Primary stability of pedicle screws depends on the screw positioning and alignment

Background contextThere is no universal consensus regarding the biomechanical aspects and relevance on the primary stability of misplaced pedicle screws.PurposeThe study is aimed to the determination of the correlation between axial pullout forces of pedicle screws with the possible screw misplacement, including mild and severe cortical violations.MethodsEighty-eight monoaxial pedicle screws were implanted into 44 porcine lumbar vertebral bodies, paying attention on trying to obtain a wide range of placement accuracy. After screw implantation, all specimens underwent a spiral computed tomography scan, and the screw placements were graded following the scales of Laine et al. and Abul Kasim et al. Axial pullout tests were then performed on a servohydraulic material testing system.ResultsDecreasing pullout forces were determined for screws implanted with increasing cortical violation. A smaller influence of cortical violations in the medial direction with respect to the lateral direction was observed. Screws implanted with a large cortical violation and misplacement in the craniocaudal direction were found to be significantly less stable than screws having comparable cortical violation but in a centered sagittal position.ConclusionsThese results provide adjunctive criteria to evaluate more accurately the fate of a spine instrumentation. Particular care should be placed in the screw evaluation regarding the craniocaudal positioning and alignment.     

Radiographic criteria for placement of translaminar facet screws

Background contextScrew fixation of the facet joint has been reported to stabilize the lumbar spine and facilitate spinal fusion. Accurate placement of translaminar facet screws (TLFSs) requires identification of the posterior spinal elements, and the facet joints in particular, which may be facilitated by intra-operative fluoroscopy.PurposeThe purpose of this study was to define the radiographic parameters that will allow for successful placement of TLFSs.Study designEighteen TLFSs were placed in three human cadaver spines using fluoroscopic guidance, with screws placed from L3–L4 to L5–S1 bilaterally. After screw placement, the spines were dissected and examined for the accuracy of screw placement.MethodsIn the first cadaver spine, TLFSs were placed with direct visualization of the posterior lumbar spine, during which a fluoroscopic image intensifier was used to define the appropriate radiographic parameters for safe placement of the screws. Bilateral TLFSs were then placed percutaneously using the radiographic parameters developed, after which the spines were dissected to examine the positions of the screws.ResultsThe radiographic views identified to achieve proper TLFS placement were a true lateral, anteroposterior (AP), a 45-degree oblique and an AP view with the X-ray beam at a 30- to 45-degree cephalad angle (“spinal outlet” view). Using these views and the defined radiographic criteria, proper positioning of percutaneously placed TLFSs was achieved, with no spinal canal breaches found.ConclusionsThis study defines intra-operative radiographic criteria that will assist in placement of TLFSs. The use of this technique may allow for screw placement with less extensive exposure of the posterior spine.     

A biomechanical analysis of the self-retaining pedicle hook device in posterior spinal fixation

Regular hooks lack initial fixation to the spine during spinal deformity surgery. This runs the risk of posterior hook dislodgement during manipulation and correction of the spinal deformity, that may lead to loss of correction, hook migration, and post-operative junctional kyphosis. To prevent hook dislodgement during surgery, a self-retaining pedicle hook device (SPHD) is available that is made up of two counter-positioned hooks forming a monoblock posterior claw device. The initial segmental posterior fixation strength of a SPHD, however, is unknown. A biomechanical pull-out study of posterior segmental spinal fixation in a cadaver vertebral model was designed to investigate the axial pull-out strength for a SPHD, and compared to the pull-out strength of a pedicle screw. Ten porcine lumbar vertebral bodies were instrumented in pairs with two different instrumentation constructs after measuring the bone mineral density of each individual vertebra. The instrumentation constructs were extracted employing a material testing system using axial forces. The maximum pull-out forces were recorded at the time of the construct failure. Failure of the SPHD appeared in rotation and lateral displacement, without fracturing of the posterior structures. The average pull-out strength of the SPHD was 236 N versus 1,047 N in the pedicle screws (P < 0.001). The pull-out strength of the pedicle screws showed greater correlation with the BMC compared to the SPHD (P < 0.005). The SPHD showed to provide a significant inferior segmental fixation to the posterior spine in comparison to pedicle screw fixation. Despite the beneficial characteristics of the monoblock claw construct in a SPHD, that decreases the risk of posterior hook dislodgement during surgery compared to regular hooks, the SPHD does not improve the pull-out strength in such a way that it may provide a biomechanically solid alternative to pedicle screw fixation in the posterior spine.     

