Polyaxial Pedicle Screws Research Articles

Biomechanical analysis of spinal pedicle screws under static compression and tensile bending

Spinal implants or instrumentation are commonly used for the re-alignment and treatment of spinal injuries. Pedicle screws are the type of bone screw designed for implementation into vertebral pedicle during the treatment of the disorders. Numerous variations in design of the pedicle screw have been developed to increase its load carrying capacity and thus resist the forces acting on the vertebral column. The main purpose of this study is to investigate the strengths of the commonly used pedicle screws i.e. mono axial and poly axial pedicle screws under bending conditions. For this purpose, static compression bending and static tension bending tests were carried out according to the ASTM standards and the behaviour of the pedicle screw implant was investigated. Furthermore, FE model was developed as identical as the experimentation setup and numerical simulation was carried out to study the fracture behaviour of the pedicle subassembly under static compression and tensile bending. This combined biomechanical and numerical analysis opens up the ways to improve the strength and load carrying capacity of the pedicle screw.

Biomechanical Comparison of Polymethylmethacrylate Augmentation Methods in Failed Pedicle Screw Revision.

To compare biomechanical results between different polymethylmethacrylate (PMMA) augmentation methods on failed lumbar pedicle screw models of animal vertebrae. Thirty lumbar vertebrae were harvested from six calves, and their bone mineral density was measured. 60 Polyaxial pedicle screws were inserted to all vertebrae. Pull-out tests were performed to all specimens on an Instron machine. The specimens were randomly divided into four groups. The same screws used in primary screwing process were labeled and used in revision. Screws in the first group were augmented by injecting PMMA into the failed screw hole with a syringe; screws in the second group by inserting bone graft and roll-shaped PMMA, screws in the third group by inserting bone graft and injecting PMMA with a syringe; and the fourth group by inserting bone graft and injecting PMMA through a fenestrated pedicle screw. The pull-out strength (POS) results of all specimens were recorded and compared with statistical analyses. The mean BMD of the vertebrae was 1.31 ± 0.225 g/cm < sup > 2 < /sup > and no significant difference was found between the groups (p > 0.05). The mean POS of the primary screws in the first, second, third, and fourth groups were 2166,5 N/m < sup > 2 < /sup > , 2183,5 N/m2, 2508,5 N/m < sup > 2 < /sup > , and 2005c N/m < sup > 2 < /sup > respectively. After the augmentation, the mean POS in the first, second, third and fourth groups were 3839 N/m < sup > 2 < /sup > , 2874 N/m < sup > 2 < /sup > , 2929 N/m < sup > 2 < /sup > and 3826 N/m < sup > 2 < /sup > respectively. No statistical difference was found between the groups in post-revision POS values (p > 0.05). There was no significant statistical difference found in POS between the augmentation methods.

Repair of spondylolysis using a pedicle screw U-shaped rod construct: A preliminary study of 25 young patients with a mean follow-up of 24 months.

Study Design:Prospective case series, therapeutic Level IV.Objectives:Functional and radiographic outcome evaluation of patients with spondylolysis treated with pars interarticularis defect repair with iliac bone grafting and application of a construct consisting of a pair of polyaxial pedicle screws connected by a U-shaped rod passing beneath the spinous process.Methods:Twenty-five patients (27 operated lumbar levels) with an average of 20 months of follow-up (range 12–24 m) with spondylolysis who met our inclusion criteria were treated with the above-mentioned technique. Functional assessment was by the Visual Analog Score (VAS) for low back pain (LBP) and Oswestry Disability Index (ODI). Fusion was confirmed with plain x-rays and when indicated with computed tomography scan. Return to activities of daily living (ADL) was also assessed.Results:There were 16 males (64%) and 9 females (36%), with a mean age of 18 ± 3 years at surgery, with a mean operating time of 79 ± 13 min and a mean blood loss of 186 ± 57 ml. ODI significantly improved from a mean of 63 ± 7 preoperatively to 10 ± 4 at 12 months postoperatively (P < 0.001). The mean preoperative LBP VAS score 8 ± 1 showed also a statistically significant decrease of values to 1 ± 1 at 12 months, (P < 0.001). At 12 m, all patients returned to unrestricted ADL. Pars healing was present in 19 patients (76%) at 6 months and in all patients at 12 months.Conclusions:Polyaxial pedicular screws with a U-shaped rod offer an effective and reproducible treatment for spondylolysis with an appropriate fusion rate, predictable return to daily activities, and good pain relief in young adults.

