Peak Pullout Force Research Articles

Primary stability of two uncemented acetabular components of different geometry: hemispherical or peripherally enhanced?

ObjectiveThis study compared the primary stability of two commercially available acetabular components from the same manufacturer, which differ only in geometry; a hemispherical and a peripherally enhanced design (peripheral self-locking (PSL)). The objective was to determine whether altered geometry resulted in better primary stability.MethodsAcetabular components were seated with 0.8 mm to 2 mm interference fits in reamed polyethylene bone substrate of two different densities (0.22 g/cm3 and 0.45 g/cm3). The primary stability of each component design was investigated by measuring the peak failure load during uniaxial pull-out and tangential lever-out tests.ResultsThere was no statistically significant difference in seating force (p = 0.104) or primary stability (pull-out p = 0.171, lever-out p = 0.087) of the two components in the low-density substrate. Similarly, in the high-density substrate, there was no statistically significant difference in the peak pull-out force (p = 0.154) or lever-out moment (p = 0.574) between the designs. However, the PSL component required a significantly higher seating force than the hemispherical cup in the high-density bone analogue (p = 0.006).ConclusionsHigher seating forces associated with the PSL design may result in inadequate seating and increased risk of component malpositioning or acetabular fracture in the intra-operative setting in high-density bone stock. Our results, if translated clinically, suggest that a purely hemispherical geometry may have an advantage over a peripherally enhanced geometry in high density bone stock.Cite this article: Bone Joint Res 2013;2:264–9.

Biomechanical Assessment of Reduction Forces Measured During Scoliotic Instrumentation Using Two Different Screw Designs

Study DesignBiomechanical finite element models simulated deformity correction using pedicle screw instrumentation and measured forces at the screw–vertebra interface. ObjectivesCompare 2 different screw designs with respect to reaction forces at screw–vertebra interfaces during scoliosis correction maneuvers. Summary of Background DataPedicle screw developments strive to enhance surgical techniques and improve patient safety. It is believed that a screw with increased lateral angulation and reduction tabs enables a more gradual correction, more effectively distributes corrective forces over multiple levels, and reduces forces at screw–vertebra interfaces compared with standard polyaxial screws. MethodsWe selected 3 scoliotic patients and reconstructed their preoperative spinal profiles as finite element models using radiographic clinical measures. The osteoligamentous models were programmed and validated with mechanical properties from published literature. We used postoperative radiographs to determine instrumented levels and calibrate disc properties to corroborate simulated results with clinical data. We alternatively examined favored angle (FA) screws and polyaxial (PA) screws using correction steps characteristic to their design. We also explored sensitivity of screw forces consequent to misalignment with adjacent screws. ResultsSimulated postoperative spinal profiles on average adhered to clinical measures within 5°. We observed no significant differences in simulated corrective profiles between screw types (5° or less). Compared with PA screws, FA screws reduced peak pullout and lateral forces by 27% and 35%, respectively, and correspondingly reduced mean pullout and lateral forces by 48% and 40%, respectively. Changes in peak and average pullout forces resulting from screw misalignment were 56% and 82% less, respectively, with FA screws. ConclusionsThis analysis demonstrated reduced screw–vertebra peak and mean forces when using a pedicle screw with a favored angle bias and reduction tabs to correct scoliosis. Compared with PA screws, FA screws provide similar correction, decrease forces applied at the screw–vertebra interface, and are more forgiving if misaligned.

