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In biomechanical studies, the thorax is often considered rigid. Since it's a well-known simplification, usually more than three markers are used to describe its movement in motion analyses. However, there is uncertainty about how many markers are advisable and which landmarks should be used. The results of the present study describe the expected error depending on the number of markers used. Furthermore, a recommendation is given for the landmarks with the least errors. This recommendation is valid for men and women as well as for different movements. The recommendations roughly reduce the error to about 50% and are beneficial especially in case only a small number of markers were used. For general motion capture, we recommend to use at least six thoracic markers., (Copyright © 2022 Elsevier Ltd. All rights reserved.)

Lumbo-pelvic parameters in the sagittal plane are normally measured from lateral radiographs obtained at a single time point during upright standing with arms held forward to expose anatomical bony structures. However, the human trunk naturally sways during still standing, which potentially alters the targeted parameters. We therefore aimed to investigate the effect of postural sway on lumbo-pelvic parameters during still standing at different arm positions. A non-radiological back measurement device was used to determine the absolute changes of back lordosis and sacral orientation during one-minute still standing while participants (10 males and 10 females without low back pain) held their arms at eight different positions. When the arms were freely hanging down at both sides, males displayed median values of 25.2° (range: 15.6-45.0°) and 7.4° (range: 2.0-26.7°) for back lordosis and sacral orientation, which were smaller than those of 33.1° (range: 11.9-41.9°) and 16.1° (range: 0.8-22.8°) for females, respectively (P < 0.05). At the same arm position, the median values were 2.7° (range: 1.3-5.2°) and 2.9° (range: 1.6-4.5°) for change of back lordosis and change of sacral orientation, respectively. Sex displayed no significant influence for both measures. Different arm positions non-significantly affected the change of back lordosis. When hands rested on horizontal bars, the change of sacral orientation was significantly less than during other arm positions (P < 0.05). Hence, back lordosis and sacral orientation inherently change during still standing, independently of sex and arm position, which could compromise the reliability of measurements at a single time point. When categorizing subjects into groups with normal or abnormal lumbo-pelvic balance, this variability should be taken into consideration., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 Elsevier Ltd. All rights reserved.)

Several skin surface-based techniques exist to non-invasively determine the spinal kinematics. However, the accuracy of these techniques is limited by soft-tissue artefacts. Furthermore, structures like the thorax are frequently assumed to be rigid but display considerable mobility within itself. This study aimed to quantify the accuracy at different thoracic landmarks for measuring mobility in healthy individuals during different activities to provide a recommendation for the best suited measurement location. The locations of 29 landmarks were continuously captured on 19 individuals (age: 25-59 years) during sitting, standing, walking, jumping, intra-thoracic motions, and different breathing depths using reflective markers. Marker triplets were used at every landmark to calculate their orientations by first backtracking the rigid-body motion (RBM) of the thorax in general, and subsequently calculating the RBM of each rigid marker triplet. Of the latter, the maximum axis angle for each exercise was statistically evaluated. Landmarks at the middle of the clavicles displayed the largest overall errors (approximately 90° during worst case scenario). However, the variability of errors among the investigated exercises was large. Landmarks at the cranial sternal region (particularly at the "Louis angle") and at the T3 spinous process showed the smallest errors for all subjects and tasks (e.g., <5° and <11°, respectively, during normal breathing). When only one sensor is to be used, it is recommended to use the cranial sternal region to assess the thoracic orientation. Study results highly sensitive to thoracic orientation should be considered with care or performed using more appropriate methods., Competing Interests: Declaration of Competing Interest All authors declare that there are no financial or personal relationships with other persons or organisations that could have inappropriately influenced this study., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

