Increase In Pullout Strength Research Articles

Intraoperative lateral wall breach simulation in the cadaveric spine and the impact of thread designs of screws on pullout strength in the osteoporotic thoracic vertebrae: A biomechanical study in human cadavers.

This study aimed (1) to simulate pedicle screw pullout after intraoperative external wall perforation and (2) to assess restoration strength with different thread designs in the pedicle screw instrumentation for osteoporotic thoracic vertebrae. Twenty fresh-frozen human cadaveric thoracic vertebra bodies were prepared and divided into 4 groups: group 1, 5.5 mm × 45 mm polyaxial single thread pedicle screws (PASTS); group 2, after wall injury 5.5 mm × 45 mm PASTS; group 3, 6.5 mm × 45 mm PASTS after wall injury; and group 4: 6.5 mm × 45 mm polyaxial mixed-threaded screws after wall injury. While group 1 was the control group, groups 2, 3, and 4 were used as study groups after the lateral wall breach. All prepared screw units were placed on a universal pullout measurement testing device. The mean bone mineral density for 20 thoracic vertebrae was 0.57 ± 0.12 g/cm2 (range 0.53-0.6 g/cm2 ). The mean pullout strength was 474.90 Newtons (N) for group 1, 412.85 N for group 2, 475.4 N for group 3, and 630.74N for group 4. The lateral wall breach caused a 14.1 % decrease in average pullout strength compared with the initial screw pullout. Mixed (double)-threaded screws increased pullout strength compared to 6.5 mm screws (P=.036) Conclusion: Using a 1 mm thicker polyaxial pedicle screw or mixed (double)-threaded pedicle screw seems to increase pullout strength; however, this was statistically significant only for group 4. In the thoracic spine, the redirection possibility of the pedicle screw is limited, and augmentation with cement will not be appropriate due to the risk of wall injury-related leakage. Therefore, care should be taken to avoid violating the lateral cortex by using appropriate pedicle entry points and trajectories.

Biomechanical and Biological Assessment of Polyglycelrolsebacate-Coupled Implant with Shape Memory Effect for Treating Osteoporotic Fractures.

Poly(glycerol sebacate) is a biocompatible elastomer that has gained increasing attention as a potential biomaterial for tissue engineering applications. In particular, PGS is capable of providing shape memory effects and allows for a free form, which can remember the original shape and obtain a temporary shape under melting point and then can recover its original shape at body temperature. Because these properties can easily produce customized shapes, PGS is being coupled with implants to offer improved fixation and maintenance of implants for fractures of osteoporosis bone. Herein, this study fabricated the OP implant with a PGS membrane and investigated the potential of this coupling. Material properties were characterized and compared with various PGS membranes to assess features such as control of curing temperature, curing time, and washing time. Based on the ISO 10993-5 standard, in vitro cell culture studies with C2C12 cells confirmed that the OP implant coupled with PGS membrane showed biocompatibility and biomechanical experiments indicated significantly increased pullout strength and maintenance. It is believed that this multifunctional OP implant will be useful for bone tissue engineering applications.

Cement Augmentation of Pedicle Screw Instrumentation: A Literature Review.

This literature review aimed to review the current understanding, indications, and limitations of pedicle screw instrumentation cement augmentation. Since they were first reported in the 1980s, pedicle screw cement augmentation rates have been increasing. Several studies have been published to date that describe various surgical techniques and the biomechanical changes that occur when cement is introduced through the screw-bone interface. This article provides a concise review of the uses, biomechanical properties, cost analysis, complications, and surgical techniques used for pedicle screw cement augmentation to help guide physician practices. A comprehensive review of the current literature was conducted, with key studies, and contributions from throughout history being highlighted. Patients with low bone mineral density are the most well-studied indication for pedicle screw cement augmentation. Many studies show that cement augmentation can improve pullout strength in patients with low bone mineral density; however, the benefit varies inversely with pathology severity and directly with technique. The various screw types are discussed, with each having its own mechanical advantages. Cement distribution is largely dependent on the filling method and volume of cement used. Cement composition and timing of cement use after mixing are critical considerations in practice because they can significantly alter the bone-cement and screw-cement interfaces. Overall, studies have shown that pedicle screw cement augmentation has a low complication rate and increased pullout strength, justifying its universal use in patients with a suboptimal bone-implant interface.

