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.     

Fatigue and biocompatibility properties of a poly(methyl methacrylate) bone cement with multi-walled carbon nanotubes

Composites of multi-walled carbon nanotubes (MWCNT) of varied functionality (unfunctionalised and carboxyl and amine functionalised) with polymethyl methacrylate (PMMA) were prepared for use as a bone cement. The MWCNT loadings ranged from 0.1 to 1.0 wt.%. The fatigue properties of these MWCNT-PMMA bone cements were characterised at MWCNT loading levels of 0.1 and 0.25 wt.% with the type and wt.% loading of MWCNT used having a strong influence on the number of cycles to failure. The morphology and degree of dispersion of the MWCNT in the PMMA matrix at different length scales were examined using field emission scanning electron microscopy. Improvements in the fatigue properties were attributed to the MWCNT arresting/retarding crack propagation through the cement through a bridging effect and hindering crack propagation. MWCNT agglomerates were evident within the cement microstructure and the degree of agglomeration was dependent on the level of loading and functionality of the MWCNT. The biocompatibility of the MWCNT-PMMA cements at MWCNT loading levels upto 1.0 wt.% was determined by means of established biological cell culture assays using MG-63 cells. Cell attachment after 4h was determined using the crystal violet staining assay. Cell viability was determined over 7 days in vitro using the standard colorimetric MTT assay. Confocal scanning laser microscopy and SEM analysis was also used to assess cell morphology on the various substrates.     

Modification of Oligomeric Cyanate Ester Polymer Properties with Multi‐Walled Carbon Nanotube‐Containing Particles

The properties of an oligomeric cyanate ester polymer were modified by the addition of 0.01–3 wt.‐% multi‐walled carbon nanotube (MWNT) containing particles. The dynamic mechanical behavior and thermal properties of the cyanate ester/MWNT nanocomposites were evaluated. The storage modulus, G′, of the nanocomposite with 1 wt.‐% MWNT particles was nearly 60 and 600% higher than the neat polymer at 100 and 200 °C, respectively. The glass transition temperature of the nanocomposite was also raised by 30 °C and its thermal stability in air and nitrogen was increased by 58 and 25 °C, respectively. The property improvements are attributed to reinforcement of the cyanate ester as a result of good nanotube dispersion and effective polymer‐MWNT interaction.

     

A new bioresorbable polymer for screw augmentation in the osteosynthesis of osteoporotic cancellous bone: a biomechanical evaluation

The aim of the study was to assess the mechanical efficacy of a new resorbable polymer developed on the basis of alkylene bis(dilactoyl)-methacrylate to improve the anchorage of osteosynthesis material in cancellous bone. Cancellous bone screws were inserted in bovine as well as in human vertebrae and human femoral condyles and were augmented with the new polymer or polymethylmethacrylate (PMMA), respectively. Nonaugmented screws were used as controls. A removal torque test, a dynamic fatigue test, and a pullout test were performed. Augmentation with the new polymer increased the removal torque by 84% in human femoral bone. In the dynamic fatigue test of bovine vertebrae, the removal torque after cyclic loading was 115% higher for the new polymer compared to the nonaugmented controls. In the human vertebrae, the reinforcement with the new polymer increased the removal torque after dynamic loading by 114%. The augmentation with the new polymer increased the pullout force by 88% in bovine vertebrae and by 118% in human vertebrae in comparison to nonaugmented screws. It was concluded that augmentation by the new resorbable polymer significantly enhanced the anchorage of bone screws in cancellous bone. The mechanical efficiency of the new polymer was comparable to that of PMMA cement.     

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.     

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.     

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.     