Posterior instrumentation improves the stabilities of Brantigan and Bagby and Kuslich (BAK) methods of posterior lumbar interbody fusion across the L4–L5 segments in a cadaveric model

BackgroundThe Brantigan and Bagby and Kuslich (BAK) cages for posterior lumbar interbody fusion have different geometric characteristics. However, both cage designs have been demonstrated to be helpful in restoring disc space across spinal motion segments in clinical observations. This study was designed to compare the biomechanical performance of these devices at one-motion segments and to determine the effects of posterior instrumentation on their stabilities.MethodsEight intact fresh human cadaver spines (L2-S1) were affixed within a testing frame for in vitro biomechanical testing: four randomly assigned spines for the BAK cage group and four for the Brantigan cage group. For each spine, the three-dimensional load-displacement behavior of each vertebra was quantified using the Selspot II Motion measurement system during the following steps: (1) intact state; (2) destabilization after laminectomy and discectomy across L4-L5; (3) stabilization using a pair of BAK cages or Brantigan cages; and (4) additional stabilization using variable screw plates (VSP) across L4-L5.ResultsThe Brantigan cage alone did not show satisfactory results in improving the stability of one-motion segment destabilized spines in left and right axial rotation. However, the BAK cages appeared to provide significant stability in extension, flexion, left and right lateral bending, and left axial rotation. After implanting the additional posterior instrumentation, both cages provided similar and significantly improved stabilities.ConclusionAlthough the results indicate that the Brantigan cage did not provide satisfactory improvement in the stabilities as the BAK cage in the one-motion segment model, implantation with additional posterior instrumentation may significantly improve the stabilities and reduce the differences between the two cage designs.     

Facet joint violation after open and percutaneous posterior instrumentation: a comparative study

Purpose

Posterior instrumentation is the state-of-the-art surgical treatment for fractures of the thoracic and lumbar spine. Options for pedicle screw placement comprise open or minimally invasive techniques. Open instrumentation causes large approach related muscle detachment, which minimally invasive techniques aim to reduce. However, concerns of accurate pedicle screw placement are still a matter of debate. Beside neurological complications due to pedicle screw malplacement, also affection of the facet joints and thus motion dependent pain is known as a complication. The aim of this study was to assess accuracy of pedicle screw placement concerning facet joint violation (FJV) after open- and minimally invasive posterior instrumentation.

Methods

A retrospective data analysis of postoperative computer tomographic scans of 219 patients (1124 pedicle screws) was conducted. A total of 116 patients underwent open screw insertion (634 screws) and 103 patients underwent minimally invasive, percutaneous screw insertion (490 screws).

Results

In the lumbar spine (segments L3, L4, L5), there were significantly more and higher grade (open = 0.55 vs. percutaneous = 1.2; p = 0.001) FJV’s after percutaneously compared to openly inserted screws. In the thoracic spine, no significant difference concerning rate and grade of FJV was found (p > 0.56).

Conclusion

FJV is more likely to occur in percutaneously placed pedicle screws. Additionally, higher grade FJV’s occur after percutaneous instrumentation. However, in the thoracic spine we didn’t find a significant difference between open and percutaneous technique. Our results suggest a precise consideration concerning surgical technique according to the fractured vertebrae in the light of the individual anatomic structures in the preop CT.