Novel Polyethylene Terephthalate Screw Sleeve Implant: Salvage Treatment in a Case of Spine Instability after Vertebroplasty Failure

The management of osteoporotic fractures is sometimes rather challenging for spinal surgeons, and considering the longer life expectancy induced by improved living conditions, their prevalence is expected to increase. At present, the approaches to osteoporotic fractures differ depending on their severity, location, and the patient's age. State-of-the-art treatments range from vertebroplasty/kyphoplasty to hardware-based spinal stabilization in which screw augmentation with cement is the gold standard. We describe the case of a 74-year-old man with an L5 osteoporotic fracture. The patient underwent a vertebroplasty (VP) procedure, which was complicated by a symptomatic cement leakage in the right L4-L5 neuroforamen. We urgently decompressed the affected pedicle via hemilaminectomy. At that point, the column required stability. The extravasation of cement had ruled out the use of cement-augmented pedicle screws but leaving the pedicular screws alone was not considered sufficient to achieve stability. We decided to cover the screws with a polyethylene terephthalate sleeve (OGmend®) to avoid additional cement leakage and to reinforce the screw strength required by the poor bone quality. In the evolving technologies used for spinal surgery, screws sleeve implants such as OGmend® are a useful addition to the surgeon's armamentarium when an increased pull-out strength is required and other options are not available.

Mechanical and Geometric Analysis of Fenestration Design for Polymethylmethacrylate-Augmented Pedicle Screw Fixation.

The practice of cement augmentation in pedicle screw fixation is well established. However, there is a lack of consensus regarding the optimal screw design or cement type. This remains a clinically important question given the incidence of cement augmentation-associated complications. While fenestrated screws have become widely used in clinical practice, the relationship between fenestration placement along the screw axis and cement plume geometry and pullout strength have yet to be clarified. This study was designed to evaluate the mechanical and geometric properties of different fenestrated screw designs and cement viscosities in pedicle screw fixation. Three different screw fenestration configurations and 2 different cement viscosities were examined in this study. Axial pullout tests were conducted in both foam blocks and cadaveric vertebrae. All vertebral specimens underwent tests of bone mineral density. In the foam blocks, 6 tests were conducted for each augmentation combination and also for nonaugmented controls. In the cadaveric testing, 36 lumbar vertebrae were instrumented with a cemented and uncemented control screw to compare features of fixation. Computed tomography (CT) images were taken to assess the geometric profile of the cement plumes in both the foam blocks and the cadaveric vertebrae. In both foam blocks and vertebral specimens, cementation was shown to confer a significant increase in pullout strength. Significant correlations existed between the anterior-posterior and lateral cement plume diameters and pullout strength in cadaveric vertebra and foam blocks, respectively. Within instrumented vertebra, variables such as the width of the vertebral body and screw insertion were found to significantly correlate with enhanced fixation. CT analysis of the instrumented vertebra demonstrated that a centrally distributed pattern of fenestrations was found to result in a cement plume with consistently predictable distribution within the vertebral body, without evidence of leak. Cementation of fenestrated pedicle screws increases overall pullout forces; however, there is an unclear relationship between the geometric properties of the cement plume and the overall strength of the screw-bone interface. This study demonstrates that the plume diameter, vertebral body width, and angle of screw insertion are correlated with enhanced pullout strength. Furthermore, varying the fenestration design of injectable screws resulted in a set of predictable plume patterns, which may be associated with fewer complications. Further investigation is required to clarify the optimal geometric and biomechanical properties of injectable pedicle screws and their role in establishing the cement-bone interface. This study is relevant to currently practicing spinal surgeons and biomechanical engineers.

Laser rapid drilling of bone tissue in minimizing thermal injury and debris towards orthopedic surgery

Traditional mechanical bone drilling is prone to heat generation and debris accumulation which will cause tissue damage in orthopedic surgeries. Therefore, research on contactless laser bone drilling has become a mainstream trend in the development of bone surgery. This study presents a laser rapid drilling strategy for bone surgery with minimizing thermal osteonecrosis and debris based on the design of dynamic focusing and spreading droplet cooling. The laser drilled hole suffered from low temperature (Tmax = 46.2 ○C) under clinically accepted threshold of 47 ○C, presenting osteocyte-filled lacunas, visible Haversian canals and increased pullout strength when compared with that following mechanically drilling. An intact bone column without debris was extracted from a 4-mm-deep hole in vitro on a sheep tibia which provides a clean surgical method with increased drilling rate of 0.94 mm3/s. The criteria dimensionless We constant related to spreading droplets leading to effective heat transfer is determined as2.8 × 103 × Re-1/2 ≤ We ≤ 1.9 × 10-2 × Re7, yielding a high convective coefficient of 0.11 W/mm2·K for cooling. The comprehensive results of laser bone drilling proved the effectiveness of the presented technology and setup, which is significant in realizing rapid, debris-free orthopedic surgery.

