Effect of multilevel lumbar disc arthroplasty on the operative- and adjacent-level kinematics and intradiscal pressures: an in vitro human cadaveric assessment

Background contextWith lumbar arthroplasty gaining popularity, limited data are available highlighting changes in adjacent-level mechanics after multilevel procedures.PurposeCompare operative– and adjacent-segment range of motion (ROM) and intradiscal pressures (IDPs) after two-level arthroplasty versus circumferential arthrodesis.Study designCadaveric biomechanical study.MethodsTen human cadaveric lumbar spines were used in this investigation. Biomechanical testing was performed according to a hybrid testing protocol using an unconstrained spine simulator under axial rotation (AR), flexion extension (FE), and lateral-bending (LB) loading. Specimens were tested in the following order: 1) Intact, 2) L3–L5 total disc replacement (TDR), 3) L3–L5 anterior interbody cages+pedicle screws. IDP was recorded at proximal and distal adjacent levels and normalized to controls (%intact). Full ROM was monitored at the operative and adjacent levels and reported in degrees.ResultsKinematics assessment revealed L3–L5 ROM reduction after both reconstructions versus intact controls (p<.05). However, global quality of segmental motion distributed over L2–S1 was preserved in the arthroplasty group but was significantly altered after circumferential fixation. Furthermore, adjacent-level ROM was increased for the arthrodesis group under LB at both segments and during AR at L2–L3 relative to controls (p<.05). FE did not reveal any intergroup statistical differences. Nonetheless, after arthrodesis IDPs were increased proximally under all three loading modalities, whereas distally a significant IDP rise was noted during AR and LB (p<.05). No statistical differences in either biomechanical parameter were recorded at the adjacent levels between intact control and TDR groups.ConclusionsOur results indicate no significant adjacent-level biomechanical changes between arthroplasty and control groups. In contrast, significant alterations in ROM and IDP were recorded both proximally (ROM=LB & AR; IDP=AR, FE, LB) and distally (ROM=LB; IDP=AR & LB) after circumferential arthrodesis. Therefore, two-level lumbar arthroplasty maintains a more favorable biomechanical environment at the adjacent segments compared with the conventional transpedicular fixation technique. This, in turn, may have a positive effect on the rate of the transition syndrome postoperatively.     

Revision strategies for single- and two-level total disc arthroplasty procedures: a biomechanical perspective

Background contextThe utilization of motion-preserving implants versus conventional instrumentation systems, which stabilize the operative segments, necessitates improved understanding of their comparative biomechanical properties and optimal biomechanical method for surgical revision.PurposeUsing an in vitro human cadaveric model, the primary objective was to compare the multidirectional flexibility properties of single- versus two-level total disc arthroplasty procedures and determine the acute in vitro biomechanical characteristics of two methods of surgical revision—posterior transpedicular instrumentation alone or circumferential spinal arthrodesis.Study designThis in vitro biomechanical study was undertaken to compare the multidirectional flexibility kinematics of single- versus two-level lumbar total disc arthroplasty reconstructions using an in vitro model.MethodsA total of seven human cadaveric lumbosacral spines (L1-sacrum) were biomechanically evaluated under the following L4–L5 reconstruction conditions: intact spine; discectomy alone; Charité total disc replacement; Charité with pedicle screws; two-level Charité (L4–S1); two-level Charité with pedicle screws (L4–S1); Charité L4–L5 with pedicle screws and femoral ring allograft (FRA) (L5–S1); and pedicle screws with FRA (L4–S1). Multidirectional flexibility testing used the Panjabi Hybrid Testing protocol, which includes pure moments for the intact condition with the overall spinal motion replicated under displacement control for subsequent reconstructions. Hence, changes in adjacent level kinematics can be obtained compared with pure moment testing strategies. Unconstrained intact moments of ±7.5 Nm were used for axial rotation, flexion-extension, and lateral bending testing with quantification of the operative- and adjacent-level range of motion (ROM). All data were normalized to the intact spine condition (intact=100%).ResultsIn axial rotation, single- and two-level Charité reconstructions produced significantly more motion than pedicle screw constructs combined with the Charité or FRA (p<.05). There were no differences between the Charité augmented with pedicle screws or pedicle screws with FRA (p>.05). The two-level annulus lumbar resection required for multilevel Charité implantation had an added destabilizing effect, resulting in a 140% to 160% ROM increase over the intact condition. Under two-level reconstructions, rotational motion at the L4–L5 level increased from 160±26% to 263±65% with the implantation of the second Charité at L5–S1. Flexion-extension and lateral bending conditions with the Charité reconstructions in this group of seven spines demonstrated no significant differences compared with the intact spine (p>.05). The Charité combined with pedicle screws or pedicle screws with FRA significantly reduced motion at the operative level compared with the Charité reconstruction (p<.05). The most pronounced changes in adjacent level kinematics and intradiscal pressures were observed under flexion-extension loading. The addition of pedicle screw fixation increased segmental motion and intradiscal pressures at the proximal and distal adjacent levels compared with the intact and Charité reconstruction groups (p<.05).ConclusionsThe findings highlight a variety of important trends at the operative and adjacent levels. In terms of revision strategies, posterior pedicle screw reconstruction combined with an existing Charité was not found acutely to be statistically different from pedicle screws combined with FRA.     

