Axial Pullout Tests Research Articles

Pullout strength of monocortical and bicortical screws in metaphyseal and diaphyseal regions of the canine humerus.

Monocortical screws are commonly employed in locking plate fixation, but specific recommendations for their placement are lacking and use of short monocortical screws in metaphyseal bone may be contraindicated. Objectives of this study were to evaluate axial pullout strength of two different lengths of monocortical screws placed in various regions of the canine humerus compared to bicortical screws, and to derive cortical thickness and bone density values for those regions using quantitative computed tomography analysis (QCT). The QCT analysis was performed on 36 cadaveric canine humeri for six regions of interest (ROI). A bicortical, short monocortical, or 50% transcortical 3.5 mm screw was implanted in each ROI and axial pullout testing was performed. Bicortical screws were stronger than monocortical screws in all ROI except the lateral epicondylar crest. Short monocortical metaphyseal screws were weaker than those placed in other regions. The 50% transcortical screws were stronger than the short monocortical screws in the condyle. A linear relationship between screw length and pullout strength was observed. Cortical thickness and bone density measurements were obtained from multiple regions of the canine humerus using QCT. Use of short monocortical screws may contribute to failure of locking plate fixation of humeral fractures, especially when placed in the condyle. When bicortical screw placement is not possible, maximizing monocortical screw length may optimize fixation stability for distal humeral fractures.

Biomechanical Comparison of Expansive Pedicle Screw and Polymethylmethacrylate-augmented Pedicle Screw in Osteoporotic Synthetic Bone in Primary Implantation: An Experimental Study.

Expansive pedicle screws (EPS) and polymethylmethacrylate-augmented pedicle screws (PMMA-PS) were inserted into osteoporotic synthetic bones, which were then tested by radiographic and biomechanical examinations. To compare the stability of EPS and PMMA-PS with that of a conventional pedicle screw (CPS) in an osteoporotic synthetic bone. It is a significant challenge for orthopedic surgeons performing transpedicular fixation in the osteoporotic spine. Prior studies have suggested that both EPS and PMMA-PS can increase the screw stability effectively. However, there are no biomechanical comparisons of EPS and PMMA-PS, especially in primary spinal surgery in osteoporosis. Thirty osteoporotic synthetic bone blocks were divided into 3 groups randomly. A pilot hole was prepared in advance in all samples by the same method. Then, the CPS was inserted directly into the pilot hole in the CPS group; the hole in the PMMA-PS group was first filled with polymethylmethacrylate (PMMA; 2.5 mL) and then inserted with CPS, and the EPS was inserted directly into the blocks in the EPS group. Twenty-four hours later, x-ray and computed tomography examination and axial pullout tests were performed on all samples; the block destructions were then recorded, and the hole diameters were measured. In the CPS group, the screw was surrounded directly by the synthetic bone without any other materials, whereas in the PMMA-PS group, the screw was totally wrapped up by PMMA, and the PMMA was evenly distributed in the synthetic bone around the screw, indicating obvious improvement of the local density around the track. In the EPS group, the anterior part of the EPS presented an obvious expansion in synthetic bone and formed an unguiform structure pressing the surrounding synthetic bone. Screw stabilities in both the PMMA-PS and the EPS groups were significantly enhanced compared with those in the CPS group, and the screw stability in the PMMA-PS group was significantly higher than that in the EPS group. After the pullout tests, the block destructions were the most severe in the PMMA-PS group and the lightest in the CPS group. Hole diameters in the PMMA-PS and the EPS groups were significantly larger than that in the CPS group, whereas the diameter of the hole in the PMMA-PS group was significantly greater than that in the EPS group. EPS can significantly increase the strength of screw fixation compared with CPS in osteoporotic synthetic bone. Although EPS shows a weaker fixation strength compared with PMMA-PS in the osteoporotic synthetic bone, it may still provide an alternative option to prevent screw loosening in the clinical treatment of osteoporosis.

