P. Westerhoff

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.

Validation of the Delft Shoulder and Elbow Model using in-vivo glenohumeral joint contact forces

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.

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.

An instrumented implant for in vivo measurement of contact forces and contact moments in the shoulder joint.

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%.

Implantable sensor technology: From research to clinical practice

Abstract For decades, implantable sensors have been used in research to provide comprehensive understanding of the biomechanics of the human musculoskeletal system. These complex sensor systems have improved our understanding of the in vivo environment by yielding in vivo measurements of force, torque, pressure, and temperature. Historically, implants have been modified to be used as vehicles for sensors and telemetry systems. Recently, microfabrication and nanofabrication technology have sufficiently evolved that wireless, passive sensor systems can be incorporated into implants or tissue with minimal or no modification to the host implant. At the same time, sensor technology costs per unit have become less expensive, providing opportunities for use in daily clinical practice. Although diagnostic implantable sensors can be used clinically without significant increases in expense or surgical time, to date, orthopaedic smart implants have been used exclusively as research tools. These implantable sensors can facilitate personalized medicine by providing exquisitely accurate in vivo data unique to each patient.

Design and Calibration of Load Sensing Orthopaedic Implants

Contact forces and moments act on orthopaedic implants such as joint replacements. The three forces and three moment components can be measured by six internal strain gauges and wireless telemetric data transmission. The accuracy of instrumented implants is restricted by their small size, varying modes of load transfer, and the accuracy of calibration. Aims of this study were to test with finite element studies design features to improve the accuracy, to develop simple but accurate calibration arrangements, and to select the best mathematical method for calculating the calibration constants. Several instrumented implants, and commercial and test transducers were calibrated using different loading setups and mathematical methods. It was found that the arrangement of flexible elements such as bellows or notches between the areas of load transfer and the central sensor locations is most effective to improve the accuracy. Increasing the rigidity of the implant areas, which are fixed in bones or articulate against joint surfaces, is less effective. Simple but accurate calibration of the six force and moment components can be achieved by applying eccentric forces instead of central forces and pure moments. Three different methods for calculating the measuring constants proved to be equally well suited. Employing these improvements makes it possible to keep the average measuring errors of many instrumented implants below 1-2% of the calibration ranges, including cross talk. Additional errors caused by noise of the transmitted signals can be reduced by filtering if this is permitted by the sampling rate and the required frequency content of the loads.

Loads acting on orthopaedic implants. Measurements and practical applications

ZusammenfassungMittels 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.Die technischen Grundlagen für derartige Messungen werden kurz geschildert, und es werden Beispiele für das Design von instrumentierten, die Belastung messenden orthopädischen Implantaten vorgestellt. Anhand der bislang vorgenommenen Messungen an Hüftgelenk, Schultergelenk, internen Fixateuren für die Wirbelsäule, Wirbelkörperersatz und Kniegelenk werden die wichtigsten Resultate aufgeführt. Hieraus werden auch praktische Rückschlüsse für die Praxis gezogen. Aufgrund der vielen Ergebnisse für das Hüftgelenk können die meisten praktischen Hinweise für Patienten mit Ersatz oder Erkrankungen dieses Gelenks gegeben werden.AbstractThe loads measured at instrumented joint replacements and other orthopaedic implants allow the optimization of their stability, wear properties, fixation stability and kinematic properties prior to clinical applications. The data obtained also indicate which activities cause very high loads and should be avoided by the patients in order not to endanger the long-term success of the implant. In addition, physiotherapy after joint arthroplasty and fractures can be further improved on the basis of these data.The technical principles for such measurements are summarized and examples for the design of load measuring instrumented implants are presented. The most important results are presented based on the measurements taken at the hip and shoulder joints, internal spinal fixation devices, vertebral body replacements and knee joints. Using this data, many practical conclusions are drawn. Due to the huge amount of data obtained from the hip, most practical advise can be provided to patients with replacement or disorders involving this joint.

Die Belastung orthopädischer Implantate

ZusammenfassungMittels 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.Die technischen Grundlagen für derartige Messungen werden kurz geschildert, und es werden Beispiele für das Design von instrumentierten, die Belastung messenden orthopädischen Implantaten vorgestellt. Anhand der bislang vorgenommenen Messungen an Hüftgelenk, Schultergelenk, internen Fixateuren für die Wirbelsäule, Wirbelkörperersatz und Kniegelenk werden die wichtigsten Resultate aufgeführt. Hieraus werden auch praktische Rückschlüsse für die Praxis gezogen. Aufgrund der vielen Ergebnisse für das Hüftgelenk können die meisten praktischen Hinweise für Patienten mit Ersatz oder Erkrankungen dieses Gelenks gegeben werden.AbstractThe loads measured at instrumented joint replacements and other orthopaedic implants allow the optimization of their stability, wear properties, fixation stability and kinematic properties prior to clinical applications. The data obtained also indicate which activities cause very high loads and should be avoided by the patients in order not to endanger the long-term success of the implant. In addition, physiotherapy after joint arthroplasty and fractures can be further improved on the basis of these data.The technical principles for such measurements are summarized and examples for the design of load measuring instrumented implants are presented. The most important results are presented based on the measurements taken at the hip and shoulder joints, internal spinal fixation devices, vertebral body replacements and knee joints. Using this data, many practical conclusions are drawn. Due to the huge amount of data obtained from the hip, most practical advise can be provided to patients with replacement or disorders involving this joint.

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.

Comparison of Two Methods for In Vivo Estimation of the Glenohumeral Joint Rotation Center (GH-JRC) of the Patients with Shoulder Hemiarthroplasty

Determination of an accurate glenohumeral-joint rotation center (GH-JRC) from marker data is essential for kinematic and dynamic analysis of shoulder motions. Previous studies have focused on the evaluation of the different functional methods for the estimation of the GH-JRC for healthy subjects. The goal of this paper is to compare two widely used functional methods, namely the instantaneous helical axis (IHA) and symmetrical center of rotation (SCoRE) methods, for estimating the GH-JRC in vivo for patients with implanted shoulder hemiarthroplasty. The motion data of five patients were recorded while performing three different dynamic motions (circumduction, abduction, and forward flexion). The GH-JRC was determined using the CT-images of the subjects (geometric GH-JRC) and was also estimated using the two IHA and SCoRE methods. The rotation centers determined using the IHA and SCoRE methods were on average 1.47±0.62 cm and 2.07±0.55 cm away from geometric GH-JRC, respectively. The two methods differed significantly (two-tailed p-value from paired t-Test ∼0.02, post-hoc power ∼0.30). The SCoRE method showed a significant lower (two-tailed p-value from paired t-Test ∼0.03, post-hoc power ∼0.68) repeatability error calculated between the different trials of each motion and each subject and averaged across all measured subjects (0.62±0.10 cm for IHA vs. 0.43±0.12 cm for SCoRE). It is concluded that the SCoRE appeared to be a more repeatable method whereas the IHA method resulted in a more accurate estimation of the GH-JRC for patients with endoprostheses.