Propagation of fatigue cracks in acrylic bone cements containing different radiopaque agents

In this work three iodine-containing monomers were proposed as new radiopaque agents for acrylic bone cements. In previous studies the addition of iodine-containing methacrylate monomers provided a statistically significant increase in tensile stress, fracture toughness and ductility, with respect to the barium sulphate (BaSO4)-containing cement. However, since fatigue resistance is one of the main properties required to ensure a good long-term performance of permanent prostheses, it is important to compare the fatigue properties of these new bone cement formulations with the radiolucent and BaSO4-containing bone cements. Because the acrylic cements have initial cracks, fatigue crack propagation studies were performed. It can be observed that these acrylic cements followed the Paris-Erdogan model. The results showed that the addition of some organic radiopacifiers (DISMA, TIBMA) increased the fatigue crack propagation resistance as compared to the radiolucent cement, being similar to the BaSO4-containing cement. The radiolucent cement showed a low crack propagation resistance.     

The effect on the fatigue strength of bone cement of adding sodium fluoride

New bone cements that include several additives are currently being investigated and tested. One such additive is sodium fluoride (NaF), which promotes bone formation, facilitating implant integration and success. The influence of NaF on the fatigue performance of the cement as used in biomedical applications was tested in this paper. In fact fatigue failure of the cement mantle is a major factor limiting the longevity of a cemented implant. An experimental bone cement with added NaF (12 wt%) was investigated. The fatigue strength of the novel bone cement was evaluated in comparison with the cement without additives; fatigue tests were conducted according to current standards. The load levels were arranged based on a validated, statistically based optimization algorithm. The curve of stress against number of load cycles and the endurance limit were obtained and compared for both formulations. The results showed that the addition of NaF (12 wt%) to polymethylmethacrylate (PMMA) bone cement does not affect the fatigue resistance of the material. Sodium fluoride can safely be added to the bone cement without altering the fatigue performance of the PMMA bone cement.     

Incorporation of large amounts of gentamicin sulphate into acrylic bone cement: effect on handling and mechanical properties, antibiotic release, and biofilm formation

Bacterial infection remains a significant complication following total joint replacement. If infection is suspected when revision surgery is being performed, a large dose of antibiotic, usually gentamicin sulphate, is often blended with the acrylic bone cement powder in an attempt to reduce the risk of recurrent infection. In this in-vitro study the effect of small and large doses of gentamicin sulphate on the handling and mechanical properties of the cement, gentamicin release from the cement, and in-vitro biofilm formation by clinical Staphylococcus spp. isolates on the cement was determined. An increase in gentamicin loading of 1, 2, 3, or 4 g, in a cement powder mass of 40 g, resulted in a significant decrease in the compressive and four-point bending strength, but a significant increase in the amount of gentamicin released over a 72h period. When overt infection was modelled, using Staphylococcus spp. clinical isolates at an inoculum of 1 x 10(7) colony-forming units/ml, an increase in the amount of gentamicin (1, 2, 3, or 4 g) added to 40 g of poly(methyl methacrylate) cement resulted in an initial decrease in bacterial colonization but this beneficial effect was no longer apparent by 72 h, with the bacterial strains forming biofilms on the cements despite the release of high levels of gentamicin. The findings suggest that orthopaedic surgeons should carefully consider the clinical consequences of blending large doses (1 g or more per 40 g of poly(methyl methacrylate)) of gentamicin into Palacos R bone cement for use in revision surgery as the increased gentamicin loading does not prevent bacterial biofilm formation and the effect on the mechanical properties could be important to the longevity of the prosthetic joint.     

Radiopacity of tantalum-loaded acrylic bone cement

Radiopacifying agents are commonly added to bone cements to enhance the visibility of the cement in radiography. The radiopacifiers usually employed may, however, have undesired effects on the mechanical properties of the cement. A potentially new radiopacifier is tantalum, which in the present work was evaluated in terms of radiopacity. Bone cements containing different percentages of tantalum were compared with plain bone cement as well as with formulations containing different percentages of the commonly used radiopacifier barium sulphate. The radiopacity was assessed quantitatively and qualitatively, by measuring with a digital densitometer the optical density of the cement on X-ray films, and consulting the expertise of ten orthopaedic surgeons. It was found that tantalum does present radiopacity, but not as high as barium sulphate under the specific conditions applied to this study.     

