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 test specimen cross-sectional shape on the in vitro fatigue life of acrylic bone cement

Constant-amplitude uniaxial tension-compression fatigue tests were conducted on specimens fabricated from 12 sets of acrylic bone cements, covering cement formulations with three different viscosities (so-called "high-", "medium-" and "low-viscosity" varieties), two different methods of mixing the cement constituents (so-called "hand-" and "vacuum-mixed" methods) and two test specimen shapes (rectangular-cross-sectioned or "flat" and circular-cross-sectioned or "round"). The test results-namely, the number of fatigue stress cycles, N(f)-were analyzed using the linearized transformation of the three-parameter Weibull relationship, allowing the values of the Weibull mean, N(WM), to be determined for each set. Values ranged from 14,300 to 1,284,331 for the round specimen sets and from 2898 to 72,960 for the flat specimen sets. Statistical analysis of the ln N(f) data, together with an examination of the N(WM) values, showed that, for any combination of cement formulation and mixing method, round specimens had significantly longer fatigue lives compared to flat ones. These results are explained in terms of two factors. The first is the smaller surface area of the waisted zone in the round specimens compared to that in the flat specimens (nominal value of 157mm(2) versus nominal value of 185mm(2)), leading to the possibility of fewer crack initiation sites on the round specimens compared to the flat ones. Secondly, it is postulated that the crystallinity of the round specimens was higher than that of the flat ones, a consequence of the significantly lower measured residual liquid monomer contents of the former compared to the latter (3.40+/-1.28wt%/wt compared to 3.81+/-1.48wt%/wt). The significance of the present finding is that it indicates that, for a set of bone cement formulation and experimental conditions, discriminating fatigue test results are more likely to be obtained if flat, rather than round, test specimens are used.     

The influence of the viscosity classification of an acrylic bone cement on its in vitro fatigue performance

The goal of the present work was to investigate the influence of the viscosity classification of an acrylic bone cement on its in vitro fatigue performance, as determined in fully-reversed tension-compression (+/-15 MPa) fatigue tests. The test matrix comprised six commercially available bone cements [Orthoset1, (OS1), Orthoset(R)3 (OS3), CemexRX (CRX), Cemex XL (CXL), Palacos R (PR) and Osteopal (OP)], two methods of mixing the cement constituents (hand-mixing and vacuum-mixing), two methods of fabricating the test specimens (direct molding and molding followed by machining), two specimen cross-sectional shapes (rectangular or "flat" and circular or "round"), and four test frequencies (1, 2, 5, and 10 Hz). In total, 185 specimens, distributed among 20 sets, were tested. The test results (number of fatigue stress cycles, N_f) were processed using the linearized transformation of the three-parameter Weibull distribution, whence estimates of the Weibull mean, N_[WM], were obtained. Statistical analysis of the ln N_f results (Mann-Whitney test; alpha<0.05) and a comparison of the N_[WM] estimates for specimen sets in which the formulations have essentially the same composition but different viscosity classification (namely, OS1 versus OS3, CRX versus CXL, and PR versus OP) showed that, in the majority of the comparisons carried out, the viscosity classification of a bone cement does not exert a significant influence on its in vitro fatigue performance.     

Fatigue and fracture toughness of acrylic bone cements modified with long-chain amine activators

The composition of acrylic bone cement has been identified as one of the important parameters affecting its mechanical properties and may, in turn, ultimately influence the longevity of a cemented arthroplasty. Our aim in this study was to determine the influence of change of one compositional variable, the activator, on the fatigue performance and fracture toughness of specimens of the fully cured cement. To that end, three sets of cements were prepared, containing either the conventional activator, 4-N,N dimethyl p-toluidine (DMPT), or novel ones that are tertiary amines based on long-chain fatty acids, that is, 4-N,N dimethylaminobenzyl oleate (DMAO) and 4-N,N dimethylaminobenzyl laurate (DMAL). In the fatigue tests, the specimens were subjected to tension-tension loading, and the results (number of cycles to failure, Nf) were analyzed using the linearized form of the three-parameter Weibull equation. The fracture toughness (KIc) tests were conducted with rectangular compact tension specimens. All fracture surfaces were subsequently examined with scanning electron microscopy. We found that the Weibull mean fatigue lives for specimens fabricated using the DMPT, DMAL, and DMAO containing cements were 272,823, 453,551, and 583,396 cycles, respectively. The corresponding values for KIc were 1.94 +/- 0.05, 2.06 +/- 0.09, and 2.00 +/- 0.07 MPa radical m, respectively. Statistical analyses showed that for both the DMAL- and DMAO-containing cements, the mean values of Nf were significantly higher compared to the corresponding value for the DMPT-containing cement (Mann-Whitney test; alpha < 0.10). This result is attributed to the higher molecular weights of the former cements compared to the latter. The same trend was found for the mean KIc values (Mann-Whitney test; alpha < 0.05), with the trend being explained in terms of the differences seen in the crack morphologies. These results thus demonstrate that these novel amines are viable alternatives to DMPT for incorporation into acrylic bone cement formulations in the future.     