Biomechanical comparison of pedicle screw augmented with different volumes of polymethylmethacrylate in osteoporotic and severely osteoporotic cadaveric lumbar vertebrae: an experimental study

Background ContextPolymethylmethacrylate (PMMA) is widely used for pedicle screw augmentation in osteoporosis. Intriguingly, there have been no biomechanical comparisons of the stability of pedicle screws augmented with different volumes of PMMA or studies of the relationship between screw stability and volume of PMMA, especially in different degrees of osteoporosis. PurposeThe purposes of the study reported here were to compare screw stability by different volumes of PMMA augmentation, to analyze the relationship between screw stability and PMMA volume, and to make a preliminary determination of the optimum volume of PMMA augmentation for different degrees of osteoporosis. Study DesignThis study is a biomechanical comparison of pedicle screws augmented with various volumes of PMMA in cadaveric lumbar vertebrae. MethodsThirty-six pedicles from 18 osteoporotic lumbar vertebrae were randomly divided into groups A0 through A5, and 36 pedicles from 18 severely osteoporotic lumbar vertebrae were randomly divided into groups B0 through B5. A different volume of PMMA was injected into each one of groups A0 through A5 (0, 0.5, 1.0, 1.5, 2.0, and 2.5 mL, respectively) and into each one of groups B0 through B5 (0, 1.0, 1.5, 2.0, 2.5, and 3.0 mL, respectively), and then pedicle screws were inserted in all vertebrae. After complete solidification of the PMMA, we examined pedicle X-rays, performed axial pullout tests, and determined the maximum axial pullout strength (Fmax) for all samples. ResultsNo PMMA was found around the screws in groups A0 and B0. In groups A1 to A5 and B1 to B5, screws were wrapped by gradually increasing amounts of PMMA. There was no PMMA leakage or screw malpositioning in any samples. The Fmax in groups A1 through A5 increased by 32.40%, 64.42%, 116.02%, 174.07%, and 207.42%, respectively, compared with that in group A0. There were no significant differences in Fmax between groups A0 and A1, A1 and A2, A2 and A3, A3 and A4, and A4 and A5 (p>.05), but there were significant differences in Fmax between any other two groups (p<.05). The Fmax in groups B1 through B5 increased by 23.48%, 48.40%, 106.60%, 134.73%, and 210.04%, respectively, compared with that in group B0. There were no significant differences in Fmax between groups B0 and B1, B0 and B2, B1 and B2, B2 and B3, B3 and B4 (p>.05), but there were significant differences in Fmax between any other two groups (p<.05). There was a significant positive correlation between Fmax and volume of PMMA in both osteoporotic and severely osteoporotic lumbar vertebrae (p<.05). ConclusionsPolymethylmethacrylate can significantly enhance stability of pedicle screws in both osteoporotic and severely osteoporotic lumbar vertebrae. There is a significant positive correlation between screw stability and volume of PMMA. Within a certain range, nevertheless, increasing the volume of PMMA does not significantly improve screw stability. We suggest that 1.5 and 3 mL, respectively, are the volumes of injected PMMA that will optimize pedicle screw stability in osteoporotic and severely osteoporotic lumbar vertebrae.

Augmented PMMA distribution: improvement of mechanical property and reduction of leakage rate of a fenestrated pedicle screw with diameter-tapered perforations.

OBJECTIVE This study investigated the optimum injection volume of polymethylmethacrylate (PMMA) to augment a novel fenestrated pedicle screw (FPS) with diameter-tapered perforations in the osteoporotic vertebral body, and how the distribution characteristics of PMMA affect the biomechanical performance of this screw. METHODS Two types of FPSs were designed (FPS-A, composed of 6 perforations with an equal diameter of 1.2 mm; and FPS-B, composed of 6 perforations each with a tapered diameter of 1.5 mm, 1.2 mm, and 0.9 mm from tip to head. Each of 28 human cadaveric osteoporotic vertebrae were randomly assigned to 1 of 7 groups: FPS-A1.0: FPS-A+1.0 ml PMMA; FPS-A1.5: FPS-A+1.5 ml PMMA; FPS-A2.0: FPS-A+2.0 ml PMMA; FPS-B1.0: FPS-B+1.0 ml PMMA; FPS-B1.5: FPS-B+1.5 ml PMMA; FPS-B2.0: FPS-B+2.0 ml PMMA; and conventional pedicle screws (CPSs) without PMMA. After the augmentation, 3D CT was performed to assess the cement distribution characteristics and the cement leakage rate. Axial pullout tests were performed to compare the maximum pullout force thereafter. RESULTS The CT construction images showed that PMMA bone cement formed a conical mass around FPS-A and a cylindrical mass around FPS-B. When the injection volume was increased from 1.0 ml to 2.0 ml, the distribution region of the PMMA cement was enlarged, the PMMA was distributed more posteriorly, and the risk of leakage was increased. When the injection volume reached 2.0 ml, the risk of cement leakage was lower for screws having diameter-tapered perforations. The pullout strengths of the augmented FPS-A groups and FPS-B groups were higher than that of the CPS group (p < 0.0001). All FPS-B groups had a higher pullout strength than the FPS-A groups. CONCLUSIONS The diameter of the perforations affects the distribution of PMMA cement. The diameter-tapered design enabled PMMA to form larger bone-PMMA interfaces and achieve a relatively higher pullout strength, although statistical significance was not reached. Study results indicated 1.5-ml of PMMA was a conservative volume for PMMA augmentation; more cement injection would significantly increase the risk of cement leakage.

