In this experiment all 3 distraction conditions temporarily significantly reduced nucleus pressure compared to simulated standing and lying. The largest effect was observed during axial-distraction without flexion or extension which reduced pressure to near zero. Degenerated discs responded differently than relatively normal discs; they had greater temporary net reductions in nucleus pressure. Although not examined quantitatively, the distribution of stress among disc regions in normal or minimally degenerated discs (grade 1 or 2) was similar in flexion-distraction and extension-distraction. This could be due to the nucleus being pressurized and efficiently distributing the stress. In discs with higher amounts of degeneration (grades 3 and 4) the nucleus had much less pressure when extension or flexion was introduced indicating that stress distribution may have been dependent on the moment applied to the segment. Flexion-distraction resulted in compressive stress being temporarily qualitatively lower in the posterior region compared to the nucleus and anterior regions. Conversely extension-distraction of degenerated discs yielded similar vertical stress in all 3 regions.
Nucleus pulposus pressure has been used to calculate axial loads on the spine [25
]. This is appropriate because the normal nucleus acts as a fluid with the stress being hydrostatic or isotropic (equal in all directions). As such it is a scalar quantity that can be measured with strain gauge technology. Quantifying stress in the anulus is more problematic. Annular stress is not isotropic but anisotropic with different vertical and horizontal components [5
]. Pressure and stress have the same SI unit of measure (Pascal). Although strain gauge transducers have been used to estimate stress in the anulus it is debatable exactly what the measurements represent. Rao et al interpreted the output from strain gauges placed in the anulus (to detect vertical stress) to be “intradiscal pressure in the axial direction” [28
] despite the fact that pressure is non-directional. McMillan et al attempted to determine the validity of strain gauge transducer measures in the anulus and found the output of their transducer to be linearly proportional to the vertical force applied to the disc. They reasoned that the output was also proportional to the compressive stress perpendicular to the transducer membrane [21
]. Interestingly, they found that the same calibration coefficient was applicable to liquids, nucleus pulposus, and all but the outer 2 to 4 mm of the anulus fibrosus. Although we recorded both horizontal and vertical stress components in this experiment, we were primarily interested in the ability of distraction to ‘unload’ the disc, i.e., reduce the vertical or compressive stress. Therefore we have referred to the vertical measures as vertical stress. Vertical stress measures in the nucleus were essentially the same as the horizontal measures, and therefore were interpreted as nucleus pressure.
The normal lumbar nucleus is displaced anteriorly by extension and posteriorly by flexion when lying [29
] but changes in nucleus pressure and position in degenerated discs are not as predictable [29
] and degenerated discs have been noted to bulge posteriorly with extension [30
]. Our findings are consistent with reports that degenerated discs may respond differently than healthy discs to flexion and extension [30
] and extends that observation to include flexion and extension combined with distraction. The qualitative differences we observed in stress distribution between relatively healthy and degenerated discs might be due to the degenerated discs being unable to generate or maintain nucleus pressure. They may also be explained in part by anatomy. When the motion segment is extended the facet joints contact each other and the center of rotation moves posteriorly toward the facets, causing the anterior disc space to widen. This effectively shields the posterior disc from further compression [32
]. Conversely, flexion-distraction of degenerated discs may result in anterior compression and an anterior shift of the center of rotation. This appears to produce a stress distribution with the least compressive force in the posterior anulus. These observations suggest that the normal response of lumbar discs to flexion and extension is dependent to some extent on the health of the disc.
The primary mechanical theory underlying the use of distraction therapies for disc herniation is that they reduce nucleus pressure and pull peripheral nucleus tissue toward the center of the disc [34
]. Distraction has been shown to produce temporary
negative pressure in the nucleus of living patients [18
]. Nucleus pressure in the present experiment became negative during axial-distraction in 4 of 8 low degeneration discs but in only 1 of 7 high degeneration discs. Gudavalli et al [19
177], recorded negative pressures during flexion-distraction in a whole cadaver model but we did not observe that in this study. This may have been due to violation of the annular ‘seal’ with the transducer, but that is unlikely considering the instruments used by Gudavalli et al were similar to the ones we used. Other possible explanations include dissimilar forces used during flexion-distraction or the difference between whole cadaver and single motion segment models. Gudavalli used intermittently applied, short-duration forces and continuous measurement. We measured pressures 1 to 2 minutes after the force was applied which might also explain this difference.
This study has several weaknesses that should be considered. First, a cadaver model may not accurately represent the response of the disc to loading in vivo
. At this time there is no safe and acceptable method of obtaining similar in vivo
measurements in humans. The age of tissue donors was generally older than persons presenting with discogenic back pain. The effects of freezing and thawing lumbar spine tissues is not thought to significantly affect the physical properties of human spine specimens [37
]. Yet, dehydration and prolonged exposure to room temperatures are known to affect their material properties. The specimens in this experiment were kept moist [38
] and the exposure to room temperature minimized. Our results were not likely affected by soft-tissue changes due to exposure. Second, the method we used to simulate treatments is most consistent with intermittent traction and lasting 1 to 2 minutes. It may not reflect the exact time course of stress change during shorter treatments such as distraction-manipulation. Third, although the output of the transducer we used has been shown to be proportional to the applied compressive stress (perpendicular to the sensing element), it may not provide a highly accurate measure of compressive stress. Nonetheless, it provides a reasonable measure of stress change within specimens [21
]. Fourth, we excluded all L5-S1 motion segments from our data. The L5-S1 segment has different ligamentous anatomy and slightly different kinematics than the other lumbar segments. Further, it can be difficult to secure and test. We did encounter difficulties with potting and as a result elected to exclude the single L5-S1 motion segment with usable data from analysis. Fifth, the results must be considered carefully in light of the small sample size and risk of error. Yet, the study was designed as a repeated measures study to maximize the power.
Our findings provide insight into the mechanical effects of distraction therapies but they do not establish a mechanism by which distraction might benefit those with back pain or sciatica due to
disc injury. It is possible that the motion or change in stress results in mechanobiological events that lead to pain relief or promote disc health [39
]. Studies using both animal and in vitro models have demonstrated that mechanical stress may play a role in the regulation of both degradative and anabolic processes in discs [41
]. Kroeber et al [42
] using a rabbit model found that degenerated discs (created by compression) treated with distraction had restoration of disc height and histological evidence of regeneration. Although the method of producing degeneration in that model can be questioned, the results provide preliminary evidence
that distraction might potentially have a beneficial affect on disc physiology. Distraction might reduce local stress peaks in the anulus fibrosus which are thought to produce lower back
]. Further studies are needed to establish a clear clinical benefit of distraction therapies. Additionally studies are needed to examine the relationship between stress distribution and clinical markers of disc biology such as the degree of nucleus hydration [45