Pull-out Strength Of The Pedicle Screw In Pedicle-lengthening Osteotomy

Objective:correlation analysis of the pull-out strength and the diameter of the extensible pediclescrew；Biomechanical characteristics evaluation of pedicle screw in pedicle-lengtheningosteotomy before and after the fatigue.Material and Methods:1.Material:Specimen preparation:For biomechanical testing,7lumbar spine,segment(L2-5)，mean BMD was0.794±0.078g/cm2，were harvested from formalin-preservedfrom cadavers obtained from the Suzhou University Department of Anatomy. Specimenswith previous spinal surgery, posttraumatic abnormalities and structural disorders wereexcluded.The para-vertebral musculature and fascia were removed and the individuallumbar vertebrae separated by incising the ligaments and annulus. They were wrapped indouble plastic bags and frozen at-4℃. Prior to testing, specimens were thawed over nightat room temperature and prepared at room temperature right before testing.2.Method:the specimen（s28vertebral body and56pedicles）were assigned randomlyinto groups：1.Group of pull out force testing：group A（fixation with6mm screw）andgroup B（fixation with7mm screw），both group A and B are divided into2subgroups：A1，A2and B1，B2；2. Group of pull out force testing after fatigue：group C（fixation with6mm screw）and group D（fixation with7mm screw），both group A and B are divided into2subgroups：C1，C2and D1，D2.Results:Pull-out force of the6mm screw in the vertebral and pedicle are:281.57±43.16N,444.91±41.8N, pull-out force after fatigue test in vertebral and pedicle are:117.08±20.56N,263.84±39.62N;Pull-out force of the7mm screw in the vertebral and pedicleare:317.20±34.98N,467.48±33.08N, pull-out force after fatigue test in vertebral and pedicle are:184.69±26.64N、290.37±64.41N..No statistically difference in pull-out forcebetween the groups A1and B1(F=1.050, P>1.050);Significant difference was testedbetween groups A2and B2(F=5.960, P <0.05).Also,statistically difference can be foundin comparison of group A1and C1, A2and C2, B1and D1, B2and D2, shows thatpull-out force of both6mm and7mm screw reduced after fatigue test in vertebral body andpedicle, the pull out force of the6mm to7mm group in vertebral body decreased58.3%、43.6%;Respectively,the pull out force of both6mm and7mm group within the pedicledecreased41.3%、39.9%.Conclusions:No significant difference in pull out strength between6mm screw and7mm screwfixation in vertebral body in pedicle-lengthening osteotomy.During the procedure ofpedicle-lengthening osteotomy7mm screw can provide stronger internal fixation in pediclethan6mm screw.Pull-out strength of both6mm screw and7mm screw decreased a lot afterfatigue test,these results indicate the the6mm screw and7mm screw have weak fatigueresistance.