Antibiotic-free antimicrobial poly (methyl methacrylate) bone cements: A state-of-the-art review.

Prosthetic joint infection (PJI) is the most serious complication following total joint arthroplasty, this being because it is associated with, among other things, high morbidity and low quality of life, is difficult to prevent, and is very challenging to treat/manage. The many shortcomings of antibiotic-loaded poly (methyl methacrylate) (PMMA) bone cement (ALBC) as an agent for preventing and treating/managing PJI are well-known. One is that microorganisms responsible for most PJI cases, such as methicillin-resistant S. aureus, have developed or are developing resistance to gentamicin sulfate, which is the antibiotic in the vast majority of approved ALBC brands. This has led to many research efforts to develop cements that do not contain gentamicin (or, for that matter, any antibiotic) but demonstrate excellent antimicrobial efficacy. There is a sizeable body of literature on these so-called "antibiotic-free antimicrobial" PMMA bone cements (AFAMBCs). The present work is a comprehensive and critical review of this body. In addition to summaries of key trends in results of characterization studies of AFAMBCs, the attractive features and shortcomings of the literature are highlighted. Shortcomings provide motivation for future work, with some ideas being formulation of a new generation of AFAMBCs by, example, adding a nanostructured material and/or an extract from a natural product to the powder and/or liquid of the basis cement, respectively.

Properties of nanofiller-loaded poly (methyl methacrylate) bone cement composites for orthopedic applications: a review.

There is a large body of literature on new generations of poly (methyl methacrylate) bone cements that address one or more of the material's shortcomings. Among these are cements in which one of the constituents is a nanofiller, such as nano-sized barium sulfate, multiwalled carbon nanotubes, natural nanoclay, mesoporous silica nanoparticles, or oleic acid-capped silver nanoparticles. This article is a review of the literature on the properties of these nanofiller-loaded bone cements (NFLBCs). Some key characteristics of the literature are that (1) in a number of studies, clinically relevant properties were determined, examples being maximum exotherm, setting time, fatigue life, and compressive modulus; (2) in some studies, properties were not determined in accordance with approved bone cement testing specifications, an example being fatigue life; and (3) there are a number of clinically relevant properties that were not determined in any of the studies, examples being fatigue crack propagation rate and dynamic compression creep life. These observations, as well as other considerations, suggest 12 areas for future study, such as determination of dynamic creep compliance (using nanoindentation), determination of compressive fatigue life for cements to be used in vertebral compression fracture augmentation, elucidation of toughening mechanism(s) in each type of NFLBC, and conducting well-designed clinical trials. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 105B: 1260-1284, 2017.

Influence of assigned material combination in a simulated total cervical disc replacement design on kinematics of a model of the full cervical spine: A finite element analysis study.

BACKGROUND: Although cervical total disc replacement (TDR) is becoming popular, there are no finite analysis (FEA) studies involving a model of the full spine cervical (C1-C7) and determination of the influence of materials assigned to different parts of a specified TDR design on biomechanics of the model when TDR implantation is simulated. OBJECTIVE: To determine the influence of assigned material combination, for a given cervical TDR design, on the kinematics of a model of the full cervical spine. METHODS: A three-dimensional solid model of the full cervical spine was constructed, a finite element mesh was obtained (INT Model), after which FEA was used to determine range of motion (ROM) at each of the intersegmental positions under three clinically-relevant loadings. INT model was then modified by simulated implantation of a notional endplates-and-mobile insert TDR design, at C5-C6 (TDR Model), and six clinically-relevant applied loadings were applied. Four variants of TDR Model were used, the difference between them being in the materials assigned to the endplates and the mobile insert. Under each of the loadings, principal motions at each of the intersegmental positions were determined and compared to counterpart motions when INT Model was used. RESULTS: Comparison of ROM results of INT Model with relevant experimental results reported in the literature showed that the model was validated. With TDR Model, the smallest overall mean of the absolute values of the % change in principal intersegmental motions (relative to corresponding results in INT Model) was when the material assigned to both the endplates and the mobile insert was poly(ether-ether-ketone). CONCLUSION: In a simulated implantation of a notional endplates-and-mobile-insert TDR design in a model of the full cervical spine, material combination assigned to the parts of the design exerts a marked influence on the kinematics of the model.

Not all approved antibiotic-loaded PMMA bone cement brands are the same: ranking using the utility materials selection concept.

