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 10kg in a backpack led to an average force increase of only 35N. 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.

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.

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 media-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 media-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. (c) 2012 Elsevier Ltd. All rights reserved.

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.

This paper aims to develop an EMG-driven model of the shoulder that can consider possible muscle co-contractions. A musculoskeletal shoulder model (the original model) is modified such that measured EMGs can be used as model-inputs (the EMG-driven model). The model is validated by using the in-vivo measured glenohumeral-joint reaction forces (GH-JRFs). Three patients carrying instrumented hemi-arthroplasty were asked to perform arm abduction and forward-flexion up to maximum possible elevation, during which motion data, EMG, and in-vivo GH-JRF were measured. The measured EMGs were normalized and together with analyzed motions served as model inputs to estimate the GH-JRF. All possible combinations of input EMGs ranging from a single signal to all EMG signals together were tested. The 'best solution' was defined as the combination of EMGs which yielded the closest match between the model and the experiments. Two types of inconsistencies between the original model and the measurements were observed including a general GH-JRF underestimation and a GH-JRF drop above 90° elevation. Both inconsistencies appeared to be related to co-contraction since inclusion of EMGs could significantly (p < .05) improve the predicted GH-JRF (up to 45%). The developed model has shown the potential to successfully take the existent muscle co-contractions of patients into account.

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. (C) 2011 Elsevier Ltd. All rights reserved.

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.

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.

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.

The Delft Shoulder and Elbow Model (DSEM), a large-scale musculoskeletal model, is used for the estimation of muscle and joint reaction forces in the shoulder and elbow complex. Although the model has been qualitatively verified using EMG-signals, quantitative validation has until recently not been feasible. The development of an instrumented shoulder endoprosthesis has now made this possible. To this end, motion data, EMG-signals, external forces, and in-vivo glenohumeral joint reaction forces (GH-JRF) were recorded for two patients with an instrumented shoulder hemi-arthroplasty, during dynamic tasks (including abduction and anteflexion) and force tasks with the arm held in a static position. Motions and external forces served as the model inputs to estimate the GH-JRF. In the modeling process, the effect of two different (stress and energy) optimization cost functions and uniform size and mass scaling were evaluated. The model-estimated GH-JRF followed the in-vivo measured force for dynamic tasks up to about 90° arm elevations, but generally underestimates the peak forces up to 31%; whereas a different behavior (ascending measured but descending estimated force) was found for angles above 90°. For the force tasks the model generally overestimated the peak GH-JRF for most directions (on average up to 34%). Applying the energy cost function improved model predictions for the dynamic anteflexion task (up to 9%) and for the force task (on average up to 23%). Scaling also led to improvement of the model predictions during the dynamic tasks (up to 26%), but had a negligible effect (<2%) on the force task results. Although results indicated a reasonable compatibility between model and measured data, adjustments will be necessary to individualize the generic model with the patient-specific characteristics.

The Delft Shoulder and Elbow Model (DSEM), a large-scale musculoskeletal model, allows for estimation of individual muscle and joint reaction forces in the shoulder and elbow complex. Although the model has been qualitatively verified previously using EMG signals, quantitative validation has not yet been feasible. In this paper we report on the validation of the DSEM by comparing the GH-joint contact forces estimated by the DSEM with the in-vivo forces measured by a recently developed instrumented shoulder endoprosthesis, capable of measuring the glenohumeral (GH) joint contact forces in-vivo [1]. To validate the model, two patients with instrumented shoulder hemi-arthroplasty were measured. The measurement process included the collection of motion data as well as in-vivo joint reaction forces. Segment and joint angles were used as the model inputs to estimate the GH-joint contact forces. The estimated and recorded GH-joint contact forces for Range of Motion (RoM) and force tasks were compared based on the magnitude of the resultant forces. The results show that the estimated force follows the measured force for abduction and anteflexion motions up to 80 and 50 degrees arm elevations, respectively, while they show different behaviors for angles above 90 degrees (decrease is estimated but increase is measured). The DSEM underestimates the peak force for RoM (up to 38% for abduction motion and 64% for anteflexion motion), while overestimates the peak forces (up to 90%) for most directions of performing the force tasks.

