Time dependent mechanical properties of bone cement. Anin vitro study over one year

Influence of Tranexamic Acid on Elution Characteristics and Compressive Strength of Antibiotic-Loaded PMMA-Bone Cement with Gentamicin

PURPOSE

The topical application of tranexamic acid (TXA) into the joint space during total joint arthroplasty (TJA) with no increase of complications, has been widely reported. We investigated the influence of TXA on antibiotic release, activity of the released antibiotic against a clinical isolate of S. aureus, and compressive strength of a widely used commercially prepared gentamicin-loaded cement brand (PALACOS R + G).

METHOD

12 bone cement cylinders (diameter and height = 6 and 12 mm, respectively) were molded. After curing in air for at least 1 h, six of the cylinders were completely immersed in 5 mL of fetal calf serum (FCS) and the other six were completely immersed in a solution consisting of 4.9 mL of FCS and 0.1 mL (10 mg) of TXA. Gentamicin elution tests were performed over 7 d. Four hundred µL of the gentamicin eluate were taken every 24 h for the first 7 d without renewing the immersion fluid. The gentamicin concentration was determined in a clinical analyzer using a homogeny enzyme immuno-assay. The antimicrobial activity of the eluate, obtained after day 7, was tested. An agar diffusion test regime was used with Staphylococcus aureus. Bacteria were grown in a LB medium and plated on LB agar plates to get a bacterial lawn. Fifty µL of each eluate were pipetted on 12-mm diameter filter discs, which were placed in the middle of the agar gel. After 24 h of cultivation at 37 °C, the zone of inhibition (ZOI) for each specimen was measured. The compressive strength of the cements was determined per ISO 5833.

RESULTS

At each time point in the gentamicin release test, the difference in gentamicin concentration, obtained from specimens immersed in the FCS solution only and those immersed in the FCS + TXA solution was not significant (p = 0.055-0.522). The same trend was seen in each of the following parameters, after 7 d of immersion: (1) Cumulative gentamicin concentration (p < 0.297); (2) gentamicin activity against S. aureus (strongly visible); (3) ZOI size (mostly > 20 mm) (p = 0.631); and (4) compressive strength (p = 0.262).

CONCLUSIONS

For the PALACOS R + G specimens, the addition of TXA to FCS does not produce significant decreases in gentamicin concentration, in the activity of the gentamicin eluate against a clinical isolate of S. aureus, the zone of inhibition of S. aureus, and in the compressive strength of the cement, after 7 d of immersion in the test solution.

Long-term mechanical properties of a novel low-modulus bone cement for the treatment of osteoporotic vertebral compression fractures

In spite of the success of vertebroplasty (VP) and balloon kyphoplasty (BKP), which are widely used for stabilizing painful vertebral compression fractures, concerns have been raised about use of poly(methyl methacrylate) (PMMA) bone cements for these procedures since the high compressive modulus of elasticity (E) of the cement is thought to be one of the causes of the higher number of adjacent-level vertebral fractures. Therefore, bone cements with E comparable to that of cancellous bone have been proposed. While the quasi-static compressive properties of these so-called "low-modulus" cements have been widely studied, their fatigue performance remains underassessed. The purpose of the present study was to critically compare a commercial bone cement (control cement) and its low-modulus counterpart on the basis of quasi-static compressive strength (CS), E, fatigue limit under compression-compression loading, and release of methyl methacrylate (MMA). At 24 h, mean CS and E of the low-modulus material were 72% and 77% lower than those of the control cement, whereas, at 4 weeks, mean CS and E were 60% and 54% lower, respectively. The fatigue limit of the control cement was estimated to be 43-45 MPa compared to 3-5 MPa for the low-modulus cement. The low-modulus cement showed an initial burst release of MMA after 24 h followed by a plateau, similar to many other commercially available cements, whereas the control cement showed a much lower, stable release from day 1 and up to 1 week. The low-modulus cement may be a promising alternative to currently available PMMA bone cements, with the potential for reducing the incidence of adjacent fractures following VP/BKP.

