Push-out Force Research Articles

Bone–implant interface strength and osseointegration: Biodegradable magnesium alloy versus standard titanium control

Previous research on the feasibility of using biodegradable magnesium alloys for bone implant applications mainly focused on biocompatibility and corrosion resistance. However, successful clinical employment of endosseous implants is largely dependent on biological fixation and anchorage in host bone to withstand functional loading. In the present study, we therefore aimed to investigate whether bone–implant interface strength and osseointegration of a novel biodegradable magnesium alloy (Mg–Y–Nd–HRE, based on WE43) is comparable to that of a titanium control (Ti–6Al–7Nb) currently in clinical use. Biomechanical push-out testing, microfocus computed tomography and scanning electron microscopy were performed in 72 Sprague–Dawley rats 4, 12 and 24 weeks after implantation to address this question. Additionally, blood smears were obtained from each rat at sacrifice to detect potential systemic inflammatory reactions. Push-out testing revealed highly significantly greater maximum push-out force, ultimate shear strength and energy absorption to failure in magnesium alloy rods than in titanium controls after each implantation period. Microfocus computed tomography showed significantly higher bone–implant contact and bone volume per tissue volume in magnesium alloy implants as well. Direct bone–implant contact was verified by histological examination. In addition, no systemic inflammatory reactions were observed in any of the animals. We conclude that the tested biodegradable implant is superior to the titanium control with respect to both bone–implant interface strength and osseointegration. These results suggest that the investigated biodegradable magnesium alloy not only achieves enhanced bone response but also excellent interfacial strength and thus fulfils two critical requirements for bone implant applications.

The effect of strontium-substituted hydroxyapatite coating on implant fixation in ovariectomized rats

This study was designed to investigate the effects of strontium-substituted hydroxyapatite coatings with 10 mol% Ca 2+ replaced by Sr 2+ (10% SrHA) on implant fixation in ovariectomized (OVX) rats. Coatings of HA and 10%SrHA were prepared on the surface of titanium implant using sol-gel dip methods, and then characterized by Scanning Electron Microscope, Atomic Force Microscope, X-ray diffraction, X-ray Photoemission Spectroscopy, and an automatic scratch tester. Twelve weeks after bilateral ovariectomy, twenty OVX rats accepted implant insertion in the proximal tibiae, half with HA-coated implants and the other half with 10% SrHA coated implants. After 12-week healing period, 10% SrHA coated implants revealed improved osseointegration compared to HA, with the bone area ratio and bone-to-implant contact increased by 70.9% and 49.9% in histomorphometry, the bone volume ratio and percent osseointegration by 73.7% and 45.2% in micro-CT evaluation, and the maximal push-out force and ultimate shear strength by 107.2% and 132.9% in push out test. These results demonstrated that 10% SrHA coatings could enhance implant osseointegration in OVX rats, and suggested the feasibility of using SrHA coatings to improve implant fixation in osteoporotic bone.

Additive effects of estrogen replacement therapy and bisphosphonates on osseointegration of hydroxyapatite-coated titanium screws in ovariectomized rats

This study investigated the effect of locally immobilized bisphosphonates and systemic estrogen replacement therapy on fixation of hydroxyapatite-coated screws in ovariectomized rats. Twelve weeks after being ovariectomized, each of the 20 rats received 2 hydroxyapatite-coated screws in the proximal tibia, with or without immobilized bisphosphonates. Half of the animals also received subcutaneous injections of 17beta-estradiol. Thus, there were 4 groups: control, estrogen, bisphosphonates, and estrogen plus bisphosphonates. Tibiae with screws were measured by microscopic computerized tomography and push-out test 12 weeks after implantation. Combined treatment had the strongest effects, with the bone volume ratio around screws and the percentage of osteointegration significantly increased by 2.3-fold and 1.9-fold and the push-out force by 3.3-fold compared with untreated control samples. Monotherapy also enhanced the fixation of screws, although less. Systemic estrogen replacement therapy and locally immobilized bisphosphonates had additive effects on fixation of hydroxyapatite-coated screws in ovariectomized rats.

