Effect of pedicle screw diameter on screw fixation efficacy in human osteoporotic thoracic vertebrae

The selection of an ideal screw size plays a crucial role in the success of spinal instrumentation as larger diameter screws are thought to provide better fixation strength but increase the risk of pedicle failure during insertion. On the other hand, smaller diameter screws are with lesser risk of pedicle breakage but are thought to compromise the stability of the instrumentation. By investigating the relationship between screw diameter and the pullout strength of pedicle screws after fatigue loading, this study seeks to find quantitative biomechanical data for surgeons in determining the most ideal diameter size screws when performing surgical implementations on osteoporotic vertebrae.Twenty-seven osteoporotic (BMD ranged: 0.353–0.848 g/cm²) thoracic vertebrae (T3-T8) were harvested from 5 human cadavers. Two sizes of poly-axial screws (5.0 mm × 35 and 4.35 mm × 35) were implanted into each pedicles of the vertebrae by an experienced surgeon. Specimens were randomly distributed into control group, fatigue group of 5000 and 10,000 cycles with peak-to-peak loadings of 10–100 N at 1 Hz. Each specimen was then axial pullout tested at a constant rate of 5 mm/min. The ultimate pullout strength (N) & stiffness (N/mm) were obtained for analysis.The results showed that although the larger diameter screws achieved superior pullout strength immediately after the implantation, both sizes of screws exhibited comparable pullout strengths post fatigue loading. This indicates that the smaller diameter screws may be considered for surgical techniques performed on osteoporotic vertebrae for reduced risk of pedicle breakage without sacrificing fixation strength.     

Treatment of Upper Cervical Spine Instability with Posterior Fusion Plus Atlantoaxial Pedicle Screw

Clinical results of posterior fusion plus pedicle screw fixation in the treatment of upper cervical spine instability were taken under consideration. 24 patients with atlantoaxial instability were treated with C1-2 pedicle screws and rods fixation under general anesthesia. There were 18 males and 6 females with mean age of 49.8 years (age range 17–69 years). The postoperative radiographs verified good position of all screws, with satisfactory atlantoaxial reduction. Follow-up for 3–45 months (average 23 months) showed no spinal cord and vertebral artery injury or interfixation failure. Atlantoaxial alignment and stability were restored without complication due to instrumentation. In conclusion, posterior atlantoaxial pedicle screw and rod fixation provide immediate three-dimensional rigid fixation of atlantoaxial joint and are more effective techniques compared with previously reported techniques.     

Spinal pedicle finder for transpedicular screw fixation--design and early clinical result

Spinal transpedicular fixation has gained widespread popularity in the past 5 years. In biomechanical studies, the deeply-inserted transpedicular screws withstood the largest number of cycles in the cephalad-caudad and medial-lateral direction before failure. However, in clinical practice, the risk of screw placement which is too far medially or too far laterally do exist. The optimization of increasing screw depth to avoid complication is of significant clinical importance. A Spinal Pedicle Finder (S.P.F) has been designed for transpedicular screws and a prototype has been completed. It is composed of an I-shaped body with a pair of front rails and a pair of rear rails. The front rail comprises two positioning arms that fit against the laminal bony crest, and the rear rail comprises two guiding bases that provide the transpedicular pin inserted with a specific angle. Both positioning arms and both guiding bases can be adjusted synchronously, and the specific angle over the guiding bases can be pre-set preoperatively according to the angle of pedicle axis. To date, in 7 cases (5 fracture and 2 spondylolisthesis) transpedicular screw fixation has been applied by aid of the S.P.F. Two-level fixation was applied in a fracture group and three-level fixation was applied in a spondylolisthesis group. The position of the transpedicular screw has been checked by CT scan postoperatively. A total of 32 transpedicular screws were inserted and all were in the pedicle and vertebral body except in one instances. One transpedicular screw was malpositioned on one side, partially lateral to the pedicle. However, this malposition did not cause any neurologic problem, such as dural tear, nerve root injury or other. Clinical experience has demonstrated its efficacy and safety.     