Supraspinatus deficiency is the most frequent and important problem associated to rotator cuff pathologies. It reduces shoulder stability and can lead to osteoarthritis. The goal of this study was to develop a numerical model of the shoulder to analyse the biomechanical consequences of this pathology.A 3D finite element model of the shoulder was developed from a normal cadaver specimen. It included the scapula, the humerus and the major abduction muscles. Instead of the usual ball-socket assumption, which prevents the natural translation of the humerus, shoulder stability was actively achieved by muscles. A feedback algorithm was developed to synchronise muscle forces during abduction. The numerical algorithm was validated against an algebraic model, and the calculated muscle moment arms were compared to the literature. Two cases were considered: a normal shoulder and the same one without supraspinatus.For the normal shoulder, the model predicted the initial upward migration of the humeral head. The maximal humerus translation occurred at 30° of abduction and was 0.75 mm above its ideal centered position. Without supraspinatus, it was 1.6 times higher and the contact point in the glenoid fossa was more eccentric. For the normal shoulder, the maximal glenohumeral force was 81% of the body weight, at 82° of abduction. Without supraspinatus, it increased by 8%, while the increase of muscle forces was 30%.Supraspinatus deficiency increased the upward migration of the humerus, the eccentric loading, and the joint and muscle forces, which may cause a limitation of active abduction and degenerative glenohumeral changes (osteoarthritis and the rotator cuff tear).

The state of stress of an intact femur was analysed using a three-dimensional finite element model. One of a pair of femora was used for determination of data for the 3-D model. The other was instrumented with 34 rosette strain gauges for experimental measurements. Good agreement of analytical and experimental data was achieved. For six modes of loading, the deflections and the principal and comparison stresses were determined and compared. The upper one third and the diaphysis of the femur are differently affected in their state of stress, by different modes of loading including simulation of the abductor muscles and the iliotibial tract. For qualitative stress studies of the diaphysis, loading by a single force on the femoral head parallel to the shaft axis may be adequate.

After total knee replacement many patients aim to resume some sport activities. Cycling is often recommended as an activity with low joint loads [Kuster, 2000]. However these recommendations are mainly based on theoretic estimations and mathematical models [Ericson, 1986]. In order to measure the joint loads in vivo, a telemetrized knee implant was developed which enables sixcomponent load measurements in a primary total knee replacement [Heinlein, 2007]. Two metallic plates with hollow stems are separated by a small gap allowing load-dependent deformation of the inner stem. The deformation is measured by six semiconductor strain gages.

To improve implant design, fixation and preclinical testing, implant manufacturers depend on realistic data of loads acting on the shoulder joint. Furthermore, these data can help to optimize physiotherapeutic treatment and to advise patients in their everyday living conditions. Calculated shoulder joint loads vary extremely among different authors [Anglin C, Wyss UP, Pichora DR. Glenohumeral contact forces. Proc Inst Mech Eng [H] 2000;214:637-44]. Additionally the moments acting in the joint caused by friction or incongruent articular surfaces, for example, are not implemented in most models.An instrumented shoulder joint implant was developed to measure the contact forces and the contact moments acting in the glenohumeral joint. This article provides a detailed description of the implant, containing a nine-channel telemetry unit, six load sensors and an inductive power supply, all hermetically sealed inside the implant. The instrumented implant is based on a clinically proven BIOMET Biomodular shoulder replacement and was calibrated before implantation by using complex mathematical calculation routines in order to achieve an average measuring precision of approximately 2%. (C) 2008 IPEM. Published by Elsevier Ltd. All rights reserved.

The forces acting during physiotherapy in both hip joints of one patient have been measured in vivo using a telemeterized joint prosthesis. During supported exercises the loads are up to 50% of the body weight (BW). Active movement of the leg against resistance in a lying position let the forces rise up to 250% BW. During symmetric two-legged-stance the load is typically 70% BW, while during walking peak values of about 300% BW were measured. By use of one crutch a 25% reduction of the joint force can be achieved.

Mittels instrumentierter Gelenkendoprothesen und anderer Implantate gemessene Belastungen erlauben es, Stabilität, Verschleißeigenschaften, Verankerungsfestigkeit und Bewegungsverhalten der Implantate noch vor deren klinischem Einsatz zu optimieren. Die gewonnenen Daten geben auch Hinweise darauf, welche Aktivitäten zu besonders hohen Belastungen führen und von den Patienten vermieden werden sollten, um den dauerhaften Implantationserfolg nicht zu gefährden. Außerdem kann mithilfe dieser Daten die Physiotherapie nach Gelenkersatz und Frakturen weiter verbessert werden.

With instrumented implants it is possible to measure joint loads directly in the body, which is important if we are to improve the stability, wear properties and fixation technology. The data can also indicate what activities lead to particularly high loads and should be avoided in the immediate postoperative period, which can also be helpful to physiotherapists. The article explains the technical principles of the measurements and gives examples of designs of instrumented implants. Measurements so far recorded with the aid of such implants in hip, knee and shoulder joints, of internal fixation devices in the spinal column and of vertebral body prostheses are presented and the practical conclusions these results allow are discussed. In view of the huge body of data recorded in hip joints, most of the advice that can be given here relate to the hip.