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.     

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。     

山羊腰椎内固定术中椎弓根螺钉植入深度对邻椎的影响

目的：观察脊柱内固定术中不同椎弓根螺钉植入深度对邻椎生物力学环境和退行性变的影响,探讨椎体内固定强度与邻椎病发生的关系。方法：雄性10月龄玻尔杂交山羊16只,体重为25～30 kg,随机分为对照组(N组)和3组内固定组,每组4只。内固定组建立L₄脊柱不稳定骨折病理模型,以椎弓根螺钉内固定脊柱L₃-L₅节段,按螺钉长度不同分为：长钉组(L组),内固定螺钉长度为25 mm; 中钉组(M组),螺钉长度为20 mm; 短钉组(S组),螺钉长度为15 mm,内固定后进行后路小关节融合;对照组不作任何处理。术后24周,制备对照组与内固定组L₃-S₁腰椎标本,测量上位未融合椎体(L₂)和椎间盘应变和应力,进行影像学、组织学观察,计算MRI指数。结果：与N组相比,L组标本L₂节段椎体、椎间盘的活动和应力均增加(P<0.05),MRI指数未见显着改变(P>0.05),髓核中出现多数退变细胞。M、S组标本上位邻椎受力和髓核退行性改变发生情况不显着(P>0.05).结论：在山羊腰椎内固定融合术中,局部坚强固定可能增加上位未融合节段的活动和承受的应力,促进椎间盘退行性变,导致邻椎病的发生。     

聚醚醚酮棒杂交手术治疗腰椎退行性疾病的中长期疗效

目的 探讨聚醚醚酮（PEEK）棒杂交手术治疗腰椎退行性疾病的中长期临床疗效。方法 回顾性分析2014年4月—2016年1月在中国人民解放军联勤保障部队第九六〇医院因腰椎退行性疾病接受PEEK棒杂交手术治疗的27例患者临床资料,术前与末次随访时采用疼痛视觉模拟量表（VAS）评分和Oswestry功能障碍指数（ODI）评估腰腿痛程度及腰椎功能,记录并发症发生情况,在术前及末次随访时的影像学资料上测量相关参数评估临床疗效。结果 所有手术顺利完成,患者随访60～89(77.2±9.8)个月。27例患者共59个节段行手术治疗,其中非融合节段32个,融合节段27个。末次随访时腰腿痛VAS评分、ODI及影像学指标与术前相比明显改善,差异均有统计学意义（P <0.05）。27个融合节段中有23个获得骨性融合,融合率为85.2%。27例患者共置入172枚椎弓根螺钉及54根PEEK棒,1例发生L5左侧椎弓根螺钉断裂,其余未发生螺钉松动或PEEK棒断裂等并发症。结论 PEEK棒杂交手术治疗腰椎退行性疾病的中长期临床疗效满意,更好地保留了固定节段活动度及非融合节段的椎间高度,融合率高且并发症少,可有效保...     

