Fracture and fatigue properties of acrylic bone cement

The purpose of this study was to characterize the relative and combined effects of sterilization, molecular weight, and mixing method on the fracture and fatigue performance of acrylic bone cement. Palacos® R brand bone cement powder was sterilized using ethylene oxide gas (EtO) or gamma irradiation. Nonsterile material was used as a control. Molecular weights of the bone-cement powders and cured cements were measured using gel permeation chromatography. Hand and vacuum mixing were employed to mold single edge-notched bend specimens for fracture toughness testing. Molded dog-bone specimens were used for fatigue tests. Electron microscopy was used to study fracture mechanisms. Analysis of variance and Student t-tests were used to compare fracture and fatigue performance between sterilization and mixing groups. Our results indicate that vacuum mixing improved significantly the fracture and fatigue resistance (P < .05, P < .07) over hand mixing in radiation-sterilized and EtO-sterilized groups. In vacuum-mixed cement, the degradation in molecular weight resulting from gamma irradiation decreased fracture resistance significantly when compared with EtO sterilization and control (P < .05). A corresponding decrease in fatigue resistance was observed in the cement that was degraded severely by a radiation dose of 10 MRad (P < .05). In contrast, EtO sterilization did not result in a significantly different fracture resistance when compared with unsterilized controls for vacuum-mixed cement (P > .1). For hand-mixed cement, fracture and fatigue resistance appeared to be independent of sterilization method. This independence is believed to be the result of higher porosity that compromised the mechanical properties and obscures any effect of sterilization. Our results indicate that a combination of nonionizing sterilization and vacuum mixing resulted in the best mechanical performance and is most likely to contribute to enhanced longevity in vivo.     

Differences in bone-cement porosity by vacuum mixing,centrifugation, and hand mixing

The mean pore size and percent porosity of vacuum-mixed cement were compared with centrifuged cement and cement hand mixed by skilled specialized operating room technicians. Centrifuged cement samples had the smallest mean pore size when compared with vacuum-mixed specimens. The mean pore size for the hand-mixed specimens was intermediate and not significantly different from the other 2 mixing techniques. Results were reversed, however, for mean percent porosity. Centrifuged cement had the highest percent porosity; vacuum-mixed cement, the lowest; and hand-mixed cement, intermediate. The porosity of vacuum-mixed Simplex P (Howmedica, Rutherford, NJ) bone-cement was similar from the initial to the remnant cement extruded from the cement gun. There was no reduced cement porosity with vacuum mixing or centrifugation as anticipated. Reversion to hand mixing by highly skilled technicians could result in a significant cost savings without negative effects on cement porosity.     

Optimization and comparison of three vacuum mixing systems for porosity reduction of Simplex P cement

Simplex P bone cement was prepared in three commercially available vacuum mixing systems, the Enhancement Mixer, the Mixevac II High Vacuum System, and the Mitab Vacuum System, to determine the improvement in fatigue strength associated with porosity reduction of the cement in all three systems. The results of the fatigue tests of vacuum-mixed Simplex P were also compared to the fatigue strength of Simplex P prepared by centrifugation of the cement immediately after mixing. Vacuum mixing one pack of Simplex P per syringe in all three systems was not effective in complete removal of all the large voids from the cement. Fatigue failure occurred very early in those specimens containing the large voids. There was no significant difference in fatigue life between one pack of cement per syringe mixed under vacuum in the three systems and the control cement (no vacuum, uncentrifuged). Vacuum mixing two packs of cement per syringe was more effective than one pack per syringe, and all three systems significantly increased the cycles to failure of Simplex P over the control cement. However, the Enhancement and Mitab vacuum mixing systems still produced some very weak specimens in fatigue. Two packs of cement per syringe prepared in the Mixevac II vacuum mixing system were significantly stronger in fatigue than two packs mixed in either the Enhancement or Mitab vacuum system. The Mixevac II vacuum mixing system was the most effective technique of the three vacuum mixing systems tested. Centrifugation of one or two packs of Simplex P per syringe produced a more uniform cement that was free of large voids and thus eliminated the very weak specimens.(ABSTRACT TRUNCATED AT 250 WORDS)     

