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