     

腰椎后路手术对其稳定性的影响

目的 从生物力学角度探讨不同的腰椎后路手术对其稳定性的影响。方法 采用新鲜入体脊椎骨制作成双侧开窗、小关节内侧半切除、小关节全切、全椎板减压及全椎板减压合并小关节全切共5种手术模型，利用脊柱三维运动实验系统进行稳定性的测试和分析，并与正常组进行比较。结果 脊椎全椎板切除及双侧开窗对腰椎稳定性影响不显著，小关节部分或者全部切除则显著影响腰椎的侧弯、前屈和旋转活动度。结论 腰椎手术应尽量避免损伤小关节，以维护腰椎稳定性，减少术后并发症的发生。     

钉棒及钩棒系统治疗胸腰椎多节段脊柱骨折

目的评价钉棒及钩棒系统治疗胸腰椎多节段脊柱骨折的临床疗效。方法23例多节段胸腰椎骨折患者，后路切开复位，选择性椎管减压．钉棒或钩棒系统内固定及后外侧植骨融合进行手术治疗。其中相邻多节段型13例，非相邻多节段型8例，混合型2例。结果全组病例平均随访14个月，未发现内固定物松动、断离，无继发性脊柱后凸畸形加重。椎体高度由术前平均48．4％恢复至术后平均92．4%。2例完全性及11例不完全性脊髓损伤者．脊髓神经功能获改善。结论在椎管进行充分减压的基础上．钉棒及钩棒系统能有效复位椎体骨折，重建脊柱稳定性，是多节段胸腰椎不稳定性骨折合并脊髓神经损伤后路手术的理想选择。     

徒手置钉技术在脊柱畸形矫正中的准确性与安全性研究

目的 探究徒手置钉技术在脊柱畸形矫正中的准确性与安全性.方法 回顾性分析本院2012年4月-2012年8月所有因脊柱畸形而接受脊柱后路三维矫形椎弓根内固定手术的36名病例.所有病例术中仅采用徒手置钉技术,均不使用C形臂X线机透视或导航系统等计算机辅助设备,较严重病例或重度后凸需要截骨者使用神经电生理监测.术后行脊柱全长正侧位X线片及CT扫描,通过影像归档和通信系统（picture archiving and communication systems,PACS）查看影像资料,记录皮质穿破的类型及距离.术后密切观察有无血管及神经源性并发症直至出院.结果 36名患者置入螺钉总数为550枚.其中210枚螺钉发生误置,总发生率为38.18%（210/550）.内侧皮质穿破（medial cortical perforation,MCP）的发生率为5.27%（29/550）;外侧皮质穿破（lateral cortical perforation,LCP）的发生率为27.09%（149/550）;椎体前壁穿破（anterior cortical perforation,ACP）的发生率为6.55%（36/550）;上终板穿破（endplate perforation,EPP）的发生率为0.55%（3/550）;椎间孔穿破（foramen perforation,FP）的发生率为0.91%（5/550）.上、中、下胸椎及腰椎的MCP发生率分别为3.39%（2/59）、3.90%（51/128）、5.81%（10/172）、6.28%（12/191）,LCP发生率分别为44.07%（26/59）、50.78%（65/128）、25.00%（43/172）、7.85%（15/191）.MCP在胸椎及腰椎的发生率相近,分别为4.7%（17/359）、6.3%（12/191）,差异无统计义意义（P＞0.05）,而胸椎较腰椎更易发生LCP,发生率分别为 37.3%（134/359）、7.9%（15/191）,差异有统计学意义（P＜0.01）.所有病例术后均无神经、血管及内脏相关并发症出现.结论 徒手置钉技术在脊柱畸形矫正中的应用是安全的,可以避免使用术中透视、计算机导航等带来的不利影响.LCP是最常见的穿破类型,在胸椎的发生率比在腰椎高,因而在胸椎使用计算机图像系统等协助置钉可能更有应用价值.     