Vacuum-mixing cement does not decrease overall porosity in cemented femoral stems: an in vitro laboratory investigation

The role of vacuum mixing on the reduction of porosity and on the clinical performance of cemented total hip replacements remains uncertain. We have used paired femoral constructs prepared with either hand-mixed or vacuum-mixed cement in a cadaver model which simulated intra-operative conditions during cementing of the femoral component. After the cement had cured, the distribution of its porosity was determined, as was the strength of the cement-stem and cement-bone interfaces. The overall fraction of the pore area was similar for both hand-mixed and vacuum-mixed cement (hand 6%; vacuum 5.7%; paired t-test, p = 0.187). The linear pore fractions at the interfaces were also similar for the two techniques. The pore number-density was much higher for the hand-mixed cement (paired t-test, p = 0.0013). The strength of the cement-stem interface was greater with the hand-mixed cement (paired t-test, p = 0.0005), while the strength of the cement-bone interface was not affected by the conditions of mixing (paired t-test, p = 0.275). The reduction in porosity with vacuum mixing did not affect the porosity of the mantle, but the distribution of the porosity can be affected by the technique of mixing used.     

骨水泥的真空搅拌实验研究

目的 :比较骨水泥经过真空搅拌和普通搅拌的机械性能变化。方法 :将一种国产骨水泥分成普通搅拌和真空搅拌 2组分别制成标本 ,按照国际标准组织 (ISO)的标准测定抗静态压力强度 ,静态抗弯曲强度和弹性模量 ,按照德国工业标准 (DIN)检测抗疲劳强度。结果 :应用真空搅拌处理 ,抗弯曲强度和抗疲劳强度分别提高了 13 %和 15 % ,骨水泥的静态压力耐受强度稍降低 ,弹性模量稍提高 ,二者在统计学上无变化。结论 :采用真空搅拌骨水泥的方法 ,提高了国产骨水泥的抗疲劳强度及静态抗弯曲强度 ,在临床应用有较大的积极意义。     

The effect of gamma radiation sterilization on the fatigue crack propagation resistance of human cortical bone

BACKGROUND: Clinical evidence has suggested that the rate of fracture in allografts sterilized with gamma radiation may be higher than that in controls. Gamma radiation sterilization has been shown to affect the post-yield properties of bone but not the elastic modulus. Since most allograft fractures occur with subcritical loads during activities of daily living, it may be that the fatigue properties of irradiated allografts are diminished. In this study, the fatigue crack propagation behavior of cortical bone sterilized with gamma radiation was compared with that of gender and age-matched controls. We hypothesized that gamma radiation significantly reduces the resistance of cortical bone to fatigue crack growth. METHODS: Specimens for fatigue crack propagation testing were machined from four pairs of fresh-frozen human femora obtained from four individuals (a younger male, younger female, older male, and older female donor). Half of the specimens were sterilized with 31.7 kGy of gamma radiation. The specimens were cyclically loaded to failure in a servohydraulic testing system, and crack growth was monitored. The cyclic stress intensity factor and the fatigue crack growth rate were calculated to examine the kinetics of fatigue crack growth. Following testing, the damage zone around the fracture plane was analyzed histologically. RESULTS: The morphology and kinetics of crack growth in irradiated specimens differed from the control data. Overall, the irradiated bone was significantly less resistant to fatigue crack growth than was control tissue (p < 0.05). There was less microdamage associated with fracture in the irradiated specimens than in the control specimens, with the exception of the bone from the older female donor. CONCLUSIONS: Gamma radiation sterilization significantly reduces the fatigue crack propagation resistance of cortical bone. Irradiated specimens also demonstrate a smaller amount of microdamage along the fracture plane. These findings may be due to ultrastructural alterations in the collagen matrix caused by radiation. CLINICAL RELEVANCE: This study suggests that, despite having pre-yield mechanical properties that are similar to those of nonirradiated bone, gamma-radiation-sterilized allograft may be more predisposed to fracture even under the subcritical loads that occur during the activities of daily living.     