Improved fatigue life of acrylic bone cements reinforced with zirconia fibers

Poly(methyl methacrylate) (PMMA) bone cements have a long and successful history of use for implant fixation, but suffer from a relatively low fracture and fatigue resistance which can result in failure of the cement and the implant. Fiber or particulate reinforcement has been used to improve mechanical properties, but typically at the expense of the pre-cured cement viscosity, which is critical for successful integration with peri-implant bone tissue. Therefore, the objective of this study was to investigate the effects of zirconia fiber reinforcement on the fatigue life of acrylic bone cements while maintaining a relatively low pre-cured cement viscosity. Sintered straight or variable diameter fibers (VDFs) were added to a PMMA cement and tested in fully reversed uniaxial fatigue until failure. The mean fatigue life of cements reinforced with 15 and 20 vol% straight zirconia fibers was significantly increased by ～40-fold, on average, compared to a commercial benchmark (Osteobond?) and cements reinforced with 0–10 vol% straight zirconia fibers. The mean fatigue life of a cement reinforced with 10 vol% VDFs was an order of magnitude greater than the same cement reinforced with 10 vol% straight fibers. The time-dependent viscosity of cements reinforced with 10 and 15 vol% straight fibers was comparable to the commercial benchmark during curing. Therefore, the addition of relatively small amounts of straight and variable diameter zirconia fibers was able to substantially improve the fatigue resistance of acrylic bone cement while exhibiting similar handling characteristics compared to current commercial products.     

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 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.     

Efficient and Robust Reactions for Polyethylene Covalently Grafted Carbon Nanotubes

Surface modification of carbon nanotubes (CNTs) with long polymer chains has attracted great attention since it promises high performance nanomaterials. However, due to the chemical inertness of both polyethylene (PE) and CNT, it still remains a challenge to covalently conjugate CNT with PE in a facile manner. In this report, trimethoxysilane terminated PE (TMS‐PE) and trimethoxysilane modified multiwall carbon nanotube (TMS‐MWNT) is prepared through radical mediated thiol‐ene addition reaction between (3‐mercaptopropyl)trimethoxysilane and vinyl terminated PE or pristine carbon nanotube, respectively. Subsequently, organotin‐catalyzed hydrolytic co‐condensation of TMS‐PE and TMS‐MWNT lead to PE covalently grafted mutliwall CNT (PE‐Si‐MWNT) with high graft ratio (17.5 wt%). Such reactions are robust and efficient and the reaction conditions are mild. In addition, the authors provide the very first evidence that end‐grafted PE chains in PE‐Si‐MWNT, which are clearly observed in TEM images, can render high compatibility and good interfacial interactions between CNT and low density PE (LDPE) matrix and thus promote homogeneous dispersion of CNT into LDPE matrix as shown by scanning electron microscopy (SEM) characterization of brittle fracture surfaces of LDPE/PE‐Si‐MWNT nanocomposites.

     

Effects of aging on the mechanical behavior of human dentin

An experimental study on the mechanical behavior of human dentin and the influence of age was conducted. Beams with rectangular cross-section were sectioned from the coronal dentin of virgin extracted molars (N = 76) that were obtained from (N = 70) patients between 17 and 80 years of age. The beams were loaded in either quasi-static 4-point flexure or 4-point flexural fatigue to failure and the stiffness, strength and fatigue properties were evaluated. In characterizing the fatigue response the beams were divided into two age groups that were regarded as young (17 < or = age < or = 30, mean +/- std. dev. = 25 +/- 5 years) and old (50 < or = age < or = 80, mean +/- std. dev. = 64 +/- 9 years) dentin. Results from monotonic loading showed that both the flexural strength and strain to fracture of dentin decreased significantly with age. The fatigue life of dentin increased with a reduction in cyclic stress amplitude and the fatigue strength of young dentin was greater than that of old dentin at all cyclic stress amplitudes. The endurance strength of young dentin (at 10(7) cycles) was approximately 44 MPa, whereas the old dentin exhibited an endurance strength of approximately 23 MPa.Based on differences in the mechanical behavior and microscopic features of the fracture surfaces from the young and old specimens, aging appears to result in an increase in both the rate of damage initiation and propagation in dentin.     