Pedicle Screw Fixation Under Nonaxial Loads: A Cadaveric Study.

An experimental study of pedicle screw fixation in human cadaveric vertebrae. The aim of this study was to experimentally characterize pedicle screw fixation under nonaxial loading and to analyze the effect of the surgeons' screw and placement choices on the fixation risk of failure. Pedicle screw fixation performance is traditionally characterized with axial pullout tests, which do not fully represent the various tridimensional loads sustained by the screws during correction maneuvers of severe spinal deformities. Previous studies have analyzed the biomechanics of nonaxial loads on pedicle screws, but their effects on the screw loosening mechanisms are still not well understood. A design of experiment (DOE) approach was used to evaluate 2 screw thread designs (single-lead and dual-lead threads), 2 insertion trajectories in the transverse and sagittal planes, and 2 loading directions (lateral and cranial). Pedicle screws were inserted in both pedicles of 12 cadaveric lumbar vertebrae for a total of 24 tests. Four sinewave loading cycles (0-400 N) were applied, orthogonally to the screw axis, at the screw head. The resulting forces, displacements, and rotations of the screws were recorded. In comparison to the other cycles, the first loading cycle revealed important permanent deformation of the bone (mean permanent displacement of the screw head of 0.79 mm), which gradually accumulated over the following cycles to 1.75 mm on average (plowing effect). The cranial loading direction caused significantly lower (P < 0.05) bone deformation than lateral loading. The dual-lead screw had a significantly higher (P < 0.05) initial stiffness than the single-lead thread screw. Nonaxial loads induce screw plowing that lead to bone compacting and subsequent screw loosening or even bone failure, thus reducing the pedicle screw fixation strength. Lateral loads induce greater bone deformation and risks of failure than cranial loads. N/A.

Biomechanical Comparison of Pedicle Screw Augmented with Different Volumes of Polymethylmethacrylate in Osteoporotic and Severely Osteoporotic Synthetic Bone Blocks in Primary Implantation: An Experimental Study.

This study was designed to compare screw stabilities augmented with different volumes of PMMA and analyze relationship between screw stability and volume of PMMA and optimum volume of PMMA in different bone condition. Osteoporotic and severely osteoporotic synthetic bone blocks were divided into groups A0-A5 and B0-B5, respectively. Different volumes of PMMA were injected in groups A0 to A5 and B0 to B5. Axial pullout tests were performed and F max was measured. F max in groups A1-A5 were all significantly higher than group A0. Except between groups A1 and A2, A3 and A4, and A4 and A5, there were significant differences on F max between any other two groups. F max in groups B1-B5 were all significantly higher than group B0. Except between groups B1 and B2, B2 and B3, and B4 and B5, there were significant differences on F max between any other two groups. There was significantly positive correlation between F max and volume of PMMA in osteoporotic and severely osteoporotic blocks. PMMA can significantly enhance pedicle screw stability in osteoporosis and severe osteoporosis. There were positive correlations between screw stability and volume of PMMA. In this study, injection of 3 mL and 4 mL PMMA was preferred in osteoporotic and severely osteoporotic blocks, respectively.