Functional implications of ontogenetic and sexual dimorphic dentition in the eastern shovelnose ray Aptychotrema rostrata (Shaw & Nodder, 1794)

Unlike other elasmobranchs, batoids exhibit sexually dimorphic dentition. The functional implications of such dentition, however, remain understudied. For the present study, ontogenetic and sexual dimorphism in tooth and jaw structure, together with the functional implications of this dimorphism, were determined in the eastern shovelnose ray, Aptychotrema rostrata. Sexually dimorphic dentition and jaw structure was first observed in sub-adult age classes, with males developing a pronounced lower jaw at the symphysis. Monognathic heterodonty was prominent in mature males, with teeth in the symphyseal region developing significantly greater heights and sharpness ratios in comparison to females. Ex vivo mechanical grip strength tests were used to determine simulated bite-grip tenacity. The mean peak pull-out forces required to withdraw a section of a dissected pectoral fin from between jaws closed with a constant occlusal force was highest for mature males, intermediate for mature females and lowest for immature females and males. Although the species exhibits ontogenetic variations in diet, these were unrelated to sex. Rather, the larger and highly cuspidate teeth of mature males increased the bite-grip tenacity, which likely aids in copulation.

Experimental study and modeling of single yarn pull-out behavior of kevlar® 49 fabric

Single yarn pull-out behavior of Kevlar® 49 fabric with varying lengths and pre-applied transverse tensile forces is studied. Results indicate that both peak pull-out force and energy needed to pull an individual yarn increase with specimen length and transverse pre-load. An analytical model is used to investigate the frictional shear properties diagram between the orthogonal warp and fill yarns. Maximum static and dynamic frictional shear strength of contact interfaces of warp and fill yarns are obtained by fitting the experimental data. The periodic nature of the post-peak behavior of pull-out response is simulated by representing the contact area as a sinusoidal function with a constant period. Finally, a three-dimensional finite element model is used to simulate the single yarn pull-out behavior and the results show that pull-out force is significantly influenced by the friction between yarns and transverse pre-load.

Evaluation of a Lumbar Intervertebral Spacer With Integrated Screws as a Stand-alone Fixation Device

An in vitro cadaveric biomechanics study. To evaluate the biomechanical contribution of the integration of screws into a polyether-ether-ketone (PEEK) spacer, and to compare the resulting construct to standard anterior lumbar interbody fusion constructs. Pedicular fixation is often performed to increase the rigidity of anterior lumbar interbody fusion constructs but also increases the risk of morbidity. Integration of screws into a PEEK spacer (investigational device, ID) may increase construct rigidity and prevent migration without the drawbacks associated with supplementary fixation. Twenty cadaveric motion segments were potted and tested under pure moments of + or - 7.5 Nm in flexion-extension, lateral bending, and axial torsion. Discectomies were performed and specimens were instrumented with the ID + or - screws, dual threaded cages, structural graft+anterior plate, and graft+pedicle screws. The ID+screws and threaded cage constructs were then pulled out at a constant rate. All constructs reduced the mean range of motion (ROM) below the intact level in flexion-extension and lateral bending (P<0.001) and for all devices except cages in torsion (P < or = 0.05). The median flexion-extension/bending/torsion ROM was 9.5/9.4/4.1 degrees for the intact segments, 6.1/5.1/1.8 degrees with the ID+screws, 4.9/5.2/2.4 degrees with threaded cages, 3.3/4.4/1.1 degrees with plates and 1.4/1.6/1.7 degrees with pedicle screws, respectively. The addition of the integrated screws decreased the mean ROM of the ID constructs by 0.9 degrees in flexion-extension, 1.8 degrees in bending, and 2.8 degrees in flexion-extension. The peak pullout force was 962 N for the ID and 337 N for threaded cages. The incorporation of screws into a PEEK interbody device was evaluated alongside traditional constructs in terms of construct rigidity and resistance to pullout. The greatest contributions of the integrated screws are in limiting torsional motion and in the containment of the interbody device. Pedicular fixation produces the most rigid constructs, but integrating screws into a PEEK spacer produces a stand-alone construct that is similar to threaded cages and provides more resistance to anterior displacement.