Augmentation of acrylic bone cement with multiwall carbon nanotubes

Acrylic bone cement, based on polymethylmethacrylate (PMMA), is a proven polymer having important applications in medicine and dentistry, but this polymer continues to have less than ideal resistance to mechanical fatigue and impact. A variety of materials have been added to bone cement to augment its mechanical strength, but none of these augmentative materials has proven successful. Carbon nanotubes, a new hollow multiwalled tubular material 10-40 nm in diameter, 10-100 microm long, and 50-100 times the strength of steel at 1/6 the weight, have emerged as a viable augmentation candidate because of their large surface area to volume ratio. The objective of this study was to determine if the addition of multiwall carbon nanotubes to bone cement can alter its static or dynamic mechanical properties. Bar-shaped specimens made from six different (0-10% by weight) concentrations of multiwall carbon nanotubes were tested to failure in quasi-static 3-point bending and in 4-point bending fatigue (5 Hz). Analyses of variance and the 3-Parameter Weibull model were used to analyze the material performance data. The 2 wt % MWNT concentration enhanced flexural strength by 12.8% (p=0.003) and produced a 13.1% enhancement in yield stress (p=0.002). Bending modulus increased slightly with the smaller (<5 wt % MWNT) concentrations, but increased 24.1% (p<0.001) in response to the 10 wt % loading. While the 2 wt % loading produced slightly improved quasi-static test results, it was associated with clearly superior fatigue performance (3.3x increase in the Weibull mean fatigue life). Weibull minimum fatigue life (No), Weibull modulus (alpha), and characteristic fatigue life (beta) for bone cement augmented with carbon nanotubes were enhanced versus that observed in the control group. These data unambiguously showed that the bone cement-MWNT polymer system has an enhanced fatigue life compared to "control" bone cement (no added nanotubes). It is concluded that specific multiwall carbon nanotube loadings can favorably improve the mechanical performance of bone cement.     

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.     

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.     

Estimation of the minimum number of test specimens for fatigue testing of acrylic bone cement

In the literature on fatigue testing of acrylic bone cements, data sets of various sizes have been used in different test series for the same cement formulation. There are two important consequences of this situation. First, it means that some test series last much longer than others, with all the implications for the cost of testing. Second, it makes drawing conclusions about the fatigue performance of a cement, based on the results of different literature series, a problematic issue. Clearly then, a recommendation as to what should be the minimum number of test specimens to use that would allow for confidence in the results of the statistical treatment of the test results (Gmin) would be desirable. In the present work, a method that could be used to culminate in such a recommendation is described. This method involves (i) obtaining experimental fatigue test results and (ii) analyzing those results using the Weibull probability distribution function and other statistical methods. This methodology is illustrated using fatigue life results obtained from uniaxial tension-compression fatigue tests on specimens fabricated from the polymerizing dough of one commercially available acrylic bone cement. For a tolerable error of 5%, we estimated Gmin to be either 7 (if the fatigue life results are treated using the two-parameter Weibull distribution function) or 11 (if the fatigue life results are treated using the three-parameter Weibull distribution function). To be on the conservative side, we therefore recommend that Gmin be 11. Three key limitations of the methodology presented here are discussed.     

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.     