In-vivo comparison of chemically polished vs. unpolished magnesium-based screws in ovine tibiae

Event Abstract Back to Event In-vivo comparison of chemically polished vs. unpolished magnesium-based screws in ovine tibiae Leopold Berger1, Johannes Eichler2, Martina Cihova3, Elisabeth Martinelli2, Claudia Kleinhans2, Jörg Löffler3 and Annelie-Martina Weinberg2 1 TU Wien, Institute for Building Construction and Technology, Austria 2 Medical University of Graz, Department of Orthopaedic and Orthopaedic Surgery, Austria 3 ETH Zurich, Laboratory of Metal Physics and Technology, Department of Materials, Switzerland Introduction: Magnesium (Mg) and its alloys possess ideal characteristics in mecha­nical properties and biological acceptance, and thus are of great promise for deployment as implant material, particularly in surgical stabi­lisation of bone fractures. The relatively fast degradation ability avoids second surgeries for removal of the implant [1]. Essential for their applicability is a sufficient load baring capacity for a clinically relevant time frame, which is directly linked to a reliable degradation rate and good bone ingrowth. , The purpose of this study was therefore to (1) analyse the general applicability of Mg-based screws for their use in cortical bone (2) study whether an improved degradation and therefore biomechanical performance can be achieved by chemical surface treatment. Methods: Screws (diameter: 3.5 mm, length: 16 mm) were dry-machined from the biodegradable Mg alloy ultrahigh-purity (XHP) ZX00 (Mg-0.3Zn-0.4Ca). The surface of 4 screws was chemically polished (mixture of ethanol and phosphoric acid), whereas the others remained untreated. Seven screws (4 chemically polished and 3 unpolished) were gamma-sterilized and inserted into two ovine right tibiae (experimental, N=2 animals), and six further screws were implanted in the cadaveric right tibiae (control, N=2) after sacrificing the animal six weeks after implantation. These control screws were placed exactly on contralateral positions. All animal studies were performed in accordance with the Austrian Ministry of Science and Research (accreditation number BMWFW-66.010/0190-WF/V/3b/2014). An axial pull-out test was performed on the screws of the in vivo groups, and the same method was also applied on the cadaveric control bones. Fig. 1: Pull-out test method performed on XHP-ZX00 screws in sheep tibiae. Results: The pull-out tests revealed lower strength of screws that stayed in the tibia for 6 weeks in comparison to the control group (Fig. 1). Chemically polished screws exhibited lower pull-out strength than untreated samples (sample size not sufficient for statistical comparison). [MC1] Analysis of bone cross section after pull-out tests revealed that the screws of both groups partly failed by shearing-off the thread flanks and partly by failure of the bone tissue. Fig. 2: Pull-out test of XHP ZX00 screws (chemically polished and untreated) inserted in ovine tibiae (diaphysis and distal epiphysis) for 6 weeks vs. ex vivo control group. Discussions and Conclusions: These first experiments show that screws made of XHP-Mg-alloy ZX00 revealed good bone integration. For all screws tested, after 6 weeks of implantation the pull-out test reveals lower load-bearing capacity in comparison to the non-degraded control. This loss in strength is associated with initial degradation of the screws and according to expectations. The lower pull-out strength of chemically polished screws may be due to volume loss resulting from the chemical surface treatment. Facing the importance of surface finish for medical implants, results emphasize the need of (1) careful adjustment of the polishing protocol and (2) thorough evaluation of its in vivo response. The interface strength still appears to be sufficient for clinical applications. Obtained data is subject of high volatility. Thus, further studies need to be performed to evaluate degradation, ingrowth and tissue reactions of Mg-screws in more detail. Laura Bassi BRIC, Austrian Promotion Agency

