In vitro biomechanical comparison of transpedicular versus translaminar C-2 screw fixation in C2-3 instrumentation

OBJECT: In instrumentation of the upper cervical spine, placement of pedicle screws into C-2 is generally safe, although there is the potential for injury to the vertebral arteries. Owing to this risk, translaminar screws into C-2 have been used. The aim of this study was to compare the stability of the in vitro cadaveric spine using C-2 laminar compared with C-2 pedicle screws in C2-3 instrumentation. METHODS: Eight fresh frozen human cadaveric cervical spines (C1-6) were potted at C1-2 and C5-6. Pure moments in increments of 0.3 Nm to a maximum of 1.5 Nm were applied in flexion, extension, right and left lateral bending, and right and left axial rotation. Each specimen was tested sequentially in three modes: 1) intact; 2) C2 pedicle screw-C3 lateral mass fixation; and 3) C2 laminar screw-C3 lateral mass fixation. The sequence of fixation testing was randomized. Motion was tracked with reflective markers attached to C-2 and C-3. RESULTS: Spinal levels with instrumentation showed significantly less motion than the intact spine in all directions and with all loads greater than 0.3 Nm (p < 0.05). Although there was no significant difference between C2 pedicle screw-C3 lateral mass fixation and C2 laminar screw-C3 lateral mass fixation, generally the former type of fixation was associated with less motion than the latter. CONCLUSIONS: When pedicle screws in C-2 are contraindicated or inappropriate, laminar screws in C-2 offer a safe and acceptable option for posterior instrumentation.     

经寰枢关节间隙螺钉和寰椎椎板钩内固定的力学稳定性

目的评价双侧经寰枢关节间隙螺钉和寰椎椎板钩内固定的力学稳定性。方法将6具新鲜尸体颈椎标本（包括枕骨基底部和C1-C4颈椎节段）置于1．5Nm载荷下，测量C1，2节段的三维运动范围（range of motion，ROM）。标本依Gallie内固定、双侧经寰枢关节间隙螺钉和Gallie内固定、双侧经寰枢关节间隙螺钉内固定、双侧经寰枢关节间隙螺钉和寰椎椎板钩内固定、双侧寰椎侧块螺钉和枢椎椎弓根螺钉内固定的顺序实施固定，每次固定后测量三维运动范围。结果包含经寰枢关节间隙螺钉的内固定组在旋转和侧屈方向上具有最小的ROM，其中双侧经寰枢关节间隙螺钉和寰椎椎板钩内固定组在屈伸运动方向上也具有最小的ROM。寰椎侧块螺钉和枢椎椎弓根螺钉内固定组在旋转方向上ROM大于单纯经寰枢关节间隙螺钉内固定组，但在侧屈和屈伸方向上接近经寰枢关节间隙螺钉，差异无统计学意义；其在侧屈和旋转方向上ROM均小于Gallie内固定组，差异有统计学意义。结论双侧经寰枢关节间隙螺钉和寰椎椎板钩“三点”内固定具有最强的生物力学稳定性。虽然双侧寰椎侧块螺钉和枢椎椎弓根螺钉内固定在生物力学稳定性上不及“三点”内固定，但明显优于Gallie内固定。     

寰椎椎板钩联合枢椎椎弓根螺钉内固定的力学稳定性评价

目的 评价寰椎椎板钩联合枢椎椎弓根螺钉内固定的生物力学稳定性.方法 取6具新鲜尸体颈椎标本置于1.5 N·m载荷下,测量C_(1-2)节段的三维运动范围(ROM).标本按随机顺序,依次行完整状态(完整状态组)、不稳状态(齿状突周围韧带切除,为不稳状态组)、经寰枢关节间隙螺钉联合Gallic内固定(固定A组)、寰椎椎板钩联合枢椎椎弓根螺钉内固定(固定B组)、寰枢椎椎弓根螺钉内固定(固定C组)5种状态下的三维ROM值测量.比较各组标本的屈伸、侧屈、旋转ROM值.结果 完整状态组、不稳状态组、固定A、B、C组的平均屈伸ROM值分别为17.78°、30.69°、2.25°、2.93°、2.73°,组间比较差异有统计学意义(F=216.69,P=0.000);平均侧屈ROM值分别为9.56°、17.18°、1.91°、2.30°、2.05°,组间比较差异有统计学意义(F=122.75,P=0.000);平均旋转ROM值分别为44.19°、57.30°、1.22°、2.88°、2.07°,组间比较差异有统计学意义(F=154.54,P=0.000).固定A、B、C组较完整状态组和不稳状态组各个方向的ROM值均明显减少,差异均有统计学意义(P<0.05),但固定A、B、C组之间符个方向的ROM值比较差异均无统计学意义(P>0.05).结论 寰椎椎板钩联合枢椎椎弓根螺钉内固定可提供与经寰枢关节间隙螺钉联合Gallic内固定和寰枢椎椎弓根螺钉内固定相当的力学稳定性.在以上两种方法无法实施时,可作为一种安全的替代.     