Some failure modes of four clinical bone cements

The fracture or failure behaviours of four commercial acrylic-based bone cements have been examined in tensile, bending and compression modes, and their mechanical properties are reviewed. It was found that Palacos R-40 bone cement had high radiopaque agent concentration, with high surface hardness. It exhibited a much lower bending strength and bending modulus compared with the other three bone cements (CMW1, CMW2000 and Simplex P). The textures of tensile fracture surfaces produced were similar for the four bone cements studied. The fracture surface was fragmented by crevices, which developed through the matrix and around large undissolved polymethylmethacrylate (PMMA) beads. Three bands with different features existed on the bending fracture surfaces, with an abrupt transition between them. It appears that the agglomerates of zirconium dioxide particles are implicated in Palacos R-40 bone cement fracture surface. The examination of compressive failed specimens revealed that a 'yielded crack band' existed across the transverse section. Plastic deformation resulted in the PMMA beads being squashed in the longitudinal direction and dilated in the transverse direction.     

Revision of cemented fixation and cement-bone interface strength.

Interfacial shear strength between poly(methyl methacrylate) (PMMA) bone cement and cancellous bone was measured in bone samples from human proximal femora. Samples were prepared with fresh cement-bone, fresh cement inside a mantle of existing cement and with fresh cement-revised bone surfaces. Push-out tests to measure shear strength caused failure only at bone-cement interfaces; revised bone interfaces were 30 per cent weaker (P < 0.02) than primary interfaces. The clinical relevance is that revision of cemented joint arthroplasties may necessitate removal of components with sound cement-bone fixation. The practice of removing all traces of PMMA cement may not yield the optimal fixation; adhesion of fresh cement to freshly prepared surfaces of the existing cement might also be considered where circumstances are favourable.     

Three-body wear of UHMWPE acetabular cups by PMMA particles against CoCr, alumina and zirconia heads in a hip joint simulator

Three-body abrasive wear of ultra-high molecular weight polyethylene (UHMWPE) acetabular cups by loose polymethyl methacrylate (PMMA) bone cement particles is an important mechanism responsible for elevated wear debris generation in total hip arthroplasty. The resistance of the femoral head material to third-body damage has been considered critical for the wear performance of the polyethylene component. This study examines the effect of loose bone cement particles on the wear rate of UHMWPE acetabular cups against both metal and ceramic counterfaces in a hip joint simulator. Against the CoCr head, the UHMWPE cup showed a strong dependence of wear rate on the concentration of the PMMA particles in the lubricant. At a concentration less than 5 g/l, the presence of the PMMA particles had no detrimental effect on the wear rate; higher concentrations of the PMMA particles greater than 5 g/l led to an accelerated wear of the acetabular cups. Mild scratching damage was observed on the CoCr heads after testing with all PMMA-containing lubricants. However, no increased UHMWPE wear rate was found against these damaged femoral heads in a fresh lubricant without PMMA particles, indicating that femoral head scratching was not a major cause for the elevated wear observed under the three-body abrasive conditions. Against both alumina and zirconia ceramic heads, the wear rate of the UHMWPE was independent of the concentration of the PMMA particles. It was observed that a significant portion of the CoCr heads was covered with loose patches of PMMA particles. The higher the concentration of the PMMA particles, the greater the area of the head covered with PMMA particles. The attachment of PMMA particles to the ceramic heads was much reduced compared to the CoCr heads. It is therefore concluded that ceramic femoral heads are effective against potential run-away wear of the UHMWPE acetabular cups when an excessive amount of loose PMMA particles are present in the lubricant.     

An investigation into the effect of cyclic loading and frequency on the temperature of PMMA bone cement in hip prostheses

A titanium alloy hip prosthesis was inserted in a Tufnol tube representing the upper part of the femur. The prosthesis was cemented in the model femur using PMMA bone cement. Five thermocouples were embedded in the bone cement and the assembly was subjected to cyclic loading with a range of 0.3-4.5 kN at a frequency of 6 Hz. Temperature measurements over a 48 hour period indicated that the temperature rise in the bone cement was less than 4 degrees C. It is concluded that such tests can be carried out at 6 Hz without significantly affecting the mechanical properties of PMMA bone cement.     

