Statistical distribution of the fatigue strength of porous bone cement

This paper reports on the damaging effects of different percentage porosities on the fatigue life of acrylic bone cement as used in the fixation of orthopaedic implants. Both hand-mixed (HM) and vacuum-mixed (VM) specimens containing different levels of porosity were fatigue tested to failure. A negative correlation between porosity level and fatigue life was demonstrated for both techniques. Considerable scatter was present in the data. Using the pore size distributions for HM and VM cement virtual HM and VM specimens were created containing various levels of porosity. Incorporating the effect of pore size and pore clustering quantified previously using the theory of critical distances a fatigue life prediction could be obtained for the virtual specimens. The virtual data agreed strongly with the experimental findings, predicting the correlation and more significantly the scatter in the experimental results. Using the virtual porosity failure model, it was demonstrated that given a constant porosity the fatigue life can vary by over an order of magnitude in both HM and VM cement. This suggests that not only porosity level but pore size distribution is extremely important in controlling the fatigue life of bone cement. It was verified that pore clustering and pore size are the major contributors to failure in HM and VM cement respectively. Furthermore, given the beneficial effects of porosity it has been proposed that an even distribution of small pores would provide an optimal bone cement mantle. Using the virtual model, it was determined that neither technique was capable of achieving such a distribution indicating a need for a new more reliable technique. The TCD based virtual porosity failure model should prove to be a powerful tool in the design of such a technique.     

Effect of mixing method and storage temperature of cement constituents on the fatigue and porosity of acrylic bone cement.

The influence of the storage temperature of the cement constituents prior to mixing (21 vs. 4 degrees C) and the mixing method (hand mixing vs. vacuum mixing) on the uniaxial tension-compression fatigue performance and porosity of Palacos R acrylic bone cement was studied. The fatigue results were analyzed using the three-parameter Weibull equation. The fatigue performance was expressed as an index I, which was defined as the product of the Weibull characteristic fatigue life and the square root of the Weibull slope. Statistical analyses of these results show that although the mixing method (for a given storage temperature) exerts a significant influence on the fatigue performance and areal porosity, the effect of storage temperature (for a given mixing method) on either of these parameters is not significant.     

A fractographic investigation of PMMA bone cement focusing on the relationship between porosity reduction and increased fatigue life.

Fracture surfaces of both monotonic and fatigue loaded bone cement samples were examined to investigate the fractographic characteristics of PMMA. Classic cleavage step river patterns were observed on all monotonically loaded samples, running downstream in the direction of crack propagation. All fatigue cracks initiated at internal pores and the direction of crack propagation of many cracks was discernible. Porosity, pore size, and pore size distribution were found to affect the crack initiation and fatigue behavior of bone cement. Statistical analysis revealed a strong negative correlation between two-dimensional porosity present on the fracture surfaces and the cycles to failure. The fractographic observations of these fatigue samples elucidate one reason why porosity reduction by centrifugation or vacuum mixing increases the fatigue life of PMMA bone cement.     

Bioactive bone cement: effect of surface curing properties on bone-bonding strength

The fact that bisphenol-a-glycidyl methacrylate (bis-GMA)-based cements contain an uncured surface is believed to play an important role when determining the surface curing properties of the cements. Therefore, in the present study, the bone-bonding strength of cement plates having an uncured surface on one side and a cured surface on the other side has been evaluated. These cement plates were composites of a bis-GMA-based resin with either an apatite- and wollastonite-containing glass-ceramic (AW-GC) powder or a hydroxyapatite (HA) powder, respectively designated AWC and HAC. The amount of each of these powders in a composite cement was 70 wt %. We formulate the hypothesis that the uncured surface of a cement plate is bioactive having bone-bonding properties. The goal of the present study was to indicate the bone-bonding strength of the uncured surfaces of AWC and HAC and compare the strength with the respective cured surfaces by a detaching in vivo test, as well as to histologically examine the bone-cement interface. Each plate has been implanted into the tibiae of male Japanese white rabbits, taking care to retain the surface properties, and the so-called "failure load has been measured using a detaching test followed 8 weeks after implantation. The failure load for AWC-plates at the uncured surface (2.05 +/- 1.11 kgf, n = 8) was significantly higher than AWC at its cured surface side (0.28 +/- 0.64 kgf, n = 8). The failure load for HAC-plates at the uncured surfaces (1.40 +/- 0.68 kgf, n = 8) was significantly higher than HAC at its cured surface (0.00 +/- 0.00 kgf, n = 8). Failure loads for AWC at its uncured and cured surfaces were both higher than for HAC, although not significantly. Direct bone formation has been observed histologically for both AWC and HAC on the uncured surfaces, and a Ca-P-rich layer was observed only at the uncured surface of AWC. These findings strongly suggest that uncured surfaces are useful for exposing a bioactive filler on a surface of composites, being very effective in inducing bone bonding.     

