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ABSTRACT: BACKGROUND: The spinal load reduction by an orthosis is still a matter of debate. Some studies predicted a load reduction while others found no effect. The aim of this study was to measure the in vivo effect of the Lumbo TriStep brace and the hyperextension orthosis medi 3C on the spinal implant loads. METHODS: Telemeterized vertebral body replacements were implanted in 5 patients suffering from a severe fracture of the L1 or L3 vertebral body. The implant allows the measurement of 6 load components acting on it. For several activities during standing, sitting and walking, implant loads were measured in patients with and without an orthosis. FINDINGS: The average resultant force on the vertebral body for 26 activities was reduced by 9% with the Lumbo TriStep brace, and by 19% with the hyperextension orthosis. The force reduction is usually more pronounced for activities performed during sitting than it is for those performed while standing. However, considerable inter- and intra-individual variation was observed. In several cases, the measured implant forces were even higher when the patients were wearing an orthosis. INTERPRETATION: In some patients, for certain activities, an orthosis may reduce the force on a vertebral body replacement and thus on the anterior column of the spine. However, in other patients for the same activities, an orthosis may increase the force. The measurements do not allow a clear recommendation to wear an orthosis since the clinically relevant reduction of implant forces is unknown.
Clinical biomechanics (Bristol, Avon) 04/2013; · 1.76 Impact Factor
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ABSTRACT: Knowledge about in vivo spinal compressive forces is a basic requirement for spinal biomechanics. Their direct measurement is not yet possible. Therefore, compressive forces are estimated from in vivo measured intradiscal pressure values. However, it is still not evident how precise these estimations are. A finite element model of the spine was employed to simulate elementary body positions and the compressive force at level L4-5 was calculated. This value was compared with different estimations calculated by multiplying the intradiscal pressure with the disc's cross-sectional area and with a correction factor. A model specific and different previously employed correction factors were used. Separately, in vivo forces were estimated from previously measured pressure values. A model specific correction factor leads for all body positions to a good estimation (error <4%) of the force except for extension (error >27%). Non-model specific correction factors lead to estimation errors of up to 44%. When accounting for these limitations, in vivo forces were estimated e.g. for standing between 430N and 600N. Compressive forces can be estimated for non-extended body positions when the individual correction factor is known. In vivo forces can be estimated from intradiscal pressure values within a certain range.
Medical Engineering & Physics 04/2013; · 1.62 Impact Factor
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ABSTRACT: Mostly simplified loads were used in biomechanical finite element (FE) studies of the spine because of a lack of data on muscular physiological loading. Inverse static (IS) models allow the prediction of muscle forces for predefined postures. A combination of both mechanical approaches - FE and IS - appears to allow a more realistic modeling. However, it is unknown what deviations are to be expected when muscle forces calculated for models with rigid vertebrae and fixed centers of rotation, as generally found in IS models, are applied to a FE model with elastic vertebrae and discs. The aim of this study was to determine the effects of these disagreements. Muscle forces were estimated for 20° flexion and 10° extension in an IS model and transferred to a FE model. The effects of the elasticity of bony structures (rigid vs. elastic) and the definition of the center of rotation (fixed vs. non-fixed) were quantified using the deviation of actual intervertebral rotation (IVR) of the FE model and the targeted IVR from the IS model. For extension, the elasticity of the vertebrae had only a minor effect on IVRs, whereas a non-fixed center of rotation increased the IVR deviation on average by 0.5° per segment. For flexion, a combination of the two parameters increased IVR deviation on average by 1° per segment. When loading FE models with predicted muscle forces from IS analyses, the main limitations in the IS model - rigidity of the segments and the fixed centers of rotation - must be considered.
