and the rate of clearance of the particles from the joint. Prosthesis wear particles may be produced by wear at the articulating surface, particularly if there is three-body wear, by abrasion due to movement at the prosthesis-bone interface of uncemented prostheses, by abrasion at the prosthesis-cement and cement-bone interfaces of cemented prostheses, or at any articulation of the components of modular prostheses. Future in vivo and in vitro studies of the effects of wear particles should focus on quantitative measurement of the effects of particles and on the use of particles of comparable size and in concentrations similar to those found in the tissues surrounding failed prostheses.

The differences in toxicity and release of bone-resorbing mediators induced by titanium and cobalt-chromium-alloy wear particles

We investigated the relationship between the toxic effects of metal wear particles and their ability to stimulate the release of inflammatory mediators implicated in bone resorption. In vitro studies were carried out with use of rat peritoneal macrophages, which were exposed to either cobalt-chromium-alloy or titanium-aluminum-vanadium particles, milled from the metal components of hip prostheses. The particles were in the size-range of, and at concentrations similar to, those found in the tissues surrounding failed prostheses in humans. The titanium-aluminum-vanadium particles showed little toxicity even at high concentrations, while the cobalt-chromium particles were very toxic. The titanium-aluminum-vanadium particles induced significantly more release of prostaglandin E2 than did the cobalt-chromium particles, and this was true for a wide range of concentrations. Exposure to titanium-aluminum-vanadium increased the release of prostaglandin E2, interleukin-1, tumor necrosis factor, and interleukin-6. In contrast, exposure to cobalt-chromium particles was associated with a decreased release of prostaglandin E2 and interleukin-6, and it had little effect on the release of interleukin-1 and tumor necrosis factor.

Measurement of the range and coupled movements observed in the lumbar spine

The aim of this study was to set up a data base, using the Polhemus Navigation Sciences 3Space Isotrak system, of the range and coupled movements expected to be seen in the lumbar spine of people not experiencing spinal problems. Measurements were taken from groups of volunteers who had not previously experienced any form of serious back pain and who were not suffering from any pathology known to affect the spine. The 'normal' group was split into sections determined by age an dsex. Age was split into five categories, those aged 20-29 years, 30-39 years, 40-49 years, 50-59 years and 60-69 years. Range of motion was seen to be affected by both the age and sex of subjects. Lateral bend and axial rotation, flexion and lateral bend and flexion and axial rotation were all strongly coupled. The younger age groups tested exhibited a greater degree of coupled movement than the older groups.

The effects of flexion on the geometry and actions of the lumbar erector spinae

A modeling study was undertaken to determine the effects of flexion on the forces exerted by the lumbar back muscles. Twenty-nine fascicles of the lumbar multifidus and erector spinae were plotted onto tracings of radiographs of nine normal volunteers in the flexion position. Moment arms and force vectors of each fascicle were calculated. The model revealed that moment arms decreased slightly in length resulting in no more than an 18% decrease in maximum extensor moments exerted across the lumbar spine. Compression loads were not significantly different from those generated in the upright posture. However, there were major changes in shear forces, in particular a reversal from a net anterior to a net posterior shear force at the L5/S1 segment. Flexion causes substantial elongation of the back muscles, which must therefore reduce their maximum active tension. However, if increases in passive tension are considered it emerges that the compression forces and moments exerted by the back muscles in full flexion are not significantly different from those produced in the upright posture.

The axial torque of the lumbar back muscles: torsion strength of the back muscles

The maximal, axial torque generated by the lumbar back muscles was determined by modelling the action of the 49 fascicles of longissimus thoracis, iliocostalis lumborum and the lumbar multifidus on radiographs of the lumbar spine of nine young male subjects in upright standing and in full lumbar flexion. No single fascicle exerted more than 2 Nm of axial torque in the upright posture, and the collective torque of all muscles acting a segment did not exceed 5 Nm. All torques were considerably less in full flexion. The lumbar back muscles exert very little torque on the lumbar spine, and contribute only about 5% of the total torque involved in trunk rotation. None of the lumbar back muscles can be considered a rotator. The oblique abdominal muscles are the principal rotators of the trunk. Preventative and rehabilitation programmes concerned with torsion injuries should focus on the abdominal muscles rather than the back muscles for stability in axial rotation.

