Prospective quantitative assessment of spinal range of motion before and after minimally invasive surgical treatment of vertebral body fractures.

INTRODUCTION: Randomized clinical trials have generated doubts regarding the therapeutic effectiveness of spinal kyphoplasty to reduce pain and improve quality of life in patients with vertebral fractures. There is a paucity of data on the influence of kyphoplasty on spinal range of motion. To quantify early postoperative changes following kyphoplasty in spinal motion, a noninvasive, radiation-free measurement method was used and results related to clinical and radiological parameters. METHODS: The study group included 30 patients with an overall number of 54 symptomatic pathological vertebral compression fractures. All patients were treated with balloon kyphoplasty. Clinical results were recorded using the visual analog scale, SF 36, Roland Morris Score and the Oswestry Disability Index, at three time points; preoperative, 2 days postoperative, and at 12 weeks postoperative. The kyphosis angle/sagittal index were determined with biplanar X-rays. Amplitude/velocity of motion in extension/flexion was measured at each time point by use of the EpionicsSPINE(©) system (Epionics Medical GmbH; Potsdam, Germany) using two external sensor strips. RESULTS: Preoperative magnetic resonance imaging scans showed bone marrow edema in all vertebral bodies indicative of a recent, non-consolidated fracture. Pain and quality of life was significantly improved by kyphoplasty, both for the immediate postoperative period, as well as at 12 weeks postoperative. Radiological parameters also showed significant improvement following surgery. Total ROM did not significantly change 2 days after kyphoplasty, but amplitude and velocity were found to be increased 12 weeks postoperatively. Significant positive correlations were observed between increased range of motion and improved clinical/radiological scores. CONCLUSION: Significant clinical and radiological improvement following kyphoplasty supports the rational for cement augmentation in patients with pathological vertebral body fractures. To the knowledge of the authors, no prior study has assessed the influence of preservation and improvement of spinal range of motion on clinical outcome following kyphoplasty.

Comparison of eight published static finite element models of the intact lumbar spine: predictive power of models improves when combined together.

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.

How does the way a weight is carried affect spinal loads?

People often have to carry a weight which increases the spinal load. Few in vivo measured spinal loading data exist for carrying a weight. The aim of this study was to measure the force increase on a vertebral body replacement (VBR) caused by carrying weights in different ways. A telemeterised VBR allowing the measurement of six load components was implanted in five patients suffering from lumbar vertebral body fractures. The patients carried different weights laterally in one or both hands, in front of the body and in a backpack. The force increase with respect to standing was more than twice as high for carrying a weight in front of the body compared with carrying it laterally. A weight of 10 kg in a backpack led to an average force increase of only 35 N. The position of the carried weight relative to the spine strongly affected the spinal load. PRACTITIONER SUMMARY: Carrying weights increases spinal loads. The loads on a telemeterised VBR were measured in five patients carrying weights in different ways. Holding a weight in front of the body strongly increased the force, while carrying it in a backpack led to only a minor load increase.

The effect of design parameters of interspinous implants on kinematics and load bearing: an in vitro study.

