| Objective To discuss the affordable maximum distraction strength in vertebral body,the distraction strength needed for recovery of vertebral height, and the effects ofdistraction and compression device to vertebral screws and its clinical significantthrough biomechanical study.Methods1. Thirty-two individual vertebrae specimens(T11-L2) and eight of thoracolumbarspine specimens(T12-L4) were captured from sixteen fresh adult cadavericthoracolumbar spine specimens. Thirty-two vertebrae were randomly divided into thedestruction group(20) and control group(12). Screws were placed in the center pointof vertebral and penetrated the contralateral cortex. Bone density was measuredbefore the test. Axial load damage and axial pullout experiment was conducted in thedestruction group while only axial pullout experiment was conducted in the controlgroup. The maximum axial load was recorded in destruction group. The vertebralbody destruction and the screw channel were observed and maximum axial pulloutstrength was compared between two groups; Corpectomy and resection of adjacentdisc was done at L2level for eight thoracolumbar spine specimens. Screw was placedin the center point of L1vertebral body and surgical simulation was conducted torecover the intervertebral height and correct kyphosis. The required distractionstrength was recorded;2. Sixty-four individual vertebral specimen were captured from sixteen fresh adultcadaveric thoracolumbar spine specimens(T11-L2) and divided into normalgroup(BMD>0.8g/cm2) and osteoporosis group(BMD≤0.8g/cm2) according to the bone density. They were further sub-divided into control group and experimentalgroup1, experiment group2and experimental group3randomly, each with8specimens. Vertebral screw was placed in the center point of vertebral body andpenetrated the contralateral cortex. The axial pullout was conducted after the axialcyclic loading in the experimental group, while solo axial pullout was conducted incontrol group. The loading speed was5mm/min, with distraction load of300N andcompression load of100N in the cyclic loading test. The maximum displacement wasset at25mm. Only one cyclic loading was added in experiment group1, while twoand three cyclic loading was added in experimental group2and experiment group3.The maximum axial pull out strength was recorded.Results1. The diameter of vertebral screw channel was similar with the screw’s. Largeamount of thread bone debris was observed. Screw path was wider in the destructiongroup and vertices into fan-shaped expansion of the screw tip. Only a small amount ofbone debris was observed. The vertebral body could withstand the average maximumdistraction strength of536.6252±82.9864N, with average screw displacement of8.7874±1.0826mm. Linear regression analysis showed that the maximum distractionstrength and vertebral bone mineral density had a significant positivecorrelation(r=0.958, P<0.01), while the screw displacement and vertebral bonemineral density had a significant negative correlation(r=-0.933, P<0.01). Nosignificant difference of bone density was found between the destruction and controlgroup(P>0.05); The different of maximum axial pullout strength between thedestruction and control group was significant(P<0.01). The average simulatedintraoperative distraction strength was300.5240±4.0645N. Linear regression analysisshowed no correlation between the simulated distraction strength and the averageBMD of spine specimens(r=0.534, P>0.05).2. The BMD values for each specimen were similar in the normal group(P>0.05). Innormal group, the pullout strength was indifferent among the Experiment group1,experimental group2and the control group(P>0.05), while the significant differnence was exist betweenthe experimental group3and control group(P<0.05). BMD valueswere also showed not statistically different in the osteoporosis gruop(P>0.05). Inosteoporosis group, no significant different was found between experimental group1and control group(P>0.05). Statistical differencewas found in experimental group2and experimental group3when comparing with control group(P<0.01). Themaximum pullout force between the normal groupand osteoporosis group wassignificant differences (P<0.01).Conclusions①The vertebral body can withstand the average maximum distractionstrength of approximately536.6252±82.9864N. The average intraoperative distractionstrength was about300N. Distraction should not be perfomed more than twice duringthe surgery.②Vertebral bone mineral density was positively correlated with thedistraction strength and negatively with the screw displacement, while no correlationwas with the intraoperative distraction strength which need for recovery ofintervertebral height.③Screw loosening would occur if repeated distraction andcompression was acted on the vertebral screw, especially those patients withosteoporosis. It is recommended that applying other methods for reduction or takeconsideration of remedy after distraction on screws. |
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