腰椎后路Ray-TFC椎间融合术对相邻节段活动范围的影响

目的：了解附加及不附加椎弓根内固定的后路Ray-TFC椎间融合术对相邻运动节段活动范围的即刻影响。方法：采用国人成年男性新鲜脊柱标本9具，以中性区(NZ)和运动范围(ROM)为观测指标，通过生物力学实验机测定单纯Ray—TFC植人和附加椎弓根内固定后相邻腰椎运动节段的活动范围。结果：单纯Ray-TFC植人仅使相邻节段前屈运动范围明显增加，附加RF-Ⅱ椎弓根钉内固定后相邻节段的前屈和后伸运动比单纯：Ray—FFC植人后均明显增加，前屈时的活动比完整初始状态显著增加。结论：附加及不附加椎弓根内固定的TFC椎间融合术都能明显增加相邻节段的前屈活动，有加速相邻节段退变的潜在可能。对相邻节段未出现症状但已经有退变表现的病例，选择此种术式时要慎重。     

Junction kinematics between proximal mobile and distal fused lumbar segments: biomechanical analysis of pedicle and hook constructs

Background contextBiomechanical studies have demonstrated increased motion in motion segments adjacent to instrumentation or arthrodesis. The effects of different configurations of hook and pedicle screw instrumentation on the biomechanical behaviors of adjacent segments have not been well documented.PurposeTo compare the effect of three different fusion constructs on adjacent segment motion proximal to lumbar arthrodesis.MethodsSeven human cadaver lumbar spines were tested in the following conditions: 1) intact; 2) L4–L5-simulated circumferential fusion (CF); 3) L4–L5-simulated fusion extended to L3 with pedicle screws; and 4) L4–L5-simulated fusion extended to L3 with sublaminar hooks. Rotation data at L2–L3, L3–L4, and L4–L5 were analyzed using both load limit control (±7.5 N·m) and displacement limit control (truncated to the greatest common angular motion of the segments for each specimen).ResultsBoth the L3–L4 and L2–L3 motion segments above the L4–L5-simulated CF had significantly increased motion in all loading planes compared with the intact spine, but no significant differences were found between L3–L4 and L2–L3 motion. When the L3–L4 segment was stabilized with pedicle screws, its motion was significantly smaller in flexion, lateral bending, and axial rotation than when stabilized with sublaminar hooks. At the same time, L2–L3 motion was significantly larger in flexion, lateral bending, and axial rotation in the pedicle screw model compared with the sublaminar hook construct.ConclusionsThe use of sublaminar hooks to stabilize the motion segment above a circumferential lumbar fusion reduced motion at the next cephalad segment compared with a similar construct using pedicle screws. The semiconstrained hook enhancement may be considered if a patient is at a risk of adjacent segment disorders.     

下腰椎不同融合方法的即刻与疲劳后稳定性

目的:观察下腰椎后路不同融合方法的即刻与疲劳后稳定性。方法:9具新鲜L1~S1尸体标本分别头尾端固定,在脊柱三维运动实验机上模拟人体行屈伸、左右侧弯及旋转活动,观察L4~L5节段运动范围(ROM),随后进行各种模拟手术并安装内固定,依次测定失稳腰椎、CD内固定(CD)、CD加椎体间植骨(CD-骨块)、CD加TFC(CD-TFC)状态下L4~L5的ROM值。随后在868Mini-MTS多轴生物力学实验机上对三种术式腰椎进行1500次疲劳加载,再观察L4~L5节段的ROM。结果:①失稳腰椎与正常腰椎比较在各运动方向上ROM均明显增加;②单纯CD内固定即刻稳定性好,但疲劳后呈不稳趋势,在屈伸及左右旋转方向明显失稳;③CD-骨块、CD-TFC稳定性好,疲劳后仍能维持良好的稳定性;④CD-骨块与CD-TFC疲劳组比较在屈伸及左右侧弯时ROM无明显差异,但CD-TFC在抗旋转方向优于CD-骨块组。结论:单纯后路短节段CD内固定和/或椎体间植骨融合均能提供良好的稳定性,但单纯后路短节段CD内固定抗疲劳性差,辅以椎间支撑(植骨或TFC)可有效提高抗疲劳稳定性。     

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.     

