Effects Of Simulated Weightlessness On The Degenerative Mechanism Of Lumbar Intervertebral Disc

Objective: Aviators had a high rate of back pain during the career, and resulted in ahigh aviator inflight incapacitation rate, it’s one of the main non-combat attrition ofaviators in peacetime.presumably due to the spine exposed to weightlessness.Toexplore the effects of degenerative mechanism of animal intervertebral disc exposuredto simulated weightlessness by building an animal simulated weightlessness modelwhich can simulate Aviators’ flight condition.Method: Forty healthy male New Zealand rabbits were randomly divided into4weekscontrol group,4weeks simulated weightlessness group,8weeks control group and8weeks simulated weightlessness group．The rabbits in groups except control groupswere exposed to simulated weightlessness,which were kept head down tilt by asuspensory weskit with Hindlimb Unloading for5days and resting for2daysalternately. Respectively．Radiological evaluation，Pathological changes of the LumbarIntervertebral Disc were observed under light microscope after staining withHematoxylin and eosin，and immunohistochemically stained to detect MMP-1,3．Theresults analysed by Independent Samples t Test to compare the different groups.AP-value＜0.05was considered statistically significant.Result:1) There was no degenerative change found in AP and lateral X-ray oflumbar、CT scan of the disc and MRI scan in simulated weightlessness groups andcontrol groups.2)As compared with control，proliferation of chondrocyte-like cells andcell clusters were found in nucleus pulposus of4weeks and8weeks group. In the8weeks group, fibrosis and cell death also occurred，with the presence of cells withnecrotic and apoptotic appearance at the junction of annulus fibrosus and nucleus pulposus, and proliferation of chondrocyte-like cells and cell clusters were found inannulus fibrosus of8weeks group. Proportion of cells found in cell clusters ofintervertebral discs were observed in rabbits increased as the time prolonged from4to8weeks that simulated weightlessness exposure between the same lumbar segment, andthis was statistically significant (P<0．05)；in the4weeks group,compared theproportion of cells found in cell clusters between the adjacent levels, it’s statisticallysignificant except L2-3to L3-4(P<0.05). the proportion increased followed the lumbarspinal sequence from L4to S1.But in8weeks group,there was not statisticallysignificant between the adjacent level except L3-4to L4-5and L4-5to L5-6(P<0.05).3).Immunohistochemically stained: Lower cells shows positive staining with MMP-1，3in the NP tissue in control groups，Cells in cell clusters shows MMP-1，3positivestaining in simulated weightlessness groups.The positive expression level ofMMP-1,3in4weeks and8weeks specimens was significantly higher than that controlgroups(P<0.05)，and in8weeks specimens was also significantly higher than that in4weeks specimens and this was significant compared between samesegment(P<0.05).There was not statistically significant in except L5-L6to L6-S1inthe MMP-1positive expression level between the adjacent level in4weeks(P＞0.05)，and compared the MMP-3positive expression level between the adjacent level, therewas statistically significant in except L2-L3to L3-L4(P＜0.05). There was notstatistically significant in except L3-L4to L4-L5and L4-L5to L5-L6in the MMP-1positive expression level between the adjacent level in8weeks(P＞0.05)，and comparedthe MMP-3positive expression level between the adjacent level, there was statisticallysignificant in L2-L3to L3-L4and L5-L6to L6-S1except the others(P＜0.05).Therewas no significant difference in the MMP-1,3positive expression level between4weeks control group and8weeks control group (P＞0.05).Conclusion: Short time simulated weightlessness exposures for4weeks and8weeksmay induce degenerative pathological change of animal lumbar intervertebral disc,asstart with the formation of cell clusters in annulus fibrosus．The degree of degenerativeseverity was increasing followed the exposured time extending and the lumbar spinalsequence from L4to S1in4weeks. It suggests that weightlessness was likely toparticipate and accelerate the Aviators’ lumbar intervertebral disc degenerative processas an important factor.