lumbar spinal canal, dural sac volume

A position-dependent MRI study 1 of 32 volunteer subjects found that the increased pressure from cerebrospinal fluid in the spines of the subjects who moved from supine to standing position while being imaged caused a significant expansion in their dural sac cross-sectional area.

The Study

A positional magnetic resonance imaging (MRI) study was conducted to evaluate how postural changes affected the lumbar dural sac. Each of the 32 male subjects was examined while in the supine, standing, and sitting positions. The L3/L4, L4/L5, and L5/S1 discs cross-sectional dural sac area and anteroposterior (AP) dural sac diameters were measured. The AP dural sac diameter and upper-endplate L1 and S1 angles were measured on midsagittal images, as well.

facet, lumbar spinal stenosis model, dural sac

Lumbar Spinal Stenosis Model

The Results

Forty-one percent of the subjects showed evidence of disc degeneration or protrusion, but no evident dural sac compression was found in any of the subjects. There were fluctuations in the mean dural sac cross-sectional area measurements and AP dural sac diameter that was posture-dependent. All subjects showed smaller mean dural sac cross-sectional areas when in a supine position. When changing the position from standing to supine, the dural area decreased more in the L5/S1 level, and the extended sitting position produced the largest increase at L5/S1. This held true for the AP dural sac diameter measurements as viewed on axial and midsagittal images.

Discussion

Researchers examining the cross-sectional IVDs of asymptomatic subjects noted that there was a clinically-significant difference between the dural sac areas at the L5/S1 level that was posture-dependent, with the smallest area being noted in the images of those who were supine at the time of their MRI. The lumbar cerebrospinal fluid (S-CSF) pressures were higher in those sitting and maintaining upright positions. Gravity caused an increase in the hydrostatic CSF pressure and an expansion of the dural sac in the subjects imaged when standing or sitting, which is why those in the supine position had smaller dural sac cross-sectional area measurements.

No significant dural sac cross-sectional area differences were found at the L3/4 and L4/5 IVD levels between the sitting or standing positions, though the AP dural sac diameter was much shorter in the sitting extended position than in the sitting flexed position.

There was a decrease in the dural sac volume space and craniocaudal diameter when the subjects changed their posture from the sitting flexion to the sitting extended. On bending forward, the AP dural sac diameter increased, and it decreased when the subject bent backwards. However, the dural cross-sectional area had no significant changes at this level.

Different dural sac cross-sectional changes were noted for the L5/S1 level, which showed the highest increase after the subject moved from a supine, to standing, position. The researchers posited that this was caused by expansion of the dural sac creating a gravity related hydrostatic CSF. Subjects in the sitting and extended position showed the largest dural sac cross-sectional area at L5/S1. Researchers believe the shortened dural sac may have expanded into the smaller spinal canal space in this scenario.

Overall, there were greater differences in the dural sac cross-sectional area between supine-to-standing, and supine-to-sitting images than in flexion to extension positions. This indicates the gravity related hydrostatic CSF pressure is greater in these posture changes than in flexion/extension changes.

Though there was no dural sac cross-sectional area influence on the L3/4 and L4/5 segments when the subjects were seated in flexion or extension, their lumbar spinal canal space at all levels decreased when they changed their posture from sitting in flexion to sitting in extension. The researchers postulate dynamic variations of the cross-sectional dural area during flexion and extension are created not only by the degree of IVD bulging or buckling and thickness of the ligamentum flavum, but by a variance in total lumbar spinal canal space. The difference could be more pronounced in lumbar spinal stenosis patients with no additional epidural space to moderate the dural tube.

Conclusion

The study showed that posture affected the size of the dural sac cross-sectional area in a group of asymptomatic volunteer subjects. Specifically, when the subjects changed from the supine to standing position, increased pressure of the CSF expanded their lumbar Dura sac volume. The smallest changes were noted when the subjects were in the supine position.

Total lumbar spinal canal space plays a factor in the dural sac cross-sectional area in flexion and extension. Gravity-related hydrostatic CSF pressure appears to be the most important factor in increasing the dural sac cross-sectional area in otherwise asymptomatic subjects.

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