Fracture resistance of gamma radiation sterilized cortical bone allografts.

Gamma radiation is widely used for sterilization of human cortical bone allografts. Previous studies have reported that cortical bone becomes brittle due to gamma radiation sterilization. This embrittlement raises concern about the performance of a radiation sterilized allograft in the presence of a stress concentration that might be surgically introduced or biologically induced. The purpose of this study was to investigate the effect of gamma radiation sterilization on the fracture resistance of human femoral cortical bone in the presence of a stress concentration. Fracture toughness tests of specimens sterilized at a dose of 27.5 kGy and control specimens were conducted transverse and longitudinal to the osteonal orientation of the bone tissue. The formation of damage was monitored with acoustic emission (AE) during testing and was histologically observed following testing. There was a significant decrease in fracture toughness due to irradiation in both crack growth directions. The work-to-fracture was also significantly reduced. It was observed that the ability of bone tissue to undergo damage in the form of microcracks and diffuse damage was significantly impaired due to radiation sterilization as evidenced by decreased AE activity and histological observations. The results of this study suggest that, for cortical bone irradiated at 27.5 kGy, it is easier to initiate and propagate a macrocrack from a stress concentration due to the inhibition of damage formation at and near the crack tip.     

The dynamic volume changes of polymerising polymethyl methacrylate bone cement

The Swedish hip register found an increased risk of early revision of vacuum-mixed cemented total hip replacements. The influence of cement mixing technique on the dynamic volume change in polymerising PMMA is not well understood and may be relevant to this observation. Applying Archimedes' principle, we have investigated the dynamic volume changes in polymerising cement and determined the influence of mixing technique. All specimens showed an overall volume reduction: hand-mixed 3.4% and vacuum-mixed 6.0%. Regression analysis of sectional porosity and volume reduction showed a highly significant relationship. Hand-mixed porous cement showed a transient volume increase before solidification. However, vacuum-mixed cement showed a progressive volume reduction throughout polymerisation. Transient expansion of porous cement occurs at the critical time of micro-interlock formation, possibly improving fixation. Conversely, progressive volume reduction of vacuum-mixed cement throughout the formation of interlock may damage fixation. Stable fixation of vacuum-mixed cement may depend on additional techniques to offset the altered volumetric behaviour of vacuum-mixed cement.     

Cement mantle defects in total hip arthroplasty: influence of stem size and cementing technique

The cause of isolated osteolysis in the femoral shaft around stem implants in patients with cemented THR has so far not been established. A number of factors have been considered such as torsional stability of the femoral stem implant, the time of reduction intraoperatively after cementing and iatrogenic and load-induced defects in the cement mantle. The aim of this in vitro investigation was to determine if the cementing technique or the thickness of the prosthesis stem, and thus its bending strength, influences the formation, extent and localisation of cement mantle defects. In vitro biomechanical loading tests were performed on twelve anatomically shaped femoral stem prostheses of two different thicknesses which were implanted in artificial bone. Six of the implants were fixed by conventional cementing technique, the other six by means of the vacuum technique. Compared with thicker implant stems, the slimmer stems fixed with the conventional cementing technique had a higher number of cracks in the cement mantle. Pore formation was localised predominantly in the interface area between the bone cement and the “cancellous” bone or “cortex” of the artificial bone. This was observed especially in the non-vacuum mixed cement, regardless of stem thickness. Large pores were found mainly in the cement around the thicker stems which had been mixed by the conventional method. The thickness of the stems, whether fixed with vacuum-mixed or non-vacuum mixed cement, had no significant influence on the percentage of pore area in the cement. In the nonvacuum mixed cement, there was no significant difference between the percentages of pore area in the proximal and distal parts of the shafts, whereas in the vacuum mixed cement the percentage of pore area was significantly larger in the distal than in the proximal part of the shafts. In the specimens of both stem sizes, the percentage of pore area in the vacuum mixed cement was significantly smaller than in the nonvacuum mixed cement. This explains the greater fatigue strength of vacuum mixed cement. The cyclic loading on the thicker stem prostheses, especially in those fixed with vacuum mixed cement, resulted in fracture between the prosthesis tip and the clamping device due to the local stiffness of the artificial bone. Due to this unfavourable biomechanical property of the artificial bone, further studies will be carried out on human femurs. Nevertheless, in view of the results presented here, the vacuum mixing technique has to recommended as the “state of the art” method in cemented total hip arthroplasty.     