The influence of bone and bone cement debris on counterface roughness in sliding wear tests of ultra-high molecular weight polyethylene on stainless steel

Studies of explanted hip prostheses have shown high wear rates of ultra-high molecular weight polyethylene (UHMWPE) acetabular cups and roughening of the surface of the metallic femoral head. Bone and bone cement particles have also been found in the articulating surfaces of some joints. It has been proposed that bone or bone cement particles may cause scratching and deterioration in the surface finish of metallic femoral heads, thus producing increased wear rates and excessive amounts of wear debris. Sliding wear tests of UHMWPE pins on stainless steel have been performed with particles of different types of bone and bone cement added. Damage to the stainless steel counterface and the motion of particles through the interface have been studied. Particles of bone cement with zirconium and barium sulphate additives and particles of cortical bone scratched the stainless steel counterface. The cement particles with zirconium additive produced significantly greater surface damage. The number of particles entering the contact and embedding in the UHMWPE pin was dependent on particle size and geometry, surface roughness and contact stress. Particles are likely to cause surface roughening and increased wear rates in artificial joints.     

Effect of interface conditions between ultrahigh molecular weight polyethylene and polymethyl methacrylate bone cement on the mechanical behaviour of total shoulder arthroplasty

The aim of this study was to investigate the effect of the interface condition between polymethyl methacrylate (PMMA) bone cement and the ultrahigh molecular weight (UHMWPE) glenoid component on cement stresses and glenoid component tilting in a finite element (FE) model. The background of this research is that most FE models assume bonding between the PMMA bone cement and the UHMWPE component, although it is very doubtful that this bonding is present. An FE model of a cemented glenoid component was developed and a joint compression force and subluxation force of 725 and 350 N respectively were applied. The maximal principal stresses in the cement layer ranged between 21.30 and 32.18 MPa. Glenoid component tilting ranged between 0.943 degrees and 0.513 degrees. It was found that the interface condition has a large effect on the maximal principal stresses and glenoid component tilting. Whether adhesion between the UHMWPE component and PMMA bone cement occurs is unknown beforehand and, as a result, design validation using the FE technique should be carried out both by using contact elements in combination with a coefficient of friction as well as by a full bonding at this interface.     

嵌段共聚物F68增韧改性PMMA的性能

采用嵌段共聚物F68对聚甲基丙烯酸甲酯(PMMA)进行原位增韧改性,研究了改性PMMA的力学性能、光学透明性和断面形貌,并探讨了其改性机理。结果表明:F68对PMMA有较好的增韧作用,随着F68含量的增加,改性PMMA的冲击强度不断增大,当F68的质量分数为20%时,其冲击强度提高了117.61%;改性PMMA的储能模量在玻璃化转度温度以上时随着F68含量的增加不断增大,而损耗因子的峰值温度逐渐降低;改性PMMA具有较好的光学透明性能,当F68的质量分数在20%以内时,改性PMMA透光率的降低小于2%。     

Prediction of cement volume for vertebroplasty based on imaging and biomechanical results

Control of bone cement volume (PMMA) may be critical for preventing complications in vertebroplasty, the percutaneous injection of PMMA into vertebra. The purpose of this study was to predict the optimal volume of PMMA injection based on CT images. For this, correlation between PMMA volume and textural features of CT images was examined before and after surgery to evaluate the appropriate PMMA amount. The gray level run length analysis was used to determine the textural features of the trabecular bone. Estimation of PMMA volume was done using 3D visualization with semi-automatic segmentation on postoperative CT images. Then, finite element (FE) models were constructed based on the CT image data of patients and PMMA volume. Appropriate material properties for the trabecular bone were assigned by converting BMD to elastic modulus. Structural reinforcement due to the changes in PMMA volume and BMD was assessed in terms of axial displacement of the superior endplate. A strong correlation was found between the injected PMMA volume and the area of the intertrabecular space and that of trabecular bone calculated from the CT images (r-0.90 and −0.90, respectively). FE results suggested that vertebroplasty could effectively reinforce the osteoporotic vertebra regardless of BMD or PMMA volume. Effectiveness of additional PMMA injection tended to decrease. For patients with BMD well lower than 50mg/ml, injection of up to 30% volume of the vertebral body is recommended. However, less than 30% is recommended otherwise to avoid any complications from excessive PMMA because the strength has already reached the normal level.     