Micro-CT and micro-FE analysis of pedicle screw fixation under different loading conditions

Anchorage of pedicle screw instrumentation in the elderly spine with poor bone quality remains challenging. In this study, micro finite element (µFE) models were used to assess the specific influence of screw design and the relative contribution of local bone density to fixation mechanics. These were created from micro computer tomography (µCT) scans of vertebras implanted with two types of pedicle screws, including a full region-or-interest of 10 mm radius around each screw, as well as submodels for the pedicle and inner trabecular bone of the vertebral body. The local bone volume fraction (BV/TV) calculated from the µCT scans around different regions of the screw (pedicle, inner trabecular region of the vertebral body) were then related to the predicted stiffness in simulated pull-out tests as well as to the experimental pull-out and torsional fixation properties mechanically measured on the corresponding specimens. Results show that predicted stiffness correlated excellently with experimental pull-out strength (R² > 0.92, p < .043), better than regional BV/TV alone (R2 = 0.79, p = .003). They also show that correlations between fixation properties and BV/TV were increased when accounting only for the pedicle zone (R² = 0.66–0.94, p ≤ .032), but with weaker correlations for torsional loads (R² < 0.10). Our analyses highlight the role of local density in the pedicle zone on the fixation stiffness and strength of pedicle screws when pull-out loads are involved, but that local apparent bone density alone may not be sufficient to explain resistance in torsion.     

数字化骨科理念辅助椎弓根置钉在寰枢椎不稳中的临床应用

目的:探讨3D打印辅助置钉技术用于寰枢椎不稳椎弓根置钉的安全性及准确性。方法:收集2013年1月到2015年1月西安交通大学第一附属医院收治的寰枢椎不稳病例,术前采用3D打印技术构建个体化3D打印模型,在模型上模拟置钉,获取最佳置钉点、置钉角度等个体化置钉数据,并于术中辅助椎弓根螺钉置入。通过CT扫描评价置钉准确性,测量术前、术后患者寰齿间隙判断寰枢椎复位情况,测量颈延角评价脊髓压迫改善情况,并采用日本骨科学会(JOA)评分判断患者脊髓功能改善情况。术后定期随访观察固定效果、稳定性、神经损伤等手术并发症的发生情况。结果:13例患者均采用3D打印辅助置钉技术进行内固定治疗,手术顺利,术中及术后无血管、神经损伤等并发症,复位及内固定效果满意。共置入椎弓根螺钉31枚,其中29枚完全在椎弓根内,置钉准确率为93.5%。寰枢椎较术前明显复位,术后寰齿间隙、颈延角和JOA评分较术前明显改善,差异具有统计学意义(P0.05)。结论:3D打印技术辅助上颈椎椎弓根置钉的准确性及安全性均较高。     

Trans-Endplate Pedicle Pillar System in Unstable Spinal Burst Fractures: Design,Technique, and Mechanical Evaluation

Background

Short-segment pedicle screw instrumentation (SSPI) is used for unstable burst fractures to correct deformity and stabilize the spine for fusion. However, pedicle screw loosening, pullout, or breakage often occurs due to the large moment applied during spine motion, leading to poor outcomes. The purpose of this study was to test the ability of a newly designed device, the Trans-Endplate Pedicle Pillar System (TEPPS), to enhance SSPI rigidity and decrease the screw bending moment with a simple posterior approach.

Methods

Six human cadaveric spines (T11-L3) were harvested. A burst fracture was created at L1, and the SSPI (Moss Miami System) was used for SSPI fixation. Strain gauge sensors were mounted on upper pedicle screws to measure screw load bearing. Segmental motion (T12-L2) was measured under pure moment of 7.5 Nm. The spine was tested sequentially under 4 conditions: intact; first SSPI alone (SSPI-1); SSPI+TEPPS; and second SSPI alone (SSPI-2).

Results

SSPI+TEPPS increased fixation rigidity by 41% in flexion/extension, 28% in lateral bending, and 37% in axial rotation compared with SSPI-1 (P<0.001), and it performed even better compared to SSPI-2 (P<0.001 for all). Importantly, the bending moment on the pedicle screws for SSPI+TEPPS was significantly decreased 63% during spine flexion and 47% in lateral bending (p<0.001).