A Hybrid Uniplanar Pedicle Screw System with a New Intermediate Screw for Minimally Invasive Spinal Fixation: A Finite Element Analysis.

Purpose A hybrid pedicle screw system for minimally invasive spinal fixation was developed based on the uniplanar pedicle screw construct and a new intermediate screw. Its biomechanical performance was evaluated using finite element (FE) analysis. Methods A T12-L2 FE model was established to simulate the L1 vertebral compression fracture with Magerl classification A1.2. Six fixation models were developed to simulate the posterior pedicle screw fracture fixation, which were divided into two subgroups with different construct configurations: (1) six-monoaxial/uniplanar/polyaxial pedicle screw constructs and (2) four-monoaxial/uniplanar/polyaxial pedicle screw constructs with the new intermediate screw. After model validation, flexion, extension, lateral bending, and axial rotation with 7.5 Nm moments and preloading of 500 N vertical compression were applied to the FE models to compare the biomechanical performances of the six fixation models with maximum von Mises stress, range of motion, and maximum displacement of the vertebra. Results Under four loading scenarios, the maximum von Mises stresses were found to be at the roots of the upper or lower pedicle screws. In the cases of flexion, lateral bending, and axial rotation, the maximum von Mises stress of the uniplanar screw construct lay in between the monoaxial and polyaxial screw constructs in each subgroup. Considering lateral bending, the uniplanar screw construct enabled to lower the maximum von Mises stress than monoaxial and polyaxial pedicle screw constructs in each subgroup. Two subgroups showed comparable results of the maximum von Mises stress on the endplates, range of motion of T12-L1, and maximum displacement of T12 between the corresponding constructs with the new intermediate screw or not. Conclusions The observations shown in this study verified that the hybrid uniplanar pedicle screw system exhibited comparable biomechanical performance as compared with other posterior short-segment constructs. The potential advantage of this new fixation system may provide researchers and clinical practitioners an alternative for minimally invasive spinal fixation with vertebral augmentation.

Automated Polyaxial Screw Placement Using a Commercial-Robot-Based, Image-Guided Spine Surgery System

Robotic spine surgery systems, used for placing pedicle screws, have been in clinical use since 2004; however, these systems act only as a positioner for guide tubes, which the surgeon uses to manually advance tools to prepare the site and place hardware (e.g., pedicle screws). This article presents the development, operation, and evaluation of a new, commercial-robot-based, image-guided surgery system that automates the process of bone preparation and polyaxial screw placement. The system is based on the KUKA LBR iiwa 7-axis collaborative robot. Fiducial marker and screw locations are defined on a pre-operative CT image using the 3D Slicer open source image visualization platform. The system uses the robot’s internal localization system for registration, by touching fiducial markers in a hand-guiding mode. A rotary-motor end effector with exchangeable tools is used for both hole-drilling and screw placement. A novel single-motor based mechanism was developed for placing polyaxial pedicle screws. An accuracy test was performed by placing polyaxial screws in synthetic vertebrae which mimic the mechanical properties of human bone. In placing 10 screws, the entry point accuracy was 0.49 ± 0.17 mm and the destination point accuracy was 1.49 ± 0.46 mm. The results compare favorably to evaluations of commercial robotic spine surgery systems.

Bisegmental posterior stabilisation of thoracolumbar fractures with polyaxial pedicle screws: Does additional balloon kyphoplasty retain vertebral height?