Pressurization of bioactive bone cement in vitro

We have developed a bioactive bone cement consisting of MgO-CaO-SiO2-P2O5-CaF2 glass-ceramic powder (AW glass-ceramic powder), silica glass powder as an inorganic filler, and bisphenol-a-glycidyl methacrylate (bis-GMA) based resin as an organic matrix. The efficacy of this bioactive bone cement was investigated by evaluating its pressurization in a 5-mm hole and small pores using a simulated acetabular cavity. Two types of acetabular components were used (flanged and unflanged sockets) and a commercially available polymethylmethacrylate (PMMA) bone cement (CMW 1 Radiopaque Bone Cement) was selected as a comparative control. Bioactive bone cement exerted greater intrusion volume in 5-mm holes than PMMA bone cement in both the flanged and unflanged sockets 10 minutes after pressurization (p < 0.05). In the small pores the bioactive and PMMA bone cements exerted almost identical intrusion volumes in flanged and unflanged sockets 10 min after pressurization. The intrusion volume in the flanged socket 10 minutes after pressurization was greater than that in the unflanged socket in all groups (p < 0.05). These results show that bioactive bone cement intrudes deeper into anchor holes than PMMA bone cement.     

Influence of the radiopacifier in an acrylic bone cement on its mechanical, thermal, and physical properties: barium sulfate-containing cement versus iodine-containing cement

In all acrylic bone cement formulations in clinical use today, radiopacity is provided by micron-sized particles (typical mean diameter of between about 1 and 2 microm) of either BaSO(4) or ZrO(2). However, a number of research reports have highlighted the fact that these particles have deleterious effects on various properties of the cured cement. Thus, there is interest in alternative radiopacifiers. The present study focuses on one such alternative. Specifically, a cement that contains covalently bound iodine in the powder (herein designated the I-cement) was compared with a commercially available cement of comparable composition (C-ment3), in which radiopacity is provided by BaSO(4) particles (this cement is herein designated the B-cement), on the basis of the strength (sigma(b)), modulus (E(b)), and work-to-fracture (U(b)), under four-point bending, plane-strain fracture toughness (K(IC)), Weibull mean fatigue life, N(WM) (fatigue conditions: +/-15 MPa; 2 Hz), activation energy (Q), and frequency factor (ln Z) for the cement polymerization process (both determined by using differential scanning calorimetry at heating rates of 5, 10, 15, and 20 K min(-1)), and the diffusion coefficient for the absorption of phosphate-buffered saline at 37 degrees C (D). For the B-cement, the values of sigma(b), E(b), U(b), K(IC), N(WM), Q, ln Z, and D were 53 +/- 3 MPa, 3000 +/- 120 MPa, 108 +/- 15 kJ m(-3), 1.67 +/- 0.02 MPa check mark m, 7197 cycles, 243 +/- 17 kJ mol(-1), 87 +/- 6, and (3.15 +/- 0.94) x 10(-12) m(2) s(-1), respectively. For the I-cement, the corresponding values were 58 +/- 5 MPa, 2790 +/- 140 MPa, 118 +/- 45 kJ m(-3), 1.73 +/- 0.11 MPa check mark m, 5520 cycles, 267 +/- 19 kJ mol(-1), 95 +/- 9, and (3.83 +/- 0.25) x 10(-12) m(2) s(-1). For each of the properties of the fully cured cement, except for the rate constant of the polymerization reaction, at 37 degrees C (k'), as estimated from the Q and ln Z results, there is no statistically significant difference between the two cements. k' for the I-cement was about a third that for the B-cement, suggesting that the former cement has a higher thermal stability. The influence of various characteristics of the starting powder (mean particle size, particle size distribution, and morphology) on the properties of the cured cements appears to be complex. When all the present results are considered, there is a clear indication that the I-cement is a viable candidate cement for use in cemented arthroplasties in place of the B-cement.     

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.     

Influence of a pre-blended antibiotic (gentamicin sulfate powder) on various mechanical, thermal, and physical properties of three acrylic bone cements