INTRODUCTION: A number of concepts with controversy approaches are currently discussed for interspinous stabilization (IPS). However, comparative biomechanical studies among the different systems are rare. Nevertheless, it remains unclear which biomechanical characteristics are influenced by different design features of these implants, such as implant stiffness or an additional tension band. Therefore, the aim of the present study was to compare different interspinous implants to investigate the biomechanical impact of IPS implant design on intersegmental kinematics, such as range of motion, neutral zone, center of rotation (COR), as well as load transfer like intradiscal pressure (IDP), to gain additional experience for clinical indications and limitations. MATERIAL AND METHOD: Twelve human lumbar spine specimens were tested in a spine loading apparatus. In vitro flexibility testing was performed by applying pure bending moments of 7.5 Nm without and with additional preload of 400 N in the three principal motion planes. Four interspinous implants, Coflex "COF" (Paradigm Spine, Germany), Wallis "WAL" (Abbott Laboratories, France), DIAM "DIA" (Sofamor Danek, France) and InterActiv (Aesculap AG, Germany) with two treatment options (without dorsal tensioning "IAO" and with dorsal tensioning "IAM") were consecutively tested in comparison to the native situation "NAT" and to a defect situation "DEF" of the functional spinal unit. The tested IPS devices are comprised of a compression stiffness range of 133 to 1,674 N/mm and a tensile stiffness range of 0-39 N/mm. Range of motion, neutral zone, center of rotation and intradiscal pressure were analyzed for all instrumentation steps and load cases. CONCLUSION: For the IPS, we found a correlation between compression stiffness and stabilization in extension. Here, the system with the lowest stiffness, DIA, displayed nearly no stabilization of the treated segment, whereas the system with the highest stiffness, WAL and COF, was most pronounced. This applies also for the correlation between device stiffness and IDP. In flexion only the degree of stabilization is in correlation with the tensile stiffness, whereas the IDP stays constant and is not affected by the different tensile stiffness. IPS is not able to stabilize in the frontal and transversal plane. Furthermore IPS does not substantially alter the location of the COR.

Loads on a vertebral body replacement during locomotion measured in vivo.

Walking is one of the most important activities in daily life, and walking exposes the spine to a high number of loading cycles. Little is known about the spinal loads during walking. Telemeterized spinal implants can provide data about their loading during different activities. The aim of this study was to measure the loads on a vertebral body replacement (VBR) during level and staircase walking and to determine the effects of walking speed and using walking aids. Telemeterized VBRs were implanted in five patients suffering from compression fractures of the L1 or L3 lumbar vertebral body. The implant allows measurements of three force and three moment components. The resultant force on the VBR was measured during level and staircase walking, when walking on a treadmill at different speeds, and when using a wheeled invalid walker or crutches. On average, the resultant force on the VBR for level walking was 171% of the value for standing. This force value increased to 265% of the standing force when ascending stairs and to 225% when descending stairs. Walking speed had a strong effect on the implant force. Using a walker during ambulation on level ground reduced the force on the implant to 62% of standing forces, whereas using two crutches had only a minor effect. Walking causes much higher forces on the VBR than standing. A strong force reduction can be achieved by using a walker.

Monitoring the load on a telemeterised vertebral body replacement for a period of up to 65 months.

PURPOSE: To determine the postoperative temporal course of the forces acting on a vertebral body replacement (VBR) for two well reproducible activities. METHODS: A telemeterised VBR was implanted in five patients. It allows the measurement of six load components. Implant loads were measured in up to 28 measuring sessions for different activities, including standing and walking. RESULTS: The postoperative temporal course of the resultant implant forces measured during standing and walking was similar in each patient, but the patterns varied strongly from patient to patient. In one patient, the forces decreased in the first year and then increased in the following 4 years. In another patient, the forces increased in the first few months and then decreased. In a third patient, the forces varied only slightly in the postoperative time. In two patients, there was a strong drop of the implant force in the first two postoperative months. The force was on average approximately 100 N or 71% higher for walking than for standing. CONCLUSIONS: The strong force reduction in the first 2 months is most likely caused by implant subsidence, and the force reduction over a period of more than 6 months is most likely caused by fusion of the vertebrae adjacent to the VBR. The short-term force increase could be attributed to bone atrophy at the index level, and the long-term force increase could be attributed to an increase in the thoracic spine kyphosis angle.

Parameters influencing the outcome after total disc replacement at the lumbosacral junction. Part 1: misalignment of the vertebrae adjacent to a total disc replacement affects the facet joint and facet capsule forces in a probabilistic finite element analysis.