We retrospectively evaluated single-level compression fractures (T12-L3) scheduled for a short-segment POS (posterior-only stabilization) using polyaxial screws. Patients averaged 55.7 years (range, 19–65). Patients received either POS or, concomitantly, BK (balloon kyphoplasty) of the fractured vertebrae as well. Primary endpoint was the radiological outcome at the last radiographic follow-up prior to implant removal. POS together with BK of the fractured vertebrae resulted in a significant improvement of the local kyphosis angle and vertebral body compression rates immediately post-OP. During the further course of FU, a considerable loss of correction was observed post-OP in both groups. (Local KA: pre-OP/ post-OP/ FU: 12.6±4.8/ 3.35±4.8/ 11.6±6.0; anterior vertebral body compression%: pre-OP/post-OP/ FU: 71.94±12.3/ 94.78±19.95/ 78.17±14.74). VAS was significantly improved from 7.2±1.3 pre-OP to 2.7±1.3 (P<0.001) at FU. We found a significant restoration of the vertebral body height by BK. Nevertheless, follow-up revealed a noticeable loss of reduction. Given the fact that BK used together with polyaxial screws did not maintain intra-operative reduction, our data do not support this additional maneuver when used together with bi-segmental polyaxial pedicle screw fixation.

Increasing loads and diminishing returns: a biomechanical study of direct vertebral rotation.

Biomechanical simulation of DVR and pure-moment testing on thoracic spines. Characterize load-deformation response of thoracic spines under DVR maneuvers until failure, and compare to pure-moment testing of same spines. Despite reports of surgical complications, few studies exist on increase in ROM under DVR torque. Biomechanical models predicting increases from surgical releases have consistently used "pure-moments", a standard established for non-destructive measurement of ROM. Yet, DVR torque is not accurately modeled using pure moments and, moreover, magnitudes of torque applied during DVR maneuvers may be substantially higher than pure-moment testing. Cadaveric thoracic spines (N = 11) were imaged, then prepared. Polyaxial pedicle screws were implanted at T7-T10 after surgical releases. Bilateral facetectomies and Ponte osteotomies were completed at T10-T11. A custom apparatus, mounted into an 8-dof MTS load frame, was used to attach to pedicle screws, allowing simulation of surgical DVR maneuvers. Motions of vertebrae were measured using optical motion tracking. Torque was increased until rupture of the T10-T11 disc or fracture at the pedicle screw sites at any level. The torque-rotation behavior was compared to its behavior under pure-moment testing performed prior to the DVR maneuver. Under DVR maneuvers, failure of the T10-T11 discs accompanied in most cases by pedicle screw loosening, occurred at 13.7-54.7Nm torque, increasing axial rotation by 1.4°-8.9°. In contrast, pure-moment testing (4Nm) increased axial rotation by only 0.0°-0.9°. DVR resulted in substantially greater correction potential increases compared to pure-moment testing even at the same torque. These results suggest increased flexibility obtained by osteotomies and facetectomies is underestimated using pure-moment testing, misrepresenting clinical expectations. The present study is an important and necessary step toward the establishment of a more accurate and ultimately surgically applied model. III.

Removal of poly-axial pedicular screw with fractured screw driver slot

The use of pedicular screws for spinal surgery has revolutionised the field. The use of poly-axial pedicle screws has eased the difficulties of assembling the rod and screw construct by providing a degree of freedom to the coupling component. This is also associated with increased difficulty in removal of implants, due to the mobile nature of the poly-axial tulip component. We describe a case of a 48 year old patient who had undergone discectomy and posterolateral instrumented fusion of the lumbar spine for L4L5 prolapse of intervertebral disc 3 years ago. She presented with L5 radiculopathy due to malpositioned pedicular screw in the L4 vertebra and was taken for exploration and implant removal. Intra-operatively the L4 pedicular screw was found to have a fractured screwdriver slot. It was removed by dismantling the poly-axial tulip to expose the threaded shaft, which was subsequently removed with the help of a plier.

Polyaxial pedicle screw dislocation during screw tightening for posterior spinal lumbar stabilization

Polyaxial pedicle screw dislocation during screw tightening for posterior spinal lumbar stabilization

Minimally invasive reduction of thoracolumbar burst fracture using monoaxial percutaneous pedicle screws: Surgical technique and report of radiological outcome.