Potential contribution of pedicle screw design to loosening rate in patients with degenerative diseases of the lumbar spine: An observational study

BACKGROUNDThe majority of published data report the results of biomechanical tests of various design pedicle screw performance. The clinical relevance and relative contribution of screw design to instrumentation stability have been insufficiently studied.AIMTo estimate the contribution of screw design to rate of pedicle screw loosening in patients with degenerative diseases of the lumbar spine.METHODSThis study is a prospective evaluation of 175 patients with degenerative diseases and instability of the lumbar spine segments. Participants underwent spinal instrumentation employing pedicle screws with posterior only or transforaminal interbody fusion. Follow-up was for 18 mo. Patients with signs of pedicle screw loosening on computed tomography were registered; logistic regression analysis was used to identify the factors that influenced the rate of loosening.RESULTSParameters included in the analysis were screw geometry, type of thread, external and internal screw diameter and helical pitch, bone density in Hounsfield units, number of levels fused, instrumentation without anterior support, laminectomy, and unilateral and bilateral total facet joint resection. The rate of screw loosening decreased with the increment in outer diameter, decrease in core diameter and helical pitch. The rate of screw loosening correlated positively with the number of fused levels and decreasing bone density. Bilateral facet joint removal significantly favored pedicle screw loosening. The influence of other factors was insignificant.CONCLUSIONScrew parameters had a significant impact on the loosening rate along with bone quality characteristics, the number of levels fused and the extensiveness of decompression. The significance of the influence of screw parameters was comparable to those of patient- and surgery-related factors. Pedicle screw loosening was influenced by helical pitch, inner and outer diameter, but screw geometry and thread type were insignificant factors.     

Assessment of different screw augmentation techniques and screw designs in osteoporotic spines

This is an experimental study on human cadaver spines. The objective of this study is to compare the pullout forces between three screw augmentation methods and two different screw designs. Surgical interventions of patients with osteoporosis increase following the epidemiological development. Biomechanically the pedicle provides the strongest screw fixation in healthy bone, whereas in osteoporosis all areas of the vertebra are affected by the disease. This explains the high screw failure rates in those patients. Therefore PMMA augmentation of screws is often mandatory. This study involved investigation of the pullout forces of augmented transpedicular screws in five human lumbar spines (L1–L4). Each spine was treated with four different methods: non-augmented unperforated (solid) screw, perforated screw with vertebroplasty augmentation, solid screw with vertebroplasty augmentation and solid screw with balloon kyphoplasty augmentation. Screws were augmented with Polymethylmethacrylate (PMMA). The pullout forces were measured for each treatment with an Instron testing device. The bone mineral density was measured for each vertebra with Micro-CT. The statistical analysis was performed with a two-sided independent student t test. Forty screws (10 per group and level) were inserted. The vertebroplasty-augmented screws showed a significant higher pullout force (mean 918.5 N, P = 0.001) than control (mean 51 N), the balloon kyphoplasty group did not improve the pullout force significantly (mean 781 N, P > 0.05). However, leakage occurred in some cases treated with perforated screws. All spines showed osteoporosis on Micro-CT. Vertebroplasty-augmented screws, augmentation of perforated screws and balloon kyphoplasty augmented screws show higher pullout resistance than non-augmented screws. Significant higher pullout forces were only reached in the vertebroplasty augmented vertebra. The perforated screw design led to epidural leakage due to the position of the perforation in the screw. The position of the most proximal perforation is critical, depending on screw design and proper insertion depth. Nevertheless, using a properly designed perforated screw will facilitate augmentation and instrumentation in osteoporotic spines.     

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.     

Complex multilevel lumbar spine fractures with transverse sacral fracture

We report an unusual and complex case of spinal trauma in a 17-year-old boy who presented with a transverse sacral fracture associated with multiple-level lumbar fractures, paraparesis, and bladder involvement. A two-stage surgery was performed. The lumbar spine fractures were treated with posterior instrumented correction of displacements, followed by anterior instrumentation and fusion. The sacral fracture was left untreated. At 5-year followup, the patient had complete neurological recovery except for the right L5 root function. The long-segment lumbar fusion and the untreated displaced sacral fracture contributed to spinal imbalance, due to which the patient is now able to stand only in a crouched posture. Determining the optimal treatment for the case is presented due to the relative rarity of transverse sacral fracture and paucity of evidence-based treatment approaches. In patients with associated lumbar spine fractures that require extension of instrumentation to the upper lumbar spine, it is critical to restore sacropelvic alignment to achieve spinal balance. Adequate reduction of sacropelvic anatomy can be achieved with iliac screw fixation.     