Stability analysis of an enhanced load sharing posterior fixation device and its equivalent conventional device in a calf spine model

STUDY DESIGN: An in vitro test of calf spine lumbar segments to compare biomechanical stabilization of a rigid versus a dynamic posterior fixation device. OBJECTIVES: To compare flexibility of a dynamic pedicle screw fixation device with an equivalent rigid device. SUMMARY OF BACKGROUND DATA: Dynamic pedicle screw device studies are not as prevalent in the literature as studies of rigid devices. These devices contain the potential to enhance load sharing and optimize fusion potential while maintaining stability similar to that of rigid systems. METHODS: Load-displacement tests were performed on intact and stabilized calf spines for the dynamic and rigid devices. Stability across a destabilized L3-L4 segment was restored by insertion of either a 6 mm x 40 mm dynamic or rigid pedicle screw fixation device across the L2-L4 segment. The screws then were removed, 7 mm x 45 mm pedicle screws of the opposite type were inserted, and the construct then was re-tested. Axial pull-out tests were performed to assess the likely effects of pedicle screw replacement on the load-displacement data. RESULTS: Results indicated a 65% reduction in motion in flexion-extension and a 90% reduction in lateral bending across the destabilized level for both devices, compared with intact spine values. Reduction in axial rotation motion was much smaller than in other modes. Axial pull-out tests showed no weakening of the bone-screw interface. CONCLUSIONS: Both devices provided significant stability of similar magnitudes in flexion, extension, and lateral bending. In axial rotation, the devices only could restore stability to levels similar to those in an intact spine. The dynamic device offers a design that may enhance load sharing without sacrificing construct stability.     

Dynamics of an Intervertebral Disc Prosthesis in Human Cadaveric Spines

Low-back pain is a common, disabling medical condition, and one of the major causes is disc degeneration. Total disc replacements are intended to treat back pain by restoring disc height and re-establishing functional motion and stability at the index level. The objective of this study was to determine the effect on range of motion (ROM) and stiffness after implantation of the ProDisc®-L device in comparison to the intact state. Twelve L5–S1 lumbar spine segments were tested in flexion/extension, lateral bending, and axial rotation with axial compressive loads of 600 N and 1,200 N. Specimens were tested in the intact state and after implantation with the ProDisc®-L device. ROM was not significantly different in the implanted spines when compared to their intact state in flexion/extension and axial rotation but increased in lateral bending. Increased compressive load did not affect ROM in flexion/extension or axial rotation but did result in decreased ROM in lateral bending and increased stiffness in both intact and implanted spine segments. The ProDisc®-L successfully restored or maintained normal spine segment motion.     

Hybrid dynamic stabilization: a biomechanical assessment of adjacent and supraadjacent levels of the lumbar spine

Object The object of this study was to evaluate the effect of hybrid dynamic stabilization on adjacent levels of the lumbar spine. Methods Seven human spine specimens from T-12 to the sacrum were used. The following conditions were implemented: 1) intact spine; 2) fusion of L4-5 with bilateral pedicle screws and titanium rods; and 3) supplementation of the L4-5 fusion with pedicle screw dynamic stabilization constructs at L3-4, with the purpose of protecting the L3-4 level from excessive range of motion (ROM) and to create a smoother motion transition to the rest of the lumbar spine. An industrial robot was used to apply continuous pure moment (± 2 Nm) in flexion-extension with and without a follower load, lateral bending, and axial rotation. Intersegmental rotations of the fused, dynamically stabilized, and adjacent levels were measured and compared. Results In flexion-extension only, the rigid instrumentation at L4-5 caused a 78% decrease in the segment's ROM when compared with the intact specimen. To compensate, it caused an increase in motion at adjacent levels L1-2 (45.6%) and L2-3 (23.2%) only. The placement of the dynamic construct at L3-4 decreased the operated level's ROM by 80.4% (similar stability as the fusion at L4-5), when compared with the intact specimen, and caused a significant increase in motion at all tested adjacent levels. In flexion-extension with a follower load, instrumentation at L4-5 affected only a subadjacent level, L5-sacrum (52.0%), while causing a reduction in motion at the operated level (L4-5, -76.4%). The dynamic construct caused a significant increase in motion at the adjacent levels T12-L1 (44.9%), L1-2 (57.3%), and L5-sacrum (83.9%), while motion at the operated level (L3-4) was reduced by 76.7%. In lateral bending, instrumentation at L4-5 increased motion at only T12-L1 (22.8%). The dynamic construct at L3-4 caused an increase in motion at T12-L1 (69.9%), L1-2 (59.4%), L2-3 (44.7%), and L5-sacrum (43.7%). In axial rotation, only the placement of the dynamic construct at L3-4 caused a significant increase in motion of the adjacent levels L2-3 (25.1%) and L5-sacrum (31.4%). Conclusions The dynamic stabilization system displayed stability characteristics similar to a solid, all-metal construct. Its addition of the supraadjacent level (L3-4) to the fusion (L4-5) did protect the adjacent level from excessive motion. However, it essentially transformed a 1-level lumbar fusion into a 2-level lumbar fusion, with exponential transfer of motion to the fewer remaining discs.     