A BIOMECHANICAL STUDY OF SHEAR LOAD ON BONE–SCREW INTERFACE OF THORACOLUMBAR VERTEBRAE

Vertebral screw failure by cutting through or pulling out of the vertebral body is common in intraoperative and postoperative stages of anterior thoracolumbar surgery, especially in osteoporotic patients. This biomechanical study was conducted to investigate the maximum shear force thoracolumbar vertebrae can withstand and the corresponding displacement of screws and analyze their correlation with vertebral bone mineral density. Forty individual vertebra specimens (T11-L3) were obtained from eight fresh adult cadaveric thoracolumbar spine specimens and randomly divided into an experimental group and a control group. Screws were placed in the center point of vertebrae and penetrated the contralateral cortex. Shear loading and axial pullout tests were conducted on the experimental group while only axial pullout test was conducted on the control group. The maximum shear forces and maximum axial pullout forces were recorded. The conditions of vertebral body destruction and screw channel were observed and the maximum axial pullout forces were recorded and analyzed. A large amount of thread bone debris was observed in the control group. In the experimental group, however, only a small amount of thread bone debris was observed; the widths of screw paths were larger than those in the control group and gradually increased from screw tips in the direction of loading. The vertebral bodies had an average shear strength of [Formula: see text][Formula: see text]N, and the corresponding average screw displacement was [Formula: see text][Formula: see text]mm. Linear regression analysis showed that the shear strength had a significant positive correlation with vertebral bone mineral density (BMD) (r[Formula: see text][Formula: see text][Formula: see text]0.958, P[Formula: see text][Formula: see text][Formula: see text]0.01), while the screw displacement had a significant negative correlation with vertebral BMD (r[Formula: see text][Formula: see text][Formula: see text]−0.933, P[Formula: see text][Formula: see text][Formula: see text]0.01). No significant difference in bone density was found between the destruction and the control groups (P[Formula: see text][Formula: see text][Formula: see text]0.05); the difference in maximum axial pullout strength between the destruction and the control groups was significant (P[Formula: see text][Formula: see text][Formula: see text]0.01). These results indicated that vertebral BMD positively correlated with the maximum shear force and negatively with the screw displacement. It is important that the corrective strength for spinal deformity may affect bone–screw interface in anterior thoracolumbar surgery.

Laboratory tests and thermal buckling analysis for pipes buried in Bohai soft clay

Upheaval buckling of submarine pipelines occurs due to relative movement of pipeline and surrounding soil and is often triggered by high operational temperature of the pipeline, initial imperfection of the pipeline, or a combination of both. Since buckling can jeopardize the structural integrity of a pipeline, it is a failure mode that should to be taken into account for the design and in-service assessment of trenched and buried offshore pipelines. In this study, a series of vertical (uplift) and axial pullout tests were carried out on model pipe segments buried in soft clay deposit similar to that present in Bohai Gulf, China. Pipe segments with three different diameters (= 30 mm, 50 mm and 80 mm) were buried in different depth-to-diameter ratios ranging from 1 to 8. Based on the results of laboratory tests, nonlinear force–displacement relations are proposed to model soil resistance mobilized during pipeline movement. The proposed nonlinear soil resistance models are employed in finite element analysis of buried pipelines with different amplitudes of initial geometric imperfections. Thermal upheaval buckling behavior of pipelines operating at different temperatures is studied. Results show that the capacity of pipeline against thermal buckling increases with the burial depth and decreases with the amplitude of initial imperfection.

Axial pullout strength comparison of different screw designs: fenestrated screw, dual outer diameter screw and standard pedicle screw.

BackgroundThe pullout strength of pedicle screws is influenced by many factors, including diameter of the screws, implant design, and augmentation with bone cement such as PMMA. In the present study, the pullout strength of an innovative fenestrated screw augmented with PMMA was investigated and was compared to unaugmented fenestrated, standard and dual outer diameter screw.MethodsTwenty four thoracolumbar vertebrae (T10-L5, age 60 to 70 years) from three cadavers were implanted with the four different pedicle screws. Twelve screws of each type were instrumented into either left or right pedicle with standard screw paired with unaugmented and dual outer diameter screw paired with augmented fenestrated screw in any given vertebra. Axial pullout testing was conducted at a rate of 5 mm/min. Force to failure (Newtons) for each pedicle screw was recorded.ResultsThe augmented fenestrated screws had the highest pullout strength, which represented an average increase of 149%, 141%, and 78% in comparison to unaugmented, standard, and dual outer diameter screws, respectively. Pullout strength of unaugmented screws was comparable to that of standard screws, however it was significantly lower than dual outer diameter screws.ConclusionsFenestrated screws augmented with PMMA improve the fixation strength and result in significantly higher pullout strength compared to dual outer diameter, standard and unaugmented fenestrated screws. Screws with dual outer diameter provided enhanced bone-screw purchase and may be considered as an alternative technique to increase the bone-screw interface in cases where augmentation using bone cement is not feasible. Unaugmented screws can be left in the pedicle even without cement and provide similar pullout strength to standard screws.