Biomechanical considerations of the posterior surgical approach to the lumbar spine

Background contextThe effect of the posterior midline approach to the lumbar spine, relevance of inter- and supraspinous ligament (ISL&SSL) sparing, and potential of different wound closure techniques are largely unknown despite their common use.PurposeThe aim of this study was to quantify the effect of the posterior approach, ISL&SSL resection, and different suture techniques.Study DesignBiomechanical cadaveric study.MethodsFive fresh frozen human torsi were stabilized at the pelvis in the erect position. The torsi were passively loaded into the forward bending position and the sagittal angulation of the sacrum, L4 and T12 were measured after a level-wise posterior surgical approach from L5/S1 to T12/L1 and after a level-wise ISL&SSL dissection of the same sequence. The measurements were repeated after the surgical closure of the thoracolumbar fascia with and without suturing the fascia to the spinous processes.ResultsPassive spinal flexion was increased by 0.8±0.3° with every spinal level accessed by the posterior approach. With each additional ISL&SSL resection, a total increase of 1.6±0.4° was recorded. Suturing of the thoracolumbar fascia reduced this loss of resistance against lumbar flexion by 70%. If the ISL&SSL were resected, fascial closure reduced the lumbar flexion by 40% only. In both settings, suturing the fascia to the spinous processes did not result in a significantly different result (p=.523 and p=.730 respectively).ConclusionEach level accessed by a posterior midline approach is directly related to a loss of resistance against passive spinal flexion. Additional resection of ISL&SSL multiplies it by a factor of two.Clinical SignificanceThe surgical closure of the thoracolumbar fascia can reduce the above mentioned loss of resistance partially. Suturing the fascia to the spinal processes does not result in improved passive stability.     

A new lumbar fixation device alternative to pedicle-based stabilization for lumbar spine: In vitro cadaver investigation

Context: To evaluate the stability provided by a new bilateral fixation technique using an in vitro investigation for posterior lumbar segmental instrumentation.

Design: Experimental cadaver study. In this study, we propose an alternative technique for a posterior lumbar fixation technique called “inferior-oblique transdiscal fixation” (IOTF).

Setting: Study performed at Engineering Center for Orthopedic Research Exellence (ECORE) in Toledo University-Ohio.

Participants: Six human lumbar cadaveric specimen used in this study.

Interventions: In this study, we propose an alternative technique for a posterior lumbar fixation technique called “inferior-oblique transdiscal fixation” (IOTF). As a novel contribution to the classical technique, the entry point of the screw is the supero-lateral point of the intersecting line drawn between the corpus and the pedicle of the upper vertebra. This approach enables the fixation of two adjacent vertebrae using a single screw on each side without utilizing connecting rods.

Outcome Measures: Flexion (Flex), extension (Ext), right and left lateral bending (LB & RB), and right and left axial rotation (LR & RR), and the position data were captured at each load step using the Optotrak motion measurement system and compared for IOTF and posterior transpedicular stabilization.

Results: The Posterior stabilization system (PSS) and IOTF significantly reduced the ROM of L4-L5 segment compared to intact segment’s ROM. During axial rotation (AR) IOTF fused index segment more than PSS. Besides this, addition of transforaminal lumbar interbody fusion (TLIF) cage improved the stabilization of IOTF system during flexion, extension and lateral bending. Whereas, PSS yielded better fusion results during extension compared to IOTF with and without interbody fusion cages.

Conclusions: We hypothesized that the new posterior bilateral system would significantly decrease motion compared to the intact spine. This cadaver study showed that the proposed new posterior fusion technique IOTF fused the index segment in a similar fashion to the classical pedicle screw fusion technique.     