To describe the reduction technique of thoracolumbar burst fracture using percutaneous monoaxial screws and its radiological outcomes compared to polyaxial screws. All surgeries were performed by minimally invasive technique with either percutaneous monoaxial or percutaneous polyaxial screws inserted at adjacent fracture levels perpendicular to both superior end plates. Fracture reduction is achieved with adequate rod contouring and distraction maneuver. Radiological parameters were measured during preoperation, postoperation, and follow-up. A total of 21 patients were included. Eleven patients were performed with monoaxial pedicle screws and 10 patients performed with polyaxial pedicle screws. Based on AO thoracolumbar classification system, 10 patients in the monoaxial group had A3 fracture type and 1 had A4. In the polyaxial group, six patients had A3 and four patients had A4. Total correction of anterior vertebral height (AVH) ratio was 0.30 ± 0.10 and 0.08 ± 0.07 in monoaxial and polyaxial groups, respectively (p < 0.001). Total correction of posterior vertebral height (PVH) ratio was 0.11 ± 0.05 and 0.02 ± 0.02 in monoaxial and polyaxial groups, respectively (p < 0.001). Monoaxial group achieved more correction of 13° (62.6%) in local kyphotic angle compared to 8.2° (48.0%) in polyaxial group. Similarly, in regional kyphotic angle, 16.5° (103.1%) in the monoaxial group and 8.1° (76.4%) in the polyaxial group were achieved. Monoaxial percutaneous pedicle screws inserted at adjacent fracture levels provided significantly better fracture reduction compared to polyaxial screws in thoracolumbar fractures.

Management of thoracolumbar fractures: A prospective study

Thoracolumbar fractures are breakages in the vertebrae of the spinal column in the thoracic and lumbar regions. They may be associated with disruption of the ligamentous complexes, and can result in instability or compression of neural structures. Thoracolumbar fractures are the usual outcome of thoracolumbar trauma. In the present study, Thoracolumbar spine fractures are more common in 31-40 years of life with male predominance due to outdoor activities, the commonest mode of injury was road traffic accident, In the present study, Type A fracture is more common at the level of L1. In this series 12 patients polyaxial pedicle screws were used and in 4 patients monoaxial with polyaxial pedicle screws were used. Fracture-dislocations of the thoracic and lumbar spine are caused by very high-energy trauma. They can be extremely unstable injuries that often result in serious spinal cord or nerve damage.

Biomechanical comparison of posterior intermediate screw fixation techniques with hybrid monoaxial and polyaxial pedicle screws in the treatment of thoracolumbar burst fracture: a finite element study

BackgroundTo compare the biomechanical characteristics of different posterior intermediate screw fixation techniques (ISFTs) with hybrid monoaxial pedicle screws (Mps) and polyaxial pedicle screws (Pps) used in thoracolumbar burst fractures.MethodsFixation techniques are compared with regard to the von Mises stress (VMS) of the instrumentations and intradiscal pressures (IDPs) of the adjacent segments by finite element method (FEM).ResultsThe redistributed ROM of the fixation models with Pps fixed at the lowest segment was twice of the other fixation models in flexion and extension. The largest value of maximal VMS of a pedicle screw was located at the lowest pedicle screws when Mps are fixed at the lowest segment. The largest value of maximal VMS of the rods was decreased when more Pps are fixed at the models. Maximal IDPs of the upper adjacent segments were all larger than those of the lower adjacent segments. The maximal IDPs of the fixation model with MPs fixed at the lowest segment were larger than the other fixation models in flexion and extension.ConclusionsPolyaxial pedicle screws could be placed at the upper or the median segment for the facilitated efficient application of the connecting rod. We should focus on the adjacent segmental degeneration especially the upper adjacent segment in the fixation model with Mps fixed at the lowest segment.

Fracture reduction with novel auto-adjust poly-axial pedicle screws: a cadaveric pilot study

Thoracolumbar (TL) fractures occur in 7% of blunt trauma and have a high impact on the patient’s quality of life and healthcare costs (Schouten et al. 2015). In case of instability, spinal alignmen...