In the literature on in vitro characterization of approved antibiotic-loaded poly(methyl methacrylate) bone cement brands, there is no information on the basis for selection of a given brand for use in cemented arthroplasties. This shortcoming is addressed in the present study. It involved determining four key properties (fatigue limit, fracture toughness, polymerization rate, and phosphate buffered saline diffusion coefficient) for six brands and then using the mean property values, in conjunction with a materials selection methodology, called the utility concept, to rank the brands. It is emphasized that the present work is an illustration of a rational approach to selection of a cement brand and, as such, the study findings are not intended to be recommendations regarding clinical use or otherwise of a brand.

Healthcare Engineering Defined: A White Paper.

Engineering has been playing an important role in serving and advancing healthcare. The term "Healthcare Engineering" has been used by professional societies, universities, scientific authors, and the healthcare industry for decades. However, the definition of "Healthcare Engineering" remains ambiguous. The purpose of this position paper is to present a definition of Healthcare Engineering as an academic discipline, an area of research, a field of specialty, and a profession. Healthcare Engineering is defined in terms of what it is, who performs it, where it is performed, and how it is performed, including its purpose, scope, topics, synergy, education/training, contributions, and prospects.

Direct and interactive influence of explanatory variables on properties of a calcium phosphate cement for vertebral body augmentation.

We used the response surface methodology to investigate the direct and interactive effects of three explanatory variables on three properties of a calcium phosphate cement (CPC) for use in vertebroplasty (VP) and balloon kyphoplasty (BKP). The variables were poly(ethylene glycol) content of the cement liquid (PEG), powder-to-liquid ratio (PLR), and the amount of Na2HPO4 added to an aqueous solution of 4 wt/wt% poly(acrylic acid) (as the cement liquid) (SPC). The properties were injectability (I), final setting time (F), and 5-day compressive strength (UCS). We found that (1) there was an interactive effect between the variables on I and F but not on UCS; (2) the maximum I (98%) was obtained with PEG = 20 wt/wt% and PLR = 2 g mL(-1); (3) F = 15 min (the proposed optimum value for a CPC for use in VP and BKP) was obtained with PEG = 4 wt/wt% and PLR = 2.9 g mL(-1); and (4) the maximum UCS (39 MPa) was obtained with SPC = 0 and PLR = 3.5 g mL(-1).

Design of a first metatarsophalangeal joint simulator.

BACKGROUND: The use of total first metatarsophalangeal joint (MPJ) arthroplasty to treat patients in which the pain, due to a pathological joint, has not been relieved with a conservative method or for which the disease or disorder is at an advanced stage, is popular. Although meta-analysis of clinical results indicates that this surgical option is efficacious, there are problems with implant failure due to wear of the components. Although there is a plethora of designs of this type of implant in clinical use, there are no literature reports on total first MPJ simulators, which may be used to evaluate, for example, the wear rate of a total first MPJ implant. METHODS: We designed such a simulator, guided by the biomechanics of the joint. Thus, for example, the implant under test will be articulated at least 40° dorsiflexion, under a 600 N loading, at 1 Hz. Furthermore, the testing stations will be configured to allow testing of any type of first MPJ implant. We also performed a finite element analysis (FEA) study of a model of an articulating station, subjected to a quasi-static load of 1200 N. RESULTS: For an articulating station, (1) the highest von Mises stress occurred at the implant-fixture interface; and (2), for the other parts, the minimum factor of safety, against elastic failure, is approximately 9. CONCLUSIONS: The designed joint simulator is mechanically sound and may be used for wear testing of any type of first MPJ implant.

Properties of open-cell porous metals and alloys for orthopaedic applications.

One shortcoming of metals and alloys used to fabricate various components of orthopaedic systems, such as the femoral stem of a total hip joint replacement and the tibial plate of a total knee joint replacement, is well-recognized. This is that the material modulus of elasticity (E') is substantially larger than that of the contiguous cancellous bone, a consequence of which is stress shielding which, in turn, has been postulated to be implicated in a cascade of events that culminates in the principal life-limiting phenomenon of these systems, namely, aseptic loosening. Thus, over the years, a host of research programs have focused on the synthesis of metallic biomaterials whose E' can be tailored to match that of cancellous bone. The present work is a review of the extant large volume of literature on these materials, which are called open-cell porous metals/alloys (or, sometimes, metal foams or cellular materials). As such, its range is wide, covering myriad aspects such as production methods, characterization studies, in vitro evaluations, and in vivo performance. The review also includes discussion of seven areas for future research, such as parametric studies of the influence of an assortment of process variables (such as the space holder material and the laser power in the space holder method and the laser-engineered net-shaping process, respectively) on various properties (notably, permeability, fatigue strength, and corrosion resistance) of a given porous metal/alloy, innovative methods of determining fatigue strength, and modeling of corrosion behavior.