PURPOSE: After total disc replacement with a ball-and-socket joint, reduced range of motion and progression of facet joint degeneration at the index level have been described. The aim of the study was to test the hypothesis that misalignment of the vertebrae adjacent to the implant reduces range of motion and increases facet joint or capsule tensile forces. METHODS: A probabilistic finite element analysis was performed using a lumbosacral spine model with an artificial disc at level L5/S1. Misalignment of the L5 vertebra, the gap size of the facet joints, the transection of the posterior longitudinal ligament, and the spinal shape were varied. The model was loaded with pure moments. RESULTS: Misalignment of the L5 vertebra reduced the range of motion up to 2°. A 2-mm displacement of the L5 vertebra in the anterior direction already led to facet joint forces of approximately 240 N. Extension, lateral bending, and axial rotation caused maximum facet joint forces between 280 and 380 N, while flexion caused maximum forces of approximately 200 N. A 2-mm displacement in the posterior direction led to capsule forces of approximately 80 N. Additional moments increased the maximum facet capsule forces to values between 120 and 230 N. CONCLUSIONS: Misalignment of the vertebrae adjacent to an artificial disc strongly increases facet joint or capsule forces. It might, therefore, be an important reason for unsatisfactory clinical results. In an associated clinical study (Part 2), these findings are validated.

Correlation between back shape and spinal loads.

The purpose of this study was to determine the correlation between the back shape of the lumbar region and the spinal loads during activities performed in the sagittal plane. Measurements were performed in four subjects who had suffered from a compression fracture of a lumbar vertebral body which was treated with a telemeterized vertebral body replacement that is able to measure six load components in vivo. An Epionics SPINE measurement system was used to determine the lumbar lordosis angle. The relationship between the lordosis angle and the corresponding loads was quantified with the Spearman's rank correlation coefficient method. Measurements were performed during thirteen exercises in lying, standing or sitting. During upper body flexion, the force increased on average by approximately 285N and the lordosis angle decreased by 15°. The change of the force for elevating 30N in one hand was on average approximately 190N and for the lordosis angle 2°. Correlation coefficients greater than 0.6 were found for exercises that involved both large back shape and load changes, such as upper body flexion. A strong increase in spinal load can be associated with an increase or a decrease of the lordosis angle. Only for considerable changes of the lordosis angle in an upright body position was a strong correlation between lordosis angle and implant force found.

In vivo measurement of shoulder joint loads during walking with crutches.

BACKGROUND: Following surgery or injury of the lower limbs, the use of walking aids like crutches can cause high loads on the shoulder joint. These loads have been calculated so far with computer models but with strongly varying results. METHODS: Shoulder joint forces and moments were measured during crutch-assisted walking with complete and partial unloading of the lower limbs. Using telemeterized implants in 6 subjects axillary crutches and forearm crutches were compared. A force direction was more in the direction of the long humeral axis, and slightly lower forces were assumed using axillary crutches. Similar force magnitudes as those experienced during previously measured wheelchair weight relief tasks were expected for complete unloading. The friction-induced moment was hypothesized to act mainly around the medio-lateral axis during the swing phase of the body. FINDINGS: Maximum loads of up 170% of the bodyweight and 0.8% of the bodyweight times meter were measured with large variations among the patients. Higher forces were found in most of the patients using forearm crutches. The hypothesized predominant moment around the medio-lateral axis was only found in some patients. More often, the other two moments had larger magnitudes with the highest values in female patients. The assumed different load direction could only be found during partial unloading. INTERPRETATION: In general the force magnitudes were in the range of activities of daily living. However, the number of repetitions during long-lasting crutch use could lead to shoulder problems as a long-term consequence. The slightly lower forces with axillary crutches could be caused by loads acting directly from the crutch on the scapula, thus bypassing the glenohumeral joint. The higher bending moments in the female patients could be a sign of lacking muscle strength for centring the humeral head on the glenoid.

Spinal loads during position changes.