A biomimetic engineered bone platform for advanced testing of prosthetic implants

Existing methods for testing prosthetic implants suffer from critical limitations, creating an urgent need for new strategies that facilitate research and development of implants with enhanced osseointegration potential. Herein, we describe a novel, biomimetic, human bone platform for advanced testing of implants in vitro, and demonstrate the scientific validity and predictive value of this approach using an assortment of complementary evaluation methods. We anchored titanium (Ti) and stainless steel (SS) implants into biomimetic scaffolds, seeded with human induced mesenchymal stem cells, to recapitulate the osseointegration process in vitro. We show distinct patterns of gene expression, matrix deposition, and mineralization in response to the two materials, with Ti implants ultimately resulting in stronger integration strength, as seen in other preclinical and clinical studies. Interestingly, RNAseq analysis reveals that the TGF-beta and the FGF2 pathways are overexpressed in response to Ti implants, while the Wnt, BMP, and IGF pathways are overexpressed in response to SS implants. High-resolution imaging shows significantly increased tissue mineralization and calcium deposition at the tissue-implant interface in response to Ti implants, contributing to a twofold increase in pullout strength compared to SS implants. Our technology creates unprecedented research opportunities towards the design of implants and biomaterials that can be personalized, and exhibit enhanced osseointegration potential, with reduced need for animal testing.

114. Dual thread screw design provides equivalent fixation to upsized screw diameter in revision pedicle screw instrumentation

BACKGROUND CONTEXT There are a number of clinical situations that require the replacement of pedicle screw instrumentation. In adjacent segment degeneration and other revision situations, pedicle screw instrumentation is often exchanged in order to accommodate a different sized rod or different pedicle screw system. Although the screws in these revision situations are not overtly loose, the screws have been physiologically loaded for a period of time. Traditionally, pedicle screws are salvaged by up-sizing the core diameter until an interference fit is obtained in the pedicular isthmus. PURPOSE To determine if dual thread screws allow for increased fixation when revising previous pedicle screw instrumentation. STUDY DESIGN/SETTING Cadaveric biomechanical study. PATIENT SAMPLE Not applicable. OUTCOME MEASURES Not applicable. METHODS Pedicle screws were tested in the lumbar vertebrae from ten cadavers. Every pedicle was first tested with a 5.5 mm diameter standard screw using break-in loading. Screws were removed and replaced with one of the following four screw types: 5.5 mm Standard Thread, 5.5 mm Dual Thread, 6.0 mm Standard Thread, or 6.0 mm Dual Thread. The sizes were varied in an S curve fashion over all 10 spines to distribute the screw types over the five lumbar levels (L1 through L5). Failure testing was done using a stepwise increasing cyclic loading protocol for 100 cycles. RESULTS The average pedicle screw cut out load (N) was 133.71 +/- 30.50 and 158.53 +/- 55.28 for standard and dual thread screws, respectively. There was significantly increased pullout strength with dual thread screws compared to the standard thread design (p = 0.001). When comparing 5.5 mm dual thread screws to 6.0 mm standard thread screws, there was no difference in ultimate strength (137.3 +/- 32.47 vs 142.86 +/- 56.50, respectively). CONCLUSIONS In cases requiring replacement of pedicle screws, both upsizing core diameter and utilizing dual thread design may increase bony purchase. The was no difference in upsizing core diameter by 0.5 mm compared with changing screw pitch to a dual thread design. FDA DEVICE/DRUG STATUS This abstract does not discuss or include any applicable devices or drugs.

Thermosensitive Chitosan-Gelatin-Glycerol Phosphate Hydrogels as Collagenase Carrier for Tendon-Bone Healing in a Rabbit Model.

