Peak Pullout Force Research Articles

Development and first biomechanical validation of a score to predict bone implant interface stability based on clinical qCT scans

Sufficient implant anchoring in osteoporotic bone is one major challenge in trauma and orthopedic surgery. In these cases, preoperative planning of osteosynthesis is becoming increasingly important. This study presents the development and first biomechanical validation of a bone-implant-anchorage score based on clinical routine quantitative computer tomography (qCT) scans. 10 pairs of fresh frozen femora (mean age 77.4 years) underwent clinical qCT scans after placing 3 referential screws (for matching with the second scan). Afterwards, three 4.5 mm cortical screws (DePuy Synthes, Zuchwil, Switzerland) were placed in each distal femur in the dia-metaphyseal transition followed by the second CT scan. The femur was segmented using thresholding and its outer shape was visualized as a surface model. A 3D model of the cortex screw in STL format was used to model the screw surface precisely. For each femur, the 3 cortex screw models were exactly positioned at the locations previously determined using the second CT scan. The BMD value was calculated at the center of each triangle as an interpolation from the measured values at the three vertices (triangle corners) in the CT. Scores are based on the sum of all the triangles’ areas multiplied by their BMD values. Four different scores were calculated. A screw pull-out test was performed until loss of resistance. A quadratic model adequately describes the relation between all the scores and pull-out values. The square of the best score explains just fewer than 70% of the total variance of the pull-out values and the standardized residual which were approximately normally distributed. In addition, there was a significant correlation between this score and the peak pull-out force (p < 0.001). The coefficient of determination was 0.82. The presented score has the potential to improve preoperative planning by adding the mechanical to the anatomical dimension when planning screw placement.

Homogenized finite element models can accurately predict screw pull-out in continuum materials, but not in porous materials

Background and ObjectiveBone screw fixation can be estimated with several test methods such as insertion torque, pull-out, push-in and bending tests. A basic understanding of the relationship between screw fixation and bone microstructure is still lacking. Computational models can help clarify this relationship. The objective of the paper is to evaluate homogenized finite element (hFE) models of bone screw pull-out. MethodsExperimental pull-out tests were performed on three materials: two polyurethane (PU) foams having a porous microstructure, and a high density polyethylene (HDPE) which is a continuum material. Forty-five titanium pedicle screws were inserted to 10, 20, and 30 mm in equally sized blocks of all three materials (N = 5/group). Pull-out characteristics i.e. stiffness (S), yield force (Fy), peak pull-out force (Fult) and displacement at Fult (dult) were measured. hFE models were created replicating the experiments. The screw was modeled as a rigid body and 5 mm axial displacement was applied to the head of the screw. Simulations were performed evaluating two different conditions at the bone-screw interface; once in which the screw fitted the pilot hole exactly ("free-stressed") and once in which interface stresses resulting from the insertion process were taken into account ("pre-stressed"). ResultsThe simulations representing the pre-stressed condition in HDPE matched the experimental data well; S, Fy, and Fult differed less than 11%, 2% and 0.5% from the experimental data, respectively, whereas dult differed less than 16%. The free-stressed simulations were less accurate, especially stiffness (158% higher than the pre-stressed condition) and dult (30% lower than pre-stressed condition) were affected. The simulations representing PU did not match the experiments well. For the 20 mm insertion depth, S, Fy and Fult differed by more than 104%, 89% and 66%, respectively from the experimental values. Agreement did not improve for 10 and 30 mm insertion depths. ConclusionsWe found that hFE models can accurately quantify screw pull-out in continuum materials such as HDPE, but not in materials with a porous structure, such as PU. Pre-stresses in the bone induced by the insertion process cannot be neglected and need to be included in the hFE simulations.