Journal of biomechanics 03/2013; · 2.66 Impact Factor
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ABSTRACT: Lifting up weights from a cupboard or table and putting them back are activities of daily living. Patients with spinal problems want to know whether they should avoid these activities. However, little is known about the spinal forces during these activities and about the effect of level height. Loads on a telemeterized vertebral body replacement were measured in 5 patients. The effect of level height when lifting or setting down weights of 0.01, 1.5 and 3.0kg in a standing posture were investigated. Furthermore, these weights were lifted and set down with a stretched arm while sitting at a table. No instructions were given on how to perform the task. For these activities, forces as high as 5 times the value for standing alone were measured. In 2 patients, implant loads decreased with increasing level height. In the other patients the effect of level height was small. Lifting a weight from a table with a stretched arm while sitting led to a strong increase of the maximum implant force. Setting down the weight usually caused a slightly higher maximum implant force than lifting it. Forces on a vertebral body replacement during lifting and setting down a weight varied strongly when no precise instructions were given on how to perform the activity. Thus, the measured forces are representative for such activities performed in daily life. This, however, led to wide variations in measured data. Compared to the value for standing, 5 times higher forces were measured for lifting and setting down of weights. This suggests that these activities should be avoided by patients who have spinal problems.
Journal of biomechanics 11/2012; · 2.66 Impact Factor
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ABSTRACT: Knowledge about spinal loading is required for designing and preclinical testing of spinal implants. It is assumed that loading of the spine depends upon body weight and height, as well as on the spine level, but a direct measurement of the loading conditions throughout the spine is not yet possible. Here, computer models can allow an estimation of the forces and moments acting in the spine. The objective of the present study was to calculate spinal loads for different postures and activities at several levels of the thoracolumbar spine for various combinations of body height and weight. A validated musculoskeletal model, together with commercially available software (AnyBody Technology), were used to calculate the segmental loads acting on the centre of the upper endplate of the vertebrae T12 to L5. The body height was varied between 150 and 200cm and the weight between 50 and 120kg. The loads were determined for five standard static postures and three lifting tasks. The resultant forces and moments increased approximately linearly with increasing body weight. The body height had a nearly linear effect on the spinal loads, but in almost all loading cases, the effect on spinal loads was stronger for variation of body weight than of body height. Spinal loads generally increased from cranial to caudal. The presented data now allow the estimation of the spinal load during activities of daily living on a subject specific basis, if body height and weight are known.
Medical Engineering & Physics 10/2012; · 1.62 Impact Factor
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ABSTRACT: After total sacrectomy, it is mandatory to reconstruct the continuity between the lumbar spine and the pelvis. Only few biomechanical analyses exist which compare different reconstructions. Therefore, the aim of this study was to compare the lumbo-pelvic motion and the relative risk of implant breakage for four different reconstructions after total sacrectomy.
Finite element analyses were performed for four general different reconstructions after total sacrectomy: sacral-rod reconstruction, four-rod reconstruction, bilateral fibular flaps reconstruction, and improved compound reconstruction. The rotations between L5 vertebra and ilium, the L5 shift-down displacement, and the maximum von Mises stress in the implants were calculated and evaluated for flexion, extension, lateral bending and axial rotation.
The decreasing order of the rotations between L5 vertebra and ilium as well as of the L5 shift-down displacement for the studied reconstruction methods was four-rod reconstruction>sacral-rod reconstruction>bilateral fibular flaps reconstruction>improved compound reconstruction. The decreasing order of the maximum von Mises stress in the implants was sacral-rod reconstruction>four-rod reconstruction>bilateral fibular flaps reconstruction>improved compound reconstruction.
From the mechanical point of view, improved compound reconstruction is superior to the other methods studied here as it shows the highest stability and the lowest maximum von Mises stress. However, clinical aspects must also be regarded when choosing a reconstruction method for a specific patient.