Application of minimally invasive poking reduction technique in the treatment of thoracolumbar fractures

Objective To investigate the clinical effects of minimally invasive poking reduction technique in the treatment of single segment thoracolumbar fractures without neural impairment. Methods From February 2011 to June 2015, 83 cases of thoracolumbar fractures without neural impairment underwent minimally invasive pedicle screw fixation in Linyi Central Hospital were selected and randomly divided into two groups.Group A (40 cases) was treated with poking reduction technique by percutaneous polyaxial pedical screw fixation, 43 patients in group B were treated with only percutaneous polyaxial pedicle screw fixation.The perioperative index, pre-and postoperative radiography, relief of the low back pain and general health status of the two groups were recorded and compared. Results There were no statistically significant differences in the operation time, operative blood loss, hospitalization time.All patients were followed up for 20-27months (average 24 months), the scores of visual analogue scale(VAS) and Oswestry disablity index(ODI) had no statistically significant differences between the two groups in the same period(all P>0.05). Before operation, the Cobb angle, sagittal index and anterior height of the fracture vertebral body in group A were (66.3±14.2)%, (20.4±6.5) °, (21.9±6.6) °, respectively, which in group B were (64.8±13.5)%, (14.5±7.7) °, (15.6±5.9) °, respectively, the differences were not statistically significant (all P>0.05). After operation, the Cobb angle, sagittal index and anterior height of the fracture vertebral body in group A were (93.8±9.8)%, (5.3±3.3)°, (5.4±2.0)°, respectively, which in group B were (88.0±10.6)%, (4.1±2.8)°, (8.1±4.7)°, respectively , the differences were statistically significant (t=8.893, 2.345, 3.351, all P<0.01). Conclusion The effect of poking reduction technique by percutaneous polyaxial pedical screw fixation is better than simply polyaxial pedicle screw in the treatment of thoracolumbar fracture, which is a safe and effective operation method. Key words: Spinal fractures; Fracture fixation, internal; Bone wires; Comparative effectiveness research

Comparison of the mechanical properties of two designs of polyaxial pedicle screw

Abstract In this study, the mechanical properties of a novel dual-core pedicle screw were compared with a commercially available cylindrical screw. In order to evaluate and compare their mechanical performance, a series of axial pullout, quasi-static and dynamic bend tests were conducted. In the pullout tests, three polyurethane (PU) foams (density: 0.16, 0.32 and 0.64 g/cm3) were used to compare the pullout strength between both screw types. The ultimate static strength of each screw was determined by a series of quasi-static cantilever bend tests. Dynamic tests were performed with a peak forces corresponding to 10%, 30%, 40%, 50%, 65% and 75% of the ultimate static strength of each screw type. Each specimen was subjected to a sinusoidally varying load which continued until the specimen fractured or reached 2.5 million cycles. The results of the pullout force indicated that the dual-core screws had higher pullout strength, in each PU foam, compared to cylindrical screws, however, the differences were not statistically significant. The average stiffness of dual-core screw during pullout from the 0.16 and 0.32 g/cm3 PU foams was significantly higher (p

Effect of Pedicle Fill on Axial Pullout Strength in Spinal Fixation After Rod Reduction.

Rod reduction to pedicle screws is used for a variety of spinal fixation procedures; however, it can alter the integrity of the screw-bone interface. The authors investigated the effect of pedicle fill (ratio of pedicle screw diameter to pedicle diameter) on the strength of the screw-bone interface after simulated rod reduction on 17 vertebrae (3 thoracolumbar spine specimens). Pedicle diameter was measured with standard clinical computed tomography scan protocols. The authors determined the minimum pedicle diameter for each level. Polyaxial pedicle screws were surgically placed bilaterally with a freehand technique with standard clinical anatomic landmarks. The pedicle pairs were instrumented with pedicle screws of predetermined diameter, 1 with greater than 80% fill and 1 with less than 80% fill. A simulated reduction maneuver was performed with a 5-mm gap followed by an axial pullout test to assess screw interface strength. Comparison of insertion torque between less than 80% fill and greater than 80% fill did not show significant increases. A significant difference in pullout load (P=.043) occurred with greater than 80% fill (791±637 N) compared with less than 80% fill (636±492 N). No significant difference in stiffness was noted (P=.154) with pedicle fill of greater than 80% (427±134 N/mm) compared with less than 80% (376±178 N/mm). The current findings support the use of greater than 80% pedicle fill for optimal screw anchoring in pedicle screw-based constructs involving rod reduction. Surgeons should consider placing screws that can safely fill vertebral pedicles, especially at the apex of the curve and the proximal and distal levels of constructs, where excessive forces are imparted to the screws. [Orthopedics. 2017; 40(6):e990-e995.].