Testing and modeling of yarn pull-out in plain woven Kevlar fabrics

Yarn pull-out behavior of five styles of plain woven Kevlar fabrics, K310, K706, K720, K745 and K779 from Hexcel, was studied. These fabrics were different in fabric count, yarn size, fiber type, thickness and weight. Single yarn pull-out tests showed that the pull-out forces of the five styles of fabrics differ substantially. The yarn pull-out force was found to have positive correlation to the impact performance: fabrics with higher pull-out force performed better in impact tests. A 2D finite element model was proposed to simulate the single yarn pull-out procedure and predict the maximum pull-out force. Several factors were considered in this model: fabric count, yarn size, yarn waviness, fiber modulus, fiber diameter, cross-yarn friction, parallel-fiber friction. The predicted peak pull-out forces showed good agreement with the experimental results. Finally a simple formula was proposed to estimate the yarn pull-out force as a function of fabric count, fiber diameter, fiber modulus, yarn waviness and friction.

Rebar pullout testing under dynamic Hopkinson bar induced impulsive loading

The dynamic behavior of the bond slip between a deformed reinforcing bar and plain concrete has been experimentally investigated by employing Hopkinson bar techniques. Pullout tests with various specimen types (unconfined, confined, cast-in-place, post-installed etc.) have been performed. Pullout of the steel rebar and splitting of the concrete cylinder have been the failure modes induced. Test results comprise peak pullout forces and complete bond stress–slip diagrams. They clearly show that the dynamic pullout forces and curves are well above the static ones, and that the pullout work of bond failure is considerably greater for the dynamic impact loading. Confinement, provided by a steel tube, leads to improved bonding; peak loads increase up to 2.5 times. The effects of bond length and concrete strength have also been put into evidence. Finally it has been verified that post-installed rebars, depending upon the particular adhesive employed, can achieve the same bond resistance as the cast-in-place ones.

Enhancement of Periprosthetic Bone Quality with Topical Hydroxyapatite-Bisphosphonate Composite

Implant loosening is associated with inflammatory bone loss induced by ultra-high molecular weight polyethylene wear debris. We hypothesized that a hydroxyapatite-bisphosphonate composite improves periprosthetic bone quality and osseous integration of an intramedullary implant even in the presence of ultra-high molecular weight polyethylene particles in an experimental rat femur model. A preliminary in vitro study determined the optimal concentration of zoledronate (50 microM) that would maximally decrease osteoclasts without harming osteoblasts. Hydroxyapatite-coated intramedullary nails were implanted bilaterally in the femora of sixteen rats (the control group), and hydroxyapatite-zoledronate-coated nails were implanted bilaterally in the femora of sixteen rats (the experimental group). Ultra-high molecular weight polyethylene particles were introduced into the femoral canal before implantation. Eight rats from each group were killed at six weeks, and the remaining rats were killed at six months. Periprosthetic bone mass was analyzed by dual x-ray absorptiometry and microcomputed tomography. Osseous integration was examined by biomechanical testing of pullout strength. The mean bone area (and standard deviation) in the periprosthetic bone region was significantly greater (p < 0.0001) in the hydroxyapatite-zoledronate group (2.388 +/- 0.960 mm2) than in the control group (0.933 +/- 0.571 mm2). This difference was larger in the six-week group than in the six-month group (p = 0.03). The average peak pullout force for the treated femora (241.0 +/- 95.1 N) was significantly greater (p < 0.0001) than that for the controls (55.6 +/- 49.0 N). This difference was similar in the six-week and six-month groups. The energy required for nail pullout was significantly greater (p < 0.0001) for the treated femora (521.6 +/- 293.8 N-mm) than for the controls (142.2 +/- 152.1 N-mm). This difference in energy to pullout was similar in the six-week and six-month groups. Regression analysis demonstrated a high correlation between periprosthetic bone mass and peak pullout force for both the six-week (r = 0.766, p = 0.0005) and six-month (r = 0.838, p < 0.0001) groups. Surface modification of implants with hydroxyapatite-zoledronate improves periprosthetic bone quality and osseous integration. Hydroxyapatite-based site-specific delivery of bisphosphonates may be one way of reducing ultra-high molecular weight polyethylene wear particle-induced periprosthetic osteolysis and implant loosening.

