| The purpose of the current study was to compare the biomechanical stiffness of traditional pedicle screw fixation and transdiscal longitudinal lumbosacral cannulated screws fixation in a cadaveric model of simulated L5-S1 spondylolisthesis.Summary of background dataThe surgical management of L5-S1 spondylolisthesis is a challenge because of the difficulties in achieving a reliable arthrodesis in the face of high mechanical forces. A method of lumbosacral fixation that has been used successfully in severe spondylolisthesis at our institution involves the use of 2 cannulated screws which were longitudinal inserted through lateral mass of the sacrum, sacral body, L5S1 disc to L5 vertebral body combinding lumbosacral pedicle screws. The biomechanical study related to this procedure need to be performed in detail.MethodsCadaveric MaterialSix human formaldehyde preservative cadaveric L3-S4 motion segments (mean age 62.3 years; range 32-78 years 2 females, 4 males) were harvested. Each motion segment was radiographed to ensure that no major structural abnormalities were present ( e. g. , pars defects, compression fractures). Specimens were then carefully stricped of soft tissues with the intact of bone membrane, in-tervertebral disc, facet sac and all ligaments- The end of the each motion segment was fixed to the materials testing machine using the self-made clamp and bone cement (PMMA).Biomechanical Loading SequenceThe currentstudy was to compare the biomechanical stability in cadaveric models. The specimen was positioned and clamped in a materials testing machine (BIONIX 858, Material Test System, Eden, Prairie, MN, USA). The intact specimen was then biomechanically tested as follows: 1) axial compression (500 N) , 2) flexion (7.5 Nm) , 3) extension (7.5Nm) , 4) right lateral bending (7.5 Nm) , and 5) left lateral bending (7. 5Nm). Stiffness values were calculated from the load-deflection curves obtained.After the intact specimen had been tested according to the six-step loading sequence, an L5 laminectomy, a bilateral L5-S1 facetectomy, and a radical dis-cectomy ( with resection of both anterior and posterior longitudinal ligaments) were performed. All the simulated Spondylolisthesis specimens were biomechanically tested as above, Stiffness values were calculated from the load-deflection curves obtained. The specimens with 3 different flxational techniques, transdis-cal longitudinal lumbosacral cannulated screws fixation, traditional L5S1 pedicle screws ( USS) fixation, and combining of above two fixation, were tested respectively. Load-deflection curves were obtained each time, and stiffness values were calculated from the curve. A cyclic compression force was applied as conditioning (500 ?150 N at 1 Hz for 1000 cycles) to remove excess fluid from the disc and return the disc to its predeath height.Load-Displacement CurvesThe stiffness of the motion segment was determined each time from the slope of the load-displacement curve obtained. To adjust for variations in stiffness between motion segments, a normalized stiffness ratio was calculated by dividing the value of stiffness obtained for the experimental condition ( e. g. , after transdiscal longitudinal lumbosacral cannulated screws fixation or pedicle fixation or combined fixation) by the stiffness value obtained for the intact specimen.StatisticsStatistical analysis consisted of analysis of variance and paired t test. Statistical significance was defined at the p <0.05 level.ResultsThe experiments showed that transdiscal longitudinal lumbosacral cannulated screws fixation was significantly stiffer (p <0.01) than traditional L5S1 pedicle screws in all modes tested ( compression, flexion, extension, right lateral bending, and left lateral bending). The greatest difference was noted in extension. The normalized stiffness values for the transdiscal longitudinal lumbosacral cannulated screws were 1.2-1,4 times greater than the values for the traditional L5S1 pedicle screws. In contrast, no significant differences i...
|
PDF Full Text Request