Does the Addition of a Dynamic Pedicle Screw to a Fusion Segment Prevent Adjacent Segment Pathology in the Lumbar Spine?

Study DesignRetrospective clinical cohort study.Purpose To investigate whether the combined use of dynamic pedicle screws and polyaxial pedicle screws was effective on adjacent segment pathology (ASP).Overview of LiteratureVarious screw and rod models have been recently developed for preventing adjacent segment disease, and hybrid systems have been described along with posterior instrumentation in the fusion segment. In the literature, although the success of dynamic systems has been demonstrated in non-fusion posterior instrumentation, it remains unclear whether the addition of a screw-based dynamic system to a fusion segment would successfully prevent ASP in the long term.MethodsThe study included 101 patients who underwent surgery for degenerative spine diseases between 2007 and 2014 with lumbar stabilization that used either polyaxial pedicle screws alone or polyaxial pedicle screws plus dynamic stabilization screws (with hinged screw heads). These two patient groups were compared using retrospectively obtained postoperative new clinical findings, Oswestry disability index (ODI) scores, visual analog scale (VAS) scores, and radiological data.ResultsThe proportion of patients with ASP who were radiologically assessed was low (p <0.01) in the group that underwent lumbar stabilization along with dynamic screws. Treatment outcomes were clinically successful in both groups according to ODI and VAS scores, and no significant difference was determined between the groups in terms of clinical ASP (p >0.05).ConclusionsAlthough the combined use of dynamic screws and the static system was radiologically found to be effective for preventing ASP in patients who underwent lumbar fusion with posterior instrumentation, it did not completely eliminate ASP or result in a significant improvement in clinical ASP.

Magnetic Resonance Imaging Safety of Magnetically Controlled Growing Rods in an In Vivo Animal Model.

Experimental animal study. To investigate the interaction between magnetically controlled growing rods (MCGRs) and magnetic resonance imaging (MRI). Growing rod treatment through serial operations results in adverse effects on the patient and high treatment costs. MCGRs can be lengthened noninvasively in an outpatient setting and with lower treatment costs. When MRI investigation is required, the interaction between MCGRs and MRI is an issue of concern in patients with MCGRs. This study investigated MRI compatibility of MCGRs in an in vivo setting. The study was conducted on three sheep. A standard posterior approach was used. One polyaxial pedicle screw at the ends was placed. Two sheep were instrumented unilaterally and one bilaterally with MCGRs. Temperature change was measured using MR-compatible sensors. Thoracic and lumbar MRIs were obtained using a 0.3 T MRI unit. MRI waves were applied for 45 minutes and temperature changes were recorded every 3 minutes. The lengths of the MCGRs were measured and anteroposterior and lateral spine radiographs were obtained pre- and postoperatively. No displacement in the positions of the MCGRs occurred. The lengths of the MCGRs did not change compared with the preoperative length. The ability of the MCGRs to elongate was not impaired after MRI scanning. There was a mean increase in the temperature of the MCGRs by 1.45°C (0.5-2.4°C). The MCGRs had a strong scattering effect on MRI of the related segments. This study indicated that lower magnet MRI is safe in an animal model with MCGRs, with no displacement of the rods and no changes in their length, no significant heating, and no adverse effects on the lengthening mechanism but with a significant scattering effect on visualization of the surrounding tissues. Further investigations are needed to clarify the exact distance where an MRI investigation of distant organs may be done without scattering. N/A.