Human lumbar facet joint capsule strains: II. Alteration of strains subsequent to anterior interbody fixation.

BACKGROUND CONTEXT: In cases of low back pain associated with biomechanical lumbar instability, anterior interbody fixation can be used as a surgical treatment, but its affect on facet joint capsule strains is unknown. PURPOSE: To determine the effect of a single-level anterolateral interbody fixation, the changes in lumbar facet joint capsule strains at the level of and adjacent to the fixation were evaluated. STUDY DESIGN/SETTING: Human cadaveric lumbar spine specimens were tested under displacement control before and after the addition of a single anterior thoracolumbar plate (ATLP) on the L4-L5 motion body. METHODS: Ligamentous lumbar spine specimens (n=7) were potted and actuated before and after fixation of the L4-L5 motion segment with an ATLP in motions of extension, flexion, left and right bending. Joint moments were calculated from the applied load and respective moment arms. Intervertebral angulation was measured using biaxial inclinometers mounted onto adjacent vertebrae. Plane strains of the capsules were measured by optically tracking the displacements of small, infrared reflective markers glued to capsule surfaces. Statistical differences (p<.05) in moment, intervertebral angle and capsular strain were assessed using analysis of variance and comparison of linear regression lines. RESULTS: Fixation resulted in an increase in moment at the three vertebral levels for all motions. There was also an increase in intervertebral angle at L3-L4 and L5-S1, and a decrease in intervertebral angle at L4-L5 for all motions. Plane strains in the L3-L4 and L5-S1 facet capsules increased as a result of the fixation. L4-L5 facet capsules experienced decreased and increased strains ipsilateral and contralateral, respectively, to the instrumentation. CONCLUSION: Restriction of a vertebral motion segment using a single ATLP increased adjacent capsular strains, which if suprathreshold for capsule nociceptors, could play a role in low back pain.     

腰椎新型钛合金动态内固定系统的稳定性研究

目的 检测由新型钛合金制成的非融合腰椎动态内固定系统的稳定性.方法 收集6具平均6月龄的新鲜小牛腰椎标本,在连续的5种状态下,即完整状态、失稳状态、使用DIFS Ⅰ系统(椎弓根螺钉的弹性模量为75GPa,由新型钛合金制成,其连接棒弹性模量为43GPa,直径为3.5mm)动态固定状态、使用DIFS Ⅱ系统(椎弓根螺钉的弹性模量为75GPa,由新型钛合金制成,其连接棒弹性模量为72GPa,直径为3.5mm)动态固定状态、使用对照组SINO系统(短尾"U"形椎弓根系统,连接棒直径为5.5mm)坚固固定状态,对小牛L2-6施加10N·m的纯力矩,测量标本在三维6个方向前屈、后伸、左右侧屈和左右旋转的活动范围(ROM)和中性区(NZ).结果 在失稳状态下L3,46个方向的ROM和NZ与其他各状态比较,差异均有统计学意义(P<0.05).DIFS Ⅰ系统动态同定与完整状态和SINO系统固定比较差异均无统计学意义.DIFS Ⅱ系统动态固定与完整状态各方向的ROM和NZ比较差异有统计学意义(P<0.05),DIFS Ⅱ系统动态固定与SINO系统固定的各方向ROM和NZ比较差异无统计学意义.L2,3和L4,5的各种状态下6个方向的ROM和NZ比较差异无统计学意义.结论 DIFS Ⅰ系统动态固定既可以维持腰椎的活动性,又可以获得坚强固定的稳定性;DIFS Ⅱ系统动态固定性质更接近坚强固定.     