Evaluation of a highly-radiopaque iodine-containing acrylic bone cement for use in augmentation of vertebral compression fractures

Vertebroplasty and balloon kyphoplasty are widely used for the augmentation of osteoporosis-induced vertebral compression fractures. Almost invariably, an injectable poly(methyl methacrylate) (PMMA) bone cement that contains a large amount of BaSO(4) particles is used in these procedures. The deleterious effects of this radiopacifier on various properties of PMMA cement have been detailed in the literature. The objective of the present study was therefore to avoid such high levels of inorganic contrast agent and thus to develop an all-polymeric bone cement, for which radiopacity was provided by 60 wt % of an iodine-containing methacrylic copolymer, incorporated into the powder (IO cement), ultimately leading to 6.6 wt % of iodine in the cement. A large array of properties of this cement were determined, ranging from setting time and injectability to fatigue life under fully-reversed tension-compression loading and cytotoxicity, and a comprehensive comparison with a cement containing 30 wt % BaSO(4) in the powder component (BA cement) has been made (11 wt % of Ba in the cement). Statistical analyses of the results showed that, for the majority of the properties, the difference between the means for the two cements was not significant. It is therefore suggested that the IO cement is a suitable alternative to the BA cement for use in the aforementioned procedures.     

Fatigue of acrylic bone cement--effect of frequency and environment

This study was conducted to investigate some fundamental fatigue testing variables as they apply to the response characteristics of acrylic bone cement. Cyclic loading under load control was conducted at frequencies of 1, 2, 5, 10, and 20 Hz in air at room temperature. At a tensile stress range of 0.3-20.0 MPa the fatigue life increased linearly with logarithmic frequency. The effect of conditioning and testing in saline at both room temperature and 37 degrees C at similar stress levels and a frequency of 10 Hz were also examined. When compared to dry testing at room temperature, testing in saline at 37 degrees C resulted in a reduction in fatigue life while testing in saline at room temperature produced an increase in fatigue life. Of a number of statistical distributions considered, the Weibull was found to be the most appropriate in documenting the findings of this investigation. A companion fractographic investigation of the failure surfaces demonstrated distinct regions of crack growth and fast fracture.     

Mechanical properties and biomineralization of multifunctional nanodiamond-PLLA composites for bone tissue engineering

Multifunctional bone scaffold materials have been produced from a biodegradable polymer, poly(L-lactic acid) (PLLA), and 1-10% wt of octadecylamine-functionalized nanodiamond (ND-ODA) via solution casting followed by compression molding. By comparison to pure PLLA, the addition of 10% wt of ND-ODA resulted in a significant improvement of the mechanical properties of the composite matrix, including a 280% increase in the strain at failure and a 310% increase in fracture energy in tensile tests. The biomimetic process of bonelike apatite growth on the ND-ODA/PLLA scaffolds was studied using microscopic and spectroscopic techniques. The enhanced mechanical properties and the increased mineralization capability with higher ND-ODA concentration suggest that these biodegradable composites may potentially be useful for a variety of biomedical applications, including scaffolds for orthopedic regenerative engineering.     

Mechanical behaviour of a new acrylic radiopaque iodine-containing bone cement

In total hip replacement, fixation of a prosthesis is in most cases obtained by the application of methacrylic bone cements. Most of the commercially available bone cements contain barium sulphate or zirconium dioxide as radiopacifier. As is shown in the literature, the presence of these inorganic particles can be unfavourable in terms of mechanical and biological properties. Here, we describe a new type of bone cement, where X-ray contrast is obtained via the introduction of an iodine-containing methacrylate copolymer; a copolymer of methylmethacrylate and 2-[4-iodobenzoyl]-oxo-ethylmethacrylate (4-IEMA) is added to the powder component of the cement. The properties of the new I-containing bone cement (I-cement) are compared to those of a commercially available bone cement, with barium sulphate as radiopacifier (B-cement). The composition of the I-cement is adjusted such that similar handling properties and radiopacity as for the commercial cement are obtained. In view of the mechanical properties, it can be stated that the intrinsic mechanical behaviour of the I-cement, as revealed from compression tests, is superior to that of B-cement. Concerning the fatigue behaviour it can be concluded that, though B-cement has a slightly higher fatigue crack propagation resistance than I-cement, the fatigue life of vacuum-mixed I-cement is significantly better than that of B-cement. This is explained by the presence of BaSO4 clumps in the commercial cement; these act as crack initiation sites. The mechanical properties (especially fatigue resistance) of the new I-cement warrant its further development toward clinical application.     