The relationship between porosity and fatigue characteristics of bone cements

In this study, the fatigue strengths of acrylic cement prepared by various commercially available reduced pressure mixing systems were compared with the fatigue strength of cement mixed by hand (control) under atmospheric conditions. The following observations were made from this investigation. The mean fatigue strength of reduced pressure mixed acrylic bone cement is double that of cement mixed by hand using an open bowl, 11,354+/-6,441 cycles to failure for reduced pressure mixing in comparison with 5,938+/-3,199 cycles for mixing under atmospheric conditions. However, the variability in mean fatigue strengths of reduced pressure mixed bone cement is greater for some mixing devices. The variation in fatigue strengths for the different mixing techniques is explained by the different porosity distributions. The design of the reduced pressure mixing system and the technique employed during mixing strongly contribute to the porosity distribution within the acrylic bone cement. The level of reduced pressure applied during cement mixing has an effect on the fatigue strength of bone cement, but the mixing mechanism is significantly more influential.     

The effect of a silane coupling agent on the bond strength of bone cement and cobalt-chrome alloy

Debonding of the cement-implant interface has been hypothesized to be the leading initial indicator of failed total hip prostheses. Many attempts have been made to increase the bond strength of this interface by precoating the implant, increasing the implant's surface roughness, and creating macro-grooves or channels on the implant. However, each of these approaches introduces new complications. This study introduces a unique silane coupling agent used to chemically bond the bone cement to the implant. Cylindrical cobalt-chrome samples were treated with the silane coupling agent, bonded to polymethylmethacrylate, and pushed out to failure. The mean shear strengths were compared to the failure strengths of untreated samples. Half of the specimens were tested immediately following cement curing, and the other half were tested after immersion in saline solution for 60 days. The mean shear strength of the silane-coated samples ranged from 18.2 to 24.1 MPa, and the mean shear strength of the uncoated samples ranged from 7.6 to 15.0 MPa. The increase in strength following silane coating noted in this study may increase the longevity of the implant by decreasing debonding at the interface and, therefore, subsequent failure due to loosening.     

The contradictory effects of pores on fatigue cracking of bone cement

The beneficial effect of porosity reduction on the fatigue life of bone cement has been demonstrated in numerous experimental studies. Clinically, however, it seems that the beneficial effect of porosity reduction of cement around total hip replacement components can only be found in large follow-up studies. Little is known about the actual mechanical effect of a pore on fatigue crack formation in cement mantles. We studied the effect of pores on the crack formation process in a finite element model of a transverse slice of a total hip reconstruction. We created models with a single large pore and models with multiple pores at levels of 2, 4, and 9%. The models were cyclically torque-loaded, causing macrocracks to appear in the cement mantle. In all models, we found that pores acted as microcrack initiators. However, pores could have both a detrimental and a beneficial effect on the macrocrack propagation in the cement mantle. Both effects were seen in the models with a single large pore and in the models with multiple pores. Pores would either accelerate, deviate, or decelerate the macrocrack propagation in the cement mantle. The effect of the pores depended on the location of the pores with respect to the stress intensities in the model, but was independent of the pore size or the level of porosity. The results may explain why the beneficial effect of vacuum mixing is difficult to demonstrate clinically. Stress intensities that are present in a cement mantle in an in vivo situation may overshadow the detrimental effect of a pore, while the beneficial effect may become more pronounced.     