Biomechanical study of adjacent intervertebral motion after lumbar spinal fusion and flexible stabilization using polyethylene-terephthalate bands

Thirty fresh-frozen calf cadaveric spinal specimens (L3-L6) were used to investigate the effect of flexible stabilization and fixation on the adjacent intervertebral motion segment. The intact spine that had not been subjected to injury was used for comparison as control. The destabilized spine was made up of specimens from which the bilateral facet joints and the supraspinous and interspinous ligaments were removed. The flexible stabilized spine was applied with pedicle screws and polyethylene-terephthalate bands, and the fixed spine was applied with pedicle screws and rods at the L4-L5 segment. The range of motion (ROM) was measured under flexion, extension, and bilateral bending moments, and the ROM ratio (ROM of each model versus ROM of the comparison model) was calculated at L3-L4, L4-L5, and L5-L6 segments. In the flexible stabilized spine, the restriction of motion was high under flexion and bilateral bending moments, and the mobility of the adjacent intervertebral motion segments approximated the normal lumbar vertebra. In the fixed spine, the ROM ratio increased at the cranial and caudal adjacent segments.     

Loading of the lumbar spine during transition from standing to sitting: effect of fusion versus motion preservation at L4–L5 and L5–S1

BACKGROUND CONTEXTTransition from standing to sitting significantly decreases lumbar lordosis with the greatest lordosis-loss occurring at L4–S1. Fusing L4–S1 eliminates motion and thus the proximal mobile segments maybe recruited during transition from standing to sitting to compensate for the loss of L4–S1 mobility. This may subject proximal segments to supra-physiologic flexion loading.PURPOSEAssess effects of instrumented fusion versus motion preservation at L4–L5 and L5–S1 on lumbar spine loads and proximal segment motions during transition from standing to sitting.STUDY DESIGNBiomechanical study using human thoracolumbar spine specimens.METHODSA novel laboratory model was used to simulate lumbosacral alignment changes caused by a person's transition from standing to sitting in eight T10-sacrum spine specimens. The sacrum was tilted in the sagittal plane while constraining anterior-posterior translation of T10. Continuous loading-data and segmental motion-data were collected over a range of sacral slope values, which represented transition from standing to different sitting postures. We compared different constructs involving fusions and motion preserving prostheses across L4–S1.RESULTSAfter L4–S1 fusion, the sacrum could not be tilted as far posteriorly compared to the intact spine for the same applied moment (p<.001). For the same reduction in sacral slope, L4–S1 fusion induced 2.9 times the flexion moment in the lumbar spine and required 2.4 times the flexion motion of the proximal segments as the intact condition (p<.001). Conversely, motion preservation at L4–S1 restored lumbar spine loads and proximal segment motions to intact specimen levels during transition from standing to sitting.CONCLUSIONSIn general, sitting requires lower lumbar segments to undergo flexion, thereby increasing load on the lumbar disks. L4–S1 fusion induced greater moments and increased flexion of proximal segments to attain a comparable seated posture. Motion preservation using a total joint replacement prosthesis at L4–S1 restored the lumbar spine loads and proximal segment motion to intact specimen levels during transition from standing to sitting.CLINICAL SIGNIFICANCEAfter L4–S1 fusion, increased proximal segment loading during sitting may cause discomfort in some patients and may lead to junctional breakdown over time. Preserving motion at L4–S1 may improve patient comfort and function during activities of daily living, and potentially decrease the need for adjacent level surgery.     

Effect of the Total Facet Arthroplasty System after complete laminectomy-facetectomy on the biomechanics of implanted and adjacent segments

Background contextLumbar fusion is traditionally used to restore stability after wide surgical decompression for spinal stenosis. The Total Facet Arthroplasty System (TFAS) is a motion-restoring implant suggested as an alternative to rigid fixation after complete facetectomy.PurposeTo investigate the effect of TFAS on the kinematics of the implanted and adjacent lumbar segments.Study designBiomechanical in vitro study.MethodsNine human lumbar spines (L1 to sacrum) were tested in flexion-extension (+8 to ?6 Nm), lateral bending (±6 Nm), and axial rotation (±5 Nm). Flexion-extension was tested under 400 N follower preload. Specimens were tested intact, after complete L3 laminectomy with L3–L4 facetectomy, after L3–L4 pedicle screw fixation, and after L3–L4 TFAS implantation. Range of motion (ROM) was assessed in all tested directions. Neutral zone and stiffness in flexion and extension were calculated to assess quality of motion.ResultsComplete laminectomy-facetectomy increased L3–L4 ROM compared with intact in flexion-extension (8.7±2.0 degrees to 12.2±3.2 degrees, p<.05) lateral bending (9.0±2.5 degrees to 12.6±3.2 degrees, p=.09), and axial rotation (3.8±2.7 degrees to 7.8±4.5 degrees p<.05). Pedicle screw fixation decreased ROM compared with intact, resulting in 1.7±0.5 degrees flexion-extension (p<.05), 3.3±1.4 degrees lateral bending (p<.05), and 1.8±0.6 degrees axial rotation (p=.09). TFAS restored intact ROM (p>.05) resulting in 7.9±2.1 degrees flexion-extension, 10.1±3.0 degrees lateral bending, and 4.7±1.6 degrees axial rotation. Fusion significantly increased the normalized ROM at all remaining lumbar segments, whereas TFAS implantation resulted in near-normal distribution of normalized ROM at the implanted and remaining lumbar segments. Flexion and extension stiffness in the high-flexibility zone decreased after facetectomy (p<.05) and increased after simulated fusion (p<.05). TFAS restored quality of motion parameters (load-displacement curves) to intact (p>.05). The quality of motion parameters for the whole lumbar spine mimicked L3–L4 segmental results.ConclusionsTFAS restored range and quality of motion at the operated segment to intact values and restored near-normal motion at the adjacent segments.     