Healing of an anterior cruciate ligament graft in bone tunnel yields weaker fibrous scar tissue, which may prolong an already prolonged healing process within the tendon–bone interface. In this study, gelatin molecules were added to thermosensitive chitosan/β-glycerol phosphate disodium salt hydrogels to form chitosan/gelatin/β-glycerol phosphate (C/G/GP) hydrogels, which were applied to 0.1 mg/mL collagenase carrier in the tendon–bone junction. New Zealand white rabbit’s long digital extensor tendon was detached and translated into a 2.5-mm diameter tibial plateau tunnel. Thirty-six rabbits underwent bilateral surgery and hydrogel injection treatment with and without collagenase. Histological analyses revealed early healing and more bone formation at the tendon–bone interface after collagenase partial digestion. The area of metachromasia significantly increased in both 4-week and 8-week groups after collagenase treatment (p < 0.01). Micro computed tomography showed a significant increase in total bone volume and bone volume/tissue volume in the 8 weeks after collagenase treatment, compared with the control group. Load-to-failure was significantly higher in the treated group at 8 weeks (23.8 ± 8.13 N vs 14.3 ± 3.9 N; p = 0.008). Treatment with collagenase digestion resulted in a 66% increase in pull-out strength. In conclusion, injection of C/G/GP hydrogel with collagenase improves tendon-to-bone healing in a rabbit model.

A Prospective Study on the Feasibility, Safety, and Efficacy of a Modified Technique to Augment the Strength of Pedicle Screw in Osteoporotic Spine Fixation.

Study DesignProspective case study.PurposeOsteoporotic spine fixation by pedicle screw instrumentation is complicated by screw loosening, migration, or pullout with rates of up to 62% documented in the literature. Contemporary solutions have not adequately addressed these complications. We propose a modified surgical technique of cement augmentation with bicortical pedicle screw fixation to address the issue related to implant failure in osteoporotic spine.Overview of LiteratureZindrick and his colleagues described a “windshield wiper” effect owing to the shift of center of rotation to the distal tip of the screw in the bicortical purchase of screws. An increase in pullout strength from 119% to 250% with polymethyl methacrylate augmentation has been documented in the literature. This technique has not been described in the literature.MethodsA prospective study was conducted with 40 patients who underwent surgery by the modified technique. Intraoperative and postoperative complications directly related to the procedure were assessed. Improvement in pain and functional status were assessed. Follow-up radiographs were assessed to check for appreciable screw migration, loosening, or pullout.ResultsThis technique was used in inserting 364 screws in 40 patients. We did not encounter any difficulty in inserting the screws. A total of 19 screws failed to breach the anterior cortex owing to an error in measurement. There were no complications during the procedure in any of the patients, and the postoperative period was uneventful. The mean follow-up period was 18 months. There were two patients in whom proximal junctional failure with kyphosis was noted during follow-up, who were surgically managed by extension of the fixation levels.ConclusionsBicortical fixation with cement augmentation is a technically feasible, safe, and effective technique to augment the strength of pedicle screws in osteoporotic spine fixation. It has the potential to be established as a standard of care in osteoporotic spine fixation.

Non-locking screw insertion: No benefit seen if tightness exceeds 80% of the maximum torque

BackgroundMillions of non-locking screws are manually tightened during surgery each year, but their insertion frequently results in overtightening and damage to the surrounding bone. We postulated that by calculating the torque limit of a screw hole, using bone and screw properties, the risk of overtightening during screw insertion could be reduced. Additionally, predicted maximum torque could be used to identify optimum screw torque, as a percentage of the maximum, based on applied compression and residual pullout strength. MethodsLongitudinal cross-sections were taken from juvenile bovine tibial diaphyses, a validated surrogate of human bone, and 3.5 mm cortical non-locking screws were inserted. Fifty-four samples were used to define the association between stripping torque and cortical thickness. The relationship derived enabled prediction of insertion torques representing 40 to 100% of the theoretical stripping torque (Tstr) for a further 170 samples. Screw-bone compression generated during insertion was measured, followed immediately by axial pullout testing. FindingsScrew-bone compression increased linearly with applied torque up to 80% of Tstr (R2 = 0.752, p < 0.001), but beyond this, no significant further compression was generated. After screw insertion, with all screw threads engaged, more tightening did not create any significant (R2 = 0.000, p = 0.498) increase in pullout strength. InterpretationIncreasing screw tightness beyond 80% of the maximum did not increase screw-bone compression. Variations in torques below Tstr, did not affect pullout forces of inserted screws. Further validation of these findings in human bone and creation of clinical guidelines based on this research approach should improve surgical outcomes and reduce operative costs.

