Abstract
Lumbar endplate fractures were investigated in different experimental scenarios, however the biomechanical effect of segmental alignment was not outlined. The objectives of this study were to quantify effects of spinal orientation on lumbar spine injuries during single-cycle compressive loads and understand lumbar spine endplate injury tolerance. Twenty lumbar motion segments were compressed to failure. Two methods were used in the preparation of the lumbar motion segments. Group 1 (n = 7) preparation maintained pre-test sagittal lordosis, whereas Group 2 (n = 13) specimens had a free-rotational end condition for the cranial vertebra, allowing sagittal rotation of the cranial vertebra to create parallel endplates. Five Group 1 specimens experienced posterior vertebral body fracture prior to endplate fracture, whereas two sustained endplate fracture only. Group 2 specimens sustained isolated endplate fractures. Group 2 fractures occurred at approximately 41% of the axial force required for Group 1 fracture (p < 0.05). Imaging and specimen dissection indicate endplate injury consistently took place within the confines of the endplate boundaries, away from the vertebral periphery. These findings indicate that spinal alignment during compressive loading influences the resulting injury pattern. This investigation identified the specific mechanical conditions under which an endplate breach will take place. Development of endplate injuries has significant clinical implication as previous research identified internal disc disruption (IDD) and degenerative disc disease (DDD) as long-term consequences of the axial load-shift that occurs following a breach of the endplate.
My comments:
This a great paper looking at one of the hypothesised causes of low back pain and DDD (Degenerative Disc Disease). The discussed cause is endplate fractures, the interface between the disc and the vertebral body (the bone) where the disc does not herniate posteriorly, but rather vertically, with ruptures through the vertebral endplate, either up or down into the bone marrow above or below the disc, forming what is called a Schmorl’s node in the process. The disc then becomes shorter, hence degenerated, the first D in DDD.
The gist of this paper is that discs can be herniated vertically entirely by compression. They note that this generally isn’t thought to be due to lifting technique, but more so trauma (like falls or accidents.) To test this, they took human lumbar cadaver spine segments and applied increasing axial force to lumbar discs until they broke. They did one group of discs aligned neutral (preserving the normal lordotic arch, which is in fact some extension) thus preserving the normal wedge shape of the lumbar disc. The other group did so with the discs fixed in flexion (representing a rounded, slouched, non-neutral posture) in which the same axial (compressive) force was then applied. The interesting finding was that the neutral spine position was largely protective such that only 2 of the 7 spine segments had endplate failures, while 5 out of the 7 vertebral bodies themselves failed first. In the flexed spine group, all the endplates failed first and failure was at 3703 Newtons, which was less than half of the force absorbed by the neutral spine vertebral segments, which didn’t fail until force reached 8442 Newtons. That’s a 128% increase in compression strength just by keeping the spine neutral.
It is pretty clear per a number of other studies that posterior disc herniations are caused by repeated spine flexion, which is accelerated with lumbar twisting, and worsened when alternating spine flexion with extension stretches. As such, maintaining neutral spine during the bulk of daily activities and exercise appears ultimately protective. What this paper adds is that the same neutral spine position that protective against posterior disc herniations also lessens risk of endplate fractures and vertical disc herniations. That’s what one might call a pretty cool coincidence.
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