During manual material lifting, the sagittal motion is mainly characterized through the lumbo-pelvic coordination, which is quantified by the ratio between the lumbar and hip rotations (L/P ratio). Alteration in the L/P ratio is an important indicator for low back pain (LBP). Previous studies demonstrated sex-dependent differences in LBP prevalence during lifting activities. However, the sex-dependent differences in the L/P ratio during different lifting tasks has to data not been investigated. An optoelectronic system was used to measure the sagittal lumbo-pelvic motion in 10 males and 10 females. Task A was lifting one weight from the ground in front of the body to three target heights with straight knees (A1-3: abdomen, chest and head levels, respectively). Task B was lifting two identical weights from the ground at the sides of the body to three target angles with bended knees (B1-3: arms close and 45° and 90° abducted to the trunk, respectively). Lifts of 10 kg (males and females) and 20 kg (males only) were performed and three phases were investigated: Phase 1 - pure flexion without load, Phase 2 - lifting up weights, Phase 3 - lowering down weights. Females generally displayed a smaller L/P ratio than males. In Phases 2 and 3, the L/P ratio was greater than in Phase 1 for Tasks A and B. The L/P ratio increased with a greater lifting height for Task B, but displayed no difference neither between lifting 10 kg and 20 kg, nor between weight lifting and lowering for both tasks. These results can provide indications for sex-specific recommendations for safer lifting activities., Competing Interests: Declaration of Competing Interest The authors declare that they have no conflict of interest., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

Background: Currently, an upright standing posture is normally adopted for evaluations of spinal alignment, which is however sensitive to posture variations. Thus, finding a reproducible reference is essential. This study aimed to evaluate the reproducibility of standing and sitting postures at different arm positions in five consecutive repetitions., Methods: 22 asymptomatic subjects (11 males; 11 females) aged 20-35 years were included. Subjects were repeatedly asked to adopt different arm positions in standing and sitting. The absolute reposition errors of lumbar lordosis and sacral orientation between two consecutive repetitions were assessed with a non-radiological back measurement system., Findings: During standing at the relaxed arm position, the median absolute reposition errors of lumbar lordosis and sacral orientation were 1.14° (range 0.23°-3.80°) and 0.92° (range 0.17°-3.27°), respectively, which increased to 1.75° (range 0.21-4.97°) and 1.36° (range 0.35°-4.08°) during sitting (P < 0.01). The absolute reposition error of lumbar lordosis was non-significantly lower at the relaxed and clasped arm positions than at other arm positions. Between the first two repetitions, the absolute reposition errors of both, lumbar lordosis and sacral orientation, were greater than between the remaining two consecutive repetitions (P < 0.01). Both during standing and sitting, lumbar lordosis was smallest when hands holding two bars (P < 0.05)., Interpretation: Sitting showed a worse reproducibility than standing. When assessing sagittal spinal balance, the clasped arm position during standing is recommended and an initial trial can help to reduce inception irreproducibility., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

A comprehensive knowledge of the thoracic shape and kinematics is essential for effective risk prevention, diagnose and proper management of thoracic disorders and assessment of treatment or rehabilitation strategies as well as for in silico and in vitro models for realistic applications of boundary conditions. After an extensive search of the existing literature, this study summarizes 45 studies on in vivo thoracic kyphosis and kinematics and creates a systematic and detailed database. The thoracic kyphosis over T1-12 determined using non-radiological devices (34°) was relatively less than measured using radiological devices (40°) during standing. The majority of kinematical measurements are based on non-radiological devices. The thoracic range of motion (RoM) was greatest during axial rotation (40°), followed by lateral bending (26°), and flexion (21°) when determined using non-radiological devices during standing. The smallest RoM was identified during extension (13°). The lower thoracic level (T8-12) contributed more to the RoM than the upper (T1-4) and middle (T4-8) levels during flexion and lateral bending. During axial rotation and extension, the middle level (T4-8) contributed the most. Coupled motion was evident, mostly during lateral bending and axial rotation. With aging, the thoracic kyphosis increased by about 3° per decade, whereas the RoM decreased by about 5° per decade for all load directions. These changes with aging mainly occurred in the lower region (T6-12). The influence of sex on thoracic kyphosis and the RoM has been described as partly contradictory. Obesity was found to decrease the thoracic RoM. Studies comparing standing, sitting and lying reported the effect of posture as significant., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