The aim of this work was to determine an array of mechanical, physical, and thermal properties of three pairs of commercially available acrylic bone cement brands, with the brands in each pair having the same compositions except that one contains 4.22 wt/wt% gentamicin sulfate blended with the powder by the manufacturer and the other one does not. The difference between the pairs was in the viscosity of the curing cement dough, with one pair of 'low-viscosity', one pair of 'medium-viscosity', and one pair of 'high-viscosity' brands being used. Thus, the brands studied cover the range of those used in anchoring some total joint replacements (TJRs). The properties determined were the strength, modulus, and work-to-fracture (all under four-point bending), plane-strain fracture toughness, Weibull mean fatigue life (fatigue conditions: 15 MPa; 2 Hz), activation energy and frequency factor for the cement polymerization process (both determined, using differential scanning calorimetry, at heating rates of 5, 10, 15, and 20 K min (1)), and the diffusion coefficient for the absorption of phosphate-buffered saline at 37 C by the cured cement. For each property determined, there was no significant difference in the mean values for the brands in each of the pairs. These results indicate that over the range of cement brands that are widely used in the anchoring of cemented TJRs, the addition of gentamicin sulfate powder does not degrade the properties of the cement, and, hence, may not adversely affect the in vivo longevity of the replacement.     

Damage accumulation, fatigue and creep behaviour of vacuum mixed bone cement

The behaviour of bone cement under fatigue loading is of interest to assess the long-term in vivo performance. In this study, uniaxial tensile fatigue tests were performed on CMW-1 bone cement. Acoustic emission sensors and an extensometer were attached to monitor damage accumulation and creep deformation respectively. The S-N data exhibited the scatter synonymous with bone cement fatigue, with large pores generally responsible for premature failure; at 20 MPa specimens failed between 2 x 10(3) and 2 x 10(4) load cycles, while at 7 MPa specimens failed from 3 x 10(5) load cycles but others were still intact after 3 x 10(6) load cycles. Acoustic emission data revealed a non-linear accumulation of damage with respect to time, with increasing non-linearity at higher stress levels. The damage accumulation process was not continuous, but occurred in bursts separated by periods of inactivity. Damage in the specimen was located by acoustic emissions, and allowed the failure site to be predicted. Acoustic emission data were also used to predict when failure was not imminent. When this was the case at 3 million load cycles, the tests were terminated. Creep strain was plotted against the number of load cycles and a linear relationship was found when a double logarithmic scale was employed. This is the first time a brand of cement has been characterised in such detail, i.e. fatigue life, creep and damage accumulation. Results are presented in a manner that allows direct comparison with published data for other cements. The data can also be used to characterise CMW-1 in computational simulations of the damage accumulation process. Further evidence is provided for the condition-monitoring capabilities of the acoustic emission technique in orthopaedic applications.     

Fatigue data analysis of canine femurs under four-point bending

When bone is subjected to fatigue loading, micro-cracks initiate and grow. This reduces the mechanical properties and quantitative relationships between stiffness loss and loading cycles may be derived. We developed the relationships between stiffness loss and loading cycles for whole canine femurs subjected to cyclic fatigue in four-point bending. The fatigue data from experiments followed Weibull statistics. When the stiffness loss is less than 15%, a linear relationship is best-fitted (R2 = 0.96, p < 0.0001) between the stiffness loss and loading cycles. However, when the stiffness loss is greater than 30%, a power law relationship is best-fitted (R2 = 0.97, p < 0.0001) between the stiffness loss and loading cycles. Thus, we conclude that the derived relationships between stiffness loss and loading cycles might be useful for the prediction of bone failure under cyclic bending subjected to an initial strain of 2700 microstrain.     

Effect of water storage time and composite cement thickness on fatigue of a glass-ceramic trilayer system

AIM: Static Hertzian contact tests of monolayer glass-ceramics in trilayer configurations (glass-ceramic/cement/composite) have shown that thick cement layers lower strength. This study sought to test the hypothesis that thick resin cement layers lower mouth motion fatigue reliability for flat glass-ceramic/cement/composite trilayer systems and that aging in water reduces reliability. METHODS: Dicor plates (n > or = 12 per group) (10 x 10 x 0.8 mm(3)) were aluminum-oxide abraded (50 microm), etched (60 s), silanized, and bonded (Rely X ARC) to water aged (30 days) Z100 resin blocks (10 x 10 x 4 mm(3)). Four groups were prepared: (1) thick cement layer (>100 microm) stored in water for 24-48 h, (2) thick cement layer stored for 60 days, (3) thin cement layer (< or =100 microm) stored for 24-48 h, and (4) thin cement layer stored for 60 days. The layered structures were fatigued (2 Hz) utilizing mouth motion loading with a step-stress acceleration method. A master Weibull distribution was calculated and reliability determined (with 90% confidence intervals) at a given number of cycles and load. RESULTS: The aged group (60 d) with thick cement layer had statistically lower reliability for 20,000 cycles at 150 N peak load (0.11) compared with both nonaged groups (24-48 h) (thin layer = 0.90 and thick layer = 0.82) and aged group with thin cement layer (0.89). CONCLUSION: Trilayer specimens with thick cement layers exhibited significantly lower reliability under fatigue testing only when stored for 60 days in water. The hypothesis was accepted. These results suggest that diffusion of water into the resin cement and also to the glass-ceramic interface is delayed in the thick cement specimens at 24-48 h. .     