A Novel Biomimetic Material for Engineering Postsurgical Adhesion Using the Injured Digital Flexor Tendon–Synovial Complex as an In Vivo Model

Many surgical procedures are complicated by adhesions. These restrictive fibrotic bands form between normally separate gliding tissue layers, potentially impairing function. The authors tested the adhesion-modifying effect of a novel fibronectin-derived biomimetic biomaterial in a tendon-synovial complex injury model. The deep flexor tendons of digits 2 and 4 in the right forepaw of 15 New Zealand White rabbits were subjected to 5-mm-long partial tenotomies. Animals were randomized to receive biomaterial tubes enveloping the tendon injuries or left untreated. Digits, amputated at 2 weeks, were randomized to mechanical pullout assessments of adhesion strength or to quantitative histologic cellularity and immunohistochemical proliferation (Ki67) assessments. The mean peak pullout force required to break the adhesions was reduced from 7.70 N (n = 6) in untreated digits to 0.31 N (n = 7) in biomaterial-treated digits (p = 0.001). The mean structural stiffness of the adhesions was also significantly reduced (p = 0.001). Histologically, treated and untreated digits demonstrated an equal incidence of adhesions. The treated adhesions were 55 percent less cellular at their surface than the untreated injured controls (p = 0.003). Treated tendons were 8 percent more cellular (with equal numbers of proliferating cells) at their surface and significantly more cellular within their bulk than positive controls (p <or= 0.05). This study suggests a significant reduction in the restrictive nature of postsurgical adhesions following treatment with the antiadhesive biomaterial without compromising tendon cellularity.

Effects of angle and laminectomy on triangulated pedicle screws

We aimed to demonstrate the effect of angle and laminectomy on paired pedicle screws to determine whether a 90° screw angle is optimal as has been previously suggested. According to the angle between right and left screws, 28 calf vertebrae were divided into three groups and instrumented as follows: Group I: 60° screw angle; Group II: 90° angle; Group III: 60° angle with laminectomy. The screws were connected using rods and cross-fixators and tested to peak pullout force. Triangulated pedicle screws provided 76.5% more pullout strength than single screws. Most of the specimens failed through loss of convergence angle (toggling of screws on the rods) and subsequent uni- or bilateral screw pullout. Mean ± SD peak loads were: Group I: 2071 ± 622 N; Group II: 1753 ± 497 N; Group III: 2186 ± 587 N. The differences were not significant ( p > 0.05). 90° triangulation was not associated with a superior pullout performance versus conventional 60° triangulation, suggesting that achieving additional triangulation angle is not necessary to obtain increased pullout strength. Laminectomy did not alter the effect of triangulation on fixation strength.

Soil–Nail Pullout Interaction in Loose Fill Materials

A laboratory study was conducted to investigate the behavior of soil nails embedded in loosely compacted sandy fills. By varying the overburden pressure, the peak pullout force and the load–displacement behavior were determined by carrying out pullout tests in a displacement-rate controlled manner. The test results were compared to other published ones. The present results show that the pullout resistance can be interpreted with conventional soil parameters. The effect of retrained dilatancy, which is considered to be the reason for high pullout resistance in dense materials, is negligible in loose fill materials except under very low stress level. Furthermore, pullout resistance increases with overburden pressure opposed to some field test results reported in the literature which show no systematic trend in pullout resistance with overburden pressure. A numerical model was developed to simulate the mobilization of pullout force in soil nails. It has been shown that a simple one-dimensional spring model can be used to simulate the pullout load–displacement relationship.

Discrete element modelling of geogrid-reinforced aggregates

A discrete element model has been developed for geogrid-reinforced ballast. A model for unreinforced ballast has first been developed and evaluated using simulations of large-scale triaxial experiments and comparing with available data. A model for the geogrid has also been developed by bonding many small balls together to form the desired geometry and evaluated by simulating standard tests. The discrete element method (DEM) has then been used to model the interaction between ballast and geogrid by simulating pull-out tests and comparing with experimental data. The DEM simulations have been shown to predict well the peak mobilised resistance and the displacement necessary to mobilise peak pull-out force. The effect of the ratio of the geogrid aperture size to ballast particle diameter on pull-out resistance has also been investigated. The zone of influence of the geogrid has also been examined by simulating larger-scale pull-out tests in addition to cyclic triaxial tests. The method holds much promise as a tool for investigating aggregate–geogrid composite systems with a view to choosing appropriate geometries to optimise performance.