BACKGROUND: Recommendations exist how patients should change from one body position to another in order to keep the spinal loads low. However, until now it is not clear whether the loads are in fact lower if the patients follow these recommendations. The aim was to measure the loads while changing the body position. METHODS: Telemeterized vertebral body replacements have been inserted into 5 patients who had a severe compression fracture of a lumbar vertebral body. The acting loads were measured during a changing of the body position while lying and when moving from lying to sitting, from sitting to standing and vice versa. FINDINGS: When the lying patients changed their position according to the physiotherapist's recommendations, the resultant force was nearly as high as it was during relaxed standing. Otherwise, the force was nearly twice as high. Changing from a lateral lying position to sitting and vice versa caused forces of about 180% of those seen for standing when the recommendations were heeded. Without instructions, the loads were about 70% higher. Use of a trapeze bar mounted to the bed did not increase the loads. Rising from a chair with the arms hanging down laterally led to average resultant forces of 380% related to standing. Placing the hands on armrests reduced this value to 180%. INTERPRETATION: High forces may act on the spine when changing from one body position to another. These loads can be minimized when following the physiotherapist's instructions and when supporting the upper body by the arms.

Measurement of shoulder joint loads during wheelchair propulsion measured in vivo.

BACKGROUND: Recent in vivo measurements show that the loads acting in the glenohumeral joint are high even during activities of daily living. Wheelchair users are frequently affected by shoulder problems. With previous musculoskeletal shoulder models, shoulder joint loading was mostly calculated during well-defined activities like forward flexion or abduction. For complex movements of everyday living or wheelchair propulsion, the reported loads vary considerably. METHODS: Shoulder joint forces and moments were measured with telemeterized implants in 6 subjects. Data were captured on a treadmill at defined speeds and inclinations. Additional measurements were taken in 1 subject when lifting the body from the wheelchair, using his arms only, and in 2 subjects when rapidly accelerating and stopping the wheelchair. The influence of the floor material on shoulder joint loading was accessed in 2 subjects. In general, the maximum shoulder loads did not exceed those during daily living but the time courses and magnitudes of the loads intra-individually varied much. FINDINGS: The highest forces acted during maximum acceleration and lifting from the wheelchair (128% and 188% of body weight). Grass was the only surface which led to a general load increase, compared to a smooth floor. INTERPRETATION: The increased incidence of overuse injuries in wheelchair users are probably not caused by excessive load magnitudes during regular propulsion. The high number of repetitions is assumed to be more decisive.

In vivo gleno-humeral joint loads during forward flexion and abduction.

To improve design and preclinical test scenarios of shoulder joint implants as well as computer-based musculoskeletal models, a precise knowledge of realistic loads acting in vivo is necessary. Such data are also helpful to optimize physiotherapy after joint replacement and fractures. This is the first study that presents forces and moments measured in vivo in the gleno-humeral joint of 6 patients during forward flexion and abduction of the straight arm. The peak forces and, even more, the maximum moments varied inter-individually to a considerable extent. Forces of up to 238%BW (percent of body weight) and moments up to 1.74%BWm were determined. For elevation angles of less than 90° the forces agreed with many previous model-based calculations. At higher elevation angles, however, the measured loads still rose in contrast to the analytical results. When the exercises were performed at a higher speed, the peak forces decreased. The force directions relative to the humerus remained quite constant throughout the whole motion. Large moments in the joint indicate that friction in shoulder implants is high if the glenoid is not replaced. A friction coefficient of 0.1-0.2 seems to be realistic in these cases.

[Stiffening effect of a transsacral fusion system for the lumbosacral junction. A probabilistic finite element analysis and sensitivity study]. / Versteifungseinfluss eines transsakralen Fusionssystems. Eine probabilistische Finite-Elemente-Analyse und Sensitivitätsstudie.

The novel transsacral fusion system AxiALIF allows stabilization of the lumbosacral junction. The system consists of a screw with two different diameters. With additional facet screws or internal fixation devices 360° fusion can be achieved. The effects of different parameters such as length, diameter combination and material of the transsacral screw, type of additional fixation and stiffness of the bone are unknown. In a probabilistic finite element analysis, the input parameters were randomly varied. The rotational angles and the axial forces in the various implants were calculated for four different load scenarios. In a subsequent sensitivity study the influences of single input parameters on the variance of the results were calculated. A transsacral screw significantly reduces the motion in the treated segment, except for axial rotation. An additional fixation has a strong effect on the variance of rotation angles. The other parameters usually explain less than 10% of the variance. The novel lumbosacral fusion system allows good stabilization of the segment, especially when additional fixation via facet screws or fixators is performed.