Influence of soil moisture content on pullout properties of Hippophae rhamnoides Linn. roots

Plant root system plays an important role in preventing soil erosion and improving slope stability. However, its performance is significantly affected by soil moisture content, and the role of soil moisture in root reinforcement is not fully understood. In this study, the influence of soil moisture on root pullout properties was studied by experiments. Vertical in-situ pullout tests under four different levels of soil matric suction (12 kPa, 18 kPa, 24 kPa, 30 kPa) were carried out on roots of sea buckthorn plants (Hippophae rhamnoides Linn.) which were artificially cultivated for 7 months. Diameter and length of the root system of sea buckthorn were investigated. The results showed that a very significant correlation was observed between root diameter (D) and root length (L) (P<0.01), and root diameter decreased with soil depth. When soil bulk density was constant, peak pullout force (F) and friction coefficient of root-soil interface (μ) decreased with increasing gravimetric soil moisture content in power functions. Soil moisture content significantly affected root pullout resistance because the increase of soil moisture content decreased the friction coefficient between the roots and soil. Root diameter at breakage point (Db) and length of root segment left in soil (Lb) were increased with soil moisture content. In addition, peak pullout force of the roots increased in a power function with root diameter at the soil surface (Do) and in a linear function with total root length (L). The results provided an experimental basis for quantifying the effects of soil moisture content on soil reinforcement by plant roots.

A New Inclined Pullout Device for Laboratory Tests on Geosynthetics

Abstract Geosynthetics are widely used in landfill cover systems for various purposes. The stability of these geosynthetics on slopes of landfill covers depends on the effectiveness of the anchors in which these are embedded at the top of the slope. These geosynthetics are subjected to pullout forces that are in an inclined direction. However, only two studies have been reported so far on the inclined pullout behavior of geosynthetics embedded in various types of anchors. In this study, a new inclined pullout device is developed to conduct pullout tests under different angles of inclinations. The salient features and details of various components of this device are presented in this article. Because geogrids are the preferred choice for veneer reinforcement, inclined pullout tests are conducted on geogrids embedded in three different types of anchor, namely, run-out, I-type, and L-type anchors. The tests are conducted on two different types of sands, i.e., quarry sand and river sand. The results show that the peak pullout force increases significantly as the inclination of the pullout force increases for all three types of anchor and for both the sands. Moreover, the complete pullout force-displacement response is found to be different for the three anchors. Practical implications of the outcome of the study are given.

A model for optimizing hooked end steel fibre reinforcements in cracked cement composites

The addition of metallic fibres in concrete is a widely used technique to increase its resistance to crack opening, energy absorption capacity, and durability. The properties of the resulting composite strongly depend on the geometry of the fibres, which usually dispose of an hooked end, as well as on the mechanical properties of both the fibre and the concrete. The optimization of the fibre reinforcement mechanisms can only be achieved by an approach that takes into account adequately these aspects. Therefore, in this work, a computational model incorporating the key features of a hooked end fibre embedded in a cement matrix is proposed. The fibre is modelled as a Timoshenko beam, whereas a cohesive interface is used to model the interaction with the surrounding concrete. Different failure mechanisms are defined including fibre debonding or fibre tensile rupture and concrete spalling at fibre exit point. The model is calibrated by using the results of an experimental campaign conducted by authors. A multi-step optimization algorithm is used to find the optimal geometry and model’s constitutive parameters that maximize the peak pull-out force and the energy absorption capacity in a fibre pull-out test. The analysis suggests that the use of concrete with high strength has the potential to increase both peak force and energy absorption capacity by designing the proper geometry of the fibre.

Finite-Element Modeling of Geogrids in Trenches under Inclined Pull

Three-dimensional finite-element analyses of the inclined pullout behavior of geogrids embedded in three types of anchors (run-out, I-type, and L-type) are presented in this paper. The soil behavior is represented by the elastic–perfectly plastic Mohr–Coulomb (MC) model and the geogrid behavior with linear-elastic plate elements. The geogrid is modeled using the geometrical and index properties of the geogrid used in model tests. The predicted results from the finite-element analyses are compared with the corresponding model test results. The peak pullout force values for different values of inclinations and types of anchors are predicted with reasonable accuracy by the present numerical model. However, the results show that the post-peak response for run-out and I-type anchors and the stiffness in the case of I-type and L-type anchors are not satisfactorily modeled.