The effect of hydrofluoric acid treatment on titanium implant osseointegration in ovariectomized rats

This study aimed to investigate the effects of hydrofluoric acid (HF) treatment of grit-blasted Ti implants on osseointegration in ovariectomized (OVX) rats. After blasting with aluminium oxide particles, half implants were treated with 0.2 vol.% HF, and the other half were kept non-modified as control. The topographical and chemical changes of implant surface were determined by Scanning Electron Microscope, Atomic Force Microscope, and X-ray Photoemission Spectroscopy. 12 Weeks after bilateral ovariectomy, each rat accepted two implants in distal femora, with the control implant on the left and the fluoride-modified on the right. As a result, fluoride modification induced markedly changed surface topography and chemical composition. 12 Weeks after implant insertion, the fluoride-modified implants showed improved osseointegration compared to control, with the bone area ratio and bone-to-implant contact increased by 0.9- and 1.4-fold in histomorphometry, the bone volume ratio and percent osseointegration by 0.8- and 1.3-fold in micro-CT evaluation, and the maximal push-out force and ultimate shear strength by 1.2- and 2.0-fold in biomechanical test. These promising results indicated that HF treatment of Ti surface improved implant osseointegration in OVX rats, and suggested the feasibility of using fluoride modification to improve Ti implant osseointegration in osteoporotic bone.

Effect of implant surface roughness on bone fixation: the differences between bone and metal pegs

BackgroundPegs made from cortical bone are used to fix osteochondral fractures and osteochondral dissecans. This technique has many advantages, but it requires long-term immobilization. This study examined the effect of surface roughness on fixation with bone and metal pegs. MethodsPegs with either rough or smooth surfaces were made of cortical bone from Japanese black cattle or from stainless steel (SUS316L). The mean roughness of the rough surface was 15.0 |xm, whereas that of the smooth surface was less than 0.6 |xm. Pegs were inserted into holes made in the distal femurs of 34 rabbits. At the time of surgery and 14 days later, mechanical tests and micro-computed tomography were performed. ResultsAt the time of surgery, although the push-out forces were less than 0.3 N, the rough surface had a higher value than the smooth surface (P = 0.0002). No difference was observed according to the material (P = 0.54). Fourteen days after surgery, no significant difference was detected in the push-out forces between bone pegs with rough and smooth surfaces (489.0 ± 149.6 vs 478.3 ± 134.4 N (mean ± SD), respectively), but a marked difference was seen with the metal pegs (235.7 ± 115.7 vs 2.2 ± 1.6 N). The bone pegs with rough surfaces made contact with the recipient bone at the high points on the abraded surfaces. After the mechanical tests, the fusion was broken within the new bone for bone pegs with rough or smooth surfaces, but no breakage occurred at the junction of bone peg and new bone. ConclusionThe surface roughness of bone pegs has little effect on bone-to-bone fusion 2 weeks postoperatively, unlike the effect with metal pegs.

Load bearing capacity of bone anchored fiber-reinforced composite device

The purpose of this study was to evaluate the push-out load-bearing capacity of threaded fiber-reinforced composite (FRC) devices for use as bone-anchored devices. The purpose was also to evaluate the possibility to use bioactive glass (BAG) granules on the experimental FRC devices in terms the mechanical behavior. Three experimental FRC devices (n = 15) were fabricated for the study: (a) threaded device with smooth surface; (b) threaded device with BAG granules (S53P4, Vivoxid Ltd, Turku, Finland) and supplementary retention grooves, and (c) unthreaded device with BAG granules. Threaded titanium devices were used as controls. The FRC devices were prepared from a light-polymerized dimethacrylate resin reinforced with preimpregnated unidirectional and bidirectional E-glass fibers (EverStick, StickTech Ltd, Turku, Finland). Experimental and control devices were embedded into dental plaster to simulate bone before the mechanical push-out test was carried out. ANOVA and Weibull analysis were used for the statistical evaluation. Threaded FRC devices had significantly higher push-out strength than the threaded titanium device (p < .001). The push-out forces exceeding 2,500 N were measured for threaded FRC devices with supplementary grooves and BAG coating. No thread failures were observed in any FRC devices. The unthreaded FRC devices with BAG lost 70% of glass particles during the test, while no BAG particles were lost from threaded FRC devices. It can be concluded that threaded FRC devices can withstand high push-out forces in the dental plaster without a risk of thread failure under physiological load.