Clinical biomechanics (Bristol, Avon) 06/2012; 27(8):771-6. · 1.76 Impact Factor
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ABSTRACT: In in vitro studies of the lumbar spine simplified loading modes (compressive follower force, pure moment) are usually employed to simulate the standard load cases flexion-extension, axial rotation and lateral bending of the upper body. However, the magnitudes of these loads vary widely in the literature. Thus the results of current studies may lead to unrealistic values and are hardly comparable. It is still unknown which load magnitudes lead to a realistic simulation of maximum lateral bending. A validated finite element model of the lumbar spine was used in an optimisation study to determine which magnitudes of the compressive follower force and bending moment deliver results that fit best with averaged in vivo data. The best agreement with averaged in vivo measured data was found for a compressive follower force of 700 N and a lateral bending moment of 7.8 Nm. These results show that loading modes that differ strongly from the optimised one may not realistically simulate maximum lateral bending. The simplified but in vitro applicable loading cannot perfectly mimic the in vivo situation. However, the optimised magnitudes are those which agree best with averaged in vivo measured data. Its consequent application would lead to a better comparability of different investigations.
Medical Engineering & Physics 05/2012; 34(6):777-80. · 1.62 Impact Factor
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ABSTRACT: Abstract Posterior disc bulging may lead to nerve root compression and radicular pain, and in extreme cases to a local pressure on the dural sac and thus to back pain. Compared to when standing, posterior disc bulging is increased during extension and decreased during flexion, in an uninstrumented spine. The aim of this study was to determine the effect of a pedicle-screw-based dynamic implant on posterior disc bulging. A finite element model of the lumbosacral spine was used to calculate posterior disc bulging before and after implantation of a dynamic implant for different loading cases. The elastic modulus of the longitudinal rod was varied, and the influence of distraction of the bridged segment on disc bulging was also determined. In addition, the centre of rotation (CoR) was determined. Due to a dynamic implant, the magnitude of posterior disc bulging was reduced compared to that for "standing without an implant" during extension, lateral bending, and axial rotation. During flexion, however, disc bulging was usually increased. With increasing stiffness of the dynamic implant, the CoR moved towards the longitudinal rod. This posterior shift of the CoR led to a compression of the entire intervertebral disc during flexion and thus to an increase of disc bulging.
Biomedizinische Technik/Biomedical Engineering 11/2011; 56(6):327-31. · 0.53 Impact Factor
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ABSTRACT: Pedicle-screw-based dynamic implants are intended to preserve intervertebral mobility while releasing certain spinal structures. The aim of the study was to determine the as yet unknown optimal stiffness value of the longitudinal rods that fulfils best these opposing tasks.
A finite element model of the lumbar spine was used which includes the posterior implant at level L4/5. More than 250 variations of this model were generated by varying the diameter of the longitudinal rods between 6 and 12 mm and their elastic modulus between 10 MPa and 200 MPa. The loading cases flexion, extension, lateral bending and axial rotation were simulated. Evaluated optimization criteria were the ranges of motion, forces in the facet joints, posterior bulgings of the intervertebral disc and the intradiscal pressures. Various objective functions were evaluated.
The results show that the objective values depend more on the axial stiffness of the rods than on bending and torsional stiffness, rod diameter and elastic modulus. The optimal stiffness value for most of the investigated objective functions is approximately 50 N/mm and is achieved, e.g. using a rod diameter of 6 mm and an elastic modulus of 50 MPa. The design with the least axial stiffness was the best one with regard to the mobility. The forces in the facet joints and the intradiscal pressures were reduced mostly by an implant with the highest axial stiffness. When minimal posterior disc bulging was the criterion, the optimal axial stiffness was also approximately 50 N/mm.
The optimal axial stiffness of a pedicle-screw-based motion preservation implant for the lumbar spine is approximately 50 N/mm.