Characterizing and analyzing the residual interfacial behavior of steel fibers embedded into cement-based matrices after exposure to high temperatures

Fiber reinforced cementitious materials, when exposed to a fire, can have better residual mechanical properties than plain cementitious materials, particularly the tensile property. One critical factor governing the residual bridging effect is the fiber-matrix bond performance. However, the effect of temperature on the residual interfacial bond performance remains poorly understood. Here, based on the pull-out tests, we investigate the residual interfacial behavior of aligned straight and hooked-end steel fibers embedded into nine groups of matrices exposed to temperature levels from 20 °C to 600 °C with an interval level of 100 °C. Specifically, cement paste, mortar, and concrete with three w/c ratios (0.3, 0.4, and 0.5) are considered. Meanwhile, based on thermoelastic mechanics and the pulley frictional model, we quantitatively analyze the effect of thermal shrinkage, thermal incompatibility, deformation of hook end, and fiber and matrix degradation on residual interfacial bond performance. By combining the analysis and test results, we show that the thermal shrinkage can enhance the fiber-cement paste bond performance. The thermal incompatibility can impose negative effects upon the residual bond performance of all the fiber-matrix systems. Interestingly, we find that the decrease of the angle of the hook end can to some extent improve the peak pull-out force of the hooked-end fibers. Moreover, we also find that the degradation of fiber yielding strength and non-straightening hook end are the main causes of the reduction in the residual peak pull-out force of hooked-end steel fibers.

Experimental Studies on Effect of Nailing in Cohesion-Less Soil

The anchorage in the form of soil nailing can be used for stabilizing weak natural soil slopes. It can also be used forsteepening of new or naturally existing soil slopes. The nail anchorage in the soil can be in the form ofreinforcement and in the different materials with different cross section area. The most of the failure of nail in soilhave been reported as pullout failure due to tension on the nail, particularly in sandy soil. Because of increased costand safety concerned the field test to determine pullout force is difficult. Therefore the laboratory studies werecarried out in simulated conditions. In this research work, the laboratory investigation were carried out to observethe effect of tensile (pullout force) force on the soil nail in sandy soil. The pullout force on the nail was graduallyincreased and the corresponding displacement was recorded till failure of the nail due to continues sliding. The testresults were compared to Standard values in earlier studies. It has been shown that a simple loading frame withtesting tank model can be used to simulate the pullout load–displacement relationship and to determine peakpullout force of different type of nails. The various parameters studied were embedded length of the nail, the type ofsurface, the material and cross-section of nail.

HDPE geogrid-residual soil interaction under monotonic and cyclic pullout loading

The understanding of soil-geosynthetic interaction under cyclic loading conditions is essential for the safe design of geosynthetic-reinforced soil structures subjected to repeated loads, such as those induced by road and railway traffic and earthquakes. This paper describes a series of large-scale monotonic and multistage pullout tests carried out to investigate the behaviour of an HDPE uniaxial geogrid embedded in a locally available granite residual soil under monotonic and cyclic pullout loading. The effects of the pullout load level at the start of the cyclic stage, cyclic load frequency and amplitude, number of cycles and soil density on the load-strain-displacement response of the reinforcement are evaluated and discussed. Test results show that the cumulative displacements measured along the length of the geogrid during cyclic loading increased significantly with the pre-cyclic pullout load level and the load amplitude. In contrast, the cumulative cyclic displacements were found to decrease with increasing frequency and soil density. In medium dense soil conditions, the geogrid post-cyclic pullout resistance decreased by up to 20%, with respect to the value obtained in the comparable monotonic test. However, for dense soil, the effect of cyclic loading on the peak pullout forces recorded during the tests was almost negligible.