Cervical-flexibility examination is routinely performed in neck-pain patients. However, diagnosis of cervical-flexibility impairment requires physiological reference values, which vary widely among the population. Although there is a general understanding that the cervical range of motion (RoM) alters with age and sex, the consolidated details of these variations remain lacking. A systematic review and meta-analysis was performed to evaluate the difference of cervical RoM in different age and sex populations. The quality-assessment tool for quantitative studies was applied to assess methodological quality. We identified 4,034 abstracts through a database search and 3 publications through a manual search. Thirty-four cross-sectional studies were selected for the systematic review and measuring technologies were identified. The difference in age descriptions was substantial and a strong discrepancy existed between the mobility measured by radiological and non-radiological devices. Therefore, only 11 non-radiological studies with similar age descriptions were selected for meta-analysis. Cervical RoMs varied considerably among the populations and generally decreased with age. However, this diminishment started earlier and ended later in males, and was not continuous across age in both sexes. Females normally displayed a greater RoM than males, except in lateral bending. In young subjects, the difference between males and females was not significant. For subjects in their 50s, males displayed a non-significantly greater RoM than females. The variability of cervical RoMs can be explained by different devices as well as age and sex. However, the age-dependent reduction is not continuous and differs between males and females. These findings lay the foundation for a better understanding of the incidence of age- and sex-dependent cervical disorders, and may have important implications for the long-term success of different clinical interventions., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

During level walking, lumbar spine is subjected to cyclic movements and intricate loading of the spinal discs and trunk musculature. This study aimed to estimate the spinal loads (T12-S1) and trunk muscles forces during a complete gait cycle. Six men, 24-33years walk barefoot at self-selected speed (4-5km/h). 3D kinematics and ground reaction forces were recorded using a motion capturing system and two force plates, implemented in an inverse dynamic musculoskeletal model to predict the spinal loads and trunk muscles forces. Additionally, the sensitivity of the intra-abdominal pressure and lumbar segment rotational stiffness was investigated. Peak spinal loads and trunk muscle forces were between the gait instances of heel strike and toe off. In L4-L5 segment, sensitivity analysis showed that average peak compressive, antero-posterior and medio-lateral shear forces were 130-179%, 2-15% and 1-6%, with max standard deviation (±STD) of 40%, 6% and 3% of the body weight. Average peak global muscles forces were 24-55% (longissimus thoracis), 11-23% (iliocostalis thoracis), 12-16% (external oblique), 17-25% (internal oblique) and 0-8% (rectus abdominus) of body weight whereas, the average peak local muscles forces were 11-19% (longissimus lumborum), 14-31% (iliocostalis lumborum) and 12-17% (multifidus). Maximum±STD of the global and local muscles forces were 13% and 8% of the body weight. Large inter-individual differences were found in peak compressive and trunk muscles forces whereas the sensitivity analysis also showed a substantial variation., (Copyright © 2017 Elsevier Ltd. All rights reserved.)

Flexion is the main motion of the lumbar spine. While in vitro tests with pure moments suggest larger intra-segmental rotations for the more caudal segments, in vivo results show diverging motion distributions. The present study analysed the motion distribution in vivo of 320 asymptomatic subjects. The change of the back curvature between standing and upper body flexion was determined using a non-invasive measurement device. Linear, bilinear, trilinear, quadratic, and cubic regression models were fitted to the segmental motion distribution over the lengths of the lordosis to categorise characteristic motion patterns. Simplicity and approximation quality were used to assign the motion distributions to the regression models. Seventy-seven percent of the motion distributions could be explained by a bilinear model. A further 12% and 11% could be represented by a trilinear and linear model, respectively. Less than 1% of the distributions could not satisfactorily be represented by the models. All of the bilinear models displayed maximum flexion in approximately the middle of lordosis. All linear models showed a decreasing rotation from caudal to cranial. Most of the trilinear models displayed a distribution similar to the bilinear. Age, sex, body height, and weight did not significantly affect these distributions. This in vivo study identified characteristic motion patterns in the lumbar spine during flexion. The quantitative results provide a clear description of the healthy condition and may serve to identify spinal motion abnormalities., (Copyright © 2017 Elsevier Ltd. All rights reserved.)

During level walking, arm swing plays a key role in improving dynamic stability. In vivo investigations with a telemeterized vertebral body replacement showed that spinal loads can be affected by differences in arm positions during sitting and standing. However, little is known about how arm swing could influence the lumbar spine and hip joint forces and motions during walking. The present study aims to provide better understanding of the contribution of the upper limbs to human gait, investigating ranges of motion and joint reaction forces. A three-dimensional motion analysis was carried out via a motion capturing system on six healthy males and five patients with hip instrumented implant. Each subject performed walking with different arm swing amplitudes (small, normal, and large) and arm positions (bound to the body, and folded across the chest). The motion data were imported in a commercial musculoskeletal analysis software for kinematic and inverse dynamic investigation. The range of motion of the thorax with respect to the pelvis and of the pelvis with respect to the ground in the transversal plane were significantly associated with arm position and swing amplitude during gait. The hip external-internal rotation range of motion statistically varied only for non-dominant limb. Unlike hip joint reaction forces, predicted peak spinal loads at T12-L1 and L5-S1 showed significant differences at approximately the time of contralateral toe off and contralateral heel strike. Therefore, arm position and swing amplitude have a relevant effect on kinematic variables and spinal loads, but not on hip loads during walking., (Copyright © 2017 Elsevier Ltd. All rights reserved.)