Development Strength of Headed Reinforcing Bars for Steel Fiber Reinforced Concrete by Pullout Test

In order to compare the development performance of headed reinforcing bar and straight reinforcing bar in tension for steel fiber reinforced concrete (SFRC), pullout test of specimens with reinforcing bar which was anchored on simple beam perpendicularly was conducted. The experimental variables were steel fiber volume ratio (VRsf), concrete compressive strength, and existence of head. As the result of test, splitting failure of concrete in the development direction of reinforcing bar in most specimens was observed. For development detail of headed reinforcing deformation bar, specimens with 1% VRsf showed approximately 63%~119% increase in pullout strength compare to specimens with 0% VRsf. Test result shows that SFRC is more effective in increasing pullout strength for headed reinforcing bars than increasing pullout strength of straight bars.

The fates of pedicle screws and functional outcomes ina geriatric population following polymethylmethacrylate augmentation fixation for the osteoporotic thoracolumbar and lumbar burst fractures with mean ninety fivemonth follow-up.

Polymethylmethacrylate (PMMA) augmentation is a common method to increase pullout strength fixed for osteoporotic spines. However, few papers evaluated whether these pedicle screws migrated with time and functional outcome in these geriatrics following PMMA-augmented pedicle screw fixation. From March 2006 to September 2008, consecutive 64 patients were retrospectively enrolled. VAS and ODI were used to evaluate functional outcomes. Kyphotic angle at instrumented levels and horizontal and vertical distances (HD and VD) between screw tip and anterior and upper cortexes were evaluated. To avoid bias, we used horizontal and vertical migration index (HMI and VMI) to re-evaluate screw positions with normalization by the mean of superior and inferior endplates or anterior and posterior vertebral body height, respectively. Forty-six patients with 282 PMMA-augmented screws were analyzed with mean follow-up of 95months. Nine patients were further excluded due to bed-ridden at latest follow-up. Twenty-six females and 11 males with mean T score of - 2.7 (range, - 2.6 to - 4.1) and mean age for operation of 77.6 ± 4.3years (range, 65 to 86). The serial HD and kyphotic angle statistically progressed with time. The serial VD did not statistically change with time (p= 0.23), and neither HMI nor VMI (p = 0.772 and 0.631). Pre-operative DEXA results did not correlate with kyphotic angle. Most patients (80.4%) maintained similar functional outcomes at latest follow-up. The incidence of screws loosening was 2.7% of patients and 1.4% of screws, respectively. The overall incidences of systemic post-operative co-morbidities were 24.3% with overall 20.2days for hospitalization. Most patients (80%) remained similar functional outcomes at latest follow-up in spite of kyphosis progression. The incidence of implant failure was not high, but the post-operative systemic co-morbidities were higher, which has to be informed before index surgery.

The quantity of bone cement influences the anchorage of augmented pedicle screws in the osteoporotic spine: A biomechanical human cadaveric study

Background: The aim of this comparative biomechanical human cadaveric study was to investigate the anchorage of augmented screws with two different volumes of bone cement. For this purpose the effect of cranio-caudal loadings on pedicle screws was evaluated and axial pullout tests were performed.Methods: A total of 50 pedicle screws (25 augmented/25 non-augmented) were instrumented into osteoporotic vertebra of fresh human cadavers. The augmented screws were grounded by two different volumes of bone cement (1.5cm3 vs 4cm3). Biomechanical performance was assessed by performing a cyclic loading protocol (frequency: 3Hz, load range: 20–200N, number of cycles: 100,000), followed by axial pullout (13 augmented/11 non-augmented) or by either directly measuring axial pullout strength (12 augmented/12 non-augmented).Findings: The median T-score of the specimens was −4.25 (range: −6.38 to −2.4). Pullout tests with and without cyclic preloading showed significantly increased pullout strength in augmented screws (Fmax: augmented: 1159N (SD 395N); non-augmented: 532N (SD 297N); p<0.05). No significant difference in the pullout strength was found concerning the quantity of cement (Fmax (direct pullout): 4.0cm3: 1463N (SD 307N); 1.5cm3: 1214N (SD 236N); p>0.05). The pullout strength significantly decreased in high-volume augmented screws after cyclic loading (Fmax (4.0cm3): direct pullout 1463N (SD 307N); cyclic preload: 902N (SD 435N); p<0.05).Interpretation: Biomechanical advantages of augmented pedicle screws can also be found after cyclic preload. However, our results indicate that the anchoring stability of high-volume augmented pedicle screws after cyclic loading is disadvantageous compared to moderate augmented screws; thus high-volume augmentation should be avoided.