Experimental investigations on inclined pullout behaviour of geogrids anchored in trenches

Stability of geosynthetics along slopes in covers and liners of landfills depends partly on their tensile strength and partly on the efficiency of anchors holding the geosynthetics at berms or the top of the slope. The pull induced in these geosynthetics is in an inclined direction parallel to the slope. But due to difficulty in modelling inclined pullout behaviour experimentally, very few studies have so far been conducted on geosynthetics embedded in anchor trenches. This paper presents details of a device that can perform inclined pullout tests on geosynthetics at variable inclinations. Since geogrids are widely used as veneer reinforcement, inclined pullout tests are conducted on geogrids embedded in run-out, I-type and L-type anchors in sand. The maximum pullout force in the geogrid increases by more than 20% as the pull inclination increases from 0° to 20° in all the three types of anchors. The geogrids exhibit markedly different force-displacement behaviour for the three types of anchors, though the length of reinforcement remains the same. The peak pullout force for the I-type anchor was 1.6–1.7 times higher, and that for the L-type anchor was 2 times higher than that of the run-out anchor at all pullout inclinations.

Small-Scale Pullout Test of a Geogrid-Reinforced Unsaturated Soil with Suction Monitoring

Abstract The use of local fine-grained soils in geosynthetic-reinforced structures can significantly reduce costs when granular materials are difficult to access. Another beneficial aspect of these soils is that enhanced mechanical behavior is expected with fine-grained soils under unsaturated conditions. With structures built in unsaturated conditions, stability analyses based on unsaturated soil mechanics would properly characterize and accurately predict the structural behavior with fine-grained soils. Accordingly, the interface shear strength between fine-grained soils and geosynthetics should also be assessed by unsaturated pullout or direct shear testing. In this article, the effect of matric suction on the pullout behavior of a polyester geogrid embedded in a fine-grained soil was evaluated using a small-scale testing apparatus. A miniature tensiometer located in the proximity of the soil-geogrid interface enabled monitoring of the matric suction of soil during pullout to evaluate its effect on the interface shear strength. A design approach for prediction of unsaturated interface strength is also assessed in this study. Three typical values of matric suction were identified during pullout: initial, peak, and residual. In the case of interfaces under water inundation, the pore water pressures developed and increased with the increase of overburden pressures. For drier interfaces, matric suction increases with increases of overburden pressure. Significant reductions (60 %) in peak pullout forces were observed for minor increases in soil moisture contents. Analyses using the moisture reduction factor indicated up to a 70 % loss of soil-geogrid interface shear strength as a result of wetting. The increase in unsaturated soil-geogrid interface strength due to the reduction of moisture content was attributed more to adhesion than to friction for the fine-grained soil used in this research. The analytical approach proposed in this study to predict the pullout strength of unsaturated interfaces has been demonstrated to be quite consistent with the actual strength values assessed from the small-scale pullout tests, primarily for higher values of overburden pressures. Matric suction values obtained from the soil-water retention curve were shown to be reliable parameters for use in the proposed analytical approach for prediction of the pullout strength of unsaturated soil-geosynthetic interfaces.

Comparison of the Pullout Strength of Different Pedicle Screw Designs and Augmentation Techniques in an Osteoporotic Bone Model.

