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

New Research Progress of Modified Bone Cement Applied to Vertebroplasty

Percutaneous vertebroplasty and percutaneous kyphoplasty are effective methods to treat acute osteoporotic vertebral compression fractures that can quickly provide patients with pain relief, prevent further height loss of the vertebral body, and help correct kyphosis. Many clinical studies have investigated the characteristics of bone cement. Bone cement is a biomaterial injected into the vertebral body that must have good biocompatibility and biosafety. The optimization of the characteristics of bone cement has become of great interest. Bone cement can be mainly divided into 3 types: polymethyl methacrylate, calcium phosphate cement, and calcium sulfate cement. Each type of cement has its own advantages and disadvantages. In the past 10 years, the performance of bone cement has been greatly improved via different methods. The aim of our review is to provide an overview of the current progress in the types of modified bone cement and summarize the key clinical findings.

Modification and evaluation of diatrizoate sodium containing polymethyl methacrylate bone cement

Polymethyl methacrylate (PMMA) bone cement is now widely used in percutaneous vertebro plasty (PVP) and percutaneous kyphoplasty (PKP). However, studies showed that the radiopacifiers (zirconia, barium sulfate, etc.) added to PMMA will have a negative impact on its use, e.g. barium sulfate will weaken the mechanical properties of bone cement and lead to bone absorption and aseptic loosening. Iodine is an element existing in the human body and has good imaging performance. Iodine contrast agent has been used in clinic for many years and has abundant clinical data. Therefore, using iodine instead of barium sulfate may be a promising choice. In this paper, the effect of different content of diatrizoate sodium (DTA, C₁₁H₈I₃N₂NaO₄) on the properties of PMMA was studied and compared with the traditional PMMA bone cement containing 30 wt% barium sulfate. The mechanical properties, setting properties, radiopacity, and biocompatibility of bone cement were evaluated. The compressive strength of PMMA bone cement with 20 wt% DTA can reach 76.38 MPa. DTA released from bone cement up to 14 days accounted for only 2.3% of its dosage. The water contact angle was 62.3°. The contrast of bone cement on X-ray film was comparable to that of bone cement containing 30 wt% barium. The hemolysis rate was lower than 4%, and there was no obvious hemolysis. PMMA with 20 wt% DTA can maintain the relative growth rate of MC3T3-E1 and L929 cells above 80%. The results show that adding 20 wt% DTA into PMMA can obtain good radiopacity while maintaining its mechanical properties, setting properties, and biocompatibility. DTA can be used as a promising candidate material for PMMA bone cement radiopacifier.

Multifunctional epoxy resin-based composites with excellent flexural strength and X-ray imaging capacity using micro/nano structured QF-Bi2SiO5 fillers

Composites have been widely applied in various industries and are beneficial in attaining complicated functionalities. Particularly, for dental fiber posts or orthopedic implants, the composites should have excellent mechanical properties and good imaging effects for visualization in vivo. The traditional method to improve mechanical strength and visibility by adding reinforcing fillers and radiopacifiers is complicated and has poor distributions and long production times. Hence, fabricating an integrated reinforced filler with radiopacity is of considerable economic and social significance. After ball-milling and sintering quartz fiber (QF) and bismuth trioxide (Bi₂O₃), a multifunctional filler (QF-Bi₂SiO₅) is fabricated to impart excellent flexural strengths and high X-ray imaging qualities to the composites. A composite made of epoxy resin (EP) and QF-Bi₂SiO₅ has a high bending strength (126.87 ± 6.78 MPa) and bending modulus (3649.31 ± 343.87 MPa), which are attributed to the tight mechanical interlock between EP and micro/nano structures of QF-Bi₂SiO₅. The QF-Bi₂SiO₅/EP composite shows good X-ray imaging quality owing to the Bi₂SiO₅ crystal. Furthermore, the mechanical and imaging performances of various composites with commercial fillers were compared with that of the QF-Bi₂SiO₅/EP composite. No filler was found that can perform both functions as well as QF-Bi₂SiO₅. Hence, the fabricated composites containing micro/nano structured QF-Bi₂SiO₅ fillers have the potential to be used in a variety of fields requiring mechanical strength and X-ray imaging capability.