European Spine Journal 10/2011; 21(4):666-73. · 1.97 Impact Factor
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ABSTRACT: A direct measurement of the complete loads in the spine continues to remain elusive. Analytical musculoskeletal models to predict the internal loading conditions generally neglect or strongly simplify passive soft tissue structures. However, during large intervertebral motions, passive structures such as ligaments and the stiffness of the intervertebral discs are thought to play a critical role on the muscle forces required for equilibrium. The objective of the present study was to add the short segmental muscles, lumbar ligaments and disc stiffnesses to an existing base musculoskeletal model of the spine in order to establish what role passive soft tissue structures play in spinal loading, but also validate these results against experimentally determined load data. The long trunk muscles not included in previous models, short segmental muscles, lumbar ligaments and disc stiffnesses were implemented into a commercially available musculoskeletal spine model construct. For several activities of daily living, the loads acting on the vertebral bodies were then calculated relative to the value for standing, and then compared to the corresponding values measured in vivo. Good agreement between calculated and measured results could be achieved in all cases, with a maximum difference of 9%. The highest muscle forces were predicted in the m. longissimus (146N) for flexion, in the m. rectus abdominis (363N) for extension, and in the m. psoas major (144N and 81N) for lateral bending and axial rotation. This study has demonstrated that the inclusion of the complete set of muscle and ligament structures into musculoskeletal models of the spine is essential before accurate spinal forces can be determined. For the first time, trend validation of spinal loading has been achieved, thus allowing confidence in the precise prediction of muscle forces for a range of activities of daily living.
Medical Engineering & Physics 10/2011; 34(6):709-16. · 1.62 Impact Factor
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ABSTRACT: Up to now, plain radiographs are not well suited to assess spinal fusion. Radiostereometric analysis performed for two postures may deliver more reliable results. However, it is unknown, which postures are most suitable for this procedure.
In a finite element study, spinal fusion at the level L4-5 was simulated assuming a posterior approach and the implantation of two cages and a spinal fixation device. The change of the distance between markers in vertebrae adjacent to the cages was calculated for moving from one of the following postures standing, flexion, extension, axial rotation, lying, and extension in a lying position to another. The changes of marker distances were calculated for the intact model, as well as for the situations: directly after surgery before fusion started, in the early-fusion-phase and in the late-fusion-phase. Differences in the marker motion between two postoperative situations were also calculated.
The most anteriorly placed markers showed the greatest motion between two postures. The greatest differences in marker motions between the two situations before-fusion and early-fusion-phase (0.54 mm) as well as between early-fusion-phase and late-fusion-phase (0.34 mm) were found for the two postures flexion while standing and extension in a lying position.
Pairs of X-rays taken while standing with maximum flexed upper body and while lying with maximum extended trunk are most suited for the assessment of spinal fusion when using radiostereometric analysis.
Clinical biomechanics (Bristol, Avon) 09/2011; 27(2):111-6. · 1.76 Impact Factor
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ABSTRACT: Sitting is frequently assumed to cause high spinal loads because people with sedentary work often suffer from low back pain. It is assumed that the posture while sitting, as well as several seat parameters, also affects the spinal loads.
To measure the loads on a spinal implant for different upper body inclinations, backrest declinations, seat heights, types of seat, and arm positions.
Loads on a vertebral body replacement during sitting were measured in five patients with telemeterized implants.
The telemeterized vertebral body replacement measures all six load components. It was implanted into five patients suffering from compression fractures of a lumbar vertebral body. Loads were measured when the patients were sitting on a stool and inclining their upper body between 15° flexion and 10° extension in steps of 5°; on a chair with an adjustable backrest that allowed declination angles between 108° and 180°; on an office chair while the seat height was varied between 40 and 60 cm in steps of 5 cm; and successively on seven different types of seats. The effect of the arm position was also studied.
The resultant implant force was increased on the average by 48% for 15° flexion and decreased by 19% for 10° extension of the trunk. When sitting on a chair with an adjustable backrest, the loads decreased with an increasing backrest declination angle. The seat height had in most cases only a minor effect on implant loads. In comparison to sitting on a stool, the loads were reduced when sitting on a bench (7%) or a stool with a padded wedge (9%), a knee stool (19%), a chair (35%), and an office chair (41%). Sitting on a physiotherapy ball increased the loads by 7%. Placing the hands on the thighs reduced the implant loads on the average by 19% in comparison to arms hanging on the sides.