Biomechanical evaluation of occipitocervicothoracic fusion: impact of partial or sequential fixation

BACKGROUND CONTEXT: Surgical instrumentation used for posterior craniocervical instability has evolved from simple wiring techniques to sophisticated implant systems that incorporate multiple means of rigid fixation for the cervical spine. Polyaxial screws and lamina hooks in conjunction with occipital plating and transitional rods for caudal fixation theoretically allow for fixation points at each vertebra along the posterior aspect of the cervical spine. However, the potential for anatomical constraints to prevent intraoperative instrumentation at the desired vertebral level exists. The biomechanical implications of such "skipped segments" have not been well documented. PURPOSE: The purpose of this study was to determine the biomechanical effects of partial three-point fixation versus sequential fixation at all levels of the cervical spine from the occiput to T1. STUDY DESIGN/SETTING: Fresh frozen human cadaveric cervical spines from the occiput (CO) to T1 were prepared and mounted on a spine simulator. Motion was assessed by a three-dimensional optoelectronic motion measurement system. Kinematic data were collected and range of motion (ROM) was analyzed and reported. METHODS: Eight human noninstrumented intact spines (Treatment 1) were tested for baseline ROM which was subclassified into axial (CO-C2), upper subaxial (C2-C4), lower subaxial (C4-T1), and total (CO-T1) ROM. Flexion extension, lateral bending, and axial torsion testing with an applied +/-3Nm moment was conducted. The same testing protocol was performed after three-point fixation in which screws were placed at the CO, C4, and T1 (Treatment 2), and also after sequential fixation at all levels from CO through T1 (Treatment 3). Fixation was achieved using an occipital plate, 12-mm lateral mass screws for C3 through C6, and 20-mm lateral mass or pedicle screws were used for C1, C2, C7, and T1. RESULTS: Intact spine testing (Treatment 1) showed statistically significant larger ROM for all segments and for overall ROM when compared with both Treatment 2 (partial fixation CO, C4, and T1) and Treatment 3 (sequential fixation at all levels from the occiput to T1). When comparing Treatment 2 with Treatment 3, no significant difference in flexion extension ROM was detected between axial, upper subaxial, lower subaxial, and total overall ROM (p > .05). Lateral bending showed statistically significant increased ROM for Treatment 2 constructs compared with Treatment 3 constructs in total overall lateral bend ROM. For axial rotation, there was significantly increased ROM for Treatment 2 at the lower subaxial segment and total overall ROM (p < .05) when compared with Treatment 3. CONCLUSIONS: There was no statistical difference between the three-point fixation treatment group and the sequential fixation group in flexion extension bending. Lateral bending and axial rotation demonstrated an increase in total overall ROM with partial fixation compared with fixation at all levels. Axial rotation in particular showed increased mobility in the lower cervical spine for the partial fixation group. In the instance where surgeons are not able to apply sequential fixation at diseased levels, especially for the lower subaxial cervical spine, particular attention to limitation of lateral bending and axial rotation by the use of external orthotics must be considered.     

The stabilizing potential of anterior, posterior and combined techniques for the reconstruction of a 2-level cervical corpectomy model: biomechanical study and first results of ATPS prototyping

Clinical studies reported frequent failure with anterior instrumented multilevel cervical corpectomies. Hence, posterior augmentation was recommended but necessitates a second approach. Thus, an author group evaluated the feasibility, pull-out characteristics, and accuracy of anterior transpedicular screw (ATPS) fixation. Although first success with clinical application of ATPS has already been reported, no data exist on biomechanical characteristics of an ATPS-plate system enabling transpedicular end-level fixation in advanced instabilities. Therefore, we evaluated biomechanical qualities of an ATPS prototype C4–C7 for reduction of range of motion (ROM) and primary stability in a non-destructive setup among five constructs: anterior plate, posterior all-lateral mass screw construct, posterior construct with lateral mass screws C5 + C6 and end-level fixation using pedicle screws unilaterally or bilaterally, and a 360° construct. 12 human spines C3–T1 were divided into two groups. Four constructs were tested in group 1 and three in group 2; the ATPS prototypes were tested in both groups. Specimens were subjected to flexibility test in a spine motion tester at intact state and after 2-level corpectomy C5–C6 with subsequent reconstruction using a distractable cage and one of the osteosynthesis mentioned above. ROM in flexion–extension, axial rotation, and lateral bending was reported as normalized values. All instrumentations but the anterior plate showed significant reduction of ROM for all directions compared to the intact state. The 360° construct outperformed all others in terms of reducing ROM. While there were no significant differences between the 360° and posterior constructs in flexion–extension and lateral bending, the 360° constructs were significantly more stable in axial rotation. Concerning primary stability of ATPS prototypes, there were no significant differences compared to posterior-only constructs in flexion–extension and axial rotation. The 360° construct showed significant differences to the ATPS prototypes in flexion–extension, while no significant differences existed in axial rotation. But in lateral bending, the ATPS prototype and the anterior plate performed significantly worse than the posterior constructs. ATPS was shown to confer increased primary stability compared to the anterior plate in flexion–extension and axial rotation with the latter yielding significance. We showed that primary stability after 2-level corpectomy reconstruction using ATPS prototypes compared favorably to posterior systems and superior to anterior plates. From the biomechanical point, the 360° instrumentation was shown the most efficient for reconstruction of 2-level corpectomies. Further studies will elucidate whether fatigue testing will enhance the benefit of transpedicular anchorage with posterior constructs and ATPS.     