A daptomycin-xylitol-loaded polymethylmethacrylate bone cement: how much xylitol should be used?

BACKGROUND: The rate of release of an antibiotic from an antibiotic-loaded polymethylmethacrylate (PMMA) bone cement is low. This may be increased by adding a particulate poragen (eg, xylitol) to the cement powder. However, the appropriate poragen amount is unclear. QUESTIONS/PURPOSES: We explored the appropriate amount of xylitol to use in a PMMA bone cement loaded with daptomycin and xylitol. METHODS: We prepared four groups of cement, each comprising the same amount of daptomycin in the powder (1.36 g/40 g dry powder) but different amounts of xylitol (0, 0.7, 1.4, and 2.7 g); the xylitol mass ratio (X) (mass divided by mass of the final dry cement-daptomycin-xylitol mixture) ranged from 0 to 6.13 wt/wt%. Eight mechanical, antibiotic release, and bacterial inhibitory properties were determined using three to 22 specimens or replicates per test. We then used an optimization method to determine an appropriate value of X by (1) identifying the best-fit relationship between the value of each property and X, (2) defining a master objective function incorporating all of the best fits; and (3) determining the value of X at the maximum master objective function. RESULTS: We found an appropriate xylitol amount to be 4.46 wt/wt% (equivalent to 1.93 g xylitol mixed with 1.36 g daptomycin and 40 g dry cement powder). CONCLUSIONS: We demonstrated a method that may be used to determine an appropriate xylitol amount for a daptomycin-xylitol-loaded PMMA bone cement. These findings will require in vivo confirmation. CLINICAL RELEVANCE: While we identified an appropriate amount of xylitol in a daptomycin-xylitol-loaded PMMA bone cement as a prophylactic agent in total joint arthroplasties, clinical evaluations are needed to confirm the effectiveness of this cement.

Nucleus pulposus replacement and regeneration/repair technologies: present status and future prospects.

Degenerative disc disease is implicated in the pathogenesis of many painful conditions of the back, chief among which is low back pain. Acute and/or chronic low back pain (A/CLBP) afflicts a large number of people, thus making it a major healthcare issue with concomitant cost ramifications. When conservative treatments for A/CLBP, such as bed rest, anti-inflammatory medications, and physical therapy, prove to be ineffectual, surgical options are recommended. The most popular of these is discectomy followed by fusion. Although there are many reports of good to excellent outcomes with this method, there are concerns, such as long-term adverse biomechanical consequences to adjacent functional spinal unit(s). A surgical option that has been attracting much attention recently is replacement or regeneration/repair of the nucleus pulposus, an approach that holds the prospect of not compromising either mobility or function and causing no adjacent-level injury. There is a sizeable body of literature highlighting this option, comprising in vitro biomechanical studies, finite element analyses, animal-model studies, and limited clinical evaluations. This work is a review of this body of literature and is organized into four parts, with the focus being on replacement technologies, regeneration/repair technologies, and detailed expositions on 14 areas for future study. This review ends with a summary of the salient points made.

An innovative multi-component variate that reveals hierarchy and evolution of structural damage in a solid: application to acrylic bone cement.

A major limitation of solid mechanics is the inability to take into account the influence of hierarchy and evolution of the inherent microscopic structure on evaluating the performance of materials. Irreversible damage and fracture in solids, studied commonly as cracks, flaws, and conventional material properties, are by no means descriptive of the subsequent responses of the microstructures to the applied load. In this work, we addressed this limitation through the use of a novel multi-component variate. The essence of this variate is that it allows the presentation of the random damage in the amplitude spectrum, probability space, and probabilistic entropy. Its uniqueness is that it reveals the evolution and hierarchy of random damage in multi- and trans-scales, and, in addition, it includes the correlations among the various damage features. To better understand the evolution and hierarchy of random damage, we conducted a series of experiments designed to test three variants of a poly (methyl methacrylate) (PMMA) bone cement, distinguished by the methods used to sterilize the cement powder. While analysis of results from conventional tension tests and scanning electron microscopy failed to pinpoint differences among these cement variants, our multi-component variate allowed quantification of the multi- and trans-scale random damage events that occurred in the loading process. We tested the statistical significance of damage states to differentiate the responses at the various loading stages and compared the damage states among the groups. We also interpreted the hierarchical and evolutional damage in terms of the probabilistic entropy (s), the applied stress (σ), and the trajectory of damage state. We found that the cement powder sterilization method has a strong influence on the evolution of damage states in the cured cement specimens when subjected to stress in controlled mechanical tests. We have shown that in PMMA bone cements, our damage state variate has the unique ability to quantify and discern the history and evolution of microstructural damage.