Thoracolumbar spinal fractures: segmental range of motion after dorsal spondylodesis in 82 patients: a prospective study

In order to study the effect of dorsal spondylodesis on intervertebral movement in patients treated for thoracolumbar fractures, we measured the sagittal range of motion (ROM) in the segments above and below the fractured vertebral body 2 years after operation. Between 1991 and 1996, 82 consecutive patients with a fracture of the thoracolumbar spine (T12, L1, L2 and L3) were treated operatively with open reduction and stabilisation using an internal fixator, combined with transpedicular cancellous bone graft and dorsal spondylodesis. Eighteen T12, 42 L1, 17 L2 and 5 L3 fractures were included. The range of motion of two segments above and two segments below fracture level was measured. This was done on plain flexion and extension radiographs. The data were compared to normal values and to the zero distribution with the Kolmogorov-Smimov test. At all fracture levels the ROM of the segment adjacent to the disturbed endplate of the fractured body was zero (K-S test). All other evaluated segments showed significant loss of ROM (P<0.05) compared to normal values, except segment L1-L2 in L3 fractures (P=0.058). Dorsal spondylodesis at the level of the disturbed endplate in thoracolumbar spinal fractures leads to immobility in this segment, measured on flexion-extension radiographs 2 years after primary operative treatment. More than 50% loss of motion in the two adjacent levels is equivalent to complete loss of ROM in a second segment.     

Use of instrumented pedicle screws to evaluate load sharing in posterior dynamic stabilization systems

Background contextDynamic stabilization is an alternative to fusion intended to eliminate or at least minimize the potential for adjacent level degeneration. Different design approaches are used in pedicle screw-based systems that should have very different effects on the loading of the posterior column and intervertebral disc. If the implant system distributes these loads more evenly, loads in the pedicle screws will be reduced, and screw loosening will be prevented.PurposeThe purpose of this study was to determine how two different design approaches to dynamic stabilization systems, Dynesys System and the Total Posterior Spine (TOPS) System, affect the load carried by the pedicle screws.Study design/settingA controlled laboratory study in which the magnitude of the moments on pedicle screws during flexion–extension and lateral bending were measured after implantation of two posterior dynamic stabilization devices into cadaveric spines.MethodsFive lumbar spines were tested in flexion–extension and lateral bending. Specimens were tested sequentially: first intact, then with the Dynesys system implanted, and finally with the TOPS system implanted. Range of motion (ROM) for each construct was measured with a 210 N and 630 N compressive load. The pedicle screws were instrumented with strain gages, which were calibrated so that the moments on the screws could be determined from the strain measurements.ResultsCompared with intact values, ROM decreased in flexion–extension and lateral bending when the Dynesys System was implanted. With implantation of the TOPS System, ROM returned to values that were not significantly different from the intact values. The moments in the screws with the Dynesys System were significantly higher than with the TOPS System with increases of as much as 56% in flexion–extension and 86% in lateral bending.ConclusionsThe design of the posterior stabilization device influences the amount of load seen by the pedicle screws and therefore the load sharing between spinal implant and bone.     

Wallis棘突间动态稳定装置的生物力学研究

目的 观察Wallis棘突间动态稳定装置对腰椎力学载荷传导及活动度的影响.方法 采用6具新鲜成人脊柱标本(L1～S1),采用自身前后对照,分为正常组、损伤组、椎弓根螺钉固定组、置入Wallis装置组,分别测量中立位、前屈后伸、左右侧弯、旋转运动加载下节段腰椎的力学载荷及活动范围.结果 W组固定节段椎间盘、关节突应力载荷明显小于Ⅰ组(P＜0.05),明显大于PS组(P＞0.05),但与N组比较差异无统计学意义(P＞0.05);临近节段椎间盘、关节突应力载荷与Ⅰ组、N组比较差异无统计学意义(P＞0.05),但明显小于PS组(P＜0.05).W组固定节段的屈伸活动范围(ROM)小于Ⅰ组及N组(P＜0.05),但明显大于PS组(P＜0.05);侧弯及旋转运动范围,W组与Ⅰ组比较差异无统计学意义(P＞0.05),但与N组及PS组比较差异有统计学意义(P＜0.05).结论 Wallis棘突间动态稳定装置限制固定节段的异常活动,降低固定及临近节段椎间盘及关节突关节应力载荷,减小邻近节段应力集中.     