Traumatic spondyloptosis resulting from high-energy trauma concurrent with a tonic-clonic seizure

Background contextTraumatic lumbosacral dislocation injuries are rare, high-energy injuries that are challenging to surgically manage.PurposeTo report a patient with a traumatic spondyloptosis of L5 on S1 as a result of bilateral pedicle fractures of L4 and L5 occurring during a motor vehicle accident, concurrent with a tonic-clonic seizure. The mechanism and treatment of the injury are discussed.Study designClinical case report and literature review.MethodsA staged circumferential fusion was performed with posterior reduction of L5 to the sacrum and instrumentation and fusion from L2 to the pelvis, followed 12 days later by anterior diskectomies and arthrodesis from L3 to S1.ResultsNear anatomic reduction and solid fusion were obtained and maintained at 3-year follow-up. The patient remained neurologically intact in all lumbosacral roots throughout the course of treatment.ConclusionsThe injury pattern described is quite rare. This case of multilevel, bilateral pedicle fracture with traumatic L5–S1 spondyloptosis was successfully treated by circumferential reduction and arthrodesis without neurological injury.     

The protective role of dynamic stabilization on the adjacent disc to a rigid instrumented level. An in vitro biomechanical analysis

Objectives

To assess the changes of intradiscal pressure at the bridged and at the adjacent levels to a lumbar two-level hybrid instrumentation.

Introduction

The elimination of motion produced by spinal fusion may have potential consequences beyond the index level overloading the juxtaposed spinal motion segments and leading to the appearance of degenerative changes. Degeneration of the segments adjacent to instrumented levels has become a topic of increasing interest in the literature over the last years. In order to prevent degenerative disc changes at the adjacent segments to a fused level, a broad scope of techniques have been developed, one of them is hybrid constructs.

Methods

In 6 human cadaveric lumbosacral specimens, pressure transducers quantified intradiscal pressure changes at three levels (L3–L4, L4–L5 and L5–S1) under axial compression (0–750 N), anterior flexion (+12°) and extension (?12°) in three different situations of spinal stability: intact, L5–S1 rigid rod pedicle screw instrumentation and L4–S1 two-level hybrid instrumentation (rigid at L5–S1 and dynamic at L4–L5).

Results

Once the L5–S1 segment had implanted the rigid instrumentation system (Diapason), the intradiscal pressure at this level decreased by 65 % while the intradiscal pressure at the disc above (L4–L5) increased 20 %. After augmenting the L5–S1 posterior construct with a dynamic stabilization device (Dynesys) at the superior adjacent level, the intradiscal pressure at this level, L4–L5, decreased by 50 % whereas intradiscal pressure at its adjacent level, L3–L4, only experienced a slight increase of 10 %.

Conclusions

The raise of intradiscal pressure at the adjacent segment to a rigid instrumented segment can be reduced when the rigid construct is augmented with a dynamic stabilization device. Hybrid constructs might have a possible protecting role preventing the occurrence of degenerative disc changes at the adjacent segment to a rigid instrumented level. Augmentation with a dynamic stabilization device might protect the disc above a rigid rod pedicle screw construct.     

Biomechanical evaluation of the Facet Wedge: a refined technique for facet fixation

Purpose

Purpose of this paper is to evaluate the primary stability of a new approach for facet fixation the so-called Facet Wedge (FW) in comparison with established posterior fixation techniques like pedicle screws (PS) and translaminar facet screws (TLS) with and without anterior cage interposition.