Finite element models are frequently used to study lumbar spinal biomechanics. Deterministic models are used to reflect a certain configuration, including the means of geometrical and material properties, while probabilistic models account for the inherent variability in the population. Because model parameters are generally uncertain, their predictive power is frequently questioned. In the present study, we determined the sensitivities of spinal forces and motions to material parameters of intervertebral discs, vertebrae, and ligaments and to lumbar morphology. We performed 1200 model simulations using a generic model of the human lumbar spine loaded under pure moments. Coefficients of determination and of variation were determined for all parameter and response combinations. Material properties of the vertebrae displayed the least impact on results, whereas those of the discs and morphology impacted most. The most affected results were the axial compression forces in the vertebral body and in several ligaments during flexion and the facet-joint forces during extension. Intervertebral rotations were considerably affected only when several parameters were varied simultaneously. Results can be used to decide which model parameters require careful consideration in deterministic models and which parameters might be omitted in probabilistic studies. Findings allow quantitative estimation of a model׳s precision., (Copyright © 2017 Elsevier Ltd. All rights reserved.)

Total disc replacement has been introduced to overcome negative side effects of spinal fusion. The amount of iatrogenic distraction, preoperative disc height and implant positioning have been considered important for surgical success. However, their effect on the postoperative range of motion (RoM) and loading of the facets merits further discussion. A validated osteoligamentous finite element model of the lumbosacral spine was employed and extended with four additional models to account for different disc heights. An artificial disc with a fixed center of rotation (CoR) was implemented in L5-S1. In 4000 simulations, the influence of distraction and the CoR's location on the RoM, facet joint forces (FJFs) and facet capsule ligament forces (FCLFs) was investigated. Distraction substantially altered segmental kinematics in the sagittal plane by decreasing range of flexion (0.5° per 1mm of distraction), increasing range of extension (0.7°/mm) and slightly affecting complete sagittal RoM (0.2°/mm). The distraction already strongly increased the FCLFs during surgery (up to 230N) and in flexion (~12N/mm), with higher values in models with larger preoperative disc heights, and increased FJFs in extension. A more anterior implant location decreased the RoM in all planes. In most loading cases, a more posterior location of the implant's CoR increased the FJFs and FCLFs, whereas a more caudal location increased the FCLFs but decreased the FJFs. The results of this study may explain the worse clinical results in patients with overdistraction after TDR. The complete RoM in the sagittal plane appears to be insensitive to detecting surgery-related biomechanical changes., (Copyright © 2015 Elsevier Ltd. All rights reserved.)

Knowledge about in vivo spinal loads is required for the identification of risk factors for low back pain and for realistic preclinical testing of spinal implants. Therefore, the aim of the present study was to measure the in vivo forces on a vertebral body replacement (VBR) during trunk flexion and to analyze in detail the typical relationship between trunk inclination and spinal load. Telemeterized VBRs were implanted in five patients. In vivo loads were measured 135 times during flexion while standing or sitting. The trunk inclination was simultaneously recorded. To reveal elementary differences between flexion while standing and sitting, the force increases at the maximal inclination, as compared to the upright position, were also determined. Approximately 90% of all standing trials showed a characteristic inclination-load relationship, with an initial increase of the resultant force followed by a plateau or even a decrease of the force at an inclination of approximately 33°. Further flexion to the average maximal inclination angle of 53° only marginally affected the implant loads (~450N). Maximal forces were measured during the return to the initial standing position (~565N). Flexion during standing led to a greater force increase (~330N) than during sitting (~200N) when compared to the respective upright positions. The force plateau at greater inclination angles might be explained by abdominal load support, complex stabilization of active and passive spinal structures or intricate load sharing within the implant complex. The data presented here aid in understanding the loads acting on an instrumented lumbar spine., (Copyright © 2015 Elsevier Ltd. All rights reserved.)