Biomechanical Analysis of a Novel Pedicle Screw Anchor Designed for the Osteoporotic Population

The biomechanical study was performed to investigate the effect of a novel pedicle screw anchor in increasing the pullout strength of pedicle screws. Ten lumbar vertebral bodies with a weighted average T-score of -2.13 were used. Pedicle screws of 4.5 mm diameter and 25 mm length were inserted in to one pedicle randomly and matched with an anchor in the corresponding pedicle. Fatigue testing was performed by applying an axial load of ±200N to the screw tulip, along the axis of the rod, at a rate of 0.5 Hz for 1,000 cycles. After fatigue loading was completed, all screws underwent axial pullout testing at a rate of 0.1 mm/sec until failure. A paired two sample for means t-test was performed to determine a significant difference between the two groups (p ≤ 0.05). Following fatigue testing, the axial displacement at the 1,000 cycle point for the anchor and non-anchor group was 1.4 ± 0.7mm and 2.9 ± 1.2mm, respectively. The anchor group had significantly lower axial displacement compared to the non-anchor group (p ≤ 0.01). The group with the anchor reached an average maximum load of 702 ± 373N. The average yield load for the non-anchor group was 421 ± 293N. The anchor group yield load was significantly greater than the non-anchor group (p ≤ 0.01). A novel anchor for standard pedicle screws resulted in significantly less axial movement during fatigue and a greater failure force compared a screw with no anchor. The anchor may provide a stronger bone-to-screw interface, than a non-anchor screw, without the complications of cement augmentation.

Bond behavior of spirally confined splice of deformed bars in grout

Abstract This paper presents the bond behavior of deformed steel reinforcement bars connected by a grouted spiral connection. The bond behavior investigated in this study comprised the mechanism of force transfer, the bond-slip relationship due to the effects of spiral diameter and the comparison of bond stresses between axial and flexural pull-out tests. A total of 36 pull-out specimens and 12 beam specimens were tested until bond failure. The results show that the bond performance between rebars and grout improved significantly due to the confinement provided by the spiral. Reducing the spiral diameter increases the confinement effect and subsequently increases the bond strength. The results also show that the bond strengths obtained from the flexural tests are within the range of 0.74–0.79 times the bond strength of the axial pull-out tests. However, both test results show similar trend of bond behavior with respect to variation in spiral configurations.

Evaluation of the Effect of Fixation Angle between Polyaxial Pedicle Screw Head and Rod on the Failure of Screw-Rod Connection.

Introduction. Polyaxial screws had been only tested according to the ASTM standards (when they were perpendicularly positioned to the rod). In this study, effects of the pedicle screws angled fixation to the rod on the mechanical properties of fixation were investigated. Materials and Method. 30 vertically fixed screws and 30 screws fixed with angle were used in the study. Screws were used in three different diameters which were 6.5 mm, 7.0 mm, and 7.5 mm, in equal numbers. Axial pull-out and flexion moment tests were performed. Test results compared with each other using appropriate statistical methods. Results. In pull-out test, vertically fixed screws, in 6.5 mm and 7.0 mm diameter, had significantly higher maximum load values than angled fixed screws with the same diameters (P < 0.01). Additionally, vertically fixed screws, in all diameters, had significantly greater stiffness according to corresponding size fixed with angle (P < 0.005). Conclusion. Fixing the pedicle screw to the rod with angle significantly decreased the pull-out stiffness in all diameters. Similarly, pedicle screw instrumentation fixed with angle decreased the minimum sagittal angle between the rod and the screw in all diameters for flexion moment test but the differences were not significant.

Influence of the screw augmentation technique and a diameter increase on pedicle screw fixation in the osteoporotic spine: pullout versus fatigue testing.