Study DesignMechanical study.PurposeTo compare the pullout strength of different screw designs and augmentation techniques in an osteoporotic bone model.Overview of LiteratureAdequate bone screw pullout strength is a common problem among osteoporotic patients. Various screw designs and augmentation techniques have been developed to improve the biomechanical characteristics of the bone–screw interface.MethodsPolyurethane blocks were used to mimic human osteoporotic cancellous bone, and six different screw designs were tested. Five standard and expandable screws without augmentation, eight expandable screws with polymethylmethacrylate (PMMA) or calcium phosphate augmentation, and distal cannulated screws with PMMA and calcium phosphate augmentation were tested. Mechanical tests were performed on 10 unused new screws of each group. Screws with or without augmentation were inserted in a block that was held in a fixture frame, and a longitudinal extraction force was applied to the screw head at a loading rate of 5 mm/min. Maximum load was recorded in a load displacement curve.ResultsThe peak pullout force of all tested screws with or without augmentation was significantly greater than that of the standard pedicle screw. The greatest pullout force was observed with 40-mm expandable pedicle screws with four fins and PMMA augmentation. Augmented distal cannulated screws did not have a greater peak pullout force than nonaugmented expandable screws. PMMA augmentation provided a greater peak pullout force than calcium phosphate augmentation.ConclusionsExpandable pedicle screws had greater peak pullout forces than standard pedicle screws and had the advantage of augmentation with either PMMA or calcium phosphate cement. Although calcium phosphate cement is biodegradable, osteoconductive, and nonexothermic, PMMA provided a significantly greater peak pullout force. PMMA-augmented expandable 40-mm four-fin pedicle screws had the greatest peak pullout force.

Laboratory study on pullout capacity of helical soil nail in cohesionless soil

Helical nails are a new alternative to conventional soil nails or tie-backs for stabilization of slopes, excavations, and embankments due to ease of installation, minimal site disturbance, and immediate loading capability. Pullout capacity of helical nails is a critical parameter for their design. This study investigates the pullout behaviour of helical soil nails installed in dry dense sand through a series of laboratory tests. The tests were performed on seven different types of helical nails in a displacement-controlled manner. The paper describes the test procedure, results and highlights the influence of different parameters on the pullout capacity. Results show that roughness of the nail shaft significantly influences the peak pullout capacity of helical soil nails. A linear relationship between peak pullout force and overburden pressure is observed for different types of helical soil nails, indicating that it satisfies the Mohr–Coulomb failure criteria. A helical soil nail having a double helix of unequal diameter and the same interspacing shows higher pullout capacity than a helical soil nail having a double helix of the same diameter. The position of the helix and spacing-to-diameter ratio of the soil nail with a double helix of unequal diameter plays a significant role on the peak pullout capacity.

Flexibility and fatigue evaluation of oblique as compared with anterior lumbar interbody cages with integrated endplate fixation

This study was undertaken to quantify the in vitro range of motion (ROM) of oblique as compared with anterior lumbar interbody devices, pullout resistance, and subsidence in fatigue. Anterior and oblique cages with integrated plate fixation (IPF) were tested using lumbar motion segments. Flexibility tests were conducted on the intact segments, cage, cage + IPF, and cage + IPF + pedicle screws (6 anterior, 7 oblique). Pullout tests were then performed on the cage + IPF. Fatigue testing was conducted on the cage + IPF specimens for 30,000 cycles. No ROM differences were observed in any test group between anterior and oblique cage constructs. The greatest reduction in ROM was with supplemental pedicle screw fixation. Peak pullout forces were 637 ± 192 N and 651 ± 127 N for the anterior and oblique implants, respectively. The median cage subsidence was 0.8 mm and 1.4 mm for the anterior and oblique cages, respectively. Anterior and oblique cages similarly reduced ROM in flexibility testing, and the integrated fixation prevented device displacement. Subsidence was minimal during fatigue testing, most of which occurred in the first 2500 cycles.

Impact of screw location and endplate preparation on pullout strength for anterior plates and integrated fixation cages

