SIB #401 - Normal Sagittal Variation and Spinal Loads
The Study: Numerical evaluation of the correlation between the normal variation in the sagittal alignment of the lumbar spine and the spinal loads.
The Facts:
a. The authors sought to determine spinal loads based on the shape of the lumbar spine in both the upright position and in the upright position while holding a weight.
b. They created 480 finite model spines which included the trunk musculature and represented “the entire range of normal variability”.
c. Their models predicted that when holding a moderate load “a lordotic and well balanced spine (e.g., type 3) could reduce the muscle activation in comparison with a more lordotic (type 4) spine, with negligible differences compared to a more straight spine (type 2).”
d. However, although the “balanced spine” configuration was not strongly “correlated with a minimization of the loading state in the intervertebral discs”, the authors do “hypothesize that simulation of higher loads may emphasize the need for a lumbar spine shape including a marked and long lordotic segment (e.g., type 3) to support at best the loading conditions arising in daily activities. (Emphasis ours)
e. However, when the subject was not holding a weight, “a less lordotic and more vertical spine was still sufficient to ensure a condition of minimal spinal loads.”
f. The authors acknowledge limits to the type of modeling used in this study which may limit its applicability to clinical interpretation.
g. One such limitation is that their models did not incorporate a thoracic spine and so the alignment of the thoracic spine and its relationship relative to the pelvis, both of which are important factors in understanding lumbar loading.
h. On a more practical note, the paper cites previous work showing that a simple vertical plumb line from C7 is the most important predictor of anterior lumbar loading.
i. There is also a tendency toward a forward shift of the C7 plumb line associated with aging, even in the absence of frank pathology, that was also not included in these models.
j. Therefore these models in this study could not determine whether or not live subjects were likely to demonstrate any “possible compensatory role of the upper trunk and head to achieve a better global spinal balance.”
k. These were all computer algorithm generated models. “Despite care taken in developing this algorithm to avoid unrealistic spinal shapes, some irregularities in the computed spine profiles, especially regarding the segmental heights, were unavoidable.”
l. However, even considering the limitations, such models remain a useful tool.
Take Home:
The shape of the sagittal spine can affect axial loading but exactly how this happens is subject to a wide range of variables. In general, more lordosis appears associated with decreased axial loading and some increase in AP shear forces in the lumbar discs however the findings are somewhat obscured by the limitations of the models used. As usual, more study is needed.
Reviewer's Comments: I included this article because I wanted you to see that sagittal alignment is important and that different spinal shapes can and do affect loading. Although this article was published in 2014, research into the importance of the global spinal postures continues.
Reviewer: Roger Coleman DC
Editor’s Comments: The importance of a healthy lordotic curve in reducing disc loading was accentuated when weight holding was added to the models. In non weighted conditions the importance did not seem as marked.
Editor: Mark R. Payne DC
Reference: Galbusera F, Brayda-Bruno M, Costa F, Wilke HJ. Numerical evaluation of the correlation between the normal variation in the sagittal alignment of the lumbar spine and the spinal loads. J Orthop Res. 2014 Apr;32(4):537-44. doi: 10.1002/jor.22569. Epub 2013 Dec 27.
Link to Abstract: https://www.ncbi.nlm.nih.gov/pubmed/24375659
Link to Full Text: https://onlinelibrary.wiley.com/doi/full/10.1002/jor.22569