Poly(propylene glycol)‐Grafted Multi‐Walled Carbon Nanotube Polyurethane

Summary: Poly(propylene glycol)‐grafted MWNT polyurethane was synthesized based on the hydroxyl functionalized MWNTs through a two‐step reaction. The grafted MWNTs can improve the rheological behavior of the polyol/MWNT dispersion, and have a better reinforcing effect on the mechanical properties of polyurethane and lower hysteresis compared to the raw carbon nanotubes. A comparison for energy dissipation in PU/carbon nanotube and exfoliated PU/organoclay nanocomposites was given. The energy dissipated in the grafted MWNT composite system is lower.

Dispersion stability of carbon nanotubes in polyether polyol after 40 min centrifugation (A: MWNT, B: MWNT‐OH, C: MWNT‐graft‐PU).     

Crosslinked poly(epsilon-caprolactone/D,L-lactide)/bioactive glass composite scaffolds for bone tissue engineering

A series of elastic polymer and composite scaffolds for bone tissue engineering applications were designed. Two crosslinked copolymer matrices with 90/10 and 30/70 mol % of epsilon-caprolactone (CL) and D,L-lactide (DLLA) were prepared with porosities from 45 to 85 vol % and their mechanical and degradation properties were tested. Corresponding composite scaffolds with 20-50 wt % of particulate bioactive glass (BAG) were also characterized. Compressive modulus of polymer scaffolds ranged from 190+/-10 to 900+/-90 kPa. Lactide rich scaffolds absorbed up to 290 wt % of water in 4 weeks and mainly lost their mechanical properties. Caprolactone rich scaffolds absorbed no more than 110 wt % of water in 12 weeks and kept their mechanical integrity. Polymer and composite scaffolds prepared with P(CL/DLLA 90/10) matrix and 60 vol % porosity were further analyzed in simulated body fluid and in osteoblast culture. Cell growth was compromised inside the 2 mm thick three-dimensional scaffold specimens as a static culture model was used. However, composite scaffolds with BAG showed increased osteoblast adhesion and mineralization when compared to neat polymer scaffolds.     

Grafting Polymer Loops onto Functionalized Nanotubes: Monitoring Grafting and Loop Formation

Polystyrene functionalized at both ends (telechelic polymer) with epoxide groups (epoxy–PS–epoxy) was reacted with carboxylated multiwall carbon nanotubes (COOH? MWNT) in solution in order to graft polymer chains at both ends onto the MWNT surface, forming loops. FT‐IR spectroscopy was employed to monitor the formation of aromatic esters and to quantify the amount of telechelic grafted to the nanotube surface as a function of reaction time. When the samples were further annealed in the melt, an increase in the aromatic ester peak was observed, indicating that the unreacted chain ends further grafted to MWNT surfaces to form loops. By reacting the grafted nanotube samples further with monocarboxy terminated poly(4‐methylstyrene) (COOH? P4MS), the amount of epoxy–PS–epoxy that had only reacted at one end was determined. Reaction rate analysis indicates that that the grafting of epoxy–PS–epoxy to the nanotube surface is reaction controlled, as the FT‐IR spectroscopy signal grows as a function of approximately t^0.3. These studies exemplify how FT‐IR spectroscopy can be used as a novel technique to quantify the amount of grafted polymer, grafting rate, and percent of difunctional polymers that form loops, and provide a method to create loop covered nanoparticles.

     

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.