Time course of peri-implant bone regeneration around loaded and unloaded implants in a rat model

The time-course of cancellous bone regeneration surrounding mechanically loaded implants affects implant fixation, and is relevant to determining optimal rehabilitation protocols following orthopaedic surgeries. We investigated the influence of controlled mechanical loading of titanium-coated polyether-ether ketone (PEEK) implants on osseointegration using time-lapsed, non-invasive, in vivo micro-computed tomography (micro-CT) scans. Implants were inserted into proximal tibial metaphyses of both limbs of eight female Sprague-Dawley rats. External cyclic loading (60 or 100 μm displacement, 1 Hz, 60 s) was applied every other day for 14 days to one implant in each rat, while implants in contralateral limbs served as the unloaded controls. Hind limbs were imaged with high-resolution micro-CT (12.5 μm voxel size) at 2, 5, 9, and 12 days post-surgery. Trabecular changes over time were detected by 3D image registration allowing for measurements of bone-formation rate (BFR) and bone-resorption rate (BRR). At day 9, mean %BV/TV for loaded and unloaded limbs were 35.5 ± 10.0% and 37.2 ± 10.0%, respectively, and demonstrated significant increases in bone volume compared to day 2. BRR increased significantly after day 9. No significant differences between bone volumes, BFR, and BRR were detected due to implant loading. Although not reaching significance (p = 0.16), an average 119% increase in pull-out strength was measured in the loaded implants. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:997-1006, 2017.

Influence of bone morphological properties on a new expandable orthopaedic fastener

Previous studies have demonstrated that bone morphological properties are a significant determinant of orthopaedic fastener fixation strength. The authors previously tested a new design of unthreaded expandable fastener (UEF) prototype against screws and demonstrated a significant increase in pull-out strength. However the effect of bone morphology on the pull-out strength of the UEF and expandable fasteners in general is unknown. This study assessed the correlation between failure force and maximum force against five microstructural parameters. The failure force of the UEF was correlated to the trabecular bone volume fraction, as with screws. Unlike screws, however, the maximum force of the UEF has an inverse relationship with cortical volume. No correlation was found between failure force and the Structural Model Index (SMI). Additionally the critical volume of interest (VOI) for the UEF is around the bottom of the fastener where the expansion occurs, whereas for the screw a full height VOI is critical. Furthermore, we observed that screw mechanical performance may be affected more by bone morphological parameters that are associated with poorer quality bone. Therefore the UEF may perform better than screws in low quality osteoporotic bone.

Comparative Analysis of Effect of Density, Insertion Angle and Reinsertion on Pull-Out Strength of Single and Two Pedicle Screw Constructs Using Synthetic Bone Model.

Study DesignBiomechanical study.PurposeTo determine the effect of density, insertion angle and reinsertion on pull-out strength of pedicle screw in single and two screw-rod configurations.Overview of LiteraturePedicle screw pull-out studies have involved single screw construct, whereas two screws and rod constructs are always used in spine fusions. Extrapolation of results using the single screw construct may lead to using expensive implants or increasing the fusion levels specifically in osteoporotic bones.MethodsSingle screw and two screw pull-out strength tests were carried out according to American Society for Testing and Materials F 543-07 on foam models to test the effect of density, insertion angle and reinsertion using poly axial pedicle screws.ResultsBone density was the most significant factor deciding the pull-out strength in both single and two screw constructs. The difference in pull-out strength between single screw and two screw configurations in extremely osteoporotic bone model (80 kg/m3) was 78%, whereas in the normal bone model it was 48%. Axial pull-out value was highest for the single screw configuration; in the two screw configuration the highest pull-out strength was at 10°–15°. There was an 18% reduction in pull-out strength due to reinsertion in single screw configuration. The reinsertion effect was insignificant in the two screw configuration.ConclusionsA significant difference in response of various factors on holding power of pedicle screw between single and two-screw configurations is evident. The percentage increase in pull-out strength between single and two screw constructs is higher for osteoporotic bone when compared to normal bone. Reinsertion has no significant effect on pull-out strength in the two screw rod configuration.

