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In vivo measurements of lumbar mechanics from ultrasound imaging
June 2005
| Principal Investigator: |
Theodore Milner (Simon Fraser University) |
For more information about this development grant project, please contact Dr. Theodore Milner.
Issue
The stability of the lower spine may be affected by different patterns of muscle activation and reflex timing. In order to explore the possible role of spine stability in lower back injury, a non-invasive method is needed for quantifying different mechanical parameters of trunk muscles and vertebral joints, that can then be incorporated into a model of the mechanics of the spinal column.
This project explored the feasibility of using ultrasound imaging for this purpose. Changes in B-mode ultrasound images of lumbar trunk muscles were analyzed to determine their relation to changes in the strength of muscle contraction, intervertebral stiffness and muscle stiffness.
Key findings
- There was a statistically significant linear trend between change in muscle thickness and trunk force, although the correlation coefficients were generally low.
- There was not a significant correlation between displacement of the L1-L2 transverse processes and trunk force, suggesting that this is not a good indicator of intervertebral stiffness.
Objectives
- To test the validity and reliability of using ultrasound imaging to quantify changes in muscle shape as a function of the external load.
- To develop a method to estimate intervertebral stiffness using measurements of relative vertebral motion from ultrasound images recorded during trunk perturbation.
Method
To assess changes in muscle shape during external loading of lower back muscles, ultrasound images were obtained while study participants exerted different degrees of force. During these contractions, the dimensions of the following muscles were measured: erector spinae, qudratus lumborum, external oblique, internal oblique and transverses abdominis muscles. The pennation angle was also measured for erector spinae muscles.
To estimate intervertebral stiffness, the researchers used a rapidly developing force produced by prestretched springs to produce a sudden perturbation of the trunk, while the subject was exerting an isometric lateral flexion force, which stretched the contracting muscles. The change in the distance between the L1-L2 transverse processes was measured to determine whether this could be used to estimate the stiffness of the intervertebral joint.
Results
Measures of muscle thickness were found to change significantly with force level, but the association was not strong for all muscles. There were no other muscle dimensions that changed significantly with force level. In particular, pennation angle of erector spinae muscles was not correlated with trunk force.
The distance between the L1-L2 transverse processes was highly variable and the measured displacement was not correlated with trunk force.
Conclusions
The association between muscle thickness and force level for most muscles is not strong enough to be used as a reliable predictor of trunk force. However, in exceptional cases such as the external oblique muscle during lateral flexion, strong correlations were found for all subjects.
The lack of a significant correlation between displacement of the L1-L2 transverse processes and trunk force suggests that this measure is not a good indicator of intervertebral stiffness.
The technique is not yet sufficiently reliable to be used to obtain accurate estimates of parameters related to spine biomechanics.
Future directions
It would be useful for further research to continue to explore the possible use of ultrasound imaging, using different potential measures. In particular, the relation between muscle activation measured by means of intramuscular electromyography and changes in muscle dimensions should be examined for the complete range of trunk force directions.
Publications and presentations
Desmoulin, G, Lessoway, V, Rohling, R and Milner, T (2005). Lumbar mechanics from ultrasound imaging: A fundamental in-vivo study of musculoskeletal mechanics. Injury Biomechanics Symposium, University of Ohio.