The effect of design parameters of dynamic pedicle screw systems on kinematics and load bearing: an in vitro study.

As an alternative treatment for chronic back pain due to disc degeneration motion preserving techniques such as posterior dynamic stabilization (PDS) has been clinically introduced, with the intention to alter the load transfer and the kinematics at the affected level to delay degeneration. However, up to the present, it remains unclear when a PDS is clinically indicated and how the ideal PDS mechanism should be designed to achieve this goal. Therefore, the objective of this study was to compare different PDS devices against rigid fixation to investigate the biomechanical impact of PDS design on stabilization and load transfer in the treated and adjacent cranial segment. Six human lumbar spine specimens (L3-L5) were tested in a spine loading apparatus. In vitro flexibility testing was performed by applying pure bending moments of 7.5 Nm without and with additional preload of 400 N in the three principal motion planes. Four PDS devices, "DYN" (Dynesys(®), Zimmer GmbH, Switzerland), "DSS™" (Paradigm Spine, Wurmlingen, Germany), and two prototypes of dynamic rods, "LSC" with a leaf spring, and "STC" with a spring tube (Aesculap AG, Tuttlingen, Germany), were tested in comparison to a rigid fixation device S(4) (Aesculap AG, Tuttlingen, Germany) "RIG", to the native situation "NAT" and to a defect situation "DEF" of the specimens. The instrumented level was L4-L5. The tested PDS devices comprising a stiffness range for axial stiffness of 10 N/mm to 230 N/mm and for bending stiffness of 3 N/mm to 15 N/mm. Range of motion (ROM), neutral zone (NZ), and intradiscal pressure (IDP) were analyzed for all instrumentation steps and load cases of the instrumented and non-instrumented level. In flexion, extension, and lateral bending, all systems, except STC, showed a significant reduction of ROM and NZ compared to the native situation (p < 0.05). Furthermore, we found no significant difference between DYN and RIG (p > 0.1). In axial rotation, only DSS and STC reduced the ROM significantly (p < 0.005) compared to the native situation, whereas DYN and LSC stayed at the level of the native intersegmental rotation (p > 0.05). A correlation was found between axial stiffness and intersegmental stabilization in the sagittal and frontal plane, but not in the transversal plane where intersegmental stabilization is mainly governed by the systems' ability to withstand shear loads. Furthermore, we observed the systems' capacity to reduce IDP in the treated segment. The adjacent segment does not seem to be affected by the stiffness of the fixation device under the described loading conditions.

Realistic loads for testing hip implants.

The aim here was to define realistic load conditions for hip implants, based on in vivo contact force measurements, and to see whether current ISO standards indeed simulate real loads. The load scenarios obtained are based on in vivo hip contact forces measured in 4 patients during different activities and on activity records from 31 patients. The load scenarios can be adapted to various test purposes by applying average or high peak loads, high-impact activities or additional low-impact activities, and by simulating normal or very active patients. The most strenuous activities are walking (average peak forces 1800 N, high peak forces 3900 N), going up stairs (average peak forces 1900 N, high peak forces 4200 N) and stumbling (high peak forces 11,000 N). Torsional moments are 50% higher for going up stairs than for walking. Ten million loading cycles simulate an implantation time of 3.9 years in active patients. The in vitro fatigue properties of cementless implant fixations are exceeded during stumbling. At least for heavyweight and very active subjects, the real load conditions are more critical than those defined by the ISO standards for fatigue tests.

Loading of the knee joint during activities of daily living measured in vivo in five subjects.