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.     

Intermediate screws in short segment pedicular fixation for thoracic and lumbar fractures: a biomechanical study

To determine the effect of adding pedicle screws at the level of a burst fracture (intermediate screws) on the stiffness of a short segment pedicle fixation, an in vitro biomechanical study was carried out. Six fresh-frozen pig lumbar spine specimens were used. The flexibility of the intact specimens was examined in flexion, extension, lateral bending, and torsion. An unstable burst fracture model was created by the dropped-mass technique. The unstable spine specimens were instrumented with pedicle screws. The flexibility was tested again with and without intermediate screws. The addition of intermediate screws provided a smaller range of motion in flexion-extension (P<0.001), torsion (P<0.001), and lateral bending (P=0.014). The slopes of the load displacement curves increased in flexion (P<0.001), extension (P=0.003), lateral bending (P=0.003), and torsion (P=0.006), signifying a decrease in flexibility. The addition of intermediate screws at the level of a burst fracture significantly increases the stiffness of a short segment pedicular fixation.     

腰椎经后路椎体间融合后单边或双边固定对邻近节段退变的影响

背景：在行后路椎体融合内固定术中,椎弓根钉置入不可避免会损伤邻近关节突关节。目前一致认为单边固定因保留一侧关节突关节可明显降低邻近节段退变的发生率,但仍缺乏对邻近节段退变的影响因素及不同节段退变发生率的长期随访研究。目的：对比经后路椎体间融合术（posterior lumbar interbody fusion,PLIF）后单边或双边椎弓根螺钉固定对邻近节段退变的影响。方法：2006年2月至2007年12月,101例行PLIF手术的L4-L5椎间盘突出症患者纳入本研究。采用单边固定42例,双边固定59例。所有患者术后随访时间均超过5年。邻近节段分为三个节段：第1个近端邻近节段、第2个近端邻近节段及远端邻近节段。依据末次随访的影像学资料评估邻近节段退变的情况,并记录末次随访时的ODI评分评价腰椎功能。结果：单边固定组第1个近端邻近节段、第2个近端邻近节段及远端邻近节段退变的发生率分别为57.1%（24/42）、45.2%（19/42）、38.1%（16/42）;双边固定组第1个近端邻近节段、第2个近端邻近节段及远端邻近节段退变的发生率分别为72.9%（43/59）、68.0%（40/59）、50.8%（30/59）。两组第1个近端邻近节段和远端邻近节段退变发生率无统计学差异,而第2个近端邻近节段退变发生率具有统计学差异。末次随访时单边固定组和双边固定组的ODI评分分别为25.6±5.9、28.4±5.2,两组具有显著统计学差异（t=-2.503,P=0.014）。结论：对于行腰椎后路减压融合术的单节段腰椎间盘突出症患者,单边固定者邻近节段退变发生率低于双边固定者,尤其对于第2个近端邻近退变节段的患者。     