Knowledge of in vivo spinal loads and muscle forces remains limited but is necessary for spinal biomechanical research. To assess the in vivo spinal loads, measurements with telemeterised vertebral body replacements were performed in four patients. The following postures were investigated: (a) standing with arms hanging down on sides, (b) holding dumbbells to subject the patient to a vertical load, and (c) the forward elevation of arms for creating an additional flexion moment. The same postures were simulated by an inverse static model for validation purposes, to predict muscle forces, and to assess the spinal loads in subjects without implants. Holding dumbbells on sides increased implant forces by the magnitude of the weight of the dumbbells. In contrast, elevating the arms yielded considerable implant forces with a high correlation between the external flexion moment and the implant force. Predictions agreed well with experimental findings, especially for forward elevation of arms. Flexion moments were mainly compensated by erector spinae muscles. The implant altered the kinematics and, thus, the spinal loads. Elevation of both arms in vivo increased spinal axial forces by approximately 100N; each additional kg of dumbbell weight held in the hands increased the spinal axial forces by 60N. Model predictions suggest that in the intact situation, the force increase is one-third greater for these loads. In vivo measurements are essential for the validation of analytical models, and the combination of both methods can reveal unquantifiable data such as the spinal loads in the intact non-instrumented situation., (Copyright © 2015 Elsevier Ltd. All rights reserved.)

Bone is a dynamic tissue which is continuously adapting not only to external mechanical stimuli but also to internal metabolic calcium demands. During normal bone remodeling, bone-resorbing osteoclasts release calcium from the bone and digest the collagenous bone matrix, after which bone-depositing osteoblasts form unmineralized collagen matrix, which subsequently mineralizes. The detailed mechanism by which calcium is deposited at the site of mineralization and removed from it during bone resorption is largely unknown. Experimental studies are difficult to conduct because in adult bone only a small fraction of bone tissue is remodeled at any moment in time. Thus, one promising approach is to study mineral deposition and resorption in model systems in which a large fraction of the bone mineral is mobilized in a relatively short period of time. We investigated the microscopic and nanoscopic alterations of avian medullary bone architecture during the egg-laying (oviposition) cycle of hens. Medullary bone forms a labile calcium reservoir for eggshell production and is characterized by an extremely rapid and high-flux calcium metabolism. It thus, provides the unique opportunity to study processes of bone remodeling in their most intensive form. We used a combination of synchrotron X-ray tomography together with small angle X-ray scattering (SAXS), wide angle X-ray diffraction (WAXD) and X-ray fluorescence (XRF) to correlate microscopic medullary bone attributes such as the mineral content, medullary bone volume fraction and medullary bone trabecular thickness with nanoscopic alterations in the mineral particle size (thickness parameter T and length parameter L) during the oviposition cycle. To identify the timing of the different stages of the cycle, ionic calcium, phosphorus and PTH concentrations in the blood of the layers were monitored. We found that the microscopic and nanoscopic architecture of avian medullary bone material changes rapidly during the oviposition cycle. During eggshell calcification, the mineral content and the size of trabeculae of medullary bone decrease markedly. Furthermore, the average mineral particle size increases during resorption, suggesting that the smaller mineral particles are preferrentially removed. Medullary bone thus formes a fast-responding system exhibiting rapid alterations of the material at the micron and nano scale. Those mechanisms are crucial to provide calcium for the high metabolic calcium demand during eggshell mineralization., (Copyright © 2014. Published by Elsevier Inc.)

Finite element (FE) model studies have made important contributions to our understanding of functional biomechanics of the lumbar spine. However, if a model is used to answer clinical and biomechanical questions over a certain population, their inherently large inter-subject variability has to be considered. Current FE model studies, however, generally account only for a single distinct spinal geometry with one set of material properties. This raises questions concerning their predictive power, their range of results and on their agreement with in vitro and in vivo values. Eight well-established FE models of the lumbar spine (L1-5) of different research centers around the globe were subjected to pure and combined loading modes and compared to in vitro and in vivo measurements for intervertebral rotations, disc pressures and facet joint forces. Under pure moment loading, the predicted L1-5 rotations of almost all models fell within the reported in vitro ranges, and their median values differed on average by only 2° for flexion-extension, 1° for lateral bending and 5° for axial rotation. Predicted median facet joint forces and disc pressures were also in good agreement with published median in vitro values. However, the ranges of predictions were larger and exceeded those reported in vitro, especially for the facet joint forces. For all combined loading modes, except for flexion, predicted median segmental intervertebral rotations and disc pressures were in good agreement with measured in vivo values. In light of high inter-subject variability, the generalization of results of a single model to a population remains a concern. This study demonstrated that the pooled median of individual model results, similar to a probabilistic approach, can be used as an improved predictive tool in order to estimate the response of the lumbar spine., (Copyright © 2014 Elsevier Ltd. All rights reserved.)