Flexural strength distribution of a PMMA-based bone cement

Polymethylmethacrylate bone cement containing either no added antibiotic or 0.5 g of Gentamicin was prepared and stored either in air at room temperature or in a 37 degree C water bath for 48 h. An additive-free cement stored in air at room temperature was also tested for purposes of comparison. Following storage the specimens were tested in flexure. Weibull statistics demonstrated to fit the flexural strength distribution of all the materials tested with regression coefficients of at least 0.98. The presence of a BaSO(4) radiopacifier markedly reduced the mean flexural strength and increased the data scatter in the air-stored specimens. On the other hand, the flexural strength of both impregnated and nonimpregnated antibiotic increased when those materials were stored in water at 37 degree C, compared with the same material stored in air, as a consequence of the water ingress. The water-stored antibiotic-impregnated cement displayed lower flexural strength, increased data scatter, and a remarkably higher number of weak specimens compared with the antibiotic-free cement. The influence of the load type on the flexural behavior was studied by testing the air-stored specimens in three-point bending and four-point bending. Cements tested in four-point bending resulted in lower flexural strength than that tested in three-point bending. The ratio of mean strength measured in the different load arrangements was satisfactory, as predicted by the Weibull model.     

Comparison of two methods of fatigue testing bone cement

Two different methods have been used to fatigue test four bone cements. Each method has been used previously, but the results have not been compared. The ISO 527-based method tests a minimum of 10 samples over a single stress range in tension only and uses Weibull analysis to calculate the median number of cycles to failure and the Weibull modulus. The ASTM F2118 test regime uses fewer specimens at various stress levels tested in fully reversed tension–compression, and generates a stress vs. number of cycles to failure (S–N) or Wöhler curve. Data from specimens with pores greater than 1 mm across is rejected. The ISO 527-based test while quicker to perform, provides only tensile fatigue data, but the material tested includes pores, thus the cement is closer to cement in clinical application. The ASTM regime uses tension and compression loading and multiple stress levels, thus is closer to physiological loading, but excludes specimens with defects obviously greater than 1 mm, so is less representative of cement in vivo. The fatigue lives between the cements were up to a factor 15 different for the single stress level tension only tests, while they were only a factor of 2 different in the fully reversed tension–compression testing. The ISO 527-based results are more sensitive to surface flaws, thus the differences found using ASTM F2118 are more indicative of differences in the fatigue lives. However, ISO 527-based tests are quicker, so are useful for initial screening.     

Effect of an in vivo environment on the strength of bone cement.

Bar-shaped polymethylmethacrylate test specimens removed from rabbits after implantation for times up to 26 months showed a significant change in fracture stress as determined by three-point bending in the period between 12 and 26 months. There were no adverse findings in the tissue which developed around the bone-cement test bars.     

Effect of porosity reduction by compaction on compressive strength and microstructure of calcium phosphate cement.

Hydroxyapatite (HA) calcium phosphate cements (CPCs) are attractive materials for orthopedic applications because they can be molded into shape during implantation. However their low strength and brittle nature limits their potential applications to principally non-load-bearing applications. Little if any use has been made of the HA cement systems as manufacturing routes for preset HA bone grafts, which although not moldable pastes, are resorbable, unlike HA sintered ceramic. It is known that the strength of cements can be increased beyond that attainable from slurry systems by compaction, and this study investigates whether compaction significantly alters the specific surface area and pore-size distribution of CPC prepared according to the method of Brown and Chow. Compaction pressures of between 18 and 106 MPa were used to decrease the porosity from 50 to 31%, which resulted in an increase in the wet compressive strength from 4 to 37 MPa. The Weibull modulus was found to increase as porosity decreased; in addition the amount of porosity larger than the reactant particle size increased as porosity decreased. It is proposed that this was caused by a combination of voids created by the aqueous solvent used in fabrication and shrinkage that occurs on reaction. The specific surface area was unchanged by compaction.     

Effect of test frequency on the in vitro fatigue life of acrylic bone cement

The goal of the present work was to test the hypothesis that test frequency, f, does not have a statistically significant effect on the in vitro fatigue life of an acrylic bone cement. Uniaxial constant-amplitude tension-compression fatigue tests were conducted on 12 sets of cements, covering three formulations with three very different viscosities, two different methods of mixing the cement constituents, and two values of f (1 and 10 Hz). The test results (number of fatigue stress cycles, N(f)) were analyzed using the linearized form of the three-parameter Weibull equation, allowing the values of the Weibull mean (N(WM)) to be determined for each set. Statistical analysis of the lnN(f) data, together with an examination of the N(WM) estimates, showed support for the hypothesis over the range of f used. The principal use and explanation of the present finding are presented.     