For posterior spinal stabilization, loosening of pedicle screws at the bone-screw interface is a clinical complication, especially in the osteoporotic population. Axial pullout testing is the standard pre-clinical testing method for new screw designs although it has questioned clinical relevance. The aim of this study was to determine the fixation strength of three current osteoporotic fixation techniques and to investigate whether or not pullout testing results can directly relate to those of the more physiologic fatigue testing. Thirty-nine osteoporotic, human lumbar vertebrae were instrumented with pedicle screws according to four treatment groups: (1) screw only (control), (2) prefilled augmentation, (3) screw injected augmentation, and (4) unaugmented screws with an increased diameter. Toggle testing was first performed on one pedicle, using a cranial-caudal sinusoidal, cyclic (1.0 Hz) fatigue loading applied at the screw head. The initial compressive forces ranged from 25 to 75 N. Peak force increased stepwise by 25 N every 250 cycles until a 5.4-mm screw head displacement. The contralateral screw then underwent pure axial pullout (5 mm/min). When compared to the control group, screw injected augmentation increased fatigue force (27 %, p = 0.045) while prefilled augmentation reduced fatigue force (-7 %, p = 0.73). Both augmentation techniques increased pullout force compared to the control (ps < 0.04). Increasing the screw diameter by 1 mm increased pullout force (24 %, p = 0.19), fatigue force (5 %, p = 0.73), and induced the least stiffness loss (-29 %) from control. For the osteoporotic spine, screw injected augmentation showed the best biomechanical stability. Although pullout testing was more sensitive, the differences observed were not reflected in the more physiological fatigue testing, thus casting further doubt on the clinical relevance of pullout testing.

Enhancing Pedicle Screw Fixation in the Lumbar Spine Using Allograft Bone Plug Interference Fixation

A within-subjects controlled laboratory study. To examine a biological alternative to cement augmentation for pedicle screw fixation comparing bilateral axial pullout tests of augmented and nonaugmented (controls) pedicle screws. Fixation in the osteoporotic spine remains a difficult challenge with failure by loosening or backout. Pedicle screw augmentation has been attempted using polymethylmethacrylate and bioabsorbable calcium cements; however, the potential for extravasation and embolization of cement are becoming increasingly concerning and merit the search for alternative methods to improve screw-anchoring strength. Twenty-four (24) fresh human lumbar vertebrae were tested to compare the pullout strength of augmented and nonaugmented pedicle screws. Two different augmentation strategies were employed using allograft bone plugs (ABPs) and evaluated using 12 specimens per group. Bone mineral density of each specimen was obtained using dual-energy x-ray absorptiometry. The augmented versus nonaugmented pedicle was randomized for each vertebra, and bilateral testing enabled paired statistical analyses. Axial pullout tests were performed using an materials testing system servohydraulic test system, and peak force, failure displacement, and stiffness was obtained for each test and correlated with bone mineral density. Augmentation using 6-mm-diameter ABPs with 6.25-mm-diameter pedicle screws resulted in statistically weaker average pullout strength (775±455 N) than the nonaugmented controls (1233±826 N). When using smaller (5 mm diameter) AGPs with the same diameter screws, there was no statistical difference between average pullout strength for the augmented pedicle screws (1772±652 N) and the nonaugmented screws (1780±575 N). Preliminary study of pedicle screw augmentation using cannulated ABPs showed no improvement of fixation with pedicles in the spine. This was even true in osteoporotic specimens, where augmentation would seem to be of considerable benefit.

Interference screw versus Endoscrew fixation for anterior cruciate ligament reconstruction: A biomechanical comparative study in sawbones and porcine knees

Interference screw fixation is one of the most common methods for ligament reconstruction. Although the advantages and clinical outcomes of this procedure have been widely reported, post-surgical complications often arise. The purpose of this study was to evaluate a new femoral fixation device, the Endoscrew, for anterior cruciate ligament (ACL) reconstruction. We performed a mechanical test in accordance with American Society for Testing and Materials (ASTM) standards and an in vitro biomechanical study. An axial pullout test was conducted to evaluate the mechanical properties of the new device and the interference screw when implanted in solid rigid polyurethane foam test blocks. The biomechanical test used porcine femora to evaluate the initial fixation strength between these two implants. The maximum pullout force of the interference screw group [722.05 ± 130.49 N (N)] was significantly greater ( p < 0.01) than the Endoscrew group (440.79 ± 26.54 N) when implanted in polyurethane foam 320 kg/m 3 density. With polyurethane foam 160 kg/m 3 density, the maximum pullout forces were (242.61 ± 37.36 N) ( p < 0.001) and (99.33 ± 30.01 N) for the interference screw group and Endoscrew group, respectively. In the in vitro mechanical study, the Endoscrew (646.39 ± 72.38 N) required a significantly greater ultimate load prior to failure ( p < 0.05) when compared with the interference screw (489.72 ± 138.64 N). With regard to pullout stiffness, there was no statistically significant difference ( p < 0.13) between the Endoscrew group (99.15 ± 12.16 N/mm) and the interference screw group (87.96 ± 11.12 N/mm). The cyclic stiffness was also not significantly different ( p < 0.44) between the Endoscrew group (93.09 ± 16.07 N/mm) and the interference screw group (85.78 ± 14.76 N/mm). The axial pullout test showed that the strength of the Endoscrew was close to the fixation strength required for daily activities, but it is inappropriate for use in osteoporotic patients. The in vitro study showed that the Endoscrew has a superior initial mechanical fixation over the biodegradable interference screw.