Background ContextNumerous integrated fixation cages (IFCs) have recently been introduced to the market with “zero-profile” designs that incorporate screw fixation through the vertebral endplate. It is unclear whether differences in bone quality and quantity in this insertion location may affect fixation compared with screws used in traditional anterior plate (AP) fixation. Moreover, endplate preparation for IFC implantation may affect fixation strength. PurposeThis study aimed to compare pullout strength of screws used in IFCs with screws used for AP implantations. Study DesignA biomechanical cadaveric study. MethodsT12 and L1 vertebrae from 13 human cadaver spines were prepared for pullout testing. End plates in T12 vertebrae were scraped according to surgical practice for fusion procedures. Conversely, endplates in L1 vertebrae were kept intact (unscraped). Integrated fixation cage screws were implanted at a 45° angle into the endplate and AP screws were implanted horizontally into the same vertebral body. Pullout testing was performed on all screws, and peak pullout force (PPF) and work were compared between groups to determine fixation strength. In addition, micro-CT imaging was used to assess bone quantity and quality parameters such as trabecular bone volume fraction, endplate and anterior cortex thickness at screw insertion location, endplate mineralization, and anterior cortex mineralization. ResultsPeak pullout force for IFC screws (176±68 N) with scraped endplates was similar (p=.26) to AP screws (192±84 N). However, PPF for IFC screws (231±75 N) with unscraped endplates was significantly greater (p<.01) than AP screws (176±50 N). Peak pullout force for IFC screws with scraped endplates was significantly lower (p=.03) than IFC screws with unscraped endplates. Scraped endplates group (0.17±0.05 mm) were thinner (p=.05) than unscraped endplates (0.21±0.06 mm) by approximately 40 µ on average. ConclusionsThese data indicate that IFC and AP screws exhibited similar fixation behavior when the endplate is prepared according to common surgical practices. However, endplate scraping reduces endplate thickness by 20% on average, resulting in a decrease in fixation strength when compared with the unscraped endplates and provides bounds for IFC screw fixation strength.

Quality control and monitoring of NSM CFRP systems: E-modulus evolution of epoxy adhesive and its relation to the pull-out force

BackgroundIn Fibre-Reinforced Polymer (FRP) applications, the mechanical behaviour of the strengthening system strongly depends on the epoxy adhesive, particularly at early ages. MethodsThe present paper describes the application of an innovative technique (termed EMM-ARM: Elasticity Modulus Monitoring through Ambient Response Method) for continuous monitoring of the stiffening process of an epoxy adhesive used in structural reinforcements applications. A simultaneous study of direct pull-out tests with concrete specimens strengthened with near-surface mounted (NSM) carbon FRP laminate strips was carried out to compare the evolution of bond performance with the E-modulus of epoxy since early ages. ResultsThe peak pull-out force and the epoxy E-modulus obtained by EMM-ARM exhibit very similar evolution kinetics. ConclusionsA relationship between the evolution of epoxy E-modulus and the maximum pull-out force is assessed, highlighting the potential of applying EMM-ARM for quality control and decision-making assistance of NSM systems.

In vitro evaluation of pedicle screw loosening mechanism: a preliminary study on animal model

Materials and methods Eighteen vertebrae were harvested from the lumbar section of six porcine spines ranged from L1 to L3. All vertebrae were instrumented on both pedicles using pedicle screws. Quantitative CT scans were used before instrumentation in order to assess the bone density of each vertebra. Cyclic bending (toggling) load of ± 1mm displacement at 3Hz for 5000 cycles were assigned in two directions including craniocaudal (CC) and transverse (TR) toggling. Twelve instrumented pedicles were selected for CC toggling, twelve pedicles for TR toggling and twelve pedicles were not toggled (NT). All toggled and non-toggled screws were then pulled out at a displacement speed of 5mm/min in longitudinal direction. The peak pullout force and stiffness were computed from the load-displacement curves. Analyses of variance (ANOVA) were performed to investigate the effects of toggling methods and vertebral levels on the pullout force and stiffness.

Biomechanical evaluation of pedicle screw loosening mechanism using synthetic bone surrogate of various densities.

Pedicle screw fixation is a well-established procedure for various spinal disorders. However, pedicle screws failures are still reported. Therefore, there is a need for a greater understanding of the pedicle screw failure mechanism. This experimental study investigates the biomechanical stability of pedicle screws using a synthetic bone surrogate with a special focus on the screw loosening mechanism. Pedicle screws have been inserted in thirty six polyurethane foam blocks of three different densities. In half of the specimens from each density group, pedicle screws were submitted to cyclic bending (toggling) before pullout. The rest of specimens were solely loaded in axial pullout. The peak pullout force and stiffness were determined from load-displacement curve of each specimen. Statistical analyses were performed to investigate on the effect of toggling and bone surrogate density on the pedicle screw's pullout force. The results suggest that the pullout force and stiffness were significantly affected by toggling and density. Higher pullout forces resulted from higher grades of density. The proposed method allowed investigating the pedicle screw loosening mechanism. However, conducing further experimental tests on animal or cadaveric vertebrae are needed to confirm these findings.