Which posterior instrumentation is better for two-level anterior lumbar interbody fusion: translaminar facet screw or pedicle screw?

Purpose

To determine whether translaminar facet screws can provide stability equivalent to pedicle screws and whether the two posterior instrumentations have the same influence on the adjacent segments in two-level anterior lumbar interbody fusion.

Methods

In a biomechanical study conducted, we used 12 fresh human lumbar spines and tested an intact spine with a stand-alone two-level anterior lumbar interbody fusion and anterior fusion augmented with pedicle screws or translaminar facet screws, under 400 N compressive preloads and 7.5 N m moments in flexion, extension, axial rotation and lateral bending, and measured the stiffness of the operated level, range of motion and intradiscal pressure at the adjacent levels.

Results

We found a significant increase in the stiffness of the segments operated, range of motion and intradiscal pressure at the adjacent superior segment in the stand-alone two-level anterior lumbar interbody fusion during flexion, axial rotation and lateral bending, but a decrease in extension, when compared with the intact spine. The stiffness of operated segments, range of motion and intradiscal pressure in the adjacent segment are significantly higher in the two-level anterior lumbar interbody fusion augmented with posterior instrumentation than in the stand-alone two-level anterior lumbar interbody fusion. There was no significant difference between the two augmented constructs except that, at the adjacent superior segment, the intradiscal pressure was more in the construction augmented with a pedicle screw than with a translaminar facet screw in flexion.

Conclusions

Translaminar facet screws can provide stability equivalent to pedicle screws, but their influence on the adjacent segments is relatively lower; therefore, we suggest that translaminar facet screws be the choice in the optimal posterior instrumentation in a two-level anterior lumbar interbody fusion.     

枕颈部后路不同内固定的生物力学比较

目的评价不同内固定重建枕颈部稳定性的生物力学性能。方法12具新鲜人体枕颈部标本,在标本完整、枕寰枢不稳、枕颈部植骨块钛缆固定(A组)、枕颈部经关节螺钉内固定(B组)、SUM-MIT枕颈部内固定系统固定(C组)五种状态下,依次用脊柱三维运动测量系统测试其OcC1、C1,2节段的运动参数。同时对固定后的OcC1经关节螺钉(OcC1TA组)、C1,2经关节螺钉(C1,2TA组)、枢椎椎弓螺钉(C2IS组)和枕骨螺钉(Oc Screw组)在生理载荷下三维六自由度运动时,运用电测法测定四种螺钉的拔出应力,并行统计学分析。结果在OcC1节段,B组在屈伸运动中的运动范围和中性区显著大于C组。在侧屈和旋转运动中,A组的运动范围和中性区均明显大于B、C组。在C1,2节段,B组各方向运动的运动范围和中性区均明显小于A组。B组在旋转运动中的运动范围和中性区均显著小于C组。电测法结果显示,侧屈状态下所有螺钉局部应变均接近0;前屈和旋转时螺钉承受不同程度的拉应力,后伸时螺钉承受压应力。Oc Screw组在屈伸和旋转运动状态下,所承受的任何载荷应力均大于其他三种置钉方法。结论枕颈部后路经关节螺钉内固定和SUMMIT枕颈内固定在控制旋转和侧屈的稳定性上有优点。枕骨螺钉承受的拉应力最大,生理环境下枕骨螺钉可能更易发生松动和断裂。     

A Biomechanical Comparison of Three Different Posterior Fixation Constructs Used for C6–C7 Cervical Spine Immobilization: A Finite Element Study