An ex-vivo model for the biomechanical assessment of cement discoplasty

Percutaneous Cement Discoplasty (PCD) is a surgical technique developed to relieve pain in patients with advanced degenerative disc disease characterized by a vacuum phenomenon. It has been hypothesized that injecting bone cement into the disc improves the overall stability of the spinal segment. However, there is limited knowledge on the biomechanics of the spine postoperatively and a lack of models to assess the effect of PCD ex-vivo. This study aimed to develop a biomechanical model to study PCD in a repeatable and clinically relevant manner. Eleven ovine functional spinal units were dissected and tested under compression in three conditions: healthy, injured and treated. Injury was induced by a papain buffer and the treatment was conducted using PMMA cement. Each sample was scanned with micro-computed tomography (CT) and segmented for the three conditions. Similar cement volumes (in %) were injected in the ovine samples compared to volumes measured on clinical PCD CT images. Anterior and posterior disc heights decreased on average by 22.5% and 23.9% after injury. After treatment, the anterior and posterior disc height was restored on average to 98.5% and 83.6%, respectively, of their original healthy height. Compression testing showed a similar stiffness behavior between samples in the same group. A decrease of 51.5% in segment stiffness was found after injury, as expected. The following PCD treatment was found to result in a restoration of stiffness—showing only a difference of 5% in comparison to the uninjured state. The developed ex-vivo model gave an adequate representation of the clinical vacuum phenomena in terms of volume, and a repeatable mechanical response between samples. Discoplasty treatment was found to give a restoration in stiffness after injury. The data presented confirm the effectiveness of the PCD procedure in terms of restoration of axial stiffness in the spinal segment. The model can be used in the future to test more complex loading scenarios, novel materials, and different surgical techniques.

Influence of the chitosan morphology on the properties of acrylic cements and their biocompatibility

Acrylic bone cements (ABC) are materials widely used in orthopedics and biomedical applications. Several active compounds have been introduced to ABC formulations to improve their mechanical properties and bifunctionality. In this research, we studied the effect of the addition of chitosan (CS) microspheres and chitosan sheets on ABC formulations. For mechanical performance optimization, the compression strength was taken as a response variable using an extreme vertices mixing design with fraction by weight of CS and poly(methyl methacrylate) (PMMA) as the variable factors. According to the statistical analysis, the control samples (without CS), samples with 7% (wt.) of CS sheets, and samples with 17% (wt.) of CS spheres presented the best compression properties: 90.6 MPa and 95.6 MPa, respectively. The study of these formulations confirmed that CS spheres allow a higher amount of loading on the formulation, maintaining comparable compression strength. By ¹H-NMR, it was observed that the residual monomer was similar in all wording. The hydrolytic degradation assay in simulated body fluid (SBF) determined that the sphere incorporation increased by 50% and 35% for the water uptake and weight loss values, respectively, when compared with the reported values with CS sheets. By morphological analysis via SEM, it was observed that the porosity increased considerably in the presence of CS spheres throughout the immersion time in SBF. The subdermal implant results demonstrated excellent compatibility between the cement studied and the biological environment.

Acrylic bone cements (ABC) are materials widely used in orthopedics and biomedical applications.

Injectable Functional Biomaterials for Minimally Invasive Surgery

Injectable materials represent very attractive ready-to-use biomaterials for application in minimally invasive surgical procedures. It is shown that this approach to treat, for example, vertebral fracture, craniofacial defects, or tumor resection has significant clinical potential in the biomedical field. In the last four decades, calcium phosphate cements have been widely used as injectable materials for orthopedic surgery due to their excellent properties in terms of biocompatibility and osteoconductivity. However, few clinical studies have demonstrated certain weaknesses of these cements, which include high viscosity, long degradation time, and difficulties being manipulated. To overcome these limitations, the use of sol-gel technology has been investigated, which has shown good results for synthesis of injectable calcium phosphate-based materials. In the last few decades, injectable hydrogels have gained increasing attention owing to their structural similarities with the extracellular matrix, easy process conditions, and potential applications in minimally invasive surgery. However, the need to protect cells during injection leads to the development of double network injectable hydrogels that are capable of being cross-linked in situ. This review will provide the current state of the art and recent advances in the field of injectable biomaterials for minimally invasive surgery.