Viscoelastic properties of injectable bone cements for orthopaedic applications: state-of-the-art review.

Injectable bone cements (IBCs) are used for a variety of orthopaedic applications, examples being poly (methyl methacrylate) (PMMA) bone cements used for anchoring total joint replacements (TJRs) (high load-bearing application), PMMA bone cements used in the vertebral body augmentation procedures of vertebroplasty (VP) and balloon kyphoplasty (BKP) (medium load-bearing application), and calcium phosphate-based and calcium sulfate-based cements used as bone void fillers/bone graft substitutes (low load-bearing application). For each of these applications, the viscoelastic properties of the cement are very important. For example, (1) creep of the cement has an influence on the longevity of a cemented TJR (for example, creep allows the cement to remodel, thereby maximizing the contact area of the cement-bone interface and, hence, minimizing stress concentration at that interface); and (2) in VP and BKP, the likelihood of cement extravasation is directly related to the profile of the viscosity-versus-time elapsed from commencement of mixing of the cement. There are a few reviews of the literature on a number of viscoelastic properties of some IBCs but a comprehensive review of the literature on all viscoelastic properties of all IBCs is lacking. The objective of this contribution is to present such a review. In addition, a number of ideas for future study in the field of viscoelastic properties of IBCs are described.

Probabilistic characteristics of random damage events and their quantification in acrylic bone cement.

The failure of brittle and quasi-brittle polymers can be attributed to a multitude of random microscopic damage modes, such as fibril breakage, crazing, and microfracture. As the load increases, new damage modes appear, and existing ones can transition into others. In the example polymer used in this study--a commercially available acrylic bone cement--these modes, as revealed by scanning electron microscopy of fracture surfaces, include nucleation of voids, cracking, and local detachment of the beads from the matrix. Here, we made acoustic measurements of the randomly generated microscopic events (RGME) that occurred in the material under pure tension and under three-point bending, and characterized the severity of the damage by the entropy (s) of the probability distribution of the observed acoustic signal amplitudes. We correlated s with the applied stress (σ) by establishing an empirical s-σ relationship, which quantifies the activities of RGME under Mode I stress. It reveals the state of random damage modes: when ds/dσ > 0, the number of damage modes present increases with increasing stress, whereas it decreases when ds/dσ < 0. When ds/dσ ≈ 0, no new random damage modes occur. In the s-σ curve, there exists a transition zone, with the stress at the "knee point" in this zone (center of the zone) corresponding to ~30 and ~35% of the cement's tensile and bending strengths, respectively. This finding explains the effects of RGME on material fatigue performance and may be used to approximate fatigue limit.

Influence of surgical treatment for disc degeneration disease at C5-C6 on changes in some biomechanical parameters of the cervical spine.

A detailed three-dimensional solid model of the full cervical spine (C1-C7 levels) and the finite element analysis method were used to investigate the extent of changes in various biomechanical properties brought about when surgical methods are used to treat condition(s) caused by or are a sequela of disc degeneration disease at the C5-C6 level. The surgical methods simulated were anterior cervical discectomy and fusion, with interbody fusion achieved using a notional brick-shaped graft only; anterior cervical discectomy alone; percutaneous nucleotomy; and three variants of nucleus replacement. The control case was a model of an intact, healthy, adult spine. Each of these seven models was subjected to (1) flexion moment, extension moment, left lateral bending moment, right lateral bending moment, clockwise-acting axial rotation moment, and counterclockwise-acting axial rotation moment, with a compression pre-load applied simultaneously with each of these loadings and (2) an axial compression force (applied as a uniform pressure) only. For each combination of model and applied loading, the maximum von Mises stress and the maximum strain energy density were determined for tissues at the treated level, at one level above the treated level, and at one level below the treated level and (2) the total principal rotation angles at each of the intersegmental positions of the entire model. In addition, for each of the study cases, we obtained the longitudinal displacement of each of the models when subjected to the axial compression force only. We found markedly fewer changes (relative to the results when the intact, healthy spine model was used) in each of the above-mentioned biomechanical parameters above a specified threshold in the case of the simulated percutaneous nucleotomy and simulated nucleus replacement models, on one hand, compared to the simulated fusion and simulated discectomy models, on the other. This finding is in consonance with the evolving clinical practice of using minimally invasive surgical methods for treating problem(s) such as soft cervical disc herniations.