The intralaminar screw construct has been recently introduced in C6–C7 fixation. The aim of the study is to compare the stability afforded by three different C7 posterior fixation techniques using a three-dimensional finite element model of a C6–C7 cervical spine motion segment. Finite element models representing three different cervical anchor types (C7 intralaminar screw, C7 lateral mass screw, and C7 pedicle screw) were developed. Range of motion (ROM) and maximum von Mises stresses in the vertebra for the three screw techniques were compared under pure moments in flexion, extension, lateral bending, and axial rotation. ROM for pedicle screw construct was less than the lateral mass screw construct and intralaminar screw construct in the three principal directions. The maximum von Misses stress was observed in the C7 vertebra around the pedicle in all the three screw constructs. Maximum von Mises stress in pedicle screw construct was less than the lateral mass screw construct and intralaminar screw construct in all loading modes. This study demonstrated that the pedicle screw fixation is the strongest instrumentation method for C6–C7 fixation. Pedicle screw fixation resulted in least stresses around the C7 pedicle-vertebral body complex. However, if pedicle fixation is not favorable, the laminar screw can be a better option compared to the lateral mass screw because the stress around the pedicle-vertebral body complex and ROM predicted for laminar screw construct was smaller than those of lateral mass screw construct.     

Pedicle screw fixation with translaminar facet joint screws for the treatment of thoracolumbar fracture

Objective:To evaluate the efficacy of Cotrel-Dubeusset (CD) instrumentation combined with translaminar facet joint screw ( TLS ) in the treatment of thoracolumbar fracture. Methods: A total of six L2-L4 spines were used to establish unstable fracture model with three-dimensional range of motion ( ROM ) of the spines measured. Fixation with CD and fixation with CD combined with translaminar facet joint screw were achieved to compare their stability. Thirty cases of thoracolumbar fracture, in whom the anterior edge of vertebral body was compressed to 59% and the posterior edge compressed to 88%, were treated by pedicle screw fixation combined with TLS. Among them, 19 received posterolateral or anterior-posterior bone grafting Results- There was significant difference in ROM between the two techniques except that in extension. In Group CD TLS, ROM was 5.38% lower, lateral bending 4.91% lower and axial rotation 11.85 % lower than those in Group CD respectively. In the clinical group, the average anterior edge restored to 97 % and posterior edge to 98%. The duration of follow-up was 5-24 months (mean, 10 months). The rate of correction loss on the anterior edge was 4.5%. Among the 19 cases of bone grafting, all of them achieved bony fusion (mean fusion time, 4.3 month) with a correction loss rate of 3.4%. Conclusions:In the treatment of thoracolumbar fracture, pedicle screw fixation combined with TLS can strengthen the stability of pedicle screws, especially antirotation stability and enhance fusion rate and reduce correction loss.     

Effect of constrained posterior screw and rod systems for primary stability: Biomechanical in vitro comparison of various instrumentations in a single-level corpectomy model

Cervical corpectomy is a frequently used technique for a wide variety of spinal disorders. The most commonly used approach is anterior, either with or without plating. The results for single-level corpectomy are better than in multilevel procedures. Nevertheless, hardware- or graft-related complications are observed. In the past, constrained implant systems were developed and showed encouraging stability, especially for posterior screw and rod systems in the lumbar spine. In the cervical spine, few reports about the primary stability of constrained systems exist. Therefore, in the present study we evaluated the primary stability of posterior screw and rod systems, constrained and non-constrained, in comparison with anterior plating and circumferential instrumentations in a non-destructive set-up, by loading six human cadaver cervical spines with pure moments in a spine tester. Range of motion and neutral zone were measured for lateral bending, flexion/extension and axial rotation. The testing sequence consisted of: (1) stable testing; (2) testing after destabilization and cage insertion; (3a) additional non-constrained screw and rod system with lateral mass screws, (3b) with pedicle screws instead of lateral mass screws; (4a) constrained screw and rod system with lateral mass screws, (4b) with pedicle screws instead of lateral mass screws; (5) 360°set-up; (6) anterior plate. The stability of the anterior plate was comparable to that of the non-constrained system, except for lateral bending. The primary stability of the non-constrained system could be enhanced by the use of pedicle screws, in contrast to the constrained system, for which a higher primary stability was still found in axial rotation and flexion/extension. For the constrained system, the achievable higher stability could obviate the need to use pedicle screws in low instabilities. Another benefit could be fewer hardware-related complications, higher fusion rate, larger range of instabilities to be treated by one implant system, less restrictive postoperative treatment and possibly better clinical outcome. From a biomechanical standpoint, in regard to primary stability the constrained systems, therefore, seem to be beneficial. Whether this leads to differences in clinical outcome has to be evaluated in clinical trials.     

Biomechanical comparison of transfacet screws to lateral mass screw-rod constructs in the lower cervical spine

Purpose

Transfacet screws have been used as an alternative posterior fixation in the cervical spine. There is lack of spinal stability of the transfacet screws either as stand-along constructs or combined with anterior plate. This study was designed to evaluate spinal stability of transfacet screws following posterior ligamentous injury and combined with anterior plate, respectively, and compare transfacet screws to lateral mass screw-rod constructs.