Conclusion

TEPPS provided strong anterior support, enhanced SSPI fixation rigidity, and dramatically decreased the load on the pedicle screws. Its biomechanical benefits could potentially improve fusion rates and decrease SSPI instrumentation failure.     

Pedicle Screw Fixation Study in Immature Porcine Spines to Improve Pullout Resistance during Animal Testing

The porcine model is frequently used during development and validation of new spinal devices, because of its likeness to the human spine. These spinal devices are frequently composed of pedicle screws with a reputation for stable fixation but which can suffer pullouts during preclinical implantation on young animals, leading to high morbidity. With a view to identifying the best choices to optimize pedicle screw fixation in the porcine model, this study evaluates ex vivo the impact of weight (age) of the animal, the level of the vertebrae (lumbar or thoracic) and the type of screw anchorage (mono- or bi-cortical) on pedicle screw pullouts. Among the 80 pig vertebrae (90- and 140-day-old) tested in this study, the average screw pullout forces ranged between 419.9N and 1341.2N. In addition, statistical differences were found between test groups, pointing out the influence of the three parameters stated above. We found that the the more caudally the screws are positioned (lumbar level), the greater their pullout resistance is, moreover, screw stability increases with the age, and finally, the screws implanted with a mono-cortical anchorage sustained lower pullout forces than those implanted with a bi-cortical anchorage. We conclude that the best anchorage can be obtained with older animals, using a lumbar fixation and long screws traversing the vertebra and inducing bi-cortical anchorage. In very young animals, pedicle screw fixations need to be bi-cortical and more numerous to prevent pullout.     

脊柱椎弓根螺钉生物力学研究进展

椎弓根螺钉内固定术是目前临床上治疗脊柱骨折的常用手术方式。生物力学特别是脊柱生物力学方面的研究是其内固定器设计和研制的基础,也是评价其固定的稳定性及实用价值性的具体标准。因此,运用生物力学的理论知识来全面分析脊柱的力学改变及内固定器的作用机制,对于合理使用内固定器械以取得最满意矫形和固定效果具有重要意义。但现有实验条件及实验方法依然存在缺陷。例如,所有现有的实验均为体外实验,仅能反映即可实验结果,而且不能反映椎弓根螺钉在体内的长期力学特性。因此,本文通过椎弓根螺钉的自身设计、螺钉的植入技术、椎弓根螺钉的强化以及其他辅助椎弓根螺钉稳定性的辅助系统来全面分析脊柱椎弓根螺钉的生物力学。     

Effect of screw position on load transfer in lumbar pedicle screws: a non-idealized finite element analysis

Angled screw insertion has been advocated to enhance fixation strength during posterior spine fixation. Stresses on a pedicle screw and surrounding vertebral bone with different screw angles were studied by finite element analysis during simulated multidirectional loading. Correlations between screw-specific vertebral geometric parameters and stresses were studied. Angulations in both the sagittal and axial planes affected stresses on the cortical and cancellous bones and the screw. Pedicle screws pointing laterally (vs. straight or medially) in the axial plane during superior screw angulation may be advantageous in terms of reducing the risk of both screw loosening and screw breakage.     

Titanium as an implant material for rods of transpedicular instrumentation of the lumbar spine]

BACKGROUND AND AIM: Titanium alloys are increasingly being used as an implant material in orthopaedics and for spinal instrumentation. In this study a metallographic analysis and mechanical testing were performed to evaluate the resistance of rods of Ti-A16-V4 in particular to tensile forces. METHOD: The surface texture of unprepared Ti-A16-V4 and a rod of the same material for spinal instrumentation were evaluated in a metallographic analysis using light microscopy and electron microscopy. Tensile strength measurements were performed on 2 rods, and the strength of the connection between rod and pedicle screws was tested in 9 cases. An electron microscopic analysis of surface changes of the connections between rod and pedicle screws after loading was performed. RESULTS: The titanium alloy Ti-A16-V4 has a mill-annealed appearance, which has a high resistance to tearing under stress. Titanium rods show high tensile strength before failure under loading. The connection between rod and pedicle screws also as high resistance to tensile loads (> 27 kN) with only little deformation of the connecting surface and no tearing. CONCLUSION: The titanium alloy Ti-A16-V4 is an appropriate material for dorsal spinal instrumentation rods because of its low weight, high biocompability and high tensile strength.     