Microtomographic evaluation of the bone-cell interactions with a silorane-based composite

The low‐shrink Silorane‐based composite could bond effectively to bone and showed the potential be used as a bone cement. Bone organ culture maintains the anatomical order, natural cell‐to‐cell and cell‐to‐matrix relationship. The purpose of this study was to evaluate the responses of bone cells to a Silorane‐based composite which was compared with a representative polymethyl methacrylate (PMMA) bone cement. The critical size defects were created through the parietal bones from one litter of mice. The paired bones were divided into two groups: Silorane‐based composite group and PMMA group. The prepared two groups of disks were put into the defects. The cultures were grown in vitro for 38 days and analyzed with microcomputed‐tomography, dissecting‐microscope, phase‐ contrast‐microscope, scanning‐electron‐microscopy, and energy‐ dispersive‐X‐ray. At the 10th day, the Silorane disk was almost fully covered by a sheet of cells but the cells hardly attached to the disk surface. The edge of the PMMA disk was covered by a sheet of cells and the migrated individual cells attached to the whole surface of the disk. At the 38th day, some cells attached to the exposed disk area of the Silorane disk while the formed tissues covered the whole surface of the PMMA disk. The collagen fibers, globular deposits and bone formation were visible in both groups. The Silorane‐based composite showed promise as a potential bone cement when compared with PMMA which is used in clinical orthopedics. However, the cell attachment to PMMA was evidently better than to Silorane‐based composite. Microsc. Res. Tech. 75:1176–1184, 2012. © 2012 Wiley Periodicals, Inc.     

Finite element analysis of the long-term fixation strength of cemented ceramic cups

Clinical studies have shown that adequate fixation of ceramic cups using bone cement is difficult to achieve. As the cement-ceramic bond strength is low, a satisfactory fixation strength requires a cup design that allows mechanical interlocking, although such a design will probably promote cement cracking and therefore cup loosening in the long term. An investigation has been carried out to establish whether a cemented ceramic cup can be designed in such a way that both a satisfactory initial fixation strength is obtained and cement cracking is reduced to levels found around PE cups functioning well in vivo. By means of finite element analysis, the fatigue loading of three geometrically different cemented acetabular cups, with ceramic and PE material properties, has been simulated, and the severity of the crack patterns produced in the cement has been analysed. Furthermore, the fixation strength has been analysed by simulating a pull-out test prior to and after fatigue testing. All ceramic cups produced much larger amounts of cement damage during fatigue testing than any PE cup, caused by stress concentrations in the cement that were attributable to the high stiffness of the ceramic. Even a completely smooth ceramic cup produced more damage than a sharp-grooved PE cup. Owing to the excessive cement cracking, the fixation strength of the ceramic cups dropped after fatigue loading. It is concluded that cemented ceramic cups have an increased risk of long-term mechanical failure by comparison with PE cups, and that a ceramic cup design that combines sufficient fixation strength with low cement failure may be difficult to achieve.     

A procedure and criterion for bone cement fracture toughness tests

Nowadays, two procedures, based on the recommendation of two American standards (ASTM E399 and ASTM D5045), are used to determine the fracture toughness, KIc, of bone cement. However, there is a lack of knowledge about the equivalence of the two testing methods applied to bone cement. Additionally, in spite of the recommendation of several authors to introduce a rejection criterion for specimens based on the size of defects found in the fracture surface, no data are available about the effect of porosity within the material on the KIc of bone cement. The aims of this study were to verify whether the KIc values calculated for bone cement using the two procedures are comparable and whether macroporosity within the tested samples affects the KIc value of bone cement, and, if so, to establish a rejection criterion for specimen selection. Samples of pure polymethyl methacrylate (PMMA) were tested by both procedures. Additionally, samples showing defects (macroporosity) of different sizes and located in different positions within the specimen were tested. The KIc value determined following the ASTM E399 procedure was 13 per cent lower than that calculated following the ASTM D5045 procedure. In the first series a lower data scatter was observed. Also, the presence of macroporosity on the fracture surface of the specimen affected the KIc value of bone cement. Therefore, the mechanical behaviour of samples was affected by defects within the material. Since it is possible to mould specimens without macroporosity, it seems recommendable to reject specimens with macroporosity on the fracture surface before calculating the KIc value of bone cement.     

Fretting corrosion of orthopaedic implant materials by bone cement

Debonding of bone cement from the stem of the femoral component of a hip prosthesis can result in local tangential oscillatory movement, i.e. fretting, between the two contacting materials as the limb is moved. Patches where such rubbing has occurred are frequently seen on removed implants. Fretting fatigue experiments have been carried out in Hanks solution on austenitic stainless steel and Ti-6Al-4V (IMI 318) with bridges of bone cement clamped to the specimens. Fretting appears to have little effect on the fatigue life of either material but the scanning electron microscope reveals the formation of thick oxide layers which subsequently give rise to loose debris particles by a process of delamination. Further experiments carried out in Hanks solution in an electrolytic cell have shown that there are potential changes when a bone cement rider is fretted against a stainless steel or titanium alloy plate although the change in potential is only one tenth that obtained with a metal-on-metal contact. Fretting by bone cement appears to be producing damage to the metal surfaces which manifests itself as mild wear rather than a diminution in fatigue strength.     