Effect of fabrication pressure on the fatigue performance of Cemex XL acrylic bone cement

During a cemented arthroplasty, the prepared polymerizing dough of acrylic bone cement is subjected to pressurization in a number of ways; first, during delivery into the freshly prepared bone bed, second, during packing in that bed (either digitally or with the aid of a mechanical device), and, third, during the insertion of the prosthesis. Only a few studies have reported on the influence of the level of pressurization experienced during these events (which, depending on the cementing technique used, has been put at between 8 and 273 kPa) on various properties of the cement. That was the focus of the present study, in which the fully reversed tension-compression (+/-15 MPa; 5 Hz) fatigue lives (expressed as number of cycles to fracture, N(f)) of rectangular cross-sectioned "dog-bone" specimens (Type V, per ASTM D 638) fabricated from Cemex XL cement, at pressure applied continuously to the cement dough during curing in the specimen mold, p=75,150, and 300 kPa, were determined. The N(f) results were analyzed using the linearized transformation of the three-parameter Weibull relationship to obtain estimates of the Weibull mean, N(WM), which was taken to be the index of fatigue performance of the specimen set. Over the range of p studied, N(WM) increased as p increased (for example, from 329,118 cycles when p was 75 kPa to 388,496 cycles when p was 300 kPa); however, the increase was not significant over any pair of p increment steps (Mann-Whitney U-test; alpha<0.05).     

A theoretical and experimental analysis of polymerization shrinkage of bone cement: A potential major source of porosity

A theoretical basis for understanding polymerization shrinkage of bone cement is presented based on density changes in converting monomer to polymer. Also, an experimental method, based on dilatometry and the Archimedes' principle is presented for highly precise and accurate measurement of unconstrained volumetric shrinkage of bone cement. Furthermore, a theoretical and experimental analysis of polymerization shrinkage in a constrained deformational state is presented to demonstrate that porosity can develop due to shrinkage. Six bone-cement conditions (Simplex-Ptrade mark vacuum and hand mixed, Endurancetrade mark vacuum mixed, and three two-solution experimental bone cements with higher initial monomer levels) were tested for volumetric shrinkage. It was found that shrinkage varied statistically (p< or = 0.05) from 5.1% (hand-mixed Simplex-Ptrade mark) to 6.7% (vacuum-mixed Simplex-Ptrade mark) to 10.5% for a 0.6:1 (polymer g/monomer mL) two-solution bone cement. Shrinkage was highly correlated with initial monomer content (R(2) = 0.912) but with a lower than theoretically expected rate. This discrepancy was due to the presence of residual monomer after polymerization. Using previously determined residual monomer levels, the theoretic shrinkage analysis was shown to be predictive of the shrinkage results with some residual monomer left after polymerization. Polymerization of a two-solution bone cement in a constrained state resulted in pores developing with volumes predicted by the theory that they are the result of shrinkage. The results of this study show that shrinkage of bone cement under certain constrained conditions may result in the development of porosity at the implant-bone cement interface and elsewhere in the polymerizing cement mantle.     

Quantitative measurement of the stresses induced during polymerisation of bone cement

When bone cement cures, residual stresses due to bulk and thermal shrinkage will result. Present finite element (FE) simulations of implanted constructs often do not account for these stresses as an initial condition; this may lead to overestimations of the fatigue life of the cement. In the present study, an instrumented stem equipped with strain gauges and a thermocouple was employed to experimentally quantify the residual stresses induced as a result of bone cement curing within a simulated bone/cement/stem construct. Residual stresses as high as 10 MPa were observed in the cement mantle. Residual stresses of this magnitude are potentially high enough to initiate damage within the cement mantle or at the stem/cement interface immediately post-implantation. The acoustic emission technique has demonstrated that cracking and sliding mechanisms are occurring during curing, resulting in partial relaxation of these stresses. The implications for FE simulations of the implanted construct are discussed.     

Effects of synergistic reinforcement and absorbable fiber strength on hydroxyapatite bone cement