Assessing the Intraoperative Accuracy of Pedicle Screw Placement by Using a Bone-Mounted Miniature Robot System through Secondary Registration

IntroductionPedicle screws are commonly employed to restore spinal stability and correct deformities. The Renaissance robotic system was developed to improve the accuracy of pedicle screw placement.PurposeIn this study, we developed an intraoperative classification system for evaluating the accuracy of pedicle screw placements through secondary registration. Furthermore, we evaluated the benefits of using the Renaissance robotic system in pedicle screw placement and postoperative evaluations. Finally, we examined the factors affecting the accuracy of pedicle screw implantation.ResultsThrough use of the Renaissance robotic system, the accuracy of Kirschner-wire (K-wire) placements deviating <3 mm from the planned trajectory was determined to be 98.74%. According to our classification system, the robot-guided pedicle screw implantation attained an accuracy of 94.00% before repositioning and 98.74% after repositioning. However, the malposition rate before repositioning was 5.99%; among these placements, 4.73% were immediately repositioned using the robot system and 1.26% were manually repositioned after a failed robot repositioning attempt. Most K-wire entry points deviated caudally and laterally.ConclusionThe Renaissance robotic system offers high accuracy in pedicle screw placement. Secondary registration improves the accuracy through increasing the precision of the positioning; moreover, intraoperative evaluation enables immediate repositioning. Furthermore, the K-wire tends to deviate caudally and laterally from the entry point because of skiving, which is characteristic of robot-assisted pedicle screw placement.     

The utility of superficial abdominal reflex in the initial diagnosis of scoliosis: a retrospective review of clinical characteristics of scoliosis with syringomyelia

Background

The incidence of spinal deformity in children with Prader-Willi syndrome (PWS) is high, with 86% of these patients found to have a significant structural scoliosis; however, there are very few case reports describing surgical treatment for this deformity.

Methods

The authors reviewed a case series consisting of 6 patients who underwent spine surgery for scoliosis. Children's mean age at index surgery was 12 years and 10 months (range, 10 to 15 yrs). Clinical evaluation revealed the typical phenotypic features of the PWS in all of the patients; 4 subjects had a karyotype confirmation of PWS. Major structural curves showed preoperative mean Cobb angles of 80.8° (range, 65° to 96°). Hybrid instrumentation with sublaminar wires, hooks and screws was used in the first 2 patients, while the remaining 4 were treated with titanium pedicle screw constructs.

Results

The mean clinical and radiological follow-up was 3 years and 10 months (range, 2 years to 9 years). Major complication rate was 50%. One patient who developed a major intraoperative complication (paraparesis) prevented spinal fusion to be obtained: the neurologic deficit resolved completely after instrumentation removal. Solid arthrodesis and deformity correction in both coronal and sagittal plane was, however, achieved in the other 5 cases and no significant curve progression was observed at follow-up. Another major short-term complication was encountered 3 months after surgery in a patient who experienced the detachment of a distally located rod and required correction through revision surgery and caudal extension by one level. Cervico-thoracic kyphosis was seen in 1 patient who did not require revision surgery.

Conclusions

Spine reconstructive surgery in patients with PWS is rare and highly demanding. The best method of reconstruction is posterior multilevel pedicle screw fixation. Moreover, even with modern techniques, the risk of complications is still high. These new techniques, however, have shown to improve the postoperative course by allowing for immediate mobilization without any brace or cast. The use of the growing rod techniques, requiring repeated surgeries, should be carefully evaluated in each single case.     

A pedicle screw bridging device for posterior segmental fixation of the spine: preliminary mechanical testing results

Mechanical assessment of a new pedicle screw bridge device for spinal surgery is reported. Results are given for a series of single tests to failure and a fatigue cyclical loading test. Comparative testing of torsional and lateral bending resistance on three surgical spinal fixation systems was carried out: Luque, wired Hartshill rectangle and pedicle screwed bridge with Hartshill rectangle and pedicle screwed bridge with Hartshill rectangle. The results show the superiority of the bridged Hartshill in both rotational and lateral bending resistance. The new bridge device could also improve the versatility of the Hartshill system to cover a wider spectrum of spinal fixations. A test to determine the axial pull-out strength of three screw designs was undertaken. The differences between the forces needed were insignificant. At failure a cylinder of bone tissues greater than the major diameter of the screw was pulled out without breaking the bone.     