Methods

Flexibility tests were conducted on eight cadaveric specimens in an intact and injury, and instrumented with the transfacet screw fixation and lateral mass screw-rod construct at C5–C7 levels either after section of the posterior ligamentous complex or combined with an anterior plate and a mesh cage for C6 corpectomy reconstruction. A pure moment of ±2.0 Nm was applied to the specimen in flexion–extension, lateral bending, and axial rotation. Ranges of motion (ROM) were calculated for the C5–C7 segment.

Results

ROM with the transfacet screws was 22 % of intact in flexion–extension, 9 % in lateral bending and 11 % in axial rotation, while ROM with the lateral mass screw-rod construct was 9 % in flexion–extension, 8 % in lateral bending and 22 % in axial rotation. The only significant difference between two constructs was seen in flexion–extension (5.8 ± 4.2° vs. 2.4 ± 1.2°, P = 0.002). When combined with an anterior plate and mesh cage, the transfacet screw fixation reduced ROM to 3.0° in flexion–extension, 1.2° in lateral bending, and 1.1° in axial rotation, which was similar to the lateral mass screw-rod construct.

Conclusions

This study identified the transfacet screw fixation, as stand-alone posterior fixation, was equivalent to the lateral mass screw-rod constructs in axial rotation and lateral bending except in flexion–extension. When combined with an anterior plate, the transfacet screw fixation was similar to the lateral mass screw-rod construct in motion constraint. The results suggested the transfacet screw fixation a biomechanically effective way as supplementation of anterior fixation.

     

A biomechanical comparison of modern anterior and posterior plate fixation of the cervical spine

STUDY DESIGN: A biomechanical study was designed to assess relative rigidity provided by anterior, posterior, or combined cervical fixation using cadaveric cervical spine models for flexion-distraction injury and burst fracture. OBJECTIVES: To compare the construct stability provided by anterior plating with locked fixation screws, posterior plating with lateral mass screws, and combined anterior-posterior fixation in clinically simulated 3-column injury or corpectomy models. SUMMARY OF BACKGROUND DATA: Anterior plating with locked fixation screws is the most recent design and is found to provide better stability than the conventional unlocked anterior plating. However, there are few data on the direct comparison of biomechanical stability provided by anterior plating with locked fixation screws versus posterior plating with lateral mass screws. Biomechanical advantages of using combined anterior-posterior fixation compared with that of using either anterior or posterior fixation alone also have not been well investigated yet. METHODS: Biomechanical flexibility tests were performed using cervical spines (C2-T1) obtained from 10 fresh human cadavers. In group I (5 specimens), one-level, 3-column injury was created at C4-C5 by removing the ligamentum flavum and bilateral facet capsules, the posterior longitudinal ligament, and the posterior half of the intervertebral disc. In group II (5 specimens), complete corpectomy of C5 was performed to simulate burst injury. In each specimen, the intact spine underwent flexibility tests, and the following constructs were tested: (1) posterior lateral mass screw fixation (Axis plate) after injury; (2) polymethylmethacrylate anterior fusion block plus posterior fixation; (3) polymethylmethacrylate block plus anterior (Orion plate) and posterior plate fixation; and (4) polymethylmethacrylate block plus anterior fixation. Rotational angles of the C4-C5 (or C4-C6) segment were measured and normalized by the corresponding angles of the intact specimen to study the overall stabilizing effects. RESULTS: Posterior plating with an interbody graft showed effective stabilization of the unstable cervical segments in all loading modes in all cases. There was no significant stability improvement by the use of combined fixation compared with the posterior fixation with interbody grafting, although combined anterior-posterior fixation tended to provide greater stability than both anterior and posterior fixation alone. Anterior fixation alone was found to fail in stabilizing the cervical spine, particularly in the flexion-distraction injury model in which no contribution of posterior ligaments is available. Anterior plating fixation provided much greater fixation in the corpectomy model than in the flexion-distraction injury model. This finding suggests that preservation of the posterior ligaments may be an important factor in anterior plating fixation. CONCLUSIONS: This study showed that the posterior plating with interbody grafting is biomechanically superior to anterior plating with locked fixation screws for stabilizing the one-level flexion-distraction injury or burst injury. More rigid postoperative external orthoses should be considered if the anterior plating is used alone for the treatment of unstable cervical injuries. It was also found that combined anterior and posterior fixation may not improve the stability significantly as compared with posterior grafting with lateral mass screws and interbody grafting.     