Twisting mobility of the human back in flexed postures

The twisting mobility of the back was measured using an electromagnetic measuring device, the 3SPACE Isotrak, in ten healthy subjects in five postures: standing upright, in two forward-leaning positions, and two seated positions. In the forward-leaning positions the subjects flexed with mean values of 28 degrees and 53 degrees and had no difference in their active twisting compared with that experienced in the upright position. The two seated positions induced mean flexion values of 32 degrees and 44 degrees and were associated with statistically significant increases in active twisting compared with the upright position of 38% and 46%, respectively, at the level of P < 0.0001. These increases suggest that the wedge morphology of the zygapophysial joints permits greater twisting when the intervertebral joints flexed, but postural muscle action in forward leaning restricts active twisting. This study suggests a mechanism for increased vulnerability of the posterior anulus to injury when twisting is combined with flexion.

A universal model of the lumbar back muscles in the upright position

A model of the lumbar back muscles was constructed incorporating 49 fascicles of the lumbar erector spinae and multifidus. The attachment sites and sizes of fascicles were based on previous anatomic studies, and the fascicles were modeled on radiographs of nine normal volunteers in the upright position. Calculations revealed that the thoracic fibers of the lumbar erector spinae contribute 50% of the total extensor moment exerted on L4 and L5; multifidus contributes some 20%; and the remainder is exerted by the lumbar fibers of erector spinae. At upper lumbar levels, the thoracic fibers of the lumbar erector spinae contribute between 70% and 86% of the total extensor moment. In the upright posture, the lumbar back muscles exert a net posterior shear force on segments L1 to L4, but exert an anterior shear force on L5. Collectively, all the back muscles exert large compression forces on all segments. A force coefficient of 46 Ncm-2 was determined to apply for the back muscles. These results have a bearing on the appreciation of the effects on the back muscles of surgery and physiotherapy.

Variation in lumbar spine mobility measured over a 24-hour period

An electronic device for the measurement of three-dimensional movements, the Polhemus Navigation Sciences 3Space Isotrak system, was used to measure the range of movement in the lumbar spine of: (1) 10 young adults pre- and post-normal night-time sleep; (2) 10 young adults tested every 2 h over a 24-h period. The results obtained for the 10 subjects tested pre-/post-sleep showed there to be significant decreases in flexion, extension and lateral bend post-sleep. Axial rotation was not seen to alter significantly. The results obtained for the 10 subjects tested over a 24-h period showed movement during the day to be significantly more than at night. A decrease in the range of all movements except extension was observed when testing subjects in the early hours of the morning (after they had been supine for a minimum of 4 h) relative to the range observed from mid-afternoon to early evening.

Axial rotation of lumbar intervertebral joints in forward flexion

This paper examined the mobility of intervertebral joints in axial rotation in a neutral and in two flexed positions. Torsion tests were conducted in vitro on specimens of isolated intervertebral joints in a rig specifically designed to apply torsion without imposing a fixed axis. This permitted the specimens to rotate about their own mobile axis of axial rotation. In addition the specimens were flexed about previously defined physiological axes of sagittal flexion in order to simulate movements as close as possible to those seen in life. It was shown that some intervertebral joints do exhibit an increased ability to rotate when in some degree of sub-maximal flexion dependent on the morphology of the zygapophysial joints. In full flexion axial rotation is limited, most probably by tightening of the posterior ligaments and zygapophysial joint capsules. This study lends evidence to the argument that torsion alone is insufficient to damage the intervertebral disc but a combination of flexion and torsion will increase its vulnerability to injury.

Bioengineering activities in the Department of Orthopaedic Surgery and Trauma, Royal Adelaide Hospital, Adelaide, south Australia

This paper outlines the main areas of bioengineering research in the Department of Orthopaedic Surgery and Trauma at the Royal Adelaide Hospital, South Australia.