Analytical Modeling of Field Axial Pullout Tests Performed on Buried Extensible Pipes

AbstractThe performance evaluation of buried extensible plastic pipelines, such as polyethylene pipes in the gas distribution industry, in areas prone to ground movement is a key consideration for many utility owners. Considering the relatively smaller deformation stiffness and nonlinear stress-strain response of these pipes compared with those of steel pipes, a good understanding of the basic soil–pipe interaction mechanisms forms a critical part in these evaluations. The increase in soil normal stress on the pipe attributable to soil dilation and gradual degradation of interface friction with axial pipe displacement was investigated. An analytical method was developed to estimate the pipe response when subject to axial frictional loads arising from soil movements, which accounts for these factors and the nonlinear stress-strain response of the pipe material. For a known relative axial soil displacement, the approach provides a convenient way to determine the axial force, strain, and length along which t...

Comparison study of the pullout strength of conventional spinal pedicle screws and a novel design in full and backed-out insertions using mechanical tests

Recently, new pedicle screw designs have been developed. However, these designs' performances are still unclear, especially when backed out after insertion. The objective of this study was to investigate the performances of different screw designs when backed out from full insertion. Seven conventional designs of the pedicle screw and one novel design were inserted into polyurethane foam (0.32 g/cm(3)). All screws were first fully inserted (43 mm) and were backed out 360°. Axial pullout tests were performed and the reaction force was measured. The results showed that the conical screw of type 1 with a small inner diameter provided the highest pullout strength in both full insertion and backed-out insertion (2401.85 and 2169.82 N, respectively). However, this screw's pullout strength significantly decreased (9.7%) when backed out from full insertion. There was no significant difference between the conical screw of type 1 with a small inner diameter and double duo core screw (p > 0.01) in backed-out insertion. The cylindrical screw with a small diameter, dual inner core screw and double dual core screw also provided good results in both full insertion (2115.44, 2182.99 and 2226.93 N, respectively) and backed-out conditions (2065.80, 2014.28 and 1941.29 N, respectively). The increased pullout strength of the conical design could be due to the effect of bone compaction. However, the screw exhibited less consistent pullout strength when backed out when compared with the other designs. The conical screw should be inserted to the precise position without turning back, especially in osteoporosis patients. The dual inner core screw and double dual core screw could provide greater stability in both conditions. Care should be taken when using both the cylindrical screw with a small thread depth and the dual outer core screw.

Designs and techniques that improve the pullout strength of pedicle screws in osteoporotic vertebrae: current status.

Osteoporosis is a medical condition affecting men and women of different age groups and populations. The compromised bone quality caused by this disease represents an important challenge when a surgical procedure (e.g., spinal fusion) is needed after failure of conservative treatments. Different pedicle screw designs and instrumentation techniques have been explored to enhance spinal device fixation in bone of compromised quality. These include alterations of screw thread design, optimization of pilot hole size for non-self-tapping screws, modification of the implant's trajectory, and bone cement augmentation. While the true benefits and limitations of any procedure may not be realized until they are observed in a clinical setting, axial pullout tests, due in large part to their reproducibility and ease of execution, are commonly used to estimate the device's effectiveness by quantifying the change in force required to remove the screw from the body. The objective of this investigation is to provide an overview of the different pedicle screw designs and the associated surgical techniques either currently utilized or proposed to improve pullout strength in osteoporotic patients. Mechanical comparisons as well as potential advantages and disadvantages of each consideration are provided herein.

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.