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., (Copyright © 2013 Elsevier Ltd. All rights reserved.)

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., (Copyright © 2013 Elsevier Ltd. All rights reserved.)

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 3.0 kg 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., (Copyright © 2012 Elsevier Ltd. All rights reserved.)

Background: 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., Method: 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., Findings: 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., Interpretation: 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., (Copyright © 2012 Elsevier Ltd. All rights reserved.)

Background: 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., Methods: 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., Findings: 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., Interpretation: 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., (Copyright © 2011 Elsevier Ltd. All rights reserved.)

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., (Copyright © 2011 Elsevier Ltd. All rights reserved.)

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., (Copyright © 2010 Elsevier Ltd. All rights reserved.)

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.

The exact loads acting on the lumbar spine during standing remain hitherto unknown. It is for this reason that different loads are applied in experimental and numerical studies. The aim of this study was to compare intersegmental rotations, intradiscal pressures and facet joint forces for different loading modes simulating standing in order to ascertain, the results for which loading modes are closest to data measured in vivo. A validated osseoligamentous finite element model of the lumbar spine ranging from L1 to the disc L5-S1, was used. Six load application modes were investigated as to how they could simulate standing. This posture was simulated by applying a vertical force of 500 N at the centre of the L1 vertebral endplate with different boundary conditions, by applying a follower load, and by applying upper body weight and muscle forces. The calculated intersegmental rotations and intradiscal pressures were compared to in vivo values. Intersegmental rotations at one level vary by up to 8 degrees for the different loading modes simulating standing. The overall rotation in the lumbar spine varies between 2.2 degrees and 19.5 degrees. With a follower load, the difference to the value measured in vivo is 3.3 degrees. For all other loading cases studied, the difference is greater than 6.6 degrees. Intradiscal pressures vary slightly with the loading mode. Calculated forces in the facet joints vary between 0 and nearly 80 N. Applying a follower load of 500 N is the only loading mode simulating standing for which the calculated values for intervertebral rotations and intradiscal pressures agreed well with in vivo data from literature.

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

Background: 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., Methods: 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., Findings: 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., Interpretations: 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.

Lung cancer is the most frequent cause of cancer deaths in the western world today. In our case, we present the history of a 62-year-old man with the diagnosis of the uncommon complication of an acute gastric pneumatosis following his palliative chemotherapy. This rare condition was first described more than 100 years ago and has since been described in several distinctive clinical settings. To our knowledge, we present the first case of chemotherapy-related pneumatosis exclusively limited to the stomach wall but involving the portal veins and the spleen.

Background: A slightly degenerated disc adjacent to a segment that has to be fused is sometimes instrumented with a dynamic fixator. The dynamic implant is assumed to reduce disc loads at that level and to preserve disc function, thus inhibiting the progression of degeneration., Methods: A three-dimensional finite element model of the lumbar spine was used to study the effect of a dynamic implant on the mechanical behavior at the corresponding level. After studying a healthy lumbar spine for comparison, a rigid fixator and a bone graft were inserted at L2/L3. Healthy and degenerated discs were assumed at the adjacent level, i.e. L3/L4. An additional paired dynamic posterior fixator was then implemented at level L3/L4. Finally, the segment with the dynamic fixator was distracted to the height of a healthy disc. The loading cases of walking, extension, flexion and axial rotation were simulated., Findings: A dynamic implant reduces intersegmental rotation for walking, extension and flexion as well as facet joint forces for axial rotation at its insertion level. Intradiscal pressure is not markedly reduced by a dynamic implant. Moreover, there are no substantial differences between the mechanical behavior of rigid and dynamic fixators., Interpretation: Our model does not predict major differences in the mechanical effects between rigid and dynamic fixators despite the extreme assumption that a dynamic implant does not transfer moments. The results do not support the assumption that disc loads are significantly reduced by a dynamic implant. For axial rotation, however, dynamic fixation devices do reduce the force in the facet joint.