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.     

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.     

Cortical bone trajectory for lumbar pedicle screws

Background contextAchieving solid implant fixation to osteoporotic bone presents a clinical challenge. New techniques and devices are being designed to increase screw–bone purchase of pedicle screws in the lumbar spine via a novel cortical bone trajectory that may improve holding screw strength and minimize loosening. Preliminary clinical evidence suggests that this new trajectory provides screw interference that is equivalent to the more traditionally directed trajectory for lumbar pedicle screws. However, a biomechanical study has not been performed to substantiate the early clinical results.PurposeEvaluate the mechanical competence of lumbar pedicle screws using a more medial-to-lateral path (ie, “cortical bone trajectory”) than the traditionally used path.Study designHuman cadaveric biomechanical study.MethodsEach vertebral level (L1–L5) was dual-energy X-ray absorptiometry (DXA) scanned and had two pedicle screws inserted. On one side, the traditional medially directed trajectory was drilled and tapped. On the contralateral side, the newly proposed cortical bone trajectory was drilled and tapped. After qCT scanning, screws were inserted into their respective trajectories and pullout and toggle testing ensued. In uniaxial pullout, the pedicle screw was withdrawn vertically from the constrained bone until failure occurred. The contralateral side was tested in the same manner. In screw toggle testing, the vertebral body was rigidly constrained and a longitudinal rod was attached to each screw head. The rod was grasped using a hydraulic grip and a quasi-static, upward displacement was implemented until construct failure. The contralateral pedicle screw was tested in the same manner. Yield pullout (N) and stiffness (N/mm) as well as failure moment (N-m) were compared and bone mineral content and bone density data were correlated with the yield pullout force.ResultsNew cortical trajectory screws demonstrated a 30% increase in uniaxial yield pullout load relative to the traditional pedicle screws (p=0.080), although mixed loading demonstrated equivalency between the two trajectories. No significant difference in construct stiffness was noted between the two screw trajectories in either biomechanical test or were differences in failure moments (p=0.354). Pedicle screw fixation did not appear to depend on bone quality (DXA) yet positive correlations were demonstrated between trajectory and bone density scans (qCT) and pullout force for both pedicle screws.ConclusionsThe current study demonstrated that the new cortical trajectory and screw design have equivalent pullout and toggle characteristics compared with the traditional trajectory pedicle screw, thus confirming preliminary clinical evidence. The 30% increase in failure load of the cortical trajectory screw in uniaxial pullout and its juxtaposition to higher quality bone justify its use in patients with poor trabecular bone quality.     

Biomechanical analysis of a novel posterior construct in a transforaminal lumbar interbody fusion model an in vitro study

Background context

Spinal fusion is a commonly performed surgical procedure. It is used to treat a variety of spinal pathologies, including degenerative disease, trauma, spondylolisthesis, and deformities. A mechanically stable spine provides an ideal environment for the formation of a fusion mass. Instrumented spinal fusion allows early ambulation with minimal need for a postoperative external immobilizer. Several biomechanical and clinical studies have evaluated the stability offered by different posterior instrumentation techniques and the effects of reduced instrumentation.

Purpose

The aim of the study was to compare the biomechanics of a novel pedicle and translaminar facet screw (TLFS) construct. Also, in this study, comparisons were made with the more common pedicle screw/TLFS constructs for posterior fixation.

Study design

Human cadaveric lumbar spines were tested in an in vitro flexibility experiment to investigate the biomechanical stability provided by a novel pedicle and TLFS construct after transforaminal lumbar interbody fusion (TLIF).