The effect of water uptake on the behaviour of hydrophilic cements in confined environments

Physiological fluids will be in contact with the implant components from the first moments after a surgery. Therefore, the study of the effect of water on the properties of the bone cements that are part of the arthroplasty procedure is of critical importance to predict the long-term performance of the whole system. In our research group, we have developed a novel concept, the hydrophilic, partially degradable and bioactive cements which uptake considerably more water than standard bone cements. In this paper, we aimed to study the effect of water uptake (WU) by these cements on their behaviour. The tests were carried out in confined cavities, which represent more accurately the in vivo situation the cement will face (constrained by the bone and prosthesis surfaces). We observed that the equilibrium WU decreased up to 60% (as compared to non-confined situations), depending of the formulation. This decrease resulted in a latent tendency of the cements to swell, and the hindering of such swelling generated a swelling pressure against the constraining walls. The pressure, and consequent press-fitting effect, could be controlled by a number of mechanisms, and resulted in higher stability of the hydrophilic cements, expressed as an increase in the push-out force, required to extract the specimens from such constrained cavities. This effect was only observed in hydrophilic cements, not in commercial, hydrophobic ones used as controls. We conclude that such cements will provide an additional and very useful source of immediate adhesion in the short-term after surgery: water induced press fitting.

Fibre-reinforced composite implant: in vitro mechanical interlocking with bone model material and residual monomer analysis

The aim of this study was to examine in vitro the mechanical interlocking of an experimental implant made of E-glass fibre-reinforced polymethyl methacrylate (PMMA)-based composite (FRC) to dental stone. FRC implants with a porous surface were embedded into the dental stone, which was chosen to simulate bone ingrowth into the porous surface of the implant, after which push-out tests were performed. PMMA cylinders with smooth and grooved surface were used as controls. In addition, the release of residual methyl methacrylate monomer (MMA) into water from FRC and control implants with different compositions and fabrication methods was determined using high performance liquid chromatography (HPLC). The highest push-out force (2149 ± 263 N) was measured for the implants with grooved surface and the lowest value for the implants with smooth surface (194 ± 68 N). The push-out forces were over five times higher for FRC implants with a porous surface (958 ± 217 N) than for implants with smooth surface. During the first day of testing, the MMA release into water was 1.4–2.8 times higher from the FRC implants than from the control PMMA implants, depending on fabrication method. With time, the difference between the implants diminished.

Bond strength of concrete plugs embedded in tubular steel piles under cyclic loading

Investigation of the load transfer of concrete plugs to tubular steel piles subjected to tension and compression and cyclic loading has been conducted at Monash University over the past 3 years. The work presented in this paper reports on the results of the combination of pull-out, push-out, and cyclic loading tests carried out on 15 steel tube specimens filled partially with reinforced concrete with variable lengths of embedment. The pull-out force was applied through steel reinforcing bars embedded in the concrete plug, and push-out forces were applied through a thick top circular plate on the top of the concrete plug. Test results included the cyclic loading, ultimate pull-out and push-out forces, slip of concrete plugs, and longitudinal and hoop strains along the piles for some specimens. The tests clearly showed that average bond strength significantly exceeds expectations and is higher than the results of previous investigations using plugs without reinforcement. The test results also indicated that cyclic loading tests reduced the bond strength due to the accumulation of damage to the plug–pile interface. The push-out and pull-out tests conducted under symmetric cyclic loading demonstrated that slip between the concrete plug and the steel tube increased with repeated loading, and the rate of slip growth increased with an increase in the peak load.Key words: tubular steel pile, reinforced concrete plug, bond, cyclic loading.

Histomorphometric and mechanical analysis of the hydroxyapatite-bone interface after electromagnetic stimulation

We investigated the effect of stimulation with a pulsed electromagnetic field on the osseointegration of hydroxyapatite in cortical bone in rabbits. Implants were inserted into femoral cortical bone and were stimulated for six hours per day for three weeks. Electromagnetic stimulation improved osseointegration of hydroxyapatite compared with animals which did not receive this treatment in terms of direct contact with the bone, the maturity of the bone and mechanical fixation. The highest values of maximum push-out force (F(max)) and ultimate shear strength (sigma(u)) were observed in the treated group and differed significantly from those of the control group at three weeks (F(max); p < 0.0001; sigma(u), p < 0.0005).