Methods

Twenty-four monosegmental fresh frozen non-osteoporotic human motion segments (L2–L3 and L4–L5) were tested in a two-arm biomechanical study using a robot-based spine tester. Facet Wedge was compared with pedicle screws and translaminar screws as a stand-alone device and in combination with an anterior fusion cage.

Results

Pedicle screws, FW and translaminar screws could stabilize an intact motion segment effectively. Facet Wedge was comparable to PS for lateral bending, extension and flexion and slightly superior for axial rotation. Facet Wedge showed a superior kinematic capacity compared to translaminar screws.

Conclusions

Facet Wedge offers a novel posterior approach in achieving primary stability in lumbar spinal fixation. The results of the present study showed that the Facet Wedge has a comparable primary stability to pedicle screws and potential advantages over translaminar screws.     

Biomechanical comparison of semirigid junctional fixation techniques to prevent proximal junctional failure after thoracolumbar adult spinal deformity correction

BACKGROUND CONTEXTAdult spinal deformity patients treated operatively by long-segment instrumented spinal fusion are prone to develop proximal junctional kyphosis (PJK) and failure (PJF). A gradual transition in range of motion (ROM) at the proximal end of spinal instrumentation may reduce the incidence of PJK and PJF, however, previously evaluated techniques have not directly been compared.PURPOSETo determine the biomechanical characteristics of five different posterior spinal instrumentation techniques to achieve semirigid junctional fixation, or “topping-off,” between the rigid pedicle screw fixation (PSF) and the proximal uninstrumented spine.STUDY DESIGNBiomechanical cadaveric study.METHODSSeven fresh-frozen human cadaveric spine segments (T8–L3) were subjected to ex vivo pure moment loading in flexion-extension, lateral bending and axial rotation up to 5 Nm. The native condition, three-level PSF (T11–L2), PSF with supplemental transverse process hooks at T10 (TPH), and two sublaminar taping techniques (knotted and clamped) as one- (T10) or two-level (T9, T10) semirigid junctional fixation techniques were compared. The ROM and neutral zone (NZ) of the segments were normalized to the native condition. The linearity of the transition zones over three or four segments was determined through linear regression analysis.RESULTSAll techniques achieved a significantly reduced ROM at T10-T11 in flexion-extension and axial rotation relative to the PSF condition. Additionally, both two-level sublaminar taping techniques (CT2, KT2) had a significantly reduced ROM at T9-T10. One-level clamped sublaminar tape (CT1) had a significantly lower ROM and NZ compared with one-level knotted sublaminar tape (KT1) at T10-T11. Linear regression analysis showed the highest linear correlation between ROM and vertebral level for TPH and the lowest linear correlation for CT2.CONCLUSIONSAll studied semirigid junctional fixation techniques significantly reduced the ROM at the junctional levels and thus provide a more gradual transition than pedicle screws. TPH achieves the most linear transition over three vertebrae, whereas KT2 achieves that over four vertebrae. In contrast, CT2 effectively is a one-level semirigid junctional fixation technique with a shift in the upper rigid fixation level. Clamped sublaminar tape reduces the NZ greatly, whereas knotted sublaminar tape and TPH maintain a more physiologic NZ. Clinical validation is ultimately required to translate the biomechanics of various semirigid junctional fixation techniques into the clinical goal of reducing the incidence of proximal junctional kyphosis and failure.CLINICAL SIGNIFICANCEThe direct biomechanical comparison of multiple instrumentation techniques that aim to reduce the incidence of PJK after thoracolumbar spinal fusion surgery provides a basis upon which clinical studies could be designed. Furthermore, the data provided in this study can be used to further analyze the biomechanical effects of the studied techniques using finite element models to better predict their post-operative effectiveness.     

Treatment of idiopathic scoliosis with CD-instrumentation: Lumbar pedicle screws versus laminar hooks in 66 patients

