Study On The Relation Between The Pedicle Screw And The Long-term Stability

This study is to investigate the biomechanics relation between theinserting depth of the pedicle screw and the long-term stability.36cases of fresh adult sheep thoracolumbar vertebra were selected asthe spine specimens. The single vertebral body compression fracture modelwas constructed in L1of27cases of vertebra and screws were inserted intodifferent depth by the upper and lower pedicle of vertebral arch of thefractured vertebral body according to the length of the nail road (insertinginto one half, three fourths and four fourths of the length).9cases ofvertebra were selected as the control group (the whole thoracolumbarvertebra).After the specimens were fixed,100000times of fatigue testwere carried out in the specimens by the HY-3080microcomputer controlelectronic universal material test machine with the frequency of1.5Hz.After the fatigue test, the effect of posterior internal fixation withdifferent inserting depth to the long-term stability of the thoracolumbarvertebra was studied by measuring the spine activity and the maximalwithdrawal force of the three kinds of internal fixation models and thecontrol.The results of this study indicated that after100000times of flexionand extension, the activity of the thoracolumbar vertebra in the group withfour fourths inserting depth weakened obviously comparing with that in thegroup with three fourths inserting depth and the difference had statisticsignificance (P<0.05); the activity of the thoracolumbar vertebra in thegroup with four fourths inserting depth weakened obviously comparingwith that in the group with one half inserting depth and the difference hadstatistic significance (P<0.05); the activity of the thoracolumbar vertebra in the group with three fourths inserting depth weakened obviously comparingwith that in the group with one half inserting depth and the difference hadstatistic significance (P<0.05). The difference also had statistic significance(P<0.05) by comparing the control with the group with four fourthsinserting depth, the group with three fourths inserting depth and the groupwith one half inserting depth respectively.The pull-out strength of the screw showed that: the pull-out strength ofthe thoracolumbar vertebra in the group with one half inserting depthweakened obviously comparing with that in the group with four fourthsinserting depth and the difference had statistic significance (P<0.05); thepull-out strength of the thoracolumbar vertebra in the group with threefourths inserting depth weakened obviously comparing with that in thegroup with four fourths inserting depth and the difference had statisticsignificance (P<0.05); the pull-out strength of the thoracolumbar vertebrain the group with one half inserting depth weakened obviously comparingwith that in the group with three fourths inserting depth and the differencehad statistic significance (P<0.05)In conclusion the depth of the screw is relevant to the stability, Thedeeper the pedicle of vertebral arch screw is inserted the better is thestability of the spine after the fatigue test.