Augmentation of osteoporotic bone: Effect of pulsed jet‐lavage on injection forces, cement distribution, and push‐out strength of implants

Demographic change in the population leads to higher incidence of fragility fractures. Fracture fixation with standard implants may lead to implant cut-out due to reduced purchase. Augmentation of the bone stock with bone cements might overcome this problem. However, cancellous bone infiltration with the viscous cement dough reveals problems of fat embolism or high pressures during application of the cement. This study investigates the improved quality of bovine cancellous bone augmentation when pulsed jet-lavage is used for fat and marrow removal. Parameters such as injection forces, cement dough distribution through cannulated implants and mechanical strength of the fixation were applied for quantification. Injection of 5 mL of acrylic bone cement required significantly lower forces in the lavaged as compared to the untreated bone (50 N vs. > 300 N). Cement distribution was much more homogeneous and push-out forces significantly higher in the pretreated bone group (8.33 +/- 1.41 kN vs. 1.66 +/- 0.63 kN). The application of pulsed jet-lavage for fat removal prior to acrylic cement augmentation led to much more controlled outcomes of the augmentation. This seems to be a relevant step towards safe and efficient injection of bone cements into cancellous bone structures.

Bonding of fibre-reinforced composite post to root canal dentin

The aim of this study was to determine bonding properties of two types of fibre-reinforced composite (FRC) posts cemented into root canals of molars. Serrated titanium posts served as reference. Prefabricated carbon/graphite FRC posts with cross-linked polymer matrix and individually formed glass FRC posts with interpenetrating polymer network (IPN) polymer matrix were compared. The crowns of extracted third molars were removed and post space (diameter: 1.5mm) was drilled, etched and bonded. The posts were treated with dimethacrylate adhesive resin, light-polymerized and cemented with a dual-polymerizing composite resin luting cement. After thermocycling (6000x) the samples were cut into discs of thicknesses: 1, 2 and 4mm (n=12/group). Push-out force was measured by pushing the post from one end. Assessment of failure mode was made under a stereomicroscope (1, adhesive failure between post and cement; 2, cohesive failure of post-system; 3, adhesive failure between cement and dentin). The push-out force increased with increased height of dentin disc in all groups (ANOVA, p<0.001). In the 4mm thick dentin discs the individually formed glass FRC posts showed highest push-out force and the difference to that of the titanium posts was significant (ANOVA, p<0.001). The other differences were not statistically significant. None of the individually formed glass FRC posts showed adhesive failures between the post and the cement. Contrary to the other posts, there were no adhesive (post-cement) failures with the individually formed glass FRC posts, suggesting better interfacial adhesion of cement to these posts.

Influence of Gliding Cable Joint on Mechanical Behavior of Suspen-Dome Structures

As a new type of spatial structure system, a suspen-dome structure shows good mechanical behavior, because its cable layers with pre-stress do not only improve the stability and stiffness of the top lattice shell of the structure but also decrease the push-out forces to its hoop beam. There are two kinds of connection forms on the common joints among hoop cable, radial cable and struts. One is continuous gliding cable joint where individual cable can move independently at the same joint, and the other one is no-gliding cable joint (discontinuous cable joint) where different cables at the same joint move in the same direction. These two forms will lead to different structural behavior, such as internal force distribution of cable layers when the structure is subjected to half-span loads. In this paper, frozen-heated method (FHM) is employed to analyze this problem. And, the results obtained show that the first joint form can adjust internal forces and decrease the peak value of internal forces of hoop cables and radial cables, while it has little influence on the mechanical behavior of the top lattice shell of the structure.

Locked internal fixator: sensitivity of screw/plate stability to the correct insertion angle of the screw.

Internal fixators with angular stability have been developed to provide high stability without compression of the plate on to the bone. Angular and axial stability of a plate-screw construct can be achieved using a conically threaded screw head undersurface and a corresponding conically threaded plate hole. Furthermore, the insertion angle of the screw must correspond precisely to the axis of the screw hole. This is not always achieved in clinical practice and may result in screw loosening. The objective of this study was to examine the relationship between the stability of the locked screw-plate on the insertion angle of the screw. Locking screws were inserted in an isolated (Point Contact Fixator, PC-Fix) or combined (Locking Compression Plate, LCP 4.5) locking hole with the use of an aiming device. The optimal insertion angle for these plates is perpendicular to the plate surface. The screws were inserted with an axis deviation of 0 degrees (optimal condition), 5 degrees , and 10 degrees respective to the optimal angle (variance +/- 1 degrees ). The samples were tested under shear or axial (push out) loading conditions until failure occurred. An Instron materials testing machine was used. Locking screws inserted in the isolated locking hole (PC-Fix) showed a significant decrease of failure load if inserted at 5 degrees and 10 degrees angle. Using an optimal insertion angle (0 degrees ), failure load was 1480 +/- 390 N, with 5 degrees axis deviation 780 +/- 160 N, P = 0.0001, and with 10 degrees axis deviation 550 +/- 110 N, P = 0.0001. Screws inserted in the combined locking hole (LCP) also showed a significant decrease of push-out force of 77% (4960 +/- 1000 N versus 1120 +/- 400 N) with 10 degrees axis deviation. Compared to optimal insertion angle (0 degrees ), bending load to failure did decrease up to 69% (1240 +/- 210 N vs. 390 +/- 100 N) with 10 degrees axis deviation. A locking head screw exhibits high stability with a moderate axis deviation in the angle of insertion of up to 5 degrees . However, there is a significant decrease in stability with increasing axis deviation (>5 degrees ). An aiming device is recommended to provide optimal fixation with angular stability.