Biomechanical evaluation of a new AxiaLIF technique for two-level lumbar fusion

Single level axial lumbar interbody fusion (AxiaLIF) using a transsacral rod through a paracoccygeal approach has been developed with promising early clinical results and biomechanical stability. Recently, the transsacral rod has been extended to perform a two-level fusion at both L4–L5 and L5–S1 levels (AxiaLIF II). No biomechanical studies have been conducted on multilevel fusion using the AxiaLIF technique. In this study, the biomechanics of L4–S1 motion segments instrumented with the AxiaLIF II transsacral rod was evaluated. Six human cadaveric lumbosacral spine segments from L4 to S1 were used (age ranges 46–74 years). Unconstrained and non-destructive pure moments in axial torsion, lateral bending, and flexion extension were applied to each specimen following intact, standalone AxiaLIF II, and AxiaLIF II with two posterior fixation options: facet screws and pedicle screws with rods. Range of motion was calculated from the raw data collected with an optical motion tracking system. The two-level transsacral rod was successfully inserted in all the specimens. At L4–L5 level in axial torsion (AT) and flexion extension (FE), none of the surgical treatments showed statistically significant difference between the procedures (all P > 0.05) although facet screws and pedicle screws had higher stability on average. In lateral bending (LB), the two posterior fixation techniques had significantly higher construct stability (P < 0.05) than the standalone rod. No significant difference was found between facet screws and pedicle screws (P = 0.821). At L5–S1 level in AT and LB, none of the surgical treatments were found to be statistically significant (all P > 0.05). In FE, standalone two-level transsacral rod had significantly higher range of motion (ROM) compared with the posterior fixation techniques (P < 0.05). In conclusion, the standalone rod reduced intact ROM significantly. Supplementary fixations including facet screws and pedicle screws are required to achieve higher construct stability for successful fusion. Further clinical studies are essential to evaluate the practical success of this technique.     

Comparative biomechanical investigation of a modular dynamic lumbar stabilization system and the Dynesys system

The goal of non-fusion stabilization is to reduce the mobility of the spine segment to less than that of the intact spine specimen, while retaining some residual motion. Several in vitro studies have been conducted on a dynamic system currently available for clinical use (Dynesys^®). Under pure moment loading, a dependency of the biomechanical performance on spacer length has been demonstrated; this variability in implant properties is removed with a modular concept incorporating a discrete flexible element. An in vitro study was performed to compare the kinematic and stabilizing properties of a modular dynamic lumbar stabilization system with those of Dynesys, under the influence of an axial preload. Six human cadaver spine specimens (L1–S1) were tested in a spine loading apparatus. Flexibility measurements were performed by applying pure bending moments of 8 Nm, about each of the three principal anatomical axes, with a simultaneously applied axial preload of 400 N. Specimens were tested intact, and following creation of a defect at L3–L4, with the Dynesys implant, with the modular implant and, after removal of the hardware, the injury state. Segmental range of motion (ROM) was reduced for flexion–extension and lateral bending with both implants. Motion in flexion was reduced to less than 20% of the intact level, in extension to approximately 40% and in lateral bending a motion reduction to less than 40% was measured. In torsion, the total ROM was not significantly different from that of the intact level. The expectations for a flexible posterior stabilizing implant are not fulfilled. The assumption that a device which is particularly compliant in bending allows substantial intersegmental motion cannot be fully supported when one considers that such devices are placed at a location far removed from the natural rotation center of the intervertebral joint.     

Biomechanical evaluation of paracoccygeal transsacral fixation

STUDY DESIGN: This is a biomechanical study using human cadaveric lumbar spine. OBJECTIVE: To evaluate the biomechanics of paracoccygeal transsacral rod fixation. SUMMARY OF BACKGROUND DATA: Various types of transsacral fixation either by posterior and paracoccygeal approaches have been described in the literature. The biomechanical advantage of transsacral rod fixation is the preservation of supporting structures at L5-S1 level. No biomechanical data on human cadavers have been reported in the literature. The aim of this study is to evaluate the biomechanics of the transsacral rod fixation. METHODS: Six fresh human cadaveric L5-S1 motion segments (mean age 67.5 y; range 46 to 82 y ) were used in the study. Unconstrained and nondestructive pure moments in axial torsion (AT), lateral bending (LB), and flexion-extension (FE) were applied to each specimen after applying transsacral rod and after additional augmentation methods, including bilateral screws, facet screws, and pedicle screw and rod system. Range of motion (ROM) was calculated for each surgical treatment. The disc space was measured with lateral plain radiographs of intact specimens and after transsacral rod insertion to evaluate the amount of distraction. RESULTS: The mean ROM of the intact specimens was 3.5, 6.4, and 11.0 degrees in AT, LB, and FE, respectively. Standalone transsacral rod reduced ROM more than 40% compared with the intact condition (P=0.002). Bilateral screws further reduced the ROM in AT (64%) and LB (70%), but not in FE (53%). Both facet screws and pedicle screw and rod system achieved high construct stability under all loading conditions. The transsacral rod augmented with facet screws reduced ROM by 70%, 80%, and 90% compared with the intact condition. When augmented with pedicle screw and rod system, the transsacral rod reduced ROM by 73%, 87%, and 88% in AT, LB, and FE, respectively. There was no statistical difference between these 2 facet screws and pedicle screw and rod system (P>0.8). CONCLUSIONS: Transsacral rod fixation provides strong ligamentotaxis due to intact annulus. Standalone transsacral rod is able to reduce ROM significantly and achieve indirect decompression by distracting L5-S1 disc space. However, additional posterior fixation, such as facet screws or pedicle screws, is required to achieve better construct stability for successful fusion.     