Effect of Physiological Saline Solution Contamination on Selected Mechanical Properties of Seasoned Acrylic Bone Cements of Medium and High Viscosity

Bone cements play a key role in present-day surgery, including the implantation of hip and knee joint endoprostheses. The correct and durable bonding of the prosthesis to the bone is affected by both the static strength characteristics determined in accordance with ISO 5833:2002 and the resistance to long-term exposure to an aggressive environment of the human body and the impurities that may be introduced into the cement during implementation. The study attempts to demonstrate statistically significant degradation of cement as a result of the seasoning of cement samples in Ringer’s solution with simultaneous contamination of the material with saline solution, which is usually present in the surgical field (e.g., during the fixing of endoprostheses). The results of statistical analysis showed the nature of changes in compressive strength and microhardness due to seasoning time and degree of contamination.

Effect of Physiological Fluids Contamination on Selected Mechanical Properties of Acrylate Bone Cement

This study analyses the degradation rate of selected mechanical properties of bone cement contaminated with human blood and saline solution. During the polymerisation stage, the PMMA cement specimens were supplemented with the selected physiological fluids in a range of concentrations from 0% to 10%. The samples were then subjected to the standardised compression tests, as per ISO 5833: 2002, and hardness tests. The obtained results were analysed statistically to display the difference in the degradation of the material relative to the degree of contamination. Subsequently, numerical modelling was employed to determine the mathematical relationship between the degree of contamination and the material strength degradation rate. The introduction of various concentrations of contaminants into the cement mass resulted in a statistically significant change in their compressive strength. It was shown that the addition of more than 4% of saline and more than 6% of blood (by weight) causes that the specimens exhibit lower strength than the minimum critical value of 70 MPa, specified in the abovementioned International Standard. It was further revealed that the cement hardness characteristics degraded accordingly. The mathematical models showed a very good fit with the results from the experiments: The coefficient of determination R2 was 0.987 in the case of the linear hardness model for blood and 0.983 for salt solution; secondly, the values of R2 for the third-degree polynomial model of compressive strength were 0.88 for blood and 0.92 for salt. From the results, it can be seen that there is a quantitative/qualitative relationship between the contamination rate and the drop in the tested mechanical characteristics. Therefore, great effort must be taken to minimise the contact of the bone cement with physiological fluids, which naturally occur in the operative field, particularly when the material cures, in order to prevent the cement material strength declining below the minimum threshold specified in the ISO standard.

Seasoning Polymethyl Methacrylate (PMMA) Bone Cements with Incorrect Mix Ratio

Cemented joint prostheses are widely used in orthopaedic surgery; however, implants/bone bonds are known to be susceptible to aseptic loosening, particularly in the case of long-term performance. The exact mechanism of this failure is under constant examination. One of the critical factors to the final mechanical functionality of bone cement can be an incorrect mix ratio of a two-component material (powdered polymer and liquid monomer). It can result in the deterioration of the final mechanical strength properties. The paper presents the results from an experimental study on the effects of the deviation from the correct mix ratio on the moisture uptake and the compression strength of cement depending on the seasoning time in Ringer's solution. The results were subjected to statistical analysis and a mathematical model was developed.

METHODS FOR THE CHARACTERIZATION OF THE LONG-TERM MECHANICAL PERFORMANCE OF CEMENTS FOR VERTEBROPLASTY AND KYPHOPLASTY: CRITICAL REVIEW AND SUGGESTIONS FOR TEST METHODS

There is a growing interest towards bone cements for use in vertebroplasty and kyphoplasty, as such spine procedures are becoming more and more common. Such cements feature different compositions, including both traditional acrylic cements and resorbable and bioactive materials. Due to the different compositions and intended use, the mechanical requirements of cements for spinal applications differ from those of traditional cements used in joint replacement. Because of the great clinical implications, it is very important to assess their long-term mechanical competence in terms of fatigue strength and creep. This paper aims at offering a critical overview of the methods currently adopted for such mechanical tests. The existing international standards and guidelines and the literature were searched for publications relevant to fatigue and creep of cements for vertebroplasty and kyphoplasty. While standard methods are available for traditional bone cements in general, no standard indicates specific methods or acceptance criteria for fatigue and creep of cements for vertebroplasty and kyphoplasty. Similarly, a large number of papers were published on cements for joint replacements, but only few cover fatigue and creep of cements for vertebroplasty and kyphoplasty. Furthermore, the literature was analyzed to provide some indications of tests parameters and acceptance criteria (number of cycles, duration in time, stress levels, acceptable amount of creep) for possible tests specifically relevant to cements for spinal applications.