Spinal loads can be reduced by leaning against the backrest, placing the arms on the armrest or the thighs, and by decreasing the flexion angle of the upper body.
The spine journal: official journal of the North American Spine Society 07/2011; 11(9):870-5. · 2.90 Impact Factor
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ABSTRACT: Simplified loading modes (pure moment, compressive force) are usually applied in the in vitro studies to simulate flexion-extension, lateral bending and axial rotation of the spine. The load magnitudes for axial rotation vary strongly in the literature. Therefore, the results of current investigations, e.g. intervertebral rotations, are hardly comparable and may involve unrealistic values. Thus, the question 'which in vitro applicable loading mode is the most realistic' remains open. A validated finite element model of the lumbar spine was employed in two sensitivity studies to estimate the ranges of results due to published load assumptions and to determine the input parameters (e.g. torsional moment), which mostly affect the spinal load and kinematics during axial rotation. In a subsequent optimisation study, the in vitro applicable loading mode was determined, which delivers results that fit best with available in vivo measurements. The calculated results varied widely for loads used in the literature with potential high deviations from in vivo measured values. The intradiscal pressure is mainly affected by the magnitude of the compressive force, while the torsional moment influences mainly the intervertebral rotations and facet joint forces. The best agreement with results measured in vivo were found for a compressive follower force of 720N and a pure moment of 5.5Nm applied to the unconstrained vertebra L1. The results reveal that in many studies the assumed loads do not realistically simulate axial rotation. The in vitro applicable simplified loads cannot perfectly mimic the in vivo situation. However, the optimised values lead to the best agreement with in vivo measured values. Their consequent application would lead to a better comparability of different investigations.
Journal of biomechanics 06/2011; 44(12):2323-7. · 2.66 Impact Factor
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ABSTRACT: Pedicle-screw-based motion preservation systems are often used to support a slightly degenerated disc. Such implants are intended to reduce intradiscal pressure and facet joints forces, while having a minimal effect on the motion patterns. In a probabilistic finite element study with subsequent sensitivity analysis, the effects of 10 input parameters, such as elastic modulus and diameter of the elastic rod, distraction of the segment, level of bridged segments, etc. on the output parameters intervertebral rotations, intradiscal pressures, and facet joint forces were determined. A validated finite element model of the lumbar spine was employed. Probabilistic studies were performed for seven loading cases: upright standing, flexion, extension, left and right lateral bending and left and right axial rotation. The simulations show that intervertebral rotation angles, intradiscal pressures and facet joint forces are in most cases reduced by a motion preservation system. The influence on intradiscal pressure is small, except in extension. For many input parameter combinations, the values for intervertebral rotations and facet joint forces are very low, which indicates that the implant is too stiff in these cases. The output parameters are affected most by the following input parameters: loading case, elastic modulus and diameter of the elastic rod, distraction of the segment, and angular rigidity of the connection between screws and rod. The designated functions of a motion preservation system can best be achieved when the longitudinal rod has a low stiffness, and when the connection between rod and pedicle screws is rigid.
Journal of biomechanics 11/2010; 43(15):2963-9. · 2.66 Impact Factor
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ABSTRACT: Osseoligamentous spinal specimens buckle under even a small vertical compressive force. To allow higher axial forces, a compressive follower load (FL) was suggested previously that approximates the curvature of the spine without inducing intervertebral rotation in both the frontal and the sagittal planes. In in vitro experiments and finite element analyses, the location of the FL path is subjected to estimation by the investigator. Such non-optimized FLs may induce bending and so far it is still unknown how this affects the results of the study and their comparability. A symmetrical finite element model of the lumbar spine was employed to simulate upright standing while applying a follower load. In analogy to in vitro experiments, the path of this FL was estimated seven times by different members of our institute's spine group. Additionally, an optimized FL path was determined and additional moments of +/-7.5Nm were applied to simulate flexion and extension. Application of the optimized 500N compressive FL causes only a marginal alteration of the curvature (cardan angle L1-S1 in sagittal plane <0.25 degrees). An individual estimation of the FL path, however, results in flexions of up to 10.0 degrees or extensions of up to 12.3 degrees. The resulting angles for the different non-optimized FL paths depend on the magnitude of the bending moment applied and whether a differential or an absolute measurement is taken. A preceding optimization of the location of the FL path would increase the comparability of different studies.