Detailed knowledge about loading of the knee joint is essential for preclinical testing of implants, validation of musculoskeletal models and biomechanical understanding of the knee joint. The contact forces and moments acting on the tibial component were therefore measured in 5 subjects in vivo by an instrumented knee implant during various activities of daily living. Average peak resultant forces, in percent of body weight, were highest during stair descending (346% BW), followed by stair ascending (316% BW), level walking (261% BW), one legged stance (259% BW), knee bending (253% BW), standing up (246% BW), sitting down (225% BW) and two legged stance (107% BW). Peak shear forces were about 10-20 times smaller than the axial force. Resultant forces acted almost vertically on the tibial plateau even during high flexion. Highest moments acted in the frontal plane with a typical peak to peak range -2.91% BWm (adduction moment) to 1.61% BWm (abduction moment) throughout all activities. Peak flexion/extension moments ranged between -0.44% BWm (extension moment) and 3.16% BWm (flexion moment). Peak external/internal torques lay between -1.1% BWm (internal torque) and 0.53% BWm (external torque). The knee joint is highly loaded during daily life. In general, resultant contact forces during dynamic activities were lower than the ones predicted by many mathematical models, but lay in a similar range as measured in vivo by others. Some of the observed load components were much higher than those currently applied when testing knee implants.

[Stabilization of the osteoporotic spine from a biomechanical viewpoint]. / Stabilisierung der osteoporotischen Wirbelsäule unter biomechanischen Gesichtspunkten.

The altered trabecular structure of the osteoporotic spine leads to an increased vulnerability of its biomechanical characteristics and reduction of load resistance. Therefore, any surgical procedure must account for these circumstances. In cement-augmented vertebrae, both the overall stability and load transfer to the adjacent structures are influenced by a variety of factors. This has been demonstrated by different findings regarding volume, special characteristics, choice of approach and application, as well as distribution of the cement within the vertebral body. Independent of the well-known good clinical results, these features leave the discussion regarding the most appropriate form of cement-augmenting technique open. In cases where implants are required, there are increasing data to allow for an appropriate choice of stabilizing devices to fit the biomechanical demands in poor bone quality. Thereby, multilevel instrumentation, additive stabilization techniques, cement-augmented pedicle screws and adapted implant designs ensure and increase patient safety. However, regardless of the procedure chosen to stabilize the osteoporotic spine, reconstruction of the column profile appears to be of pre-eminent importance.

In vivo measurement of shoulder joint loads during activities of daily living.

Until recently the contact loads acting in the glenohumeral joint have been calculated using musculoskeletal models or measured in vitro. Now, contact forces and moments are measured in vivo using telemeterized shoulder implants. Mean total contact forces from four patients during eight activities of daily living are reported here. Lifting a coffee pot (1.5kg) with straight arm caused an average force of 105.0%BW (%body weight) (range: 90-124.6%BW), while setting down the coffee pot in the same position led to higher forces of 122.9%BW on the average (105.3-153.4%BW). The highest joint contact forces were measured when the straight arm was abducted or elevated by 90 degrees or more, with a weight in the hand. Lifting up 2kg from a board up to head height caused a contact force of 98.3%BW (93-103.6%BW); again, setting it down on the board led to higher forces of 131.5%BW (118.8-144.1%BW). In contrast to previously calculated high loads, the contact force during passive holding of a 10kg weight laterally was only 12.3%BW (9.2-17.9%BW), but when lifting it up to belt height it increased to 91.5%BW (87-95%BW). The moments transferred inside the joint at our patients varied much more than did the forces both inter and intra-individually. Our data suggest that patients with shoulder problems or during the first post-operative weeks after shoulder fractures or joint replacements should avoid certain activities encountered during daily living e.g. lifting or holding a weight with an outstretched arm. Some energy-related optimization criteria used in the literature for analytical musculoskeletal shoulder models must now be reconsidered.

Applying a follower load delivers realistic results for simulating standing.

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.

Realistic loading conditions for upper body bending.

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