Multiscale characterization of acrylic bone cement modified with functionalized mesoporous silica nanoparticles

Acrylic bone cement is widely used to anchor orthopedic implants to bone and mechanical failure of the cement mantle surrounding an implant can contribute to aseptic loosening. In an effort to enhance the mechanical properties of bone cement, a variety of nanoparticles and fibers can be incorporated into the cement matrix. Mesoporous silica nanoparticles (MSNs) are a class of particles that display high potential for use as reinforcement within bone cement. Therefore, the purpose of this study was to quantify the impact of modifying an acrylic cement with various low-loadings of mesoporous silica. Three types of MSNs (one plain variety and two modified with functional groups) at two loading ratios (0.1 and 0.2wt/wt) were incorporated into a commercially available bone cement. The mechanical properties were characterized using four-point bending, microindentation and nanoindentation (static, stress relaxation, and creep) while material properties were assessed through dynamic mechanical analysis, differential scanning calorimetry, thermogravimetric analysis, FTIR spectroscopy, and scanning electron microscopy. Four-point flexural testing and nanoindentation revealed minimal impact on the properties of the cements, except for several changes in the nano-level static mechanical properties. Conversely, microindentation testing demonstrated that the addition of MSNs significantly increased the microhardness. The stress relaxation and creep properties of the cements measured with nanoindentation displayed no effect resulting from the addition of MSNs. The measured material properties were consistent among all cements. Analysis of scanning electron micrographs images revealed that surface functionalization enhanced particle dispersion within the cement matrix and resulted in fewer particle agglomerates. These results suggest that the loading ratios of mesoporous silica used in this study were not an effective reinforcement material. Future work should be conducted to determine the impact of higher MSN loading ratios and alternative functional groups.

Ageing and moisture uptake in polymethyl methacrylate (PMMA) bone cements

Bone cements are extensively employed in orthopaedics for joint arthroplasty, however implant failure in the form of aseptic loosening is known to occur after long-term use. The exact mechanism causing this is not well understood, however it is thought to arise from a combination of fatigue and chemical degradation resulting from the hostile in vivo environment. In this study, two commercial bone cements were aged in an isotonic fluid at physiological temperatures and changes in moisture uptake, microstructure and mechanical and fatigue properties were studied. Initial penetration of water into the cement followed Fickian diffusion and was thought to be caused by vacancies created by leaching monomer. An increase in weight of approximately 2% was experienced after 30 days ageing and was accompanied by hydrolysis of poly(methyl methacrylate) (PMMA) in the outermost layers of the cement. This molecular change and the plasticising effect of water resulted in reduced mechanical and fatigue properties over time. Cement ageing is therefore thought to be a key contributor in the long-term failure of cemented joint replacements. The results from this study have highlighted the need to develop cements capable of withstanding long-term degradation and for more accurate test methods, which fully account for physiological ageing.

•

Two commercial bone cements were aged in Ringer's solution at 37 °C for 60 days.

•

Moisture uptake, mechanical, fatigue and microstructural properties were studied.

•

A maximum of 2% change in weight occurred due to Fickian diffusion after 30 days.

•

Hydrolysis of PMMA and reduced mechanical and fatigue properties were observed.

•

Cement degradation is thought to contribute to the failure of cemented implants.

Experimental investigation of the effect of surface roughness on bone-cement-implant shear bond strength

Debonding of cemented bone implants is regarded as a major contributor to complications. The relationship between shear bond strength and surface roughness has been investigated, however there are inconsistencies in the trends reported in different studies. The shear strength between poly(methyl methacrylate) bone-cement and sand blasted cobalt-chromium and titanium alloy surfaces was measured to investigate the relationship between interfacial shear strength and surface topology. Surface roughness was quantified by a power law relationship fitted to Fourier spectra as well as three traditional parameters (arithmetical average roughness (Ra), volume of interdigitation (Rr), and RMS slope (Rdq)). We found that the interfacial shear strength is directly proportional to the exponent of the surfaces power spectra (P2) and Rdq, but not to Ra and Rr. However, Rdq is shown to be critically dependent on sampling frequency, making it sensitive to measurement settings. P2 was found to be a robust measure of the surface roughness being independent of sampling frequency.