Approximately a million bone grafts are performed each year in the United States, and this number is expected to increase rapidly as the population ages. Calcium phosphate cement (CPC) can intimately adapt to the bone cavity and harden to form resorbable hydroxyapatite with excellent osteoconductivity and bone-replacement capability. The objective of this study was to develop a strong CPC using synergistic reinforcement via suture fibers and chitosan, and to determine the fiber strength-CPC composite strength relationship. Biopolymer chitosan and cut suture filaments were randomly mixed into CPC. Both suture filaments and composite were immersed in a physiological solution. After 1-day immersion, cement flexural strengths (mean +/- SD; n = 6) were: (2.7 +/- 0.8) MPa for CPC control; (11.2 +/- 1.0) MPa for CPC-chitosan; (17.7 +/- 4.4) MPa for CPC-fiber composite; and (40.5 +/- 5.8) MPa for CPC-chitosan-fiber composite. They are significantly different from each other (Tukey's at 0.95). The strength increase from chitosan and fiber together in CPC was much more than that from either fiber or chitosan alone. The composite strength became (9.8 +/- 0.6) MPa at 35-day immersion and (4.2 +/- 0.7) MPa at 119 days, comparable to reported strengths for sintered porous hydroxyapatite implants and cancellous bone. After suture fiber dissolution, long macropore channels were formed in CPC suitable for cell migration and tissue ingrowth. A semiempirical relationship between suture fiber strength S(F) and composite strength S(C) were obtained: S(C) = 14.1 + 0.047 S(F), with R = 0.92. In summary, this study achieved substantial synergistic effects by combining random suture filaments and chitosan in CPC. This may help extend the use of the moldable, in situ hardening hydroxyapatite to moderate stress-bearing orthopedic applications. The long macropore channels in CPC should be advantageous for cell infiltration and bone ingrowth than conventional random pores and spherical pores.     

The strength of acrylic bone cement cured under thumb pressure

In this investigation, the static tensile strength of bone cement was quantified after mixing it in an open bowl or in a commercially available vacuum mixer and molding it under pressures consistent with values obtained by finger/digital application, as it is used in surgery. Pressure, held for a brief time span on cement in its lower viscosity state, has been demonstrated to increase penetration of the cement into bone. Clinically, bone cement is pressurized by digital pressure, specialized instruments, or by implant design.Specimens were cured under constant pressures of up to 100kPa, which is in the range reported for thumb pressurization of plugged proximal femurs and instrumented pressurization of acetabular sockets. The results showed that application of constant pressure during the polymerization of open bowl mixed bone cement significantly improved its mechanical properties. Application of 100kPa constant pressure to the open bowl mixed bone cement while it cured increased its ultimate strength to a value similar to vacuum mixed cement. Curing under pressure showed no significant effect on the tensile properties of vacuum mixed cement. Curing under pressure did not significantly reduce the size of the largest pores in the tensile specimens.     

The effect of the extracorporeal shock wave lithotriptor on bone cement

For the purpose of studying its applicability for acrylic cement removal during total hip revision surgery, experiments with an extracorporeal shock wave lithotriptor were carried out. High-energy shock waves (HESW) were focussed on discs of polymethylmethacrylate bone cement. The average discharge was 18.1 kV; the number of shock waves 0, 100, 250, 500, 1000, and 2000; the application rate was 85 shocks/min. Macroscopic or radiographic effects were not in evidence. Microscopically, typical lesions in a small concentric focal area with a diameter of 8.5 (+/- 2.5) mm were found. The individual lesions were smaller than 0.1 mm, and displayed characteristic shapes. The area porosity increased with the number of shocks. The maximal area porosity caused by the HESW, measured by quantitative microscopy, was 4% after 2000 shock waves. The lesions were also studied by scanning electron microscopy. It can be concluded that HESW causes only microscopic lesions on the frontal surface of discs of bone cement, and that these lesions are small compared to the pores normally present in bone cement, when applied clinically.     

A controlled-notch specimen to study fatigue crack initiation in bone cement

Despite the extensive literature on the mechanical characteristics and failure properties of poly(methyl methacrylate) bone cement, little is known of its fatigue crack initiation process. The most likely in vivo bone cement fatigue crack initiation sites are internal flaws and irregularities on the bone cement surface. The stress concentration created by a flaw, and subsequently the stress state at that flaw, depends on the flaw geometry. To model the fatigue crack initiation process of a flaw, it is necessary to reproduce the stress state at that flaw. In this study, a special mold was designed to introduce notches with specific tip radii into fatigue specimens. The notch was molded into the specimen to simulate the in vivo flaw formation process. The molding method allows control of the stress concentration by specifying the notch tip radius. We created notched specimens where the tip radii of the notches ranged from "sharp" (< 3 microm) to 400 microm. The results demonstrated that notched specimens created by the special mold satisfied two necessary requirements for fatigue crack initiation studies: (1) the material microstructure at the notch tip must not be disrupted by the notching process, and (2) the notch tip stress field, determined by the notch tip geometry, must be reproducible.