We studied whether the pedicle screw is better than laminar hooks for fixation of the lumbar spine in the treatment of idiopathic scoliosis. 66 consecutive patients with idiopathic scoliosis (King I and II) were studied retrospectively. Group S included 33 patients (25 females) treated with pedicle screws. Their mean age at operation was 17 (13-54) years. Group H included 33 patients (30 females) treated exclusively with hooks. Their mean age at operation was 16 (11-40) years. The preoperative mean angles of the thoracic curve in group S was 66 (42-115)°, and in group H 65 (42-121)°. The lumbar curve averaged 46 (20-85)° in group H and 53 (33-86)° in group S. All patients were fused only posteriorly with Cotrel-Dubousset instrumentation and an autogenic bone graft. The mean follow-up time was 4 (2-7) years. Mean correction of the thoracic curve was 45% in group S and 50% in group H. The lumbar curve was corrected by 50% in group S and 51% in group H. Loss of correction of the thoracic curve occurred in 5% in group S and 6% in group H and of the lumbar curve in 3% in group S and 10% in group H (p = 0.04). Group S better maintained the correction of the lateral tilt of the uninstrumented segment adjacent to the fusion (p = 0.04). Derotation, according to Perdriolle, in the distal segment adjacent to the fusion was 6% in group S and 2% in group H. We found no difference between correction of the thoracic and lumbar curves using pedicle screws and laminar hooks in the lumbar spine. Pedicle screws better maintained the correction of the lumbar curve and the lateral tilt in the distal segment adjacent to fusion.     

Biomechanical evaluation and comparison of polyetheretherketone rod system to traditional titanium rod fixation on adjacent levels

PurposeAdjacent segment degeneration or fracture of the vertebral body was commonly reported in rigid fusion. Use of semirigid instruments such as PEEK rod system could be an alternative treatment. However, the biomechanical implications of using PEEK rod systems are not well understood. Purpose of this study was to compare a PEEK rod fixation system to traditional titanium rod fixation via a finite element analysis.MethodsA lumbar spine model from L2–L5 vertebral bodies was constructed. A fusion model, created by modifying the intact lumbar model, was used to simulate anterior interbody and posterolateral lumbar fusion. Loading was applied through flexion, extension, lateral bending, torsion.ResultsThe greatest increase in stress was estimated at the upper disc adjacent to the titanium rod with interbody fusion. The lower increase in stress on adjacent segments occurred with PEEK rod fixation without fusion and noninstrumented posterolateral lumbar fusion models. With the same fusion or nonfusion procedures, the stress on discs and facet joints of adjacent segments in the PEEK rod group decreased by 5–25% of that in the titanium rod group for all loading conditions.ConclusionIn comparison with rigid fixation, some potential advantages of using PEEK rod systems include a reduced stress on adjacent segment disc and facet joint, and the elastic ability of PEEK rod fixation allows for a greater range of motion, which may reduce the incidence of clinical complications seen with rigid fusion devices.     

Biomechanics of adjacent segments after a multilevel cervical corpectomy using anterior,posterior, and combined anterior-posterior instrumentation techniques: a finite element model study

Background contextAdjacent segment degeneration (ASD) after cervical fusion is a clinical concern. Despite previous studies documenting the biomechanical effects of multilevel cervical fusion on segments immediately superior and inferior to the operative segments, the pathogenesis of the initiation of degeneration progression in neighboring segments is still poorly understood.PurposeTo test the hypothesis that changes in range of motion, disc stresses, and facet loads would be highest at the superior adjacent segment (C3–C4) after anterior C4–C7 corpectomy and fusion and that these changes would be the least in anterior fixation and the greatest in posterior or combined anterior-posterior instrumentation techniques.Study designA finite element (FE) analysis of adjacent vertebral segment biomechanics after a two-level corpectomy fusion with three different fixation techniques (anterior, posterior, and combined anterior-posterior).MethodsA previously validated three-dimensional FE model of an intact C3–T1 segment was used. From this intact model, three additional instrumentation models were constructed using anterior (rigid screw-plate), posterior (rigid screw-rod), and combined anterior-posterior fixation techniques after a C4–C7 corpectomy and fusion. Motion patterns, disc stresses, and posterior facet loads at the levels cephalad and caudal to the fusion were assessed.ResultsRange of motion, disc stresses, and posterior facet loads increased at the adjacent segments. Use of posterior fixation, whether alone or in combination with anterior fixation, infers higher changes in segmental motion, disc stresses, and posterior facet loads at adjacent segments compared with the use of anterior fixation alone. The superior C3–C4 motion was most affected during lateral bending and the inferior C7–T1 motion was most affected during flexion, whereas both superior C3–C4 and inferior C7–T1 motions were least affected during extension. However, disc stresses and facet loads were most affected during extension. Hence, it is speculated that the most remodeling changes in discs and facets might be related to the least changes in extension motion.ConclusionsBiomechanical factors such as increased mechanical demand and motion that have been associated with the development of ASD progression are highest in the segment immediately superior to the fusion. These changes are even more pronounced when the fixation technique involves the addition of posterior instrumentation, thereby supporting the hypothesis of the present study. Increased degrees of stiffening of the fused segments not only may lead to degenerative changes in the disc but may also predispose the segments to premature facet degeneration. Over subsequent time period, any remaining construct micro-motion is further eliminated with fusion of the posterior facet joints and the remaining regions in the disc space also filled in with bone, which eventually results in a circumferential type of fusion. After a circumferential fusion, authors, however, speculate that the role of instrumentation in ASD progression might not be significant. In fact, sufficient evidence to support this speculation is still lacking in the literature.     