C1侧块C2椎板螺钉固定与C1-2关节螺钉固定的生物力学性能比较

目的 比较1侧块C2椎板螺钉固定与后路C1-2关节螺钉固定的牛物力学性能,为枢椎交叉椎板螺钉的临床应用提供参考依据.方法 收集新鲜成年人尸体颈椎标本(C0-C4)10具,男6具,女4具,平均年龄58岁(50～72岁).分别制备成C1-2完整状态的模型(M0)、C1-2失稳的模型(M1)、C1,侧块螺钉C2交义椎板螺钉固定模型(M2)及后路C1-2关节螺钉固定模型(M3),进行三维生物力学测试,记录各组模型在屈伸、轴向旋转和侧屈的运动范围(ROM)和中性区(NZ).结果 屈伸模式下M0、M1、M2、M3的ROM和NZ分别为:(20.4±9.2)°、(10.4±5.8)°;(46.3±17.2))°、(34.5±14.1)°;(4.1±0.9)°、(1.9±0.8)°;(4.3±1.0)°、(2.1±0.7)°.轴向旋转模式下 M0、M1、M2、M3的ROM和NZ分别为:(65.3±12.9)°、(40.8±11.2)°;(91.7±10.5)°、(72.3±12.6)°;(8.9±2.1)°、(4.7±1.8)°;(8.4±2.5)°、(4.4±2.3)°.侧屈模式F M0、M1、M2、M3的ROM和NZ分别为:(11.5±4.3)°、(7.2±3.2)°;(35.0±12.9)°、(21.9±11.8)°;(2.9±1.1)°、(1.4±0.8)°;(2.5±1.4)°、(1.3±1.0)°.在三维运动模式下M2和M3力学稳定性均明显优于M1和M0,然而M2和M3,的差异无统计学意义(P>0.05).结论 C1侧块C2椎板螺钉固定拥有良好的生物力学稳定性,可作为C1-2关节螺钉固定的良好替代.     

枢椎棘突螺钉单侧应用联合对侧椎弓根螺钉固定的有限元分析

目的分析枢椎棘突螺钉单侧应用联合对侧椎弓根螺钉固定在寰枢和枕颈固定中的生物力学稳定性。方法构建正常枢椎解剖、椎板薄和椎动脉变异椎弓根细小3种不同解剖状态下的完整上部颈椎有限元模型作为完整模型组,然后分别模拟齿状突骨折进行寰枢固定和寰椎骨折进行枕颈固定。在寰枢固定中,比较单侧枢椎棘突螺钉+对侧椎弓根螺钉+双侧寰椎侧块螺钉固定组(棘突螺钉组)和枢椎双侧椎弓根螺钉+双侧寰椎侧块螺钉固定组(椎弓根螺钉组);在枕颈固定中,比较单侧枢椎棘突螺钉+对侧椎弓根螺钉+枕骨螺钉固定组(棘突螺钉组)和枢椎双侧椎弓根螺钉+枕骨螺钉固定组(椎弓根螺钉组)。枢椎棘突螺钉分别测试水平、斜向、垂直置钉3种不同的固定技术。模拟颈椎运动,测量枕颈的屈伸、侧屈、旋转的关节活动范围(ROM)。结果在寰枢和枕颈固定中,棘突螺钉组和椎弓根螺钉组的C1~C2屈伸、侧屈、旋转ROM均较完整模型组均明显下降。在寰枢固定中棘突螺钉组C0~C2屈伸、侧屈、旋转的ROM大于椎弓根螺钉组;在枕颈固定中,棘突螺钉组C1~C2侧屈的ROM大于椎弓根螺钉组,棘突螺钉组的C0~C2旋转的ROM大于椎弓根螺钉组。枢椎棘突螺钉分别测试水平、斜向、垂直固定间有差异,但不明显。结论在寰枢和枕颈固定中,枢椎双侧椎弓根螺钉固定和枢椎单侧棘突螺钉联合对侧椎弓根螺钉组合式固定方法均具有良好的稳定性。在寰枢固定中,相对于枢椎棘突螺钉组合式固定,枢椎双侧椎弓根螺钉固定具有更好的寰枢稳定性。在枕颈固定中,枢椎双侧椎弓根螺钉固定在侧屈和旋转活动上较枢椎棘突螺钉组合式固定稳定性更好。枢椎三种棘突螺钉置钉技术间的稳定性差异并不明显。     