A biomechanical study to evaluate the effect of PMMA augmentation and restoration of the strength of cervical vertebral screws inserted in an osteoporotic vertebral body.

An experimental study. To compare the peak pull-out force (PPF) of vertebral screws fixed in osteoporotic vertebrae versus polymethylmethacrylate (PMMA) used for augmentation or restoration, before and after fatigue. Failure of screw fixation in anterior cervical interbody fusion remains a significant clinical problem. However, little is known of the biomechanical characteristics of cervical vertebral screws before and after restoration or augmentation, especially after fatigue. Fifty fresh cervical vertebrae, of which 40 were randomly selected, from 12 adult cadavers were used. The vertebrae were divided into healthy controls, osteoporotic controls, a PMMA restoration group, and a PMMA augmentation group. In each of the control groups, 2 pilot holes on each side of 20 vertebral bodies were implanted with vertebral screws (4 mm in diameter). Each side randomly received either acute PPF or PPF beyond fatigue that was ensured using cyclic loading (2 Hz; 20,000 times). In the PMMA groups, pilot holes were drilled parallel to the sagittal plane and injected with 0.6-1.0 mL PMMA before the vertebral screw was inserted. Each side of each vertebral body randomly received either PPF or PPF beyond fatigue that was ensured using cyclic loading (2 Hz; 20,000 times). A Bose3510-AT fatigue testing machine measured the PPF of vertebral screws with or without PMMA for all specimens before and after fatigue. In all groups, the prefatigue PPF was significantly higher than the postfatigue PPF. Compared with the prefatigue PPF, postfatigue PPF was reduced by 37.40%, 43.10%, 32.08%, and 31.85% in the healthy controls, osteoporotic controls, PMMA restoration, and PMMA augmentation groups, respectively. The acute and postfatigue PPFs of the healthy controls were significantly higher from that of the osteoporotic controls. The acute and postfatigue PPFs of both control groups were significantly lower from that of both PMMA groups. There was no difference in acute and postfatigue PPFs between the PMMA restoration and augmentation groups. The results indicated that both PMMA augmentation and PMMA restoration could significantly increase cervical screw pull-out strength and antifatigue capability. The results provide a biomechanical justification for spine surgeons to use PMMA for augmentation or restoration in cases of surgeries with poor bone quality or osteoporotic vertebral bodies.

Simulation of single fiber pullout response with account of fiber morphology

A finite element model was developed at the single fiber length scale to predict the quasi-static pullout response of individual fibers from cementitious composites. The model accounts for energy dissipation through granular flow of the interfacial transition zone (ITZ) and matrix, plastic work in the fiber, and frictional dissipation at the fiber–ITZ interface. The considered fiber morphology was a triangular cross section that had been uniformly twisted along the fiber length. The model was calibrated to published experimental data for fiber pitches of 12.7 and 38.1mm/revolution pulled from cement mortar with a 44-MPa unconfined compressive strength. The model was used to investigate slip-hardening behavior, tunneling of the cement mortar, in situ pullout behavior of helically twisted fibers at a crack plane, and provide an alternate explanation for the pullout response of twisted fibers from a 84-MPa unconfined compressive strength matrix containing silica fume. Calculations show that twisted fibers can provide up to 5 times the peak pullout force and 10 times the total work compared with straight fibers and infer work-hardening behavior during fiber pullout. The findings indicate that the tailoring of fiber morphology and control of constituent properties are important avenues for achieving significant improvements in the performance of fiber-reinforced cementitious composites.