Biomechanical evaluation of five different occipito-atlanto-axial fixation techniques

STUDY DESIGN: The stabilizing effects of five different occipitocervical fixations were compared. OBJECTIVES: To evaluate the construct stability provided by five different occipito-atlanto-axial fixation techniques. SUMMARY OF BACKGROUND DATA: Few studies have addressed occipitocervical reconstruction stability and no studies to data have investigated anterior-posterior translational stiffness. METHODS: A total of 21 human cadaveric spines were used. After testing intact spines (CO-C2), a type II dens fracture was created and five different reconstructions were performed: 1) occipital and sublaminar wiring/rectangular rod, 2) occipital screws and C2 lamina claw hooks/rod, 3) occipital screws, foramen magnum screws, and C1-C2 transarticular screws/rod, 4) occipital screws and C1-C2 transarticular screws/Y-plate, and 5) occipital screws and C2 pedicle screws/rod. Biomechanical testing parameters included axial rotation, flexion/extension, lateral bending, and anterior-posterior translation. RESULTS: Pedicle screw fixation demonstrated the highest stiffness among the five reconstructions (P < 0.05). The two types of transarticular screw methods provided greater stability than hook or wiring reconstructions (P < 0.05). The C2 claw hook technique resulted in greater stability than sublaminar wiring fixation in anterior-posterior translation (P < 0.05). However, the wiring procedure did not significantly increase the stiffness levels beyond the intact condition under anterior-posterior translation and lateral bending (P > 0.05). DISCUSSION: C2 transpedicular and C1-C2 transarticular screws significantly increased the stabilizing effect compared to sublaminar wiring and lamina hooks. The improved stability afforded by C2 pedicular and C1-C2 transarticular screws offer many potential advantages including a high rate of bony union, early ambulation, and easy nursing care. CONCLUSION: Occipitocervical reconstruction techniques using C1-C2 transarticular screws or C2 pedicle screws offer biomechanical advantages compared to sublaminar wiring or lamina hooks. Pedicle screw fixation exhibited the highest construct stiffness among the five reconstructions.     

Comparison of adjacent segment degeneration after successful posterolateral fusion with unilateral or bilateral pedicle screw instrumentation: a minimum 10-year follow-up

Background context

In the instrumented fusion, adjacent segment facet joint violation or impingement by pedicle screws is unavoidable especially in cephalad segment, despite taking specific intraoperative precautions in terms of surgical approach. In such circumstances, unlike its original purpose, unilateral pedicle screw instrumentation can contribute to reduce the degeneration of cephalad adjacent segment by preventing contralateral cephalad adjacent facet joint from the unavoidable injury by pedicle screw insertion. However, to our knowledge, no long-term follow-up study has compared adjacent segment degeneration (ASD) between unilateral and bilateral pedicle screw instrumented fusion.

Purpose

To compare ASD after successful posterolateral fusion using either unilateral or bilateral pedicle screw instrumentation for patients with lumbar spinal stenosis and/or Grade 1 spondylolisthesis.

Study design

Retrospective case-control study.

Patient sample

One hundred forty-seven patients who had undergone one- or two-level posterolateral fusion with unilateral or bilateral pedicle screw instrumentation for lumbar spinal stenosis with or without low-grade spondylolisthesis and achieved successful fusion, with a minimum 10-year follow-up.

Outcome measure

The occurrence of radiologic ASD, Oswestry disability index (ODI) scores, and revision rates.

Methods

A total of 194 consecutive patients were contacted and encouraged to visit our hospital and to participate in our study. Radiologic ASD was evaluated at three motion segments: cephalad adjacent segment (first cephalad adjacent segment), one cephalad to cephalad adjacent segment (second cephalad adjacent segment), and caudal adjacent segment. Clinical outcomes were compared by ODI scores and revision rates.