Static and fatigue mechanical behavior of bone cement with elevated barium sulfate content for treatment of vertebral compression fractures

The use of bone cement to treat vertebral compression fractures in a percutaneous manner requires placement of the cement under fluoroscopic image guidance. To enhance visualization of the flow during injection and to monitor and prevent leakage beyond the confines of the vertebral body, the orthopedic community has described increasing the amount of radiopacifier in the bone cement. In this study, static tensile and compressive testing, as well as fully reversed fatigue testing, was performed on three PMMA-based bone cements. Cements tested were SimplexP with 10% barium sulfate (Stryker Orthopedics, Mahwah, NJ) which served as a control; SimplexP with 36% barium sulfate prepared according to the clinical recommendation of Theodorou et al.; and KyphX HV-R with 30% barium sulfate (Kyphon Inc., Sunnyvale, CA). Static tensile and compressive testing was performed in accordance with ASTM F451-99a. Fatigue testing was conducted in accordance with ASTM F2118-01a under fully reversed, +/-10-, +/-15-, and +/-20-MPa stress ranges. Survival analysis was performed using three-parameter Weibull modeling techniques. KyphX HV-R was found to have comparable static mechanical properties and significantly greater fatigue life than either of the two control materials evaluated in the present study. The static tensile and compressive strengths for all three PMMA-based bone cements were found to be an order of magnitude greater than the expected stress levels within a treated vertebral body. The static and fatigue testing data collected in this study indicate that bone cement can be designed with barium sulfate levels sufficiently high to permit fluoroscopic visualization while retaining the overall mechanical profile of a conventional bone cement under typical in vivo loading conditions.     

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.     

Augmentation of implant purchase with bone cements: an in vitro study of injectability and dough distribution

Vertebroplasty is widely used to treat (augment) osteoporotic fractures of the spine. This technique--with or without metallic implants--might have more widespread indications, if the mechanics of the injection and distribution of the cement dough through cannulated instruments and implants were better understood. This study was performed to investigate injectability of calcium phosphate and acrylic bone cements through implant prototypes, which featured different perforated sleeve designs. Using a custom-made capillary rheometer, the forces needed to inject 10 mL of the cement dough through standard cannulas were measured in the first series of experiments. In the second series, plastic sleeves were attached to the rheometer, simulating the implant. In both series, the dough was injected into ambient laboratory atmosphere, and in the second series, cement distribution was analyzed by means of an optical system. Injection of cement dough through the cannulas required forces between 50 and 400 N in the case of acrylic cements and between 40 and 500 N in case of the calcium phosphate cements. Using different sleeves did not have a significant influence on the distribution of the cement dough around the sleeve. The amount of cement dough injected was reduced when a perforated implant was attached to the cannula. More material was delivered through the proximal holes of the implant, leading to a V-shaped distribution of the cement dough. Among topics to be investigated in future studies is determination of the injectability of cement dough into trabecular bone or bone-like structures.     

Effect of two variables on the fatigue performance of acrylic bone cement: mixing method and viscosity

The goal of the present work was to establish the relative influence of one exogenous variable versus one endogenous variable on the fully-reversed tension-compression fatigue performance of bone cement. The method used to mix the cement constituents was the exogenous variable, while the viscosity of the mixed cement dough was the endogenous variable. Two commercial cement formulations (Palacos R and Osteopal) and two cement mixing methods (hand mixing and vacuum mixing) were used. It was found that for a given mixing method, cement viscosity exerts a marginal influence on fatigue performance. On the other hand, for a given cement formulation, vacuum mixing led to a statistically significant improvement in fatigue performance. The present results demonstrate the superior influence of mixing method over cement viscosity.