Anterior vertebral body screw pullout testing with the hollow modular anchorage system--a comparative in vitro study.

Pullout of implants at the proximal and distal ends of multilevel constructs represents a common spinal surgery problem. One goal concerning the development of new spinal implants is to achieve stable fixation together with the least invasive approach to the spinal column. This biomechanical study measures the influence of different modes of implantation and different screw designs, including a new monocortical system, on the maximum pullout strength of screws inserted ventrolaterally into calf vertebrae. The force pullout of eight different groups were tested and compared. Included were three bicortical used single screws (USS, Zielke-VDS, single KASS). To further increase pullout strength either a second screw (KASS) or a pullout-resistant nut can be added (USS with pullout nut). A completely new concept of anchorage represents the Hollow Modular Anchorage System (MACS-HMA). This hollow titanium implant has an increased outside diameter and is designed for monocortical use. Additionally two screw systems suitable for bicortical use were tested in monocortical mode of anchorage (USS, single KASS). We selected seven vertebrae equal in mean size and bone mineral density for each of the eight groups. The vertebral body and implant were connected to both ends of a servohydraulic testing machine. Displacement controlled distraction was applied until failure at the metal-bone-interface occurred. The maximum axial pullout force was recorded. Mean BMD was 312 +/- 55 mg CaHA/ml in cancellous bone and 498 +/- 98 mg CaHA/ml in cortical bone. The highest resistance to pullout found, measured 4.2 kN (KASS) and 4.0 kN (USS with pullout nut). The mean pullout strength of Zielke-VDS was 2.1 kN, of single KASS 2.5 kN, of MACS-HMA 2.6 kN and of USS 3.2 kN. There was no statistically significant difference (t-test, p > 0.05) between bicortical screws and the new monocortical implant. For the strongest fixation at the proximal or distal end of long spinal constructs the addition of a second screw or a pullout-resistant nut behind the opposite cortex offers even stronger fixation.     

A Robot-Assisted Surgical System Using a Force-Image Control Method for Pedicle Screw Insertion

Objective

To introduce a robot-assisted surgical system for spinal posterior fixation that can automatically recognize the drilling state and stop potential cortical penetration with force and image information and to further evaluate the accuracy and safety of the robot for sheep vertebra pedicle screw placement.

Methods

The Robotic Spinal Surgery System (RSSS) was composed of an optical tracking system, a navigation and planning system, and a surgical robot equipped with a 6-DOF force/torque sensor. The robot used the image message and force signals to sense the different operation states and to prevent potential cortical penetration in the pedicle screw insertion operation. To evaluate the accuracy and safety of the RSSS, 32 screw insertions were conducted. Furthermore, six trajectories were deliberately planned incorrectly to explore whether the robot could recognize the different drilling states and immediately prevent cortical penetration.

Results

All 32 pedicle screws were placed in the pedicle without any broken pedicle walls. Compared with the preoperative planning, the average deviations of the entry points in the axial and sagittal views were 0.50±0.33 and 0.65±0.40 mm, and the average deviations of the angles in the axial and sagittal views were 1.9±0.82° and 1.48±1.2°. The robot successfully recognized the different drilling states and prevented potential cortical penetration. In the deliberately incorrectly planned trajectory experiments, the robot successfully prevented the cortical penetration.

Conclusion

These results verified the RSSS’s accuracy and safety, which supported its potential use for the spinal surgery.     

A new technique for the surgical management of deformities in the growing spine.