The effect of insertion technique and surgeon experience on the pullout strength of orthopaedic screws

Background: We tested the null hypothesis that the manner of orthopaedic screw insertion (hand-driven or electrically-driven) and the experience of the screw inserter (orthopaedic trainees or experienced orthopaedic surgeons) have no effect on the pullout strength of orthopaedic screws. Methods: Four fellowship-trained orthopaedic surgeons and four orthopaedic chief residents each inserted 10 cancellous bone screws into a polyurethane foam cancellous bone substitute. Five screws were inserted with a standard orthopaedic screw driver, and five screws were inserted with a battery-powered electric driver. Samples were loaded to failure, and pullout strength was measured using a electromechanical tensile tester. Analysis of variance was used to compare experimental groups. Results: No significant difference was found between the pullout strengths of screws inserted by hand and the pullout strengths of screws inserted with an electric driver. Screws inserted by experienced orthopaedic surgeons, however, demonstrated significantly higher pullout strengths than screws inserted by orthopaedic trainees. Conclusions: Hand-driven and electrically-driven screws demonstrate similar pullout strengths in a model for orthopaedic screw insertion into bone. Screws inserted by experienced surgeons, however, did exhibit increased pullout strength. This suggests a technical nuance that allows experienced surgeons to better insert screws. If identified, this nuance would be of interest to orthopaedists in general.

Bond characteristics of straight- and headed-end, ribbed-surface, GFRP bars embedded in high-strength concrete

Glass Fiber Reinforced Polymer (GFRP) bars as a proper substitute for traditional reinforcing steel bars not only eliminate the durability problem due to corrosion of reinforcing steel, but also provide remarkably enhanced capacity due to their high tensile strength compared to that of the steel bars. This paper presents the experimental findings of 180 pullout tests conducted on GFRP bars embedded into high-strength concrete blocks covering different parameters. The studied parameters were bar diameter size (12 or 16mm), embedment length (4 or 6 times the bar diameter), bar end condition (straight and headed), and concrete cover (1.5, 2.5, and 5 or 7 times bar diameter for straight bars and 8 or 10.5 times bar diameter for headed bars) in addition to a case of no embedment length except the head length for headed-end bars. In total, 30 variables were studied, while each variable was conducted on 6 identical specimens in order to increase the reliability of the results. Based on the results of the parametric study, the bond stress was shown to be inversely proportional to the embedment length and bar diameter as expected. In addition, the smaller concrete cover appeared to have significant effect on bond stress, leading to side blow-out failure rather than bar pullout or concrete splitting in the case of headed-end GFRP bars. In addition, the GFRP bar with headed-end showed significant increase in pullout strength compared to that for the straight-end bars. Finally, an empirical expression was proposed to calculate the development length of GFRP bars with either straight or headed-end, and then compared with the available design standards such as CSA-S806-02, CSA S6-06, ACI 440-1R-06, and JSCE-97. The comparison showed that the results developed by CSA S6-06 standards was the closest to the experimental findings showed about 2% safety margin exceeding the obtained development length by the proposed expression.

Balancing Rigidity and Safety of Pedicle Screw Fixation via a Novel Expansion Mechanism in a Severely Osteoporotic Model

Many successful attempts to increase pullout strength of pedicle screws in osteoporotic bone have been accompanied with an increased risk of catastrophic damage to the patient. To avoid this, a single-armed expansive pedicle screw was designed to increase fixation strength while controlling postfailure damage away from the nerves surrounding the pedicle. The screw was then subsequently tested in two severely osteoporotic models: one representing trabecular bone (with and without the presence of polymethylmethacrylate) and the other representing a combination of trabecular and cortical bone. Maximum pullout strength, stiffness, energy to failure, energy to removal, and size of the resulting block damage were statistically compared among conditions. While expandable pedicle screws produced maximum pullout forces less than or comparable to standard screws, they required a higher amount of energy to be fully removed from both models. Furthermore, damage to the cortical layer in the composite test blocks was smaller in all measured directions for tests involving expandable pedicle screws than those involving standard pedicle screws. This indicates that while initial fixation may not differ in the presence of cortical bone, the expandable pedicle screw offers an increased level of postfailure stability and safety to patients awaiting revision surgery.