Background: Implants for vertebral body replacement are often inserted together with an additional stabilizing implant, e.g. an internal fixation device. During implantation bone grafts or milled bone is normally added to the anterior implant. Little is known about the stiffening effect of this fusion-bone mass on the mechanical behavior of the corresponding bone region, including the load distribution between the different parts., Methods: A three-dimensional finite element model of the lumbar spine was created with a vertebral body replacement at L3, a paired internal fixation device between L2 and L4, and left anterolateral fusion bone. The elastic modulus of fusion bone was varied in discrete steps between 0 MPa and 10,000 MPa. The model was loaded to simulate standing, 20 degrees flexion, 15 degrees extension and 6 degrees axial rotation in the lumbar spine., Findings: The elastic modulus of fusion bone has a considerable effect on the compressive force on vertebral body replacement and fusion bone for all loading cases studied. For extension, it also affects intersegmental rotation, the force in the erector spinae muscle, the compressive force on the internal fixator and intradiscal pressure in the adjacent discs. The elastic modulus most strongly affects the different parameters at values between 0 MPa and 500 MPa., Interpretation: Adding bone mass during vertebral body replacement reduces the loads on the ventral implant for all loading cases studied but extension when the fusion-bone stiffens. This protects the implant from fatigue. The load on the fusion bone increases with increasing elastic modulus. Thus bone grafts should be used whenever possible.

Compared to a healthy intervertebral disc, the geometry and the material properties of the involved tissues are altered in a degenerated disc. It is not completely understood how this affects the mechanical behaviour of a motion segment. In order to study the influence of disc degeneration on motion segment mechanics a three-dimensional, nonlinear finite element model of the L3/L4 functional unit was used. Different grades of disc degeneration were simulated by varying disc height and bulk modulus of the nucleus pulposus. The model was loaded with pure moments of 10Nm in the three main anatomic planes. The finite element model predicted the same trends for intersegmental rotation and intradiscal pressure as described in the literature for in vitro studies. A comparison between calculated intersegmental rotation and experimental data showed a mean difference of 1.9 degrees while the mean standard deviation was 2.5 degrees . A mildly degenerated disc increases intersegmental rotation for all loading cases. With further increasing disc degeneration intersegmental rotation is decreased. For axial rotation the decrease takes place in the final stage. Intradiscal pressure is lower while facet joint force and maximum von Mises stress in the annulus are higher in a degenerated compared to a healthy disc.

Muscle forces stabilize the spine and have a great influence on spinal loads. But little is known about their magnitude. In a former in vitro experiment, a good agreement with intradiscal pressure and fixator loads measured in vivo could be achieved for standing and extension of the lumbar spine. However, for flexion the agreement between in vitro and in vivo measurements was insufficient. In order to improve the determination of trunk muscle forces, a three-dimensional nonlinear finite element model of the lumbar spine with an internal fixation device was created and the same loads were applied as in a previous in vitro experiment. An extensive adaptation process of the model was performed for flexion and extension angles up to 20 degrees and -15 degrees, respectively. With this validated computer model intra-abdominal pressure, preload in the fixators, and a combination of hip- and lumbar flexion angle were varied until a good agreement between analytical and in vivo results was reached for both, intradiscal pressure and bending moments in the fixators. Finally, the fixators were removed and the muscle forces for the intact lumbar spine calculated. A good agreement with the in vivo results could only be achieved at a combination of hip- and lumbar flexion. For the intact spine, forces of 170, 100 and 600 N are predicted in the m. erector spinae for standing, 5 degrees extension and 30 degrees flexion, respectively. The force in the m. rectus abdominus for these body positions is less than 25 N. For more than 10 degrees extension the m. erector spinae is unloaded. The finite element method together with in vivo data allows the estimation of trunk muscle forces for different upper body positions in the sagittal plane. In our patients, flexion of the upper body was most likely a combination of hip- and lumbar spine bending.