Biomechanical Evaluation of the Pullout Strength of Cervical Screws

In the process of anterior cervical fusion, little is known about the biomechanics of anterior cervical screw pullout. In this study, three different aspects of cervical screw fixation were evaluated: self-tapping (ST) versus self-drilling (SD) screws, the effect of screw geometry (length, diameter, thread pitch), and the use of rescue screws. Nine screws consisting of different diameters, lengths, and thread pitch (cancellous and cortical) were tested in peak pullout force in an artificial bone model using an MTS 858 Mini Bionix test system. Rescue screws (4.5 mm) were then inserted in the failed holes of 4.0-mm screws and extracted to determine their holding strength. Length of screws and thread pitch both had a significant effect on the pullout force. Each 1 mm of increased screw length translates to 16 N of increased force to pullout in the foam bone model. Pullout strength did not vary significantly according to screw diameter or between SD and ST screws. However, the SD screw has an advantage because it can decrease the length of surgery. A decrease in pullout force of between 43% and 70% was found when using rescue screws. In situations in which the use of rescue/salvage screws is required, the surgeon should anticipate a significant decrease in the holding force compared with the original screw. Future directions for research include an evaluation of pullout force for screw and plate constructs.

Interaction between Reinforcing Geosynthetics and Soil-Tire Chip Mixtures

The objective of the present study was to evaluate the mechanical properties of the tire chips and soil-tire chip mixtures relevant to geosynthetic-reinforced earthworks. Tests were conducted to evaluate shear strength and pull out capacity with a woven geotextile and two geogrids. Soil-tire chip mixtures made with clean sand and sandy silt were tested. These properties were then used to assess the potential advantages of using soil-tire chip backfills for geosynthetic-reinforced retaining walls and embankments. The test results show that the geosynthetic pull-out force in tire chip and soil-tire chip backfills increases with displacement—i.e., no peak pull-out force is generally obtained, at least for displacements ≤100 mm. Pull-out interaction coefficients for tire chip backfills are typically greater than 1, whereas for soil-tire chip backfills they typically range between 0.2 and 0.7, even though the pull-out capacity for soil-tire chip backfills is generally similar to or greater than the pull-out capacity in a soil backfill. The higher strength, lower unit weight and good backfill-geosynthetic interaction obtained with soil-tire chip backfills can result in walls requiring less geosynthetic reinforcement than walls backfilled with soil. In addition, embankments can potentially be constructed with steeper slopes and a smaller volume of material when soil-tire chip fill is used, while providing greater resistance against lateral sliding and foundation settlement.

Load Transfer Mechanism in Pull-Out Tests

This paper presents a numerical method to predict soil-geotextile interface friction parameters. A strain softening model was used to simulate the relationship between shear stress and horizontal displacement at the soil-geotextile interface. Pull-out tests were performed on two types of geotextile, with different tensile stiffnesses, embedded in a granular soil. For each geotextile, pull-out tests were performed at confining pressures of 20, 50, 100, and 200 kPa. It was found that, unless breakage of the geotextile occurs, the peak pull-out force occurs after a small displacement of the free end of the geotextile. At the peak pull-out force, the maximum shear stress occurs near the free end of the geotextile while the shear stress at the loaded end is at or near a residual value. The use of an average interface friction angle overestimates pull-out resistance. The numerical technique developed in this paper provides more accurate values for soil-geotextile interface friction parameters.

