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. 2010 Nov;25(9):853-8.
doi: 10.1016/j.clinbiomech.2010.06.010. Epub 2010 Jul 23.

A biomechanical model for estimating loads on thoracic and lumbar vertebrae

Affiliations

A biomechanical model for estimating loads on thoracic and lumbar vertebrae

Sravisht Iyer et al. Clin Biomech (Bristol). 2010 Nov.

Abstract

Background: Biomechanical models are commonly used to estimate loads on the spine. Current models have focused on understanding the etiology of low back pain and have not included thoracic vertebral levels. Using experimental data on the stiffness of the thoracic spine, ribcage, and sternum, we developed a new quasi-static stiffness-based biomechanical model to calculate loads on the thoracic and lumbar spine during bending or lifting tasks.

Methods: To assess the sensitivity of the model to our key assumptions, we determined the effect of varying ribcage and sternal stiffness, maximum muscle stress, and objective function on predicted spinal loads. We compared estimates of spinal loading obtained with our model to previously reported in vivo intradiscal pressures and muscle activation patterns.

Findings: Inclusion of the ribs and sternum caused an average decrease in vertebral compressive force of 33% for forward flexion and 18% in a lateral moment task. The impact of maximum muscle stress on vertebral force was limited to a narrow range of values. Compressive forces predicted by our model were strongly correlated to in vivo intradiscal pressure measurements in the thoracic (r=0.95) and lumbar (r=1) spine. Predicted trunk muscle activity was also strongly correlated (r=0.95) with previously published EMG data from the lumbar spine.

Interpretation: The consistency and accuracy of the model predictions appear to be sufficient to justify the use of this model for investigating the relationships between applied loads and injury to the thoracic spine during quasi-static loading activities.

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Figures

Figure 1
Figure 1
At each vertebral level, the superincumbent force and the subsequent moment about the vertebral body are balanced by forces in the sternum and ribcage and muscle forces. The relative mechanical contributions of the spine, ribcage, and sternum in each vertebral cross-section were calculated based on the relationship between parallel springs, using experimentally determined stiffness data (Watkins et al., 2005)
Figure 2
Figure 2
Model compressive forces depended strongly on the predicted task. The figure above depicts task-dependent changes in compressive force (A). The tasks modeled in this figure are also shown (B). From left to right, these are: standing, standing with 10kg (5 kg on each arm), lifting 10 kg with elbows bent, 30 degrees flexion with 10kg, and 15 degrees of extension.
Figure 3
Figure 3
Compressive forces during forward flexion and lateral moment tasks. Predicted compressive forces are shown for 30 degrees of flexion (A) and for a lateral moment with 5 kg in the right hand (B). For both panels, ribcage-sternal stiffness, the load bearing contribution of the ribcage, was varied from 0 (no loadbearing by the ribs) to 200% of the baseline value. Compressive force on the vertebral body were strongly dependent on the mechanical contributions of the sternum and ribcage at thoracic levels.
Figure 4
Figure 4
Vertebral compressive force at T9 and T10 predicted by the biomechanical model showed strong correlation to reported intradiscal pressures at T9/T10 and T10/T11 in the thoracic spine for various tasks (Polga et al., 2004).

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