Data on the stiffnesses of spinal ligaments are required for analytical studies on the mechanical behavior of spinal segments. Values obtained experimentally vary widely in the literature. A finite element model of an L3/L4 functional spinal unit was used to determine the influence of ligament stiffness on intersegmental rotation and forces in the ligaments. The lowest values for ligament stiffness selected from the literature were used in one set of calculations, and the highest values were simulated in a second set. The nonlinear model was loaded with pure moments of 7.5 and 15 Nm in the three main anatomical planes. The mechanical behavior of the functional spinal unit was strongly influenced by ligament stiffness. In some cases, a ligament with low stiffness does not carry any load, while the same ligament with high stiffness has to carry a high load. This indicates that finite element models of spinal segments have to be validated and that a realistic quantitative prediction of ligament forces is extremely difficult.

Objective: To determine whether the mechanical behavior of the entire lumbar spine differs following mono- and bisegmental stabilization., Design: The mechanical behavior of the lumbar spine was studied using the finite element method., Background: Nonunion is somewhat more frequent after bi- than after monosegmental stabilization of the spine. Little is known about differences between the mechanical behavior associated with these procedures., Methods: A three-dimensional nonlinear finite element model of the lumbar spine with internal spinal fixators and bone grafts was used to study mechanical behavior after mono- and bisegmental fixation with and without stabilization of the bridged vertebra. Finite element analyses were performed to determine the influence of four different graft positions, five loading conditions, and six different pretensions in the longitudinal fixator rod. The following parameters were considered: the maximum contact pressure at the interface between the bone graft and vertebral body, the force transmitted by the bone graft, and the size of the contact area between the graft and the vertebral body., Results: Our model shows no clear differences between mono- and bisegmental fixation. Additional stabilization of the bridged vertebra exerts a partly adverse influence on the parameters studied. Pretension in the bridged region has a strong effect on the mechanical behavior., Conclusions: The mechanical behavior of the lumbar spine after mono- and bisegmental stabilization is similar. Thus biological factors and the surgical procedure are probably decisive in determining the fusion rate., Relevance: Knowledge of the mechanical behavior after stabilization of the spine may help to improve the fusion rate. Our results suggest that the mechanical factors studied have only a minor influence on fusion rate and that other factors, such as incomplete resection of cartilage plate and poor local blood supply, are more decisive.

There is little information about the influence of bone graft size, position and elasticity on the mechanical behavior of the lumbar spine. Intersegmental motion, intradiscal pressure and stresses in the lumbar spine were calculated using a three-dimensional, nonlinear finite element model which included an internal spinal fixation device and a bone graft. Cross-sectional area, position, and elastic modulus of the graft were varied in this study. Bone grafts, especially very stiff ones, increase stresses on adjacent endplates. Though larger grafts lead to less contact pressure, it is difficult to judge the quality of different bone graft positions. In general, ventral flexion results in lower maximum contact pressure than lateral bending. There is always little intersegmental rotation in the bridged region compared with that of an intact spine.A larger graft with low stiffness should be favored from a mechanical point of view. Patients should avoid lateral bending of the upper body shortly after surgery.

Objective: To estimate the muscle forces during upper-body inclination and to determine their influence on stress distribution in the annulus fibrosus of the lumbar spine discs., Design: The muscle forces and stresses were calculated using a non-linear finite element model of the lumbar spine., Background: Little is known about the influence of muscle forces on the deformation of, and stresses in, the lumbar spine. In most studies, muscle forces are neglected., Methods: Three-dimensional non-linear finite element models of the ligamentous lumbar spine, with and without internal spinal fixators, were created. They were validated by use of experimental data from in vitro measurements on cadaver specimens. In a second step, the influence of muscle forces on stresses in the annulus fibrosus of the lumbar spine discs was investigated in a parameter study. This was done for different inclination angles of the upper-body., Results: Good agreement between analytical and experimental results proved achievable when loading with pure moments in the three main planes of the lumbar spine. For inclination of the upper-body, the flexion angle clearly has a strong influence on the stresses in the lumbar spine while the influence of local muscles was small. The stress distribution in the discs differed considerably when the muscle forces are neglected and only a pure moment is applied., Conclusions: This study confirmed earlier ones that have shown that muscle forces should not be neglected when studying the stresses in the lumbar spine. The local dorsal muscles, however, have only a small influence on the stresses in the discs., Relevance: For investigations of the biomechanical effects of spinal implants and surgical procedures, experimental or analytical methods are used. Due to the complexity involved, as well as to a lack of information, muscle forces are often neglected. Our study showed that muscles do in fact have a major influence on the mechanical behaviour of the spine and should always be taken into account.