Comparison of pressure effects on adjacent disk levels after 2-level lumbar constructs: fusion, hybrid, and total disk replacement

BACKGROUND: With increasing advocacy for the use of TDR procedure as a surgical alternative to fusion in the management of lumbar DDD, intradiskal pressures at the adjacent levels of spine have generated considerable interest. The common belief is that adjacent-level disk pressures will be lower after a TDR as opposed to conventional fusion. The aim of this study is to present the effect of different constructs on adjacent-level disk pressures in lumbar spine. We hypothesized that the adjacent-segment disk pressures after 1- and 2-level TDR and/or a fusion-TDR hybrid procedure will show significant variance within physiological range of motion. METHODS: Six adult spine segments T12-S1 with intact ligaments were harvested from cadavers and held firmly in a specially designed fixture. Intradiskal pressures, in motions of flexion, extension, and lateral bending, at L2-L3 and L3-L4 were measured using needle transducers after 2-level TDR L4 through S1, hybrid procedure, and 2-level fusion L4-S1 with femoral ring allograft and pedicle screws. RESULTS: The pressures with lateral bending were not significantly lower than those with flexion and extension at both levels (P = .18). Although TDR and hybrid specimens recorded slightly lower pressures specifically during lateral bending, no statistical difference in pressures could be detected when movements were combined with various procedures. CONCLUSION: Contrary to the assumed hypothesis, the pressures at the adjacent-level disks (L3-4 and L2-3) did not depend upon the stabilization procedure (2-level disk replacement, hybrid, or 2-level fusion) performed after 2-level diskectomy in the lumbosacral spine.     

前路腰椎椎间融合术即刻稳定性的生物力学研究

目的 评价应用聚醚醚酮(poly-ether-ether-ketone,PEEK)椎间融合器行前路腰椎椎间融合的即刻生物力学稳定性.方法 采用6具成人尸体腰椎标本,均来源于男性,X线榆查排除腰椎疾患和明显退行性改变,双能X线骨密度检查证实所有标本骨密度均处于正常范围.截取L4～S2节段腰椎,用自凝型聚甲基丙烯酸甲酯将标本两端固定在夹具内.沿腰椎轴线加载500N的预负荷后,分别在生物力学试验机上检测以下三种状态的前屈、后伸及侧屈各方向活动的即刻稳定性:完整腰椎标本,L5S1椎间盘切除、PEEK椎间融合器植入,辅以后路椎弓根螺钉系统固定.结果 L5S1椎间单纯椎间融合器植入后前屈、后伸、左侧屈方向的活动范围较完整腰椎标本分别减少76.9%、66.6%与81.1%;辅以后路惟弓根螺钉系统固定后,前屈、后伸、左侧屈方向的活动范围较完整腰椎标本分别减少93.9%、90%与86.6%.完整腰椎标本的空白对照组L4,5椎间前屈、后伸与左侧屈活动度分别为4.16°±0.33°、4.02°±0.30°、3.48°±0.34.;L5S1单纯椎间融合器组则分别为5.82°±0.36°、5.38°±0.30°、4.96°±0.29°;L5S1椎间融合器辅助椎弓根螺钉固定组则分别为4.82°±0.26°、5.76°±0.31°、3.98°±0.29°.L5S1椎间单纯椎间融合器植入及附加椎弓根螺钉崮定后,L4,5椎间相对活动度显著增加,各组间差异有统计学意义(P=0.000).结论 单纯应用PEEK椎间融合器的前路腰椎椎间融合术即刻稳定性优于完整腰椎标本.辅以后路椎弓根螺钉系统固定后融合节段稳定性得到显著增强.