Early failure of a polyethylene acetabular liner cemented into a metal cup

In 2002, a patient underwent revision total hip arthroplasty for polyethylene wear. The acetabular cup was well fixed, and it was decided to cement a new polyethylene liner into the existing cup. In 2006, the patient presented with inability to weight bear and easy subluxation of the hip. Revision surgery was performed, and all components were examined postoperatively. Investigation with microcomputed tomography revealed that the liner had plastically deformed at the superior pole resulting in the hip instability. The reasons for this are suspected to be related to the abducted nature of the original cup and an uneven cement mantle. This report suggests that cementation of polyethylene liners into metal cups has limitations and is not appropriate in all circumstances.

Effect of vacuum-treatment on deformation properties of PMMA bone cement

Deformation behavior of polymethyl methacrylate (PMMA) bone cement is explored using microindentation. Two types of PMMA bone cement were prepared. Vacuum treated samples were subjected to the degassing of the material under vacuum of 270 mbar for 35 s, followed by the second degassing under vacuum of 255 mbar for 35 s. Air-cured samples were left in ambient air to cool down and harden. All samples were left to age for 6 months before the test. The samples were then subjected to the indentation fatigue test mode, using sharp Vickers indenter. First, loading segment rise time was varied in order to establish time-dependent behavior of the samples. Experimental data showed that viscous part of the deformation can be neglected under the observed test conditions. The second series of microindentation tests were realized with variation of number of cycles and indentation hardness and modulus were obtained. Approximate hardness was also calculated using analysis of residual impression area. Porosity characteristics were analyzed using CellC software. Scanning electron microscopy (SEM) analysis showed that air-cured bone cement exhibited significant number of large voids made of aggregated PMMA beads accompanied by particles of the radiopaque agent, while vacuum treated samples had homogeneous structure. Air-cured samples exhibited variable hardness and elasticity modulus throughout the material. They also had lower hardness values (approximately 65-100 MPa) than the vacuum treated cement (approximately 170 MPa). Porosity of 5.1% was obtained for vacuum treated cement and 16.8% for air-cured cement. Extensive plastic deformation, microcracks and craze whitening were produced during indentation of air-cured bone cement, whereas vacuum treated cement exhibited no cracks and no plastic deformation.

Polymethylmethacrylate: properties and contemporary uses in orthopaedics

Polymethylmethacrylate (PMMA) has been used in orthopaedics since the 1940s. Despite the development and popularity of new biomaterials, PMMA remains popular. Although its basic components remain the same, small proprietary and environmental changes create variations in its properties. PMMA can serve as a spacer and as a delivery vehicle for antibiotics, and it can be placed to eliminate dead space. Endogenous and exogenous variables that affect its performance include component variables, air, temperature, and handling and mixing. PMMA is used in hip arthroplasty and vertebral augmentation, notably, vertebroplasty and kyphoplasty. Cardiopulmonary complications have been reported.

Performance of bone cements: are current preclinical specifications adequate?

BACKGROUND

Current specifications (standards) for preclinical testing of bone cements (ISO 5833: 2002, ASTM F451-99a) require simple mechanical testing after ageing for 24 h under dry conditions at 23 degrees C. Some bone cements have fulfilled the requirements in the specifications, and yet had inferior clinical results. Clinically, bone cements are subjected to complex loading patterns in a moist or wet environment at 37 degrees C. Thus, both the validity and the robustness of current standard testing protocols can be questioned.

METHODS

We examined the influence of temperature and storage medium on the properties of bone cement. We also compared the results of storage and testing under standard conditions of 23 degrees C in dry air, with the results obtained at 37 degrees C in water or plasma.

RESULTS

The dry specimens showed an increase in strength and elastic modulus with time, while the values of the wet ones decreased. There was no difference between specimens stored in water or in plasma. Ultimate compressive strength of dry specimens after 24 h was 1.16 times higher than that of the ones stored wet, increasing to 1.34 times after 1 month, and 1.46 times after 6 months (p<0.001 for all comparisons).

INTERPRETATION

Testing under dry conditions-as required in current standards-always gave higher values for mechanical properties than did storage and testing under more physiological conditions. The sensitivity of test values to different environments implies that testing conditions for bone cements should be scrutinized in order to develop more relevant testing protocols that reflect the in vivo environment more closely.