The Biomechanical Studies Of Mono-point Correction And Three Dimensional Fixation Of The Sacral One Bicortical Pedicle Screw On The Lumbosacral Construct

Background:Sacral fixation commonly used for the lumbo-pelvic construct is accepted as one main treatment for spinal deformity, spondylolysis,sacral tumor, and infection. With the population aging, more and more patients with lumbar degenerative deformity need surgery intervention.Sacral one(S1) pedicle screw as the optimum distal anchor is utilized in lumbo-pelvic fixation. Although the lumbo-pelvic fixation in the correction of spine deformity and stability reconstruction presented with better clinical outcomes, the complications of implant failure and pseudoarthrosis have frequently been reported, such as sacral screw breakage, loosening and pull-out. Currently, bicortical fixation technique is popularized in S1 pedicle screw for its better purchase. The proper anchoring zone at anterior sacral cortex is vital for the mechanical stability of S1 bicortical pedicle screw. During instrumentation, the direction maintenance of S1 pedicle screw insertion is often disturbed by the deep wound, posterior superior iliac spine and strong muscle tissues squeezing. Therefore, on many clinical occasions,we inevitably encounter S1 bicortical pedicle screw de-orbiting from the pilot hole and misplaced, confirmed by intraoperative fluoroscopy,which occur occasionally in osteoporotic sacrum and poly-axial designed pedicle screw. Aiming at proper medial convergent angle of the S1 bicortical pedicle screw and minimizing neurovascular complications, most of spine surgeons prefer to immediate removal and correction. Intuitively, the screw after correction could beneficially acquire more length ofinsertion and more rigid purchase at anterior cortex before S1 body.However, this maneuver could impair the integrity of the screw trajectory,and the anterior cortical fixation strength significantly depends on bone mineral density. To our knowledge, no study investigated the biomechanical fate of the S1 bicortical pedicle screw following correction. Therefore, the study that the biomechanical effect of the correction of de-orbiting S1 bicortical pedicle screw on its anchoring strength was conducted under different bone conditions. This study concluded that the single anchoring point correction of the S1 pedicle screw provided no beneficial improvement in poor bone quality, and implied that extending to sacral two(S2), iliac screw, or ala screw for more distal anchoring points may be a better alternative decision. However,implant failure has still been frequently reported. Spine pedicle as a‘force core’ offers rigid fixation, instead, in sacrum, anterior and posterior cortical bone act most. In the condition of thinner sacral cortex caused by osteoporosis, sacral fixation is prone to failure.Consequently, stress concentration in lumbosacral junction, poor bone quality and untypical pedicle play main important roles in the failure of fixation techniques. Thus, the ways to augment mono-point anchor or multiple points are not effectively to improve sacral fixation. Therefore,we raise 3-dimensional(3D) fixation combining the plane fixation integrated by multiple anchor points with ‘locked’ augmentation of the untypical sacral pedicle. Based on this, a novel instrument called locked sacral pedicle screw-plate system(LSPS) is designed and biomechanically investigated in lumbo-pelvic fixation construct. Therefore, the objective of the whole current biomechanical study is to investigate the biomechanical effects of the mono-point correction and 3D fixation of S1 pedicle screw on the lumbosacral construct, and to explore an optimum sacral fixation for lumbosacral fixation construct.Methods:1. 18 fresh-frozen human lumbosacral cadavers were collected for this study. The bone mineral density(BMD) of each specimen was measured at L1-L4 using a dual-energy X-ray absorptiometry(DEXA) machine, and specimens were divided into the Normal Group(> 0.8 g/cm2) and Osteoporotic Group(< 0.8 g/cm2). On the same sacrum, the standard S1 bicortical pedicle screw was inserted on the right side(Control Group), and on the contralateral side, the pilot hole was made according to the standard S1 bicortical pedicle screw technique. And then, another screw was inserted with less medial angle and parallel to the upper endplate, and penetrated through the point vertically upper the anterior S1 foramina just located at the junction of sacral alar and body(de-orbiting S1 bicortical pedicle screw). Finally, the de-orbiting S1 pedicle screw was back out, and reinserted along the previous pilot hole until penetrated anterior cortex after palpation of the trajectory(Correction Group). Following fatigue loading test(30-250 N load on screw head over 2000 cycles), the subsidence displacement and axial pull-out strength were measured and analyzed statistically.2. 