The Luque procedure was developed to correct the deformity without the need of bracing and maintaining that correction with growth. However many authors are disappointed by their results and the complications which appear in the management of infantile scoliosis with Luque trolley alone. Besides failed implants, pseudarthrosis, modest spinal growth and protuberant rods and wires, the major problem of the Luque systems is the high incidence of loss of correction by postoperative rotation. Therefore a new application technique is recommended. A standard posterior extraperiostal approach is chosen. Sublaminar titanium cables are passed at each level except the caudal lamina. Then the rods are precontoured in shape of the planed curve correction. We use a low profile titanium instrumentation with 5.0 mm diameter rods and 4.2 mm pedicle screws. In contrast to the conventional use of two antiparallel "L"-rods we recommend the use of one reversed "U"-rod securing the laminae with sublaminar titanium cables of the upper end vertebrae. For fixation of the lower spine a dual-opening pedicle screw system is used. Using a holding forceps the distal rods are introduced and fixed into the side opening of the screws then secured by sublaminar wires. In addition both single rods are stabilized by a low profile cross link bar. This technique allows to correct pelvic obliquity and a stable anchorage of two screws reduces risk of postoperatively rotation or caudal rod slippage due to gravity forces.     

Biomechanical differences of Coflex-F and pedicle screw fixation combined with TLIF or ALIF--a finite element study

Lumbar interbody fusion is a common procedure for treating lower back pain related to degenerative disc diseases. The Coflex-F is a recently developed interspinous spacer, the makers of which claim that it can provide stabilisation similar to pedicle screw fixation. Therefore, this study compares the biomechanical behaviour of the Coflex-F device and pedicle screw fixation with transforaminal lumbar interbody fusion (TLIF) or anterior lumbar interbody fusion (ALIF) surgeries by using finite element analysis. The results show that the Coflex-F device combined with ALIF surgery can provide stability similar to the pedicle screw fixation combined with TLIF or ALIF surgery. Also, the posterior instrumentations (Coflex-F and pedicle screw fixation) combined with TLIF surgery had lower stability than when combined with ALIF surgery.     

Enhanced bone screw fixation with biodegradable bone cement in osteoporotic bone model

Purpose: The purpose of this study was to study the potential of novel biodegradable PCL bone cement to improve bone screw fixation strength in osteoporotic bone. Methods: The biomechanical properties of bone cement (ε-polycaprolactone, PCL) and fixation strength were studied using biomechanical tests and bone screws fixed in an osteoporotic bone model. Removal torques and pullout strengths were assessed for cortical, self-tapping, and cancellous screws inserted in the osteoporotic bone model (polyurethane foam blocks with polycarbonate plate) with and without PCL bone cement. Open cell and cellular rigid foam blocks with a density of 0.12 g/cm3 were used in this model. Results: Removal torques were significantly (more than six-fold) improved with bone cement for cancellous screws. Furthermore, the bone cement improved pullout strengths three to 12 times over depending on the screw and model material.?Conclusions: Biodegradable bone cement turned out to be a very potential material to stabilize screw fixation in osteoporotic bone. The results warrant further research before safe clinical use, especially to clarify clinically relevant factors using real osteoporotic bone under human body conditions and dynamic fatigue testing for long-term performance.     

A medium invasiveness multi-level patient’s specific template for pedicle screw placement in the scoliosis surgery

Background

Several methods including free-hand technique, fluoroscopic guidance, image-guided navigation, computer-assisted surgery system, robotic platform and patient’s specific templates are being used for pedicle screw placement. These methods have screw misplacements and are not always easy to be applied. Furthermore, it is necessary to expose completely a large portions of the spine in order to access fit entirely around the vertebrae.

Methods

In this study, a multi-level patient’s specific template with medium invasiveness was proposed for pedicle screw placement in the scoliosis surgery. It helps to solve the problems related to the soft tissues removal. After a computer tomography (CT) scan of the spine, the templates were designed based on surgical considerations. Each template was manufactured using three-dimensional printing technology under a semi-flexible post processing. The templates were placed on vertebras at four points—at the base of the superior-inferior articular processes on both left–right sides. This helps to obtain less invasive and more accurate procedure as well as true-stable and easy placement in a unique position. The accuracy of screw positions was confirmed by CT scan after screw placement.

Results

The result showed the correct alignment in pedicle screw placement. In addition, the template has been initially tested on a metal wire series Moulage (height 70 cm and material is PVC). The results demonstrated that it could be possible to implement it on a real patient.

Conclusions

The proposed template significantly reduced screw misplacements, increased stability, and decreased the sliding & the intervention invasiveness.