The effect of different power densities and method of exposure on the marginal adaptation of four light-cured dental restorative materials

Purpose: To determine whether marginal adhesion is sensitive to different irradiation parameters, we investigated the in vitro adhesion values of four dental resins on metal surfaces. Methods: Four groups of eight specimens each of Z250, Filtek flow, Dyract AP and Dyract flow were placed in pre-treated stainless steel cavities and irradiated using different methods of exposure. The curing lights used were a Spectrum 800 halogen curing light at settings of 800 and 450 mW/cm 2 and an Optilux 501 ramping light. The maximum amount of push-out force required to displace the resin from the metal cavity was equated with adhesive value (shear bond strength). Comparisons (ANOVA, p<0.0001) were made within the same material and between the different materials when using different curing protocols. Results: Significant lower bond strengths were recorded when curing was done by gradually increasing the intensity (ramping method) compared to curing with the fixed intensities ( p>0.0001) Comparing the fixed intensities, significant lower bond strength values were obtained at 800 mW/cm 2 compared to 450 mW/cm 2 ( p<0.0001). For all exposures, the two flowable materials demonstrated weaker values when compared to the higher filled materials. Clinical significance: The advantage of initial slow polymerization (more elasticity and less tension) obtained by the so-called “soft start” method, was offset by a rise in total polymerization shrinkage, when final curing was completed at 1130 mW/cm 2. These tests demonstrated that using halogen units, exposure for 40 s with an intensity of 450 mW/cm 2 appeared to be the most promising for light-curing dental resin composites.

Mechanical and histomorphometric evaluations of titanium implants with different surface treatments inserted in sheep cortical bone

Improvement of the implant–bone interface is still an open problem and the interest in chemical modification of implant surfaces for cementless fixation has grown steadily over the past decade. Mechanical and histomorphometric investigations were performed at different times on implants inserted into sheep femoral cortical bone to compare the in vivo osseointegration of titanium screws (∅ 3.5×7 mm length) with different surface treatments. After 8 weeks of implantation, the push-out force of anodized and hydrothermally treated implants (ANODIC) was significantly higher than that of machined implants (MACH) (36%, p<0.0005), whereas a decrease of 39% was observed for acid-etched implants (HF) when compared to other surface treatments. After 12 weeks of implantation, the push-out force values of HF implants were still significantly lower than those observed for MACH (−19%, p<0.01) and hydroxyapatite vacuum plasma-sprayed implants (HAVPS, −25%, p<0.0005), and the highest push-out force was found in HAVPS ( p<0.001) implants. After 8 and 12 weeks of implantation, the AI of HF implants was significantly ( p<0.05) lower (∼−25%) than that of MACH, HAVPS and ANODIC implants. In conclusion, results appear to confirm that there are no specific differences between ANODIC and HAVPS implants in terms of behavior. Moreover, although MACH implants show some surface contaminating agents, they appear to ensure good osseointegration within 12 weeks both mechanically and histomorphometrically, as do ANODIC and HAVPS implants. However, further studies are required to investigate bone hardness and mineralization around implants.

Dynamic Fiber Debonding and Frictional Push-out in Model Composite Systems: Experimental Observations

In the present work a modified Split Hopkinson Pressure Bar (SHPB) system is adopted to perform dynamic fiber push-out experiments on model single fiber composite systems. A tapered punch and a support connect a monofilament composite with the incident and transmitted bars of the SHPB. The tapered punch is used to apply compressive loading to a single fiber (either steel or aluminum) embedded in a surrounding matrix material (EPON 862). The SHPB allows real time measurement of relative fiber/matrix displacement and push-out force, as the debonding and push-out event progresses. Using this technique we have studied the effect of loading rate, material mismatch, fiber length, and surface roughness on the push-out event. It was seen that maximum push-out force increases with increasing loading rate. In addition dynamic interfacial strength and toughness is highly dependent on fiber surface roughness. Results from a finite element analysis incorporating a cohesive failure model were used to extract interface strength and toughness values. It was found that the particular aluminum/EPON interface used is characterized by a dynamic shear failure strength of 48±8 MPa, a mode II fracture toughness of 160±40 N/m, and a friction coefficient of 0.2 at a sliding rate of 6 m/s. For the rates tested here these quantities were found to be approximately constant.