An Approach for determining antibiotic loading for a physician-directed antibiotic-loaded PMMA bone cement formulation.

BACKGROUND: When a physician-directed antibiotic-loaded polymethylmethacrylate (PMMA) bone cement (ALBC) formulation is used in total hip arthroplasties (THAs) and total knee arthroplasties (TKAs), current practice in the United States involves arbitrary choice of the antibiotic loading (herein defined as the ratio of the mass of the antibiotic added to the mass of the cement powder). We suggest there is a need to develop a rational method for determining this loading. QUESTIONS/PURPOSES: We propose a new method for determining the antibiotic loading to use when preparing a physician-directed ALBC formulation and illustrate this method using three in vitro properties of an ALBC in which the antibiotic was daptomycin. MATERIALS AND METHODS: Daptomycin was blended with the powder of the cement using a mechanical mixer. We performed fatigue, elution, and activity tests on three sets of specimens having daptomycin loadings of 2.25, 4.50, and 11.00 wt/wt%. Correlational analyses of the results of these tests were used in conjunction with stated constraints and a nonlinear optimization method to determine the daptomycin loading to use. RESULTS: With an increase in daptomycin loading, the estimated mean fatigue limit of the cement decreased, the estimated elution rate of the antibiotic increased, and the percentage inhibition of staphylococcal growth by the eluate remained unchanged at 100%. For a daptomycin-loaded PMMA bone cement we computed the optimum amount of daptomycin to mechanically blend with 40 g of cement powder is 1.36 g. CONCLUSIONS: We suggest an approach that may be used to determine the amount of antibiotic to blend with the powder of a PMMA bone cement when preparing a physician-directed ALBC formulation, and highlighted the attractions and limitations of this approach. CLINICAL RELEVANCE: When a physician-directed ALBC formulation is selected for use in a TKA or THA, the approach we detail may be employed to determine the antibiotic loading to use rather than the empirical approach that is taken in current clinical practice.

Influence of loading cycle profile and frequency on a biomechanical parameter of a model of a balloon kyphoplasty-augmented lumbar spine segment: a finite element analysis study.

For patients who are suffering debilitating and persistent pain due to vertebral compression fracture(s) and for whom conservative therapies have not provided relief, balloon kyphoplasty (BKP) is used as a surgical option. There are only a very few literature reports on the use of the finite element analysis (FEA) method to obtain biomechanical parameters of models of spine segments that include BKP augmentation at a given level. In each of these studies, the applied loading used was quasi-static. During normal activities of daily living, the patient's spine would be subject to dynamically-applied loading. Thus, the question of the influence of the characteristics of a dynamically-applied loading cycle on biomechanical parameters of a spine that includes BKP-augmented segment(s) is germane; however, a study of this issue is lacking. We investigated this issue in the present FEA work, with the spine segment model being the L1-L3 motion segment units (MSUs) (a segment that is commonly augmented using BKP) and prophylactic BKP simulated at L2. The dynamic load was the compressive load-versus-time cycle to which the L3-L4 MSU is subjected during gait. Four cases of the cycle were considered, corresponding to slow-, normal-, fast- and very fast-paced gait. The loading cycle was applied to the superior surface of L1 while the inferior surface of L3 was fully constrained. It was found that (1) the global mean von Mises stress during the loading cycle (σVMG), in each tissue in the model increased in going from a slow-paced gait cycle to a very fast-paced gait cycle; and (2) for the slow-paced gait cycle, with increase in frequency of the cycle, f (1 ≤ f ≤ 3 Hz), σVMG in each of these tissues increased. Potential uses of the present findings are identified.

Association between extent of simulated degeneration of C5-C6 disc and biomechanical parameters of a model of the full cervical spine: a finite element analysis study.

PURPOSE: To delineate the association between extent of degeneration of the disc at C5-C6, simulated in a model of the full cervical spine, and biomechanical responses of the model. METHODS: A validated three-dimensional model of an intact, healthy, adult full cervical spine (C1-C7) was constructed. This model was then modified to create three models simulating mild, moderate, and severe grades of degeneration of the disc at C5-C6. For each of these four models, we used the finite element analysis method to obtain three biomechanical parameters at various tissues in the model, under seven different physiologically relevant loadings. RESULTS: For each of the biomechanical parameters, the results were expressed as relative change in its value when a specified combination of simulated degeneration model and applied loading was used, with respect to the corresponding value in the intact model. We introduced a new measure, which we designate the composite biomechanical performance index (CBPI), whose value incorporates all of the aforementioned relative changes. CONCLUSIONS: We found that CBPI increased with increase in the severity of disc degeneration simulated. The clinical relevance of CBPI should be addressed in future work.