Biomechanical effect of pedicle screw distribution in AIS instrumentation using a segmental translation technique: computer modeling and simulation

BackgroundEfforts to select the appropriate number of implants in adolescent idiopathic scoliosis (AIS) instrumentation are hampered by a lack of biomechanical studies. The objective was to biomechanically evaluate screw density at different regions in the curve for AIS correction to test the hypothesis that alternative screw patterns do not compromise anticipated correction in AIS when using a segmental translation technique.MethodsInstrumentation simulations were computationally performed for 10 AIS cases. We simulated simultaneous concave and convex segmental translation for a reference screw pattern (bilateral polyaxial pedicle screws with dorsal height adjustability at every level fused) and four alternative patterns; screws were dropped respectively on convex or concave side at alternate levels or at the periapical levels (21 to 25% fewer screws). Predicted deformity correction and screw forces were compared.ResultsFinal simulated Cobb angle differences with the alternative screw patterns varied between 1° to 5° (39 simulations) and 8° (1 simulation) compared to the reference maximal density screw pattern. Thoracic kyphosis and apical vertebral rotation were within 2° of the reference screw pattern. Screw forces were 76 ± 43 N, 96 ± 58 N, 90 ± 54 N, 82 ± 33 N, and 79 ± 42 N, respectively, for the reference screw pattern and screw dropouts at convex alternate levels, concave alternate levels, convex periapical levels, and concave periapical levels. Bone-screw forces for the alternative patterns were higher than the reference pattern (p < 0.0003). There was no statistical bone-screw force difference between convex and concave alternate dropouts and between convex and concave periapical dropouts (p > 0.28). Alternate dropout screw forces were higher than periapical dropouts (p < 0.05).ConclusionsUsing a simultaneous segmental translation technique, deformity correction can be achieved with 23% fewer screws than maximal density screw pattern, but resulted in 25% higher bone-screw forces. Screw dropouts could be either on the convex side or on the concave side at alternate levels or at periapical levels. Periapical screw dropouts may more likely result in lower bone-screw force increase than alternate level screw dropouts.

Novel Screw Head Design of Pedicle Screw for Reducing the Correction Loss in the Patients With Thoracolumbar Vertebral Fractures: A Biomechanical Study.

A biomechanical study. To study the different biomechanical property among fixed-axis, monoplanar and polyaxial screws in the static and dynamic tests. Correction loss is a common phenomenon in the patients with thoracolumbar vertebral fractures who underwent the posterior pedicle screw fixation. The incidence varies with the kinds of fixation instrumentation used. There is higher incidence in polyaxial pedicle screws group than in fixed-axis pedicle screws. Monoplanar pedicle screws, which are mobile in the axial plane but fixed in the sagittal plane, can be a better fixation instrumentation for thoracolumbar vertebral fractures in theory. A total of 30 porcine spinal units (L2-L4) were used for the static and dynamic tests, which were randomized into six groups (A1, A2, A3, B1, B2, and B3). Static test was performed in A1, A2, and A3. In this test, fixed-axis, monoplanar, and polyaxial screws were performed in A1, A2, and A3, respectively. The ultimate load was noted after tested. In addition, dynamic test was performed in B1, B2, and B3, used fixed-axis, monoplanar, and polyaxial screws, respectively. Correction loss (head-shank angle shift and anterior vertebral body height shift) was obtained and analyzed in each mode. In static test, fixed-axis and monoplanar screws had significantly higher ultimate load than polyaxial screws (P < 0.05) and fixed-axis screws had a little higher ultimate load than monoplanar screws (P < 0.05). In dynamic test, correction loss was minimal in fixed-axis screws, medium in monoplanar screws, and maximal in polyaxial screws. However, the differences were statistically significant in all comparisons but not in the comparison of fixed-axis and monoplanar screws (P > 0.05). The findings from the current study suggest that monoplanar screws can significantly increase the stiffness in axial direction compared with polyaxial screws, and reduce the risks of correction loss. For thoracolumbar vertebral fractures, monoplanar screw is a better optional instrumentation for minimally invasive surgery. N/A.