Fatigue crack propagation resistance of highly crosslinked polyethylene

A higher degree of cross-linking has been shown to improve wear properties of ultra-high molecular weight polyethylene in laboratory studies. However, cross-linking can also affect the mechanical properties of ultra-high molecular weight polyethylene. Fatigue crack propagation resistance was determined for electron beam cross-linked ultra-high molecular weight polyethylene and compared with gamma irradiation cross-linked and noncross-linked polyethylene fatigue specimens. Crosslinking was done with different dosages of irradiation followed by melting. For one irradiation dose (50 kGy) extrusion and molding processes were compared. A fracture mechanics approach was used to determine how the degree of cross-linking affects resistance to crack propagation in ultra-high molecular weight polyethylene. Fatigue crack propagation resistance was reduced in proportion to the irradiation dose. The type of irradiation (gamma or electron beam) or manufacturing method (extrusion or molding) did not affect fatigue crack propagation resistance. The reduced fatigue strength of highly cross-linked ultra-high molecular weight polyethylene could lead to mechanical failure in conditions that are associated with cyclic local tensile stresses.     

Does vacuum-mixing improve the fatigue properties of high-viscosity poly(methyl-methacrylate) (PMMA) bone cement?

The objective was to verify different reports in the literature which show an increase of stability using vacuum-mixing of bone cement, by testing the fatigue properties of bone-cement specimens (Palacos R). Evacuation of the poly(methyl-methacrylate) (PMMA) and evacuation with additional pressurization (Draenert system) were used to manufacture the specimens. Although we found improvement in the ultimate bending strength for evacuated Palacos R, an increase of fatigue stability could not be found either for vaccum-mixed cement or for vacuum-mixed and compressed cement. As fatigue failure is one of the most important factors leading to aseptic loosening of cemented alloarthroplasties, we conclude that the long-term results cannot be improved by vaccum-mixing this cement.     

The dynamic volume changes of polymerising polymethyl methacrylate bone cement

The Swedish hip register found an increased risk of early revision of vacuum-mixed cemented total hip replacements. The influence of cement mixing technique on the dynamic volume change in polymerising PMMA is not well understood and may be relevant to this observation.

Applying Archimedes' principle, we have investigated the dynamic volume changes in polymerising cement and determined the influence of mixing technique. All specimens showed an overall volume reduction: handmixed 3.4% and vacuum-mixed 6.0%. Regression analysis of sectional porosity and volume reduction showed a highly significant relationship.

Hand-mixed porous cement showed a transient volume increase before solidification. However, vacuummixed cement showed a progressive volume reduction throughout polymerisation.

Transient expansion of porous cement occurs at the critical time of micro-interlock formation, possibly improving fixation. Conversely, progressive volume reduction of vacuum-mixed cement throughout the formation of interlock may damage fixation. Stable fixation of vacuum-mixed cement may depend on additional techniques to offset the altered volumetric behaviour of vacuum-mixed cement.     