Journal of biomechanics 09/2010; 43(13):2625-8. · 2.66 Impact Factor
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ABSTRACT: Fractured vertebral bodies are often stabilized by vertebroplasty. Several parameters, including fracture type, cement filling shape, cement volume, elastic moduli of cement, cancellous bone and fractured region, may all affect the stresses in the augmented vertebral body and in bone cement. The aim of this study was to determine numerically the effects of these input parameters on the stresses caused. In a probabilistic finite element study, an osteoligamentous model of the lumbar spine was employed. Seven input parameters were simultaneously and randomly varied within appropriate limits for >110 combinations thereof. The maximum von Mises stresses in cancellous and cortical bone of the treated vertebral body L3 and in bone cement were calculated. The loading cases standing, flexion, extension, lateral bending, axial rotation and walking were simulated. In a subsequent sensitivity analysis, the coefficients of correlation and determination of the input parameters on the von Mises stresses were calculated. The loading case has a strong influence on the maximum von Mises stress. In cancellous bone, the median value of the maximum von Mises stresses for the different input parameter combinations varied between 1.5 (standing) and 4.5 MPa (flexion). The ranges of the stresses are large for all loading cases studied. Depending on the loading case, up to 69% of the maximum stress variation could be explained by the seven input parameters. The fracture shape and the elastic modulus of the fractured region have the highest influence. In cortical bone, the median values of the maximum von Mises stresses varied between 31.1 (standing) and 61.8 MPa (flexion). The seven input parameters could explain up to 80% of the stress variation here. It is the fracture shape, which has always the highest influence on the stress variation. In bone cement, the median value of the maximum von Mises stresses varied between 3.8 (standing) and 12.7 MPa (flexion). Up to 75% of the maximum stress variation in cement could be explained by the seven input parameters. Fracture shape, and the elastic moduli of bone cement and of the fracture region are those input parameters with the highest influence on the stress variation. In the model with no fracture, the maximum von Mises stresses are generally low. The present probabilistic and sensitivity study clearly showed that in vertebroplasty the maximum stresses in the augmented vertebral body and in bone cement depend mainly on the loading case and fracture shape. Elastic moduli of cement, fracture region and cancellous bone as well as cement volume have sometimes a moderate effect while number and symmetry of cement plugs have virtually no effect on the maximum stresses.
European Spine Journal 04/2010; 19(9):1585-95. · 1.97 Impact Factor
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ABSTRACT: Total disc arthroplasty (TDA) has been successfully used for monosegmental treatment in the last few years. However, multi-level TDA led to controversial clinical results. We hypothesise that: (1) the more artificial discs are implanted, the stronger the increases in spinal mobility and facet joint forces in flexion and extension; (2) deviations from the optimal implant position lead to strong instabilities. A three-dimensional finite element model of the intact L1-L5 human lumbar spine was created. Additionally, models of the L1-L5 region implanted with multiple Charité discs ranging from two to four levels were created. The models took into account the possible misalignments in the antero-posterior direction of the artificial discs. All these models were exposed to an axial compression preload of 500 N and pure moments of 7.5 Nm in flexion and extension. For central implant positions and the loading case extension, a motion increase of 51% for two implants up to 91% for four implants and a facet force increase of 24% for two implants up to 38% for four implants compared to the intact spine were calculated. In flexion, a motion decrease of 5% for two implants up to 8% for four implants was predicted. Posteriorly placed implants led to a better representation of the intact spine motion. However, lift-off phenomena between the core and the implant endplates were observed in some extension simulations in which the artificial discs were anteriorly or posteriorly implanted. The more artificial discs are implanted, the stronger the motion increase in flexion and extension was predicted with respect to the intact condition. Deviations from the optimal implant position lead to unfavourable kinematics, to high facet joint forces and even to lift-off phenomena. Therefore, multilevel TDA should, if at all, only be performed in appropriate patients with good muscular conditions and by surgeons who can ensure optimal implant positions.