Polymerization kinetics stability, volumetric changes, apatite precipitation, strontium release and fatigue of novel bone composites for vertebroplasty

PURPOSE

The aim was to determine effects of diluent monomer and monocalcium phosphate monohydrate (MCPM) on polymerization kinetics and volumetric stability, apatite precipitation, strontium release and fatigue of novel dual-paste composites for vertebroplasty.

MATERIALS AND METHODS

Polypropylene (PPGDMA) or triethylene (TEGDMA) glycol dimethacrylates (25 wt%) diluents were combined with urethane dimethacrylate (70 wt%) and hydroxyethyl methacrylate (5 wt%). 70 wt% filler containing glass particles, glass fibers (20 wt%) and polylysine (5 wt%) was added. Benzoyl peroxide and MCPM (10 or 20 wt%) or N-tolyglycine glycidyl methacrylate and tristrontium phosphate (15 wt%) were included to give initiator or activator pastes. Commercial PMMA (Simplex) and bone composite (Cortoss) were used for comparison. ATR-FTIR was used to determine thermal activated polymerization kinetics of initiator pastes at 50-80°C. Paste stability, following storage at 4-37°C, was assessed visually or through mixed paste polymerization kinetics at 25°C. Polymerization shrinkage and heat generation were calculated from final monomer conversions. Subsequent expansion and surface apatite precipitation in simulated body fluid (SBF) were assessed gravimetrically and via SEM. Strontium release into water was assessed using ICP-MS. Biaxial flexural strength (BFS) and fatigue properties were determined at 37°C after 4 weeks in SBF.

RESULTS

Polymerization profiles all exhibited an inhibition time before polymerization as predicted by free radical polymerization mechanisms. Initiator paste inhibition times and maximum reaction rates were described well by Arrhenius plots. Plot extrapolation, however, underestimated lower temperature paste stability. Replacement of TEGDMA by PPGDMA, enhanced paste stability, final monomer conversion, water-sorption induced expansion and strontium release but reduced polymerization shrinkage and heat generation. Increasing MCPM level enhanced volume expansion, surface apatite precipitation and strontium release. Although the experimental composite flexural strengths were lower compared to those of commercially available Simplex, the extrapolated low load fatigue lives of all materials were comparable.

CONCLUSIONS

Increased inhibition times at high temperature give longer predicted shelf-life whilst stability of mixed paste inhibition times is important for consistent clinical application. Increased volumetric stability, strontium release and apatite formation should encourage bone integration. Replacing TEGDMA by PPGDMA and increasing MCPM could therefore increase suitability of the above novel bone composites for vertebroplasty. Long fatigue lives of the composites may also ensure long-term durability of the materials.

Compressive fatigue properties of commercially available standard and low-modulus acrylic bone cements intended for vertebroplasty

Vertebroplasty (VP) is a minimally invasive surgical procedure commonly used to relieve severe back pain associated with vertebral compression fractures. The poly(methyl methacrylate) bone cement used during this procedure is however presumed to facilitate the occurrence of additional fractures next to the treated vertebrae. A reason for this is believed to be the difference in stiffness between the bone cement and the surrounding trabecular bone. The use of bone cements with lower mechanical properties could therefore reduce the risk of complications post-surgery. While intensive research has been performed on the quasi-static mechanical properties of these cements, there is no data on their long-term mechanical properties. In the present study, the in vitro compressive fatigue performance as well as quasi-static mechanical properties of two commercially available acrylic bone cements - a low-modulus cement (Resilience^®) and a standard cement (F20) from the same manufacturer - were determined. The quasi-static mechanical properties of the low-modulus and standard cements after 24 h of setting were in the range of other vertebroplastic cements (σ = 70-75 MPa; E= 1600-1900 MPa). F20 displayed similar mechanical properties over time in 37 °C phosphate buffered saline solution, while the mechanical properties of the Resilience^® cement decreased gradually due to an increased porosity in the polymeric matrix. The standard cement exhibited a fatigue limit of approx. 47 MPa, whereas the low-modulus cement showed a fatigue limit of approx. 31 MPa. In summary, the low-modulus bone cement had a lower fatigue limit than the standard cement, as expected. However, this fatigue limit is still substantially higher than the stresses experienced by vertebral trabecular bone.