Vacuum mixing of acrylic bone cement

A partial-vacuum (500-550 mmHg), slow-speed (2 Hz) system for optimal blending of the liquid and powder components of Simplex-P acrylic bone cement was developed to eliminate five different sources of porosity observed with x-ray during the course of cement preparation and specimen fabrication. The vacuum mixing system produces set specimens of less than 1% porosity that have significant improvements over specimens prepared with conventional mixing in the mechanical properties of tensile and compressive strength and uniaxial tensile fatigue life. Hence, a much stronger cement can be available in surgery without any change in original chemical composition.     

Comparison of various vacuum mixing systems and bone cements as regards reliability, porosity and bending strength

BACKGROUND: There are several vacuum mixing systems on the market which are arbitrarily used with various bone cements in clinical work. Hardly any studies have been done on the performance and handling of these systems in combination with different cement brands. MATERIAL AND METHODS: We therefore tested 6 vacuum mixing systems (Palamix, Summit, Cemvac, Optivac, Vacumix, MixOR) in combination with 6 cement brands (Palacos R, Simplex P, CWM 1, CWM 2000, Palamed G, VersaBond) concerning their reliability, user-friendliness, porosity and bending strength. RESULTS: Our study indicated that each system has weak points. The preparation of the mixed cement for gun injection can present problems. If cement collection under vacuum fails, porosity is increased. Manual collection without a vacuum carries the risk of intermixing air. For comfortable and effective retrograde cement application, cement guns should have a stable connection with the cartridge and a high piston stroke. There are marked differences between the systems as regards overall porosity when all tested cements are considered (range 2-18%), and between the cements when all tested systems are considered (range 2-17%). All test samples exceeded the required bending strength of 50 MPa, according to ISO 5833. Palaces specimens showed excessive plastic deformation in the bending test. INTERPRETATION: There are better and worse mixing system/cement combinations for a given system and a given cement. Systems with cement collection under vacuum reduce porosity best.     

New PMMA bone cements for vacuum mixing systems

In cemented total hip replacement, fractures and cracks in the cement mantle may facilitate mechanical loosening of the prosthesis. Especially large voids and flaws within the cement can cause fatigue fractures. Reduction of cement porosity is a logical step in the attempt to improve the quality and durability of bone cement. An effective reduction of pores during vacuum mixing requires prechilling of Palacos R at 4 degrees C. For easier handling, new bone cements have been developed with characteristics similar to Palacos R, but with no chilling necessary prior to mixing under vacuum. In our study two newly developed bone cements (Palamed G, VersaBond) and a bone cement used widely in clinics (Palacos R) were mixed in three different vacuum mixing systems (Optivac, MixOR, Palamix). Macro-, micro-, and total porosity and bending strength (ISO 5833) were determined. All three mixing systems proved effective in reducing porosity and showed constant mixing results. All cement specimens that we tested fulfilled the ISO requirements for bending strength. VersaBond yielded the lowest porosities, but showed lower viscosity compared to Palacos R and Palamed G. The new cements are equal in vitro to Palacos R with regard to bending strength, but further clinical studies are necessary before widespread use is advisable.     

The effect of mixing on gentamicin release from polymethylmethacrylate bone cements

We compared the release of gentamicin from 6 different commercially available, antibiotic-loaded PMMA bone cements used for vacuum- and hand-mixed cement using a Cemvac vacuum mixing system. We also measured the release of gentamicin after manual addition of the antibiotic to different commercial, unloaded bone cements after hand-mixing. The porosity of cements was reduced in all vacuum-mixed cements, as compared with hand-mixed cements, concurrent with a statistically significant reduction (3 of 6) or increase (1 of 6) in the total amounts of gentamicin released. The total gentamicin release was studied in 3 of the brands after manual addition and mixing of the antibiotics. We found that the release of antibiotics was lower than in samples made from industrial mixing. In conclusion, the manual addition and mixing of gentamicin in PMMA bone cements leads to a lower release of antibiotics than that in corresponding commercially available antibiotic-loaded cements, while vacuum-mixing only leads to a minor reduction in antibiotic release, as compared to hand-mixing.     