Results

In total, 147 of 194 (75.8%) patients were available for at least 10 years of radiologic and clinical follow-up. Adjacent segment degeneration (in first cephalad or caudal adjacent segment) was noted in 55.9% (33 of 59 patients) of the unilateral group and 72.7% (64 of 88 patients) of the bilateral group (p=.035). The occurrence of ASD in each first cephalad and caudal adjacent segment was not significantly different between groups but that in second cephalad adjacent segment was significantly different between groups (p=.004). Clinical outcomes according to ODI showed significant difference between groups (p=.016), especially when ODI scores were compared in patients with ASD (p=.004).

Conclusions

In a minimum 10-year follow-up retrospective study of posterolateral fusion for lumbar spinal stenosis and/or Grade 1 spondylolisthesis, unilateral pedicle screw instrumentation showed a lower rate of radiologic ASD, especially in second cephalad adjacent segment, and a better clinical outcome by ODI.     

Treatment of idiopathic scoliosis with CD-instrumentation: lumbar pedicle screws versus laminar hooks in 66 patients

We studied whether the pedicle screw is better than laminar hooks for fixation of the lumbar spine in the treatment of idiopathic scoliosis. 66 consecutive patients with idiopathic scoliosis (King I and II) were studied retrospectively. Group S included 33 patients (25 females) treated with pedicle screws. Their mean age at operation was 17 (13-54) years. Group H included 33 patients (30 females) treated exclusively with hooks. Their mean age at operation was 16 (1140) years. The preoperative mean angles of the thoracic curve in group S was 66 (42.115) degrees, and in group H 65 (42-121) degrees. The lumbar curve averaged 46 (20-85) degrees in group H and 53 (33-86) degrees in group S. All patients were fused only posteriorly with Cotrel-Dubousset instrumentation and an autogenic bone graft. The mean follow-up time was 4 (2-7) years. Mean correction of the thoracic curve was 45% in group S and 50% in group H. The lumbar curve was corrected by 50% in group S and 51% in group H. Loss of correction of the thoracic curve occurred in 5% in group S and 6% in group H and of the lumbar curve in 3% in group S and 10% in group H (p = 0.04). Group S better maintained the correction of the lateral tilt of the uninstrumented segment adjacent to the fusion (p = 0.04). Derotation, according to Perdriolle, in the distal segment adjacent to the fusion was 6% in group S and 2% in group H. We found no difference between correction of the thoracic and lumbar curves using pedicle screws and laminar hooks in the lumbar spine. Pedicle screws better maintained the correction of the lumbar curve and the lateral tilt in the distal segment adjacent to fusion.     

Treatment of idiopathic scoliosis with CD-instrumentation: Lumbar pedicle screws versus laminar hooks in 66 patients

We studied whether the pedicle screw is better than laminar hooks for fixation of the lumbar spine in the treatment of idiopathic scoliosis.

66 consecutive patients with idiopathic scoliosis (King I and II) were studied retrospectively. Group S included 33 patients (25 females) treated with pedicle screws. Their mean age at operation was 17 (13-54) years. Group H included 33 patients (30 females) treated exclusively with hooks. Their mean age at operation was 16 (11-40) years. The preoperative mean angles of the thoracic curve in group S was 66 (42-115)°, and in group H 65 (42-121)°. The lumbar curve averaged 46 (20-85)° in group H and 53 (33-86)° in group S. All patients were fused only posteriorly with Cotrel-Dubousset instrumentation and an autogenic bone graft. The mean follow-up time was 4 (2-7) years.

Mean correction of the thoracic curve was 45% in group S and 50% in group H. The lumbar curve was corrected by 50% in group S and 51% in group H. Loss of correction of the thoracic curve occurred in 5% in group S and 6% in group H and of the lumbar curve in 3% in group S and 10% in group H (p = 0.04). Group S better maintained the correction of the lateral tilt of the uninstrumented segment adjacent to the fusion (p = 0.04). Derotation, according to Perdriolle, in the distal segment adjacent to the fusion was 6% in group S and 2% in group H.

We found no difference between correction of the thoracic and lumbar curves using pedicle screws and laminar hooks in the lumbar spine. Pedicle screws better maintained the correction of the lumbar curve and the lateral tilt in the distal segment adjacent to fusion.