双节段后路腰椎椎体间融合术单侧椎弓根钉固定的生物力学稳定性

目的 分析对模拟双节段腰椎后路椎体间融合术(PLIF)采用单侧椎弓根钉固定(单侧固定)的生物力学稳定性.方法 将6具新鲜成人尸体腰椎标本(L2～S2)分别制备成L4～S1的PLIF模型,应用MTS 858实验机模拟产生屈伸、侧弯、轴向旋转,并按初始状态、单侧不稳、单侧不稳-单侧固定、双侧不稳-单侧固定、双侧不稳-双侧固定、双侧不稳的顺序进行测试,动态摄取记录各个节段角位移运动范围(ROM)与中性区值(NZ).结果 单侧不稳-单侧固定屈伸、侧弯、轴向旋转方向ROM值依次为2.53±1.12、4.03±2.19、2.78±1.00,NZ值依次为1.14±0.70、1.96±1.13、1.28±0.71,均显著小于初始状态(P<0.05),相比双侧不稳-双侧固定,各方向ROM与NZ值分别增加60.13%与17.52%、315.46%与243.86%、8.17%与6.20%,但差异无统计学意义(P>0.05).双侧不稳-单侧固定侧弯与旋转状态ROM与NZ值较双侧不稳-双侧固定显著增加(P<0.05).结论 单侧固定对人腰椎标本模拟双节段单侧PLIF可提供与双侧固定相似的生物力学稳定性,而对于模拟双节段双侧PLIF则单侧固定在大多数三维运动方向上不能提供足够的力学稳定性.

Abstract：

Objective To analyze the biomechanical efficacy of unilateral pedicle screw fixation on human cadaveric lumbar spine model simulated by two-level posterior lumbar interbody fusion (PLIF). Methods Six fresh-frozen adult human cadaveric lumbar spine motion segments (L2-S2) were simulated to unilateral/bilateral L4-S1 PLIF constructs augmented by unilateral/bilateral pedicle screw fixation sequentially and respectively. All configurations were tested by MTS 858 in the following sequential construct order: the intact, UI (unilateral instability), UIUF1C (unilateral instability via unilateral pedicle screw fixation plus one cage) , BIUF1C (bilateral instability via unilateral pedicle screw fixation plus one cage) , BIBF1C (bilateral instability via bilateral pedicle screw fixation plus one cage) and BI (bilateral instability without pedicle screw and cage). Each specimen was nondestructively tested in flexion/extension, lateral performed between different simulated constructs with One Way of ANOVA and Post hoc LSD tests. Results BIBF1C had the lowest ROM and NZ of L4-S1 fusion segments in all loading models, which were significantly lower than those of any uninstmmented construct (the intact, UI and BI) (P < 0. 05). In flexion/extension, lateral bending, and axial rotation, the ROM of UIUF1C was respectively 2.53 ± 1. 12, 4.03 ± 2. 19, 2. 78 ±1.00 and the NZ of UIUF1C was respectively 1.14 ±0.70, 1.96 ±1. 13, 1.28 ±0.71, which were significantly lower than those of the intact (P <0. 05). Compared to BIBF1C, the ROM and NZ were respectively increased 60.13% and 17.52% in flexion/extension, 315.46% and 243.86% in lateral bending, 8. 17% and 6. 20% in axial rotation, however, there were no significant differences between these two constructs (P > 0. 05). In lateral-bending and axial rotation, the ROM and NZ of BIUF1C were significantly higher than those of BIBF1C (P < 0. 05). In flexion/extension, the ROM and NZ of BIUF1C were higher than those of BIBF1C but there were no significant differences (P >0. 05). Compared to the intact, BIUF1C had lower ROM and NZ except for higher NZ in axial rotation, and there were significant differences only in flexion/extension (P < 0. 05). Conclusions All tested two-level unilateral fixation on simulated human cadaveric model with unilateral PLIF can achieve similar initial biomechanical stability in comparison with two-level bilateral pedicle screw fixation. However in most test modes, two-level unilateral pedicle screw fixation on simulated human cadaveric model with bilateral PLIF can not achieve enough biomechanical efficacy in comparison with two-level bilateral pedicle screw fixation.     

The biomechanical stability of a novel spacer with integrated plate in contiguous two-level and three-level ACDF models: an in vitro cadaveric study