How do material properties influence wear and fracture mechanisms?

The wear and fracture mechanisms of ultra-high-molecular-weight polyethylene (UHMWPE) hip and knee implant components are of great interest. The material properties of UHMWPE are affected by ionizing radiation as used for sterilization and cross-linking. Cross-linking with high-dose irradiation has been shown to improve the wear resistance of UHMWPE. However, cross-linking leads to a loss in properties such as ductility and resistance to fatigue crack propagation. Highly cross-linked UHMWPE may be more susceptible than conventional UHMWPE to fracture under severe clinical conditions (eg, impingement). Contemporary hip and knee simulator studies provide good information with which new UHMWPE formulations can be screened for clinical wear performance. However, comparable methodologies are lacking for screening UHMWPEs for fracture resistance. Mechanical tests as well as computational material and structural models should be developed to evaluate the combined effect of material and geometry (structure) on fracture resistance under clinically relevant loading conditions.     

The effect of mixing on gentamicin release from polymethylmethacrylate bone cements

We compared the release of gentamicin from 6 different commercially available, antibiotic-loaded PMMA bone cements used for vacuum- and hand-mixed cement using a Cemvac vacuum mixing system. We also measured the release of gentamicin after manual addition of the antibiotic to different commercial, unloaded bone cements after hand-mixing. The porosity of cements was reduced in all vacuum-mixed cements, as compared with hand-mixed cements, concurrent with a statistically significant reduction (3 of 6) or increase (1 of 6) in the total amounts of gentamicin released. The total gentamicin release was studied in 3 of the brands after manual addition and mixing of the antibiotics. We found that the release of antibiotics was lower than in samples made from industrial mixing. In conclusion, the manual addition and mixing of gentamicin in PMMA bone cements leads to a lower release of antibiotics than that in corresponding commercially available antibiotic-loaded cements, while vacuum-mixing only leads to a minor reduction in antibiotic release, as compared to hand-mixing.     

The effect of mixing on gentamicin release from polymethylmethacrylate bone cements

We compared the release of gentamicin from 6 different commercially available, antibiotic-loaded PMMA bone cements used for vacuum- and hand-mixed cement using a Cemvac vacuum mixing system. We also measured the release of gentamicin after manual addition of the antibiotic to different commercial, unloaded bone cements after hand-mixing. The porosity of cements was reduced in all vacuum-mixed cements, as compared with hand-mixed cements, concurrent with a statistically significant reduction (3 of 6) or increase (1 of 6) in the total amounts of gentamicin released. The total gentamicin release was studied in 3 of the brands after manual addition and mixing of the antibiotics. We found that the release of antibiotics was lower than in samples made from industrial mixing. In conclusion, the manual addition and mixing of gentamicin in PMMA bone cements leads to a lower release of antibiotics than that in corresponding commercially available antibiotic-loaded cements, while vacuum-mixing only leads to a minor reduction in antibiotic release, as compared to hand-mixing.     

Vacuum mixing systems for bone cement completion - comparison of different systems

INTRODUCTION: The use of vacuum mixing systems when mixing bone cement reduces its porosity and hereby significantly improves the features of the material. The currently available mixing systems are compared with regard to handling, mechanical properties and economical aspects. METHODS: In 8 vacuum mixing systems the handling, pump performance, system tightness, the used air volume, the filter efficiency, the remaining amounts in the mixing system and the porosity of the cements are shown in comparison. RESULTS: All vacuum mixing systems reduce the porosity of the cement in comparison to hand mixed cements significantly if used correctly. The individual examinations, though, show enormous differences, which can be of significance to the user in the individual choice of system and, depending on the individual circumstances, can influence the quality of the mixed cement. CONCLUSION: The results enable the user to choose and handle a vacuum mixing system which is optimally suitable for the individual circumstances in respect to the characteristics examined.