Random damage and characteristics of debris particles are two important and yet ignored factors in the mechanical integrity of the stem-cement interface of a total hip replacement: influence of the surface finish of the metal stem.

The importance of the conditions at the stem-cement interface in cemented total joint replacements (THRs) with regard to the in vivo longevity of the implant is well recognized. In the present study, we used a simplified model of one part of a cemented THR (alloy rectangular beam bonded to rectangular cement plate) to study the influence of surface finish of the alloy beam (stem) on two measures of the evolution of random damage at the alloy beam-cement plate interface (stem-cement interface), under quasi-static direct shear load. Three surface finishes of the beams were used: satin-finish, grit-blasted, and plasma-sprayed. The random damage events were monitored from the emitted acoustic signals, with the two measures computed from these signals being the intensity of random damage events (IRDE) and the mean damage event energy (MDEE). Large number of random damage events (higher values of IRDE and low value of MDEE) occurred with grit blasted specimens, suggesting a high probability for the generation of debris particles at the interface. These findings, in conjunction with details on the size and shape of the debris particles, obtained using scanning electron microscopy, lead to the suggestion that satin-finish stems are desirable for use in cemented THRs.

Evaluation of two novel aluminum-free, zinc-based glass polyalkenoate cements as alternatives to PMMA bone cement for use in vertebroplasty and balloon kyphoplasty.

Vertebroplasty (VP) and balloon kyphoplasty (BKP) are now widely used for treating patients in whom the pain due to vertebral compression fractures is severe and has proved to be refractory to conservative treatment. These procedures involve percutaneous delivery of a bolus of an injectable bone cement either directly to the fractured vertebral body, VB (VP) or to a void created in it by an inflatable bone tamp (BKP). Thus, the cement is a vital component of both procedures. In the vast majority of VPs and BKPs, a poly(methyl methacrylate) (PMMA) bone cement is used. This material has many shortcomings, notably lack of bioactivity and very limited resorbability. Thus, there is room for alternative cements. We report here on two variants of a novel, bioactive, Al-free, Zn-based glass polyalkenoate cement (Zn-GPC), and how their properties compare to those of an injectable PMMA bone cement (SIMPL) that is widely used in VP and BKP. The properties determined were injectability, radiopacity, uniaxial compressive strength, and biaxial flexural modulus. In addition, we compared the compression fatigue lives of a validated synthetic osteoporotic VB model (a polyurethane foam cube with an 8 mm-diameter through-thickness cylindrical hole), at 0-2300 N and 3 Hz, when the hole was filled with each of the three cements. A critical review of the results suggests that the performance of each of the Zn-GPCs is comparable to that of SIMPL; thus, the former cements merit further study with a view to being alternatives to an injectable PMMA cement for use in VP and BKP.

Dependence of in vitro fatigue properties of PMMA bone cement on the polydispersity index of its powder.

Four variants of poly (methyl methacrylate) (PMMA) bone cement were used, the difference being in the method used to sterilize the powder (three different dosages of gamma irradiation and ethylene oxide gas) and, hence, in the molecular weight of the powder. For each cement powder, the number-average molecular weight and weight-average molecular weight (and, hence, the polydispersity index, PDI) were determined using gel permeation chromatography. For each of the cured cements, the fatigue lives (N(f)) of specimens, at loads corresponding to stresses (S) of +/-10.0 MPa, +/-12.5 MPa, +/-15.0 MPa, and +/-20.0 MPa, were determined using the protocol detailed in ASTM F2118-03. Hence, the values of the three Weibull parameters were determined for each cement set-S combination. From these results, one index of the fatigue life of the cement, namely, the Weibull mean fatigue life (N(WM)), was computed for each combination. For each cement, the Olgive equation was fitted to the S-N(f) results, yielding an estimate of another fatigue property, the cement's fatigue limit. Best-fit empirical relationships (1) betweenlnN(WM), S, and PDI, and (2) between the estimated fatigue limit and PDI were obtained. These relationships may be used in the development of new cement powder sterilization methods.