BIOMECHANICAL EVALUATION OF LOW-MODULUS BONE CEMENT FOR ENHANCING APPLICABILITY IN VERTEBROPLASTY — AN EXPERIMENTAL STUDY IN PORCINE MODEL

Polymethylmethacrylate (PMMA) bone cement has been widely used in vertebroplasty to treat osteoporotic vertebral compression fracture. However, the high compression stiffness of PMMA is suspected to induce adjacent vertebral fracture following vertebroplasty. In the current study, modified low-modulus cement was prepared by combining PMMA with castor oil to solve this problem. The percentage of height recovery and compression stiffness of vertebral bodies was compared after injection of standard PMMA or low-modulus cement. This study aims to investigate whether low-modulus cement is as effective as standard PMMA for storing the initial vertebral height; while lowering the compression stiffness in treatment of osteoporotic vertebral compression fractures. A total of 20 fresh porcine lumbar vertebrae were assigned into two groups (10 per group): standard and low-modulus. All specimens received a four-week decalcification to mimic human osteoporotic vertebrae. The standard and low-modulus groups received a simulated compression fracture followed by treatment of standard and low-modulus cement augmentation, respectively. The low-modulus cement was prepared by combining standard PMMA with 15% weight fractions of castor oil. For all the 20 specimens, vertebral compression fracture was created by reducing the vertebral height of 25% using a material testing machine. The compression stiffness determined from the creation of compression fracture was defined as the intact group (20 specimens). The fractured vertebrae were then treated with standard and low-modulus cement augmentation. The vertebral height was measured pre- and post-treatment, and the percentage of vertebral height recovery was compared between two cementing groups. Following cement augmentation, axial compression test was conducted to compare compression stiffness among three groups. The results indicated that there is no significant difference in percentage of vertebral height between standard (83.42[Formula: see text][Formula: see text][Formula: see text]11.60%) and low-modulus (88.50[Formula: see text][Formula: see text][Formula: see text]6.15%) groups ([Formula: see text]). Moreover, the compression stiffnesses were 1166.49[Formula: see text][Formula: see text][Formula: see text]392.91 N/mm, 1795.85[Formula: see text][Formula: see text][Formula: see text]247.45[Formula: see text]N/mm and 1362.57[Formula: see text][Formula: see text][Formula: see text]236.92[Formula: see text]N/mm for intact, standard and low-modulus groups, respectively. There is significant difference among three groups ([Formula: see text]). We concluded that the modified low-modulus cement is as effective as standard PMMA for storing the initial vertebral height while lowering the compression stiffness in treatment of osteoporotic vertebral compression fractures. These reduce the risks of adjacent vertebral body fracture following vertebroplasty.

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.

Synthesis and Characterization of an Injectable and Hydrophilous Expandable Bone Cement Based on Poly(methyl methacrylate)