Background contextAnterior cervical plating increases stability and hence improves fusion rates to treat cervical spine pathologies, which are often symptomatic at multiple levels. However, plating is not without complications, such as dysphagia, injury to neural elements, and plate breakage. The biomechanics of a spacer with integrated plate system combined with posterior instrumentation (PI), in two-level and three-level surgical models, has not yet been investigated.PurposeThe purpose of the study was to biomechanically evaluate the multidirectional rigidity of spacer with integrated plate (SIP) at multiple levels as comparable to traditional spacers and plating.Study designAn in vitro cervical cadaveric model.MethodsEight fresh human cervical (C2–C7) cadaver spines were tested under pure moments of ±1.5 Nm on spine simulator test frame. Each spine was tested in intact condition, with only anterior fixation and with both anterior and PI. Range of motion (ROM) was measured using Optotrak Certus (NDI, Inc., Waterloo, Ontario, Canada) motion analysis system in flexion-extension (FE), lateral bending (LB), and axial rotation (AR) at the instrumented levels (C3–C6). Repeated-measures analysis of variance was used for statistical analysis.ResultsAll the surgical constructs showed significant reduction in motion compared with intact condition. In two-level fusion, SIP (C4–C6) construct significantly reduced ROM by 66.5%, 65.4%, and 60.3% when compared with intact in FE, LB, and AR, respectively. In three-level fusion, SIP (C3–C6) construct significantly reduced ROM by 65.8%, 66%, and 49.6% when compared with intact in FE, LB, and AR, respectively. Posterior instrumentation showed significant stability only in three-level fusion when compared with their respective anterior constructs. In both two-level and three-level fusion, SIP showed comparable stability to traditional spacer and plate constructs in all loading modes.ConclusionsThe anatomically profiled spacer with integrated plate allows treatment of cervical disorders with fewer steps and less impact to cervical structures. In this biomechanical study, spacer with integrated plate construct showed comparable stability to traditional spacer and plate for two-level and three-level fusion. Posterior instrumentation showed significant effect only in three-level fusion. Clinical data are required for further validation of using spacer with integrated plate at multiple levels.     

In vitro biomechanical comparison of an anterior and anterolateral lumbar plate with posterior fixation following single-level anterior lumbar interbody fusion

OBJECT: Anterior lumbar interbody fusion (ALIF) is often supplemented with instrumentation to increase stability in the spine. If anterior plate fixation provided the same stability as posterior pedicle screw fixation (PSF), then a second approach and its associated morbidity could be avoided. METHODS: Seven human cadaveric L4-5 spinal segments were tested under three conditions: ALIF with an anterior plate, ALIF with an anterolateral plate, and ALIF supplemented by PSF. Range of motion (ROM) was calculated for flexion/extension, lateral bending, and axial torsion and compared among the three configurations. RESULTS: There were no significant differences in ROM during flexion/extension, lateral bending, or axial torsion among any of the three instrumentation configurations. CONCLUSIONS: The addition of an anterior plate or posterior PS/rod instrumentation following ALIF provides substantially equivalent biomechanical stability. Additionally, the position of the plate system, either anterior or anterolateral, does not significantly affect the stability gained.     

Pullout strength of pedicle screws versus pedicle and laminar hooks in the thoracic spine

While the biomechanical properties of pedicle screws have proven to be superior in the lumbar spine, little is known concerning pullout strength of pedicle screws in comparison to hooks in the thoracic spine. In vitro biomechanical pullout testing was performed to evaluate the axial pullout strength of pedicle screws versus pedicle and laminar hooks in the thoracic spine with regard to surgical correction techniques in scoliosis. Nine human cadaveric thoracic spines were harvested and disarticulated. To simulate a typical posterior segmental scoliosis instrumentation, standard pedicle hooks were used between T4 and T8 and supralaminar hooks between T9 and T12 and tested against pedicle screws. The pedicle screws were loaded strictly longitudinal to their axis; the hooks were loaded perpendicular to the intended rod direction. In total, 90 pullout tests were performed. Average pullout strength of the pedicle screws was significantly higher than in the hook group (T4-T8: 531 N versus 321 N, T9-T12: 807 N versus 600 N, p < 0.05). Both screw diameter and the bone mineral density (BMD) had significant influence on the pullout strength in the screw group. For scoliosis correction, pedicle screws might be beneficial, especially for rigid thoracic curves, since they are significantly more resistant to axial pullout than both pedicle and laminar hooks.     

寰枢椎后路椎弓根螺钉固定的生物力学评价

目的：评价寰枢椎后路椎弓根螺钉固定的生物力学稳定性。方法：6具新鲜颈椎标本，按随机顺序，对每一标本先后行C1-C2椎弓根螺钉、Magerl螺钉、Brooks钢丝以及螺钉联合钢丝固定，在脊柱三维运动实验机上测量其三维运动范围。结果：Magerl螺钉或C1-C2椎弓根螺钉联合Brooks钢丝组成的固定系统的三维运动范围最小。C1-C2椎弓根螺钉固定的前后屈伸运动范围与Brooks钢丝固定无差异，但大于Magerl螺钉；其左右侧屈运动范围小于Brooks钢丝固定，大于Magerl螺钉；其轴向旋转角度明显小于Brooks钢丝固定，但与Magerl螺钉无统计学差异。结论：C1-C2椎弓根螺钉的三维稳定性与Magerl螺钉相当，联合Brooks钢丝固定可进一步提高其稳定性。     

Spine update: cervical spine internal fixation using screw and screw-plate constructs

Screw and screw-plate constructs have been used successfully in fixation of the cervical spine. This update focuses on the indications, complications, and nuances in the technique used for odontoid screws, transarticular C1-C2 screws, occipitocervical plating, posterior lateral mass screws, pedicle screws, and anterior plating.