Methods

Seven fresh human lumbar spines (L2–L5) were tested by applying pure moments of ±8 Nm. After intact specimen testing, a left-sided TLIF with a radiolucent interbody spacer was performed at L3–L4. Each specimen was then tested for the following constructs: bilateral pedicle screws (BPS) and rods at L3–L4; unilateral pedicle screws (UPS) and rods at L3–L4; UPS and rods and TLFS at L3–L4 (UPS+TLFS); and unilateral single pedicle screw and TLFS and rod at L3–L4 (V construct). The L3–L4 range of motion (ROM) and stiffness for each construct were obtained by applying pure moments in flexion, extension, lateral bending, and axial rotation.

Results

All instrumented constructs significantly reduced ROM in flexion-extension and lateral bending compared with the intact specimen. In axial rotation, only BPS constructs significantly reduced ROM compared with intact specimen. The V construct was able to achieve more reduction in ROM compared with UPS construct and was comparable to UPS+TLFS construct. Unilateral pedicle screws construct was the least stable in all loading modes and was significantly different than BPS construct in lateral bending.

Conclusions

The V construct exhibited enhanced stability compared with UPS construct in all loading modes. It provides bilateral fixation and preserves the anatomic integrity of the superior facet joint. The novel construct may offer advantages of less invasiveness, significant reduction in operation time, duration of hospitalization, and costs of implants, which would require further clinical evaluation.     

Effects of misalignment on static torsional strength of anterior cervical plate systems

Background contextThere is little understanding of cervical plate misalignment as a risk factor for plate failure at the plate-screw-bone interface.PurposeTo assess the torsional strength and mode of failure of cervical plates misaligned relative to the midsagittal vertical axis.Study designPlastic and foam model spine segments were tested using static compression and torsion to assess effects of misaligned and various lengths anterior cervical plate (ACPs).MethodsDifferent length ACPs and cancellous fixed angle screws underwent axial torsional testing on a servo-hydraulic test frame at a rate of 0.5°/s. A construct consisted of one ACP, four screws, one ultrahigh–molecular weight polyethylene inferior block, and one polyurethane foam superior block. Group 1 had ACPs aligned in the midsagittal vertical axis, group 2 plates were positioned 20° offset from the midline, and group 3 had the ACP shifted 5 mm away and 20° offset from midline. Torques versus angle data were recorded. The failure criterion was the first sign of pullout determined visually and graphically.ResultsGroup 1 had a more direct screw pullout during failure. For the misaligned plates, failure was a combination of the screws elongating the holes and shear forces acting between the plate and block. The misaligned plates needed more torque to failure. The failure torque was 50% reduced for the longer versus the shorter plates in the neutral position. Graphically shown initial screw slippage inside the block preceded visual identification of slippage in some cases.ConclusionsWe observed different failure mechanisms for neutral versus misaligned plates. Clinically, misalignment may have the benefit of needing more torque to fail. Misalignment was a risk factor for failure of the screw-bone interface, especially in longer plate constructs. These comparisons of angulations may be a solid platform for expansion toward a more applicable in vivo model.     

Avascular necrosis of lumbar facet joints associated with bilateral facet fractures

Background contextAvascular necrosis is a commonly described condition caused by a disruption of blood supply to the bones, resulting in necrosis. Although common in joints of the extremities, it is seen less often in the spine. Risk factors for avascular necrosis include steroid use, alcohol consumption, smoking, scuba diving, thrombosis, hypercoagulability, and hypertension.PurposeThe purpose was to report an unprecedented case of avascular necrosis of the lumbar facet joints and bilateral facet fractures.Study designThis is a case report.MethodsThe patient underwent L3–S1 decompression and L5–S1 discectomy, during which time avascular necrosis of the superior articular process of the L3–L4 facet joints was discovered. The patient then underwent spinal fusion with pedicle screw instrumentation. Pathologic examination of both right and left facet joints confirmed the diagnosis of avascular necrosis.ResultsAt 19-month follow up, the patient's leg and back pain had significantly improved. His spine appeared fused with no instability or implant failure.ConclusionWe have presented a case of avascular necrosis of L3–L4 facet joints resulting in fracture and instability at the L3–L4 level of the spine, which was stabilized with an L3–L4 pedicle screw spinal fusion.