Poly(methyl methacrylate) (PMMA), the most common bone cement, has been used as a graft substitute in orthopedic surgeries such as vertebroplasty. However, an undesirable minor crack in the bone-cement interface provoked by shrinkage during polymerization and high elastic modulus of conventional PMMA bone cement dramatically increases the risk of vertebral body refracture postsurgery. Thus, herein, a hydrophilous expandable bone cement was synthesized based on a PMMA commercial cement (Mendec Spine Resin), acrylic acid (AA), and styrene (St). The two synthesized cements (PMMA-PAA, PMMA-PAA-PSt) showed excellent volumetric swelling in vitro and cohesive bone-cement contact in rabbit femur cavity defect. The elastic modulus and compressive strength of the new cements were lower than PMMA. Furthermore, the in vitro analysis indicated that the new cements had lower cytotoxicity than PMMA, including superior proliferation and lower apoptotic rates of Sprague-Dawly rat-derived osteoblasts. Western blotting for protein expression and RT-PCR analysis of osteogenesis-specific genes were conducted on SD rat-derived osteoblasts from both PMMA and new cements films; the results showed that new cements enhanced the expression of osteogenesis-specific genes. Scanning electron microscopy demonstrated improved morphology and attachment of osteoblast on new cement discs compared to the PMMA discs. Additionally, the histological morphologies of the bone-cement interface from the rabbit medial femoral condyle cavity defect model revealed direct and cohesive contact with the bone in the new cement groups in contrast to a minor crack in the PMMA cement group. The sign of a new bone growing into the cement has been found in the new cements after 12 weeks, thereby indicating the osteogenic capacity in vivo. In conclusion, the synthesized hydrophilous expandable bone cements based on PMMA and poly(acrylic acid) (PAA) are promising candidates for vertebroplasty.

Compressive fatigue properties of an acidic calcium phosphate cement-effect of phase composition

Calcium phosphate cements (CPCs) are synthetic bone grafting materials that can be used in fracture stabilization and to fill bone voids after, e.g., bone tumour excision. Currently there are several calcium phosphate-based formulations available, but their use is partly limited by a lack of knowledge of their mechanical properties, in particular their resistance to mechanical loading over longer periods of time. Furthermore, depending on, e.g., setting conditions, the end product of acidic CPCs may be mainly brushite or monetite, which have been found to behave differently under quasi-static loading. The objectives of this study were to evaluate the compressive fatigue properties of acidic CPCs, as well as the effect of phase composition on these properties. Hence, brushite cements stored for different lengths of time and with different amounts of monetite were investigated under quasi-static and dynamic compression. Both storage and brushite-to-monetite phase transformation was found to have a pronounced effect both on quasi-static compressive strength and fatigue performance of the cements, whereby a substantial phase transformation gave rise to a lower mechanical resistance. The brushite cements investigated in this study had the potential to survive 5 million cycles at a maximum compressive stress of 13 MPa. Given the limited amount of published data on fatigue properties of CPCs, this study provides an important insight into the compressive fatigue behaviour of such materials.

The effect of oligo(trimethylene carbonate) addition on the stiffness of acrylic bone cement

With the increasing elderly population an increase in the number of bony fractures associated to age-related diseases such as osteoporosis also follows. The relatively high stiffness of the acrylic bone cements used in these patients has been suggested to give raise to a suboptimal load distribution surrounding the cement in vivo, and hence contribute to clinical complications, such as additional fractures. The aim of this study was to develop a low-modulus bone cement, based on currently used, commercially available poly(methyl methacrylate) (PMMA) cements for vertebroplasty. To this end, acrylate end-functionalized oligo(trimethylene carbonate) (oTMC) was incorporated into the cements, and the resulting compressive mechanical properties were evaluated, as well as the cytotoxic and handling properties of selected formulations. Sixteen wt%oTMC was needed in the vertebroplastic cement Osteopal V to achieve an elastic modulus of 1063 MPa (SD 74), which gave a corresponding compressive strength of 46.1 MPa (SD 1.9). Cement extracts taken at 1 and 12 hours gave a reduced MG-63 cell viability in most cases, while extracts taken at 24 hours had no significant effect on cell behavior. The modification also gave an increase in setting time, from 14.7 min (SD 1.7) to 18.0 min (SD 0.9), and a decrease in maximum polymerization temperature, from 41.5°C (SD 3.4) to 30.7°C (SD 1.4). While further evaluation of other relevant properties, such as injectability and in vivo biocompatibility, remains to be done, the results presented herein are promising in terms of approaching clinically applicable bone cements with a lower stiffness.

In