Relationship between liquidity index and stabilized strength of local subgrade materials in a tropical area

This paper presents the effect of soil liquidity index and cement stabilizer on strength properties for the usage of low traffic volume subgrade roads. Three types of soil were used to represent a different soil based on liquid limit value. Standard proctor tests have been conducted to determine the Optimum Moisture Content (OMC) and Maximum Dry Density (MDD) of stabilized soils with 0%, 7%, and 13% Ordinary Portland Cement (OPC). In order to study the effect of the soil Liquidity Index, various moisture contents are used based on the OMC value from the compaction test (0.9, 1.0, and 1.1 from OMC). The California Bearing Ratio (CBR) and Unconfined Compressive Strength (UCS) tests were conducted to determine the strength of all soil samples at optimum moisture content after 7 days curing period. It has been observed that the CBR and UCS values increased by increasing the percentage of cement content. This study found that 7% of cement content was the optimum percentage of cement content to be added to all tested soils to achieve the minimum required strength of 0.8 MPa and 80% CBR for low traffic volume roads. The presented results could provide a guideline for engineers as regards the property changes of the local subgrade materials in a tropical area due to the addition of cement content.     

The influence of stem insertion rate on the porosity of the cement mantle of hip joint replacements

This study investigates the effect of stem insertion rate on the porosity of the cement mantle. An experimental protocol was developed to simulate the surgical technique of cementing a prosthetic stem into the medullary canal of the femur. Cement mantle specimens were produced for three different stem insertion rates. The presence of porosity in the cement mantle was investigated. Additionally, the mechanical strength of the bone cement was assessed. Increasing the stem insertion rate did not have a significant effect on the porosity distribution within the bulk cement mantle. However, for all stem insertion rates investigated, the porosity concentration increased significantly moving from the cement/pseudofemur interface through to the stem/cement interface. In all cases, the presence of porosity significantly decreased the mechanical behaviour of the bone cement. High porosity concentration at the stem/cement interface seems to be attributed also to the rheology of the cement during implant insertion. Nevertheless, the surgeon cannot influence the formation of porosity by changing the stem insertion rate.     

Stress relaxation modelling of polymethylmethacrylate bone cement

This paper describes tests that were carried out to model the stress relaxation behaviour of polymethylmethacrylate (PMMA) bone cement. Stress relaxation of bone cement is believed to be a significant factor in the mechanism of load transfer in the femoral stem of a polished, collarless taper-fit replacement hip joints. It is therefore important that this condition and its implications are understood. Stress relaxation was carried out on PMMA samples of varying age in four-point bending configuration. It was shown that the samples stiffened with age and that the amount of stress relaxation reduced as the samples aged. The experimental results of the stress relaxation were accurately modelled on the double exponential of the Maxwell model so that long-term predictions of the stress condition could be made from short-term mechanical tests.     

Investigations on the wear behaviour of the temporary PMMA-based hip Spacer-G

Total hip replacement has become one of the most successful orthopaedic procedures. However, complications due to infections may give serious problems and have devastating consequences for the hip implant. The use of a temporary three-dimensional polymethylmethacrylate (PMMA) cement spacer may be an alternative to solve infections in hip implants, improving the lives of patients awaiting reimplantation. In order to evaluate their wear behaviour, five PMMA Spacer-G femoral heads were tested against five post-mortem pelves in a hip joint simulator with bovine calf serum as lubricant. The surface of the worn spacers was characterized by scanning electron microscopy (SEM) analysis; all the samples revealed a similar morphology, showing areas characterized by different degrees of wear. Particle debris was isolated from the lubricant and PMMA particles and bone fractions were quantified. The amount of debris was found to be higher than where no-temporary prostheses were used. However, this result is acceptable since wear debris is removed by lavage irrigation when the Spacer-G is explanted. On the basis of these data, it is considered that the use of the cement Spacer-G could be a promising approach to the treatment of complicated infections of the hip joint. Therefore, Spacer-G is worthy of further research.