11 formalin fixed human lumbo-pelvic cadavers were used in this experiment. The BMD of each specimen was measured at L1-L4 using a DEXA machine. Each specimen was instrumented bilaterally by L3, L4,L5, S1 pedicle screws, S2 screw and iliac screw(IS). Connected by the pre-bending rod, each specimen was instrumented by L3-S1 fixation(Control Group) and then biomechanical tested. Following this, three lumbo-pelvic constructs were established in the sequence of L3-S2fixation(Group A), L3-S1-IS fixation(Group B) and L3-S1-LSPS fixation(Group C). Group A, B and C were established and tested sequentially. Biomechanical testing of each L3-pelvic construct wasperformed on a material testing machine(MTS) under 800 N axial cyclic compression and 7 Nm cyclic torsion loading for 5 cycles. Compressive subsidence displacement, compressive stiffness, and torsional stiffness of each L3-pelvic construct were calculated and normalized to the corresponding data from the Control Group of the same specimen for statistical analyses.Results:1.(1)In Normal Group, the averages of the subsidence displacement of the control group and the correction group were 0.33 mm and 0.40 mm,respectively. The averages of the maximum pull-out strength of the two groups were 364 N and 352 N, respectively. The subsidence displacement of the correction group significantly increased by comparison with the control group(P < 0.05). However, no significant difference of the maximum pull-out strength(POS) between the two groups was observed. Furthermore, the ratios of the changes of subsidence displacement(ΔS) and maximum POS(ΔP) between the correction group and the control group on the same specimen showed no significantly correlation with BMD values, and the linear regression R2 values were 0.01 and 0.06, respectively.(2)In Osteoporotic Group, the averages of the subsidence displacement of the control group and the correction group were 0.74 mm and 1.16 mm, respectively. The averages of the maximum POS of the two groups were 249 N and 161 N, respectively. The subsidence of the correction group was more than that of the control group, and the significant difference between them was detected(P < 0.05). The maximum POS of the correction group showed a significant decrease comparing with the control group(P < 0.05). The ratios of the changes of subsidence displacement(ΔS) and maximum POS(ΔP) between the correction group and the control group on the same specimen were significantly negativecorrelation with BMD values, and the linear regression R2 values were0.78 and 0.77(P < 0.05), respectively.2.(1)In axial compression testing, the average compression stiffness of Group A, Group B and Group C was 116.4%、150.9% and 141.7% of the Control Group, respectively. And the percentages of Group A, Group B and Group C showed significantly more than that of Control Group(P < 0.05). Although no significantly difference between Group B and Group C was detected, the two groups showed higher than Group A(P< 0.5). The subsidence displacements of the Control Group, Group A,Group B and Group C were 3.41 mm, 2.97 mm, 2.31 mm and 2.46 mm,respectively. Among these groups, Group B and Group C was confirmed no obvious difference in the subsidence, whereas, significantly less than the Control Group and Group A(P < 0.05).(2)In torsional testing, the average torsional stiffness of Group A,Group B and Group C was 113.4%、144.6% and 120.3% of the Control Group,respectively. And the percentages of Group A, Group B and Group C showed significantly more than that of Control Group(P < 0.05). Among these groups, Group B showed significantly higher torsional stiffness than Group A and Group C(P < 0.05) and most torsional stiffness,however, no significant difference was detected between Group A and Group C.Conclusion:For normal bone quality, the de-orbiting S1 bicortical pedicle screw after correction could acquire the comparable stability. However, under osteoporotic condition, the correction maneuver could not beneficially prevent anchoring strength loss, and additional immediate augmentation fixations should be considered. For the long segmental lumbo-pelvic fixation, extending to S2 screw or iliac screw could increase mechanical stability, whereas combining with locked sacral pedicle screw-platesystem not only acquires the comparable stability, but also resists to the subsidence and loosening of S1 pedicle screw caused by cut-off.