Influence of cutting technology on initial fixation strength and thread quality of different bone pins

A comparison of the fixation strength was made for the Steinmann, Denham, and the Apex commercial bone pins, which have different drilling end geometry and self-tapping thread design. Additional tests were made to assess the effects of pre-drilling with an orthopaedic twist drill bit prior to pin insertion. Peak pull-out force and pin displacement were measured for these pins inserted into the distal metaphysis of human cadaveric tibiae. The quality of thread reproduction in the bone resulting from the cutting process was assessed from thread profiles of negative moulds cast into the threaded holes. The Apex pins had similar pull-out forces to the Denham pins, despite their smaller diameter. Values for both pins were significantly greater than those for the Steinmann pin. The thread profiles showed that the Denham pin caused substantial damage to the cortical bone and formed poor quality threads in the cancellous bone. The Apex pin formed a better thread, which was improved further by the use of a pre-drilled pilot hole.

The biomechanical efficacy of an oscillating K-wire driver.

A new oscillating drill, designed for insertional ease and safety, was biomechanically tested using an MTS 812 servohydraulic material testing machine (Instrom). Holding strength and drilling force were compared against a traditional rotary drill using rabbit tibias to approximate the diameter and cortical thickness of human metacarpals. Three differently sized (0.028-, 0.045-, and 0.062-in. diameter) K-wires were evaluated for each drill. Mean peak axial load (drilling force) was significantly different for each wire driver type in each of the three sized K-wires (p < or = 0.01). The oscillating drill used an average of 3 N less force to penetrate either cortex. Mean peak pull-out force (holding strength) was higher for the oscillating drill, but the difference was not significant (p > or = 0.5). This study indicated that the oscillating drill required less force for insertion and had a similar holding strength as the rotary drill. In addition, our drill has been shown to be less damaging to surrounding soft tissues. This new type of drill is likely to be advantageous in the fields of hand, microsurgery, orthopedic, and plastic surgery, where bone fixation is often near vital neurovascular structures.

A primate model for the evaluation of tendon adhesions

A cynomologous monkey animal model was developed for future pharmacological trials to reduce tendon adhesions. When developing an animal model, similarity to human anatomy and severance of motor units from operated tendons are stressed. Differences in vincular anatomy among various laboratory animals and incomplete postoperative tendon-motor unit immobilization in previous studies tend to distort results. Peak pull-out forces were measured on the Instron tensionmeter as operated tendons that underwent surgical procedures on Zone II were removed from their sheaths and compared to controls at various times postoperatively. Increasing pull-out force was obtained in a reproducible fashion.

A primate model for the evaluation of tendon adhesions

A cynomologous monkey animal model was developed for future pharmacological trials to reduce tendon adhesions. When developing an animal model, similarity to human anatomy and severance of motor units from operated tendons are stressed. Differences in vincular anatomy among various laboratory animals and incomplete postoperative tendon-motor unit immobilization in previous studies tend to distort results. Peak pull-out forces were measured on the Instron tensionmeter as operated tendons that underwent surgical procedures on Zone II were removed from their sheaths and compared to controls at various times postoperatively. Increasing pull-out force was obtained in a reproducible fashion.

Biomechanical Effects of Point Configuration in Kirschner-Wire Fixation

The drilling and bone holding properties of Kirschner wires of three different tip designs have been investigated. The tip designs studied were the commercially available trocar- and diamond-shaped tips and the homemade cut tip produced by cutting the pin obliquely with wire cutters. Thirteen canine metacarpals were drilled with wires of each tip design at 400 and 800 rpm. A servohydraulic test machine (Materials Test Systems, Minneapolis, MN) simultaneously measured axial loads, torque, and displacement during insertion and pull-out. Kirschner wires of each tip configuration were also placed in seven canine metacarpals for a three-week in vivo test. After the animals were killed, peak pull-out forces were measured and compared with in vitro results. The trochar-tipped wire displayed the highest required torque and axial load to penetrate bone. It also demonstrated the highest pull-out force immediately after drilling, especially when inserted at the low drill speed. After placement for three weeks, pull-out force of the diamond-tipped wire equaled the trochar. The homemade cut wire consistently yielded inferior holding properties. For immediate and sustained strong fixation in bone, a wire with a trochar tip configuration should be inserted at low drill speeds.