European Spine Journal 04/2010; 21 Suppl 5:S663-74. · 1.97 Impact Factor
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ABSTRACT: Diurnal changes of intervertebral disc height are caused by high compressive loading during the day, which expulses fluid from the disc, and by osmotic pressure, which imbibes fluid into the disc at low loading. The aim of the present study was to determine the magnitude of diurnal changes in spine flexibility, intradiscal pressures and contact forces in the facet joints. A validated osseoligamentous finite element model of the lumbar spine was used to determine these quantities for morning and evening situations. Disc height varied by 10% for these two situations. Spine flexibility and facet joint forces were markedly higher in the evening than in the morning. Intradiscal pressures were higher in the morning than in the evening. The different spine flexibilities in the morning and evening should be taken into account during kinematic measurements. Predicted facet joint forces may be used for the designing and pre-clinical testing of artificial facet joint replacements.
Computer Methods in Biomechanics and Biomedical Engineering 11/2009; 13(5):551-7. · 0.85 Impact Factor
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ABSTRACT: Large implants for vertebral body replacement (VBR) have a large footprint, and are normally supported by stronger bone at the rim of the vertebral body. But they also necessitate a greater corpectomy defect in the vertebral body concerned. In order to study the effect of implant size on contact pressure on the adjacent vertebral bodies and thus the risk of implant subsidence, an osseoligamentous finite element model of the lumbar spine was employed. The VBR was inserted at the level of L4 and additionally stabilized by posterior spinal instrumentation. Flat and curved vertebral endplates, small and large corpectomy defects, different implant positions and axial preloads as well as normal and osteoporotic vertebral bodies were simulated. Contact pressures in the vertebral body are increased for a curved vertebral endplate in comparison with a flat one, they are increased when an additional implant preload was assumed, and they are usually decreased for an osteoporotic vertebra when compared to a non-osteoporotic one. In some cases the average contact pressures were higher for the small-sized VBR, in others for the large-sized one. Our results reveal that from the mechanical point of view, a small-sized VBR is not generally disadvantageous.
Medical Engineering & Physics 09/2009; 31(10):1307-12. · 1.62 Impact Factor
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ABSTRACT: There are several different artificial discs for the lumbar spine in clinical use. Though clinically established, little is known about the biomechanical advantages of different disc kinematics.
A validated finite element model of the lumbosacral spine was used to compare the results of total disc arthroplasty at level L4/L5 performed by simulating the kinematics of three established artificial disc prostheses (Charité, ProDisc, Activ L). For flexion, extension, lateral bending, and axial torsion, the intervertebral rotations, the locations of the helical axes of rotation, the intradiscal pressures, and the facet joint forces were evaluated at the operated and adjacent levels.
After insertion of an artificial disc, intervertebral rotation is reduced for flexion and increased for extension, lateral bending, and axial torsion for all studied discs at implant level. The positions of the helical axes are altered especially for lateral bending and axial torsion. Increased facet joint contact forces are predicted for the Charité disc during extension-- influenced by the existence of anterior scar tissue--and for the ProDisc and the Activ L during lateral bending and axial torsion. The studied artificial discs have only a minor effect on the adjacent levels.
For some load cases, total disc arthroplasty leads to considerably altered kinematics and increased facet joint contact forces at implant level. The spinal kinematic alterations due to an artificial disc exceed by far the inter-implant differences, while facet joint contact force alterations are strongly implant and load case dependent. The importance of implant kinematics is often overestimated.
Clinical biomechanics (Bristol, Avon) 02/2009; 24(2):135-42. · 1.76 Impact Factor
