Elsevier

Journal of Biomechanics

Volume 48, Issue 4, 26 February 2015, Pages 578-584
Journal of Biomechanics

Spinal loads as influenced by external loads: A combined in vivo and in silico investigation

https://doi.org/10.1016/j.jbiomech.2015.01.011Get rights and content

Abstract

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 100 N; each additional kg of dumbbell weight held in the hands increased the spinal axial forces by 60 N. 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.

Introduction

Abnormal loads are often considered to be risk factors for spinal disorders and endanger the success of surgical treatments (Frymoyer et al., 1983, Hoogendoorn et al., 2000, Kelsey et al., 1984, Marras et al., 1995). Until now, the complexity of the musculo-spinal system has, however, prevented the detailed determination of spinal loads for even basic tasks.

Electromyography (Staudenmann et al., 2010), intradiscal pressure measurements (Nachemson, 1981), or stadiometry (Althoff et al., 1992), e.g., allow for empirical assessments of spinal loads. In vivo measurements provide realistic data but contain stochastic uncertainties, are limited in number, and do not provide the desired physical quantities, thus often require a conversion to spinal and muscle forces. Computational models do not suffer from these disadvantages and allow the investigation of the effect of single parameters. However, these models are idealisations of reality, contain simplifications and uncertainties, and may involve systematic errors. By combining the advantages of experimental and analytical approaches, e.g., using in vivo data to validate in silico models, questions that extend beyond measurements, such as the influence of external loads on muscle activities or the differences in statics with and without implants may be answered.

One in vivo quantity that can be used to test analytical predictions is the load acting on spinal implants. In our group, clinically used vertebral body replacements (VBRs, Fig. 1) were telemeterised, which permitted the repetitive in vivo measurements of the three moment and three force components acting on the implant. They were implanted in patients with vertebral body compression fractures. One of the available computational models, which allows comparing in vivo to in silico data, is the inverse static model (Damsgaard et al., 2006) provided by the AnyBody software (AnyBody Technology, Aalborg, Denmark). It permits calculation of spinal loads and muscle forces for known postures.

Combinations of basic mechanical or detailed computational spinal models with in vivo data for indirect spinal load estimations have been employed in several studies (e.g. Althoff et al., 1992, Arjmand and Shirazi-Adl, 2005, Dolan and Adams, 1993, El-Rich et al., 2004, Gagnon et al., 2001, Granata and Wilson, 2001, Guzik et al., 1996, Hughes et al., 1994, Iyer et al., 2010, Macintosh et al., 1993, Schultz and Andersson, 1981, Takahashi et al., 2006). However, in none of these studies were spinal forces directly measured.

The aim of the current study was to evaluate the analytical predictions of an inverse static model (Fig. 2) by comparing the calculated data to the in vivo loads measured in the spine during simple loading situations. The computer model was used to investigate the influence of external loads on muscle forces and to assess the changes of spinal loading caused by the kinematics of a VBR.

Section snippets

In vivo measurements

The in vivo measurements were performed on four patients (WP1 to WP4, Table 1) who suffered from A3 type compression fractures of the L1 vertebral body (Magerl et al., 1994). In the first of two surgeries, the patients were stabilized from the posterior using an internal spinal fixation device. In the second surgery, parts of the fractured vertebral body and the adjacent discs were removed, and the VBR was inserted in the created defect. Further details of the surgeries, the implants, their

External forces (holding)—in vivo data

The results for the ‘holding’ posture without dumbbells reflect the intra- and inter-patient variability of the implant force during ‘standing’ (Fig. 4A). In this case, the standard deviation is 18.3 N. Dumbbells increased the ΔFholding (Fig. 4B) and the standard deviation. Due to the discrete amount of weights of the dumbbells, the results were available for only discrete external forces. A linear regression for the medians of each weight for which there were more than 30 measurements available

Discussion

Numerous directly measured in vivo spinal load data were combined with data from a computational model to evaluate the extent to which external loads alter the spinal forces in people with and without lumbar VBR. In the in vivo study, the axial force on the VBR was measured in four patients holding dumbbells while their arms were either hanging down or forwardly elevated. There was large implant force variability even for the ‘standing’ posture (Fig. 4A). When ‘holding’ weights, the implant

Conflict of interest statement

There are no conflicts of interest.

Acknowledgements

The authors greatly appreciate the friendly cooperation of their patients. They thank Dr. A. Bender, J. Dymke, and Dr. F. Graichen for technical assistance and Dr. D. Belavý for his helpful comments. Funding for this study was obtained from the Deutsche Forschungsgemeinschaft (Ro581/18-1), the Arthrose-Hilfe, Frankfurt, Germany, and the German Federal Institute of Sport Science, Bonn, Germany (MiSpEx—the National Research Network for Medicine in Spine Exercise).

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