Osseointegration of sandblasted or anodised hydrothermally-treated titanium implants: mechanical, histomorphometric and bone hardness measurements.

The improvement of the implant-bone interface is still an open problem in the long-term mechanical stability of cementless fixed implants. Mechanical, histomorphometric and bone hardness measurements were performed in sheep femoral cortical bone implants at 8 and 12 weeks from surgery to compare in vivo the osseointegration of titanium screws (psi 3.5 mm x 7 mm length) with two different surface treatments: sandblasting with 70-100 microm HA followed by acid etching with HNO3 (Group A) and Ca-P anodization followed by a hydrothermal treatment (Group B). No significant differences were found for maximum push-out force and interfacial strength between groups at both experimental times. No significant difference was observed for Bone Ingrowth between groups at both experimental times, while the Affinity Index of Group B was significantly higher (7.5%, p<0.05) and lower (10.2%, p<0.05) than that of Group A at 8 and 12 weeks, respectively. Finally, a significant increase in bone microhardness measured within 200 microm from the interface and inside the thread depth of Group A was observed between the two experimental times (p<0.05). In conclusion, present findings show that osseointegration may be accelerated by adequate surface roughness and bioactive ceramic coating such as current tested treatments which enhance bone interlocking and mineralization.

Determination of the role of the cancellous bone in generation of screw holding power at metaphysis

Objective. To determine whether cancellous bone at metaphysis plays a significant role in generation of holding power of the cancellous screw. Design. Maximal holding power of the cancellous screw at distal metaphysis of the femur with and without cancellous bone inside was determined by applying a load to push out the screw. Background. Generation of screw holding power from the cancellous bone can arise by a mechanism of compression of cancellous bone between screw threads when a screw is inserted by non-tapping technique. Metaphysis has intermediate amount of cancellous and cortical bone when compared with diaphysis and intercondyle of a long bone. Methods. Eight pairs of fresh cadaveric femurs were used. One femur of a pair was removed of cancellous bone at the distal metaphysis; cancellous bone of the other was preserved. A full thread cancellous screw was inserted into the distal femoral metaphysis. An axial load was applied at the screw tip to push out the screw by using a universal testing machine. Result. Mean push-out force of the screw at distal femoral metaphysis without cancellous bone inside was 1824.76 N and stiffness was 746.76 N/mm. Mean push-out force of the screw at distal femoral metaphysis with preservation of cancellous bone was 2015.86 N and stiffiness was 853.09 N/mm. The statistical analysis of both groups showed no significant differences. Conclusions. This study confirmed that cancellous bone at metaphysis plays no significant role in generation of holding power of the cancellous screw. Relevance Because metaphyseal cancellous bone plays no role in generation of screw holding power, only a well-inserted cancellous screw into bone cortices can achieve good screw holding power at metaphysis of a long bone.

Application of debond length measurements to examine the mechanics of fiber pushout

The interface failure sequence was observed during fiber pushout tests on several model composites. Composites with varying fiber-to-matrix moduli ratio ( E f \\ E m ), sample lengths, interface bond strength, and processing residual stresses were tested to determine which composites would debond from the top and which from the bottom. The present pushout experiments combined with previous work in the literature indicate that only composites with an E f \\ E m ratio greater than four tend to debond from the bottom during pushout testing, and debonding tends to initiate at the top of the interface when the E f \\ E m ratio is less than three. The debond length as a function of force and displacement was also measured in a polariscope for two of the model composites—steel\\epoxy and polyester\\epoxy. The pushout data from a polyester\\epoxy system that debonded from the top was fit to a shear lag solution of the fiber pushout problem to obtain the mode II toughness ( G II c ) of the fiber–matrix interface. The resulting interface toughness was then used to check the predicted debond length as a function of pushout force. The debond length calculated from the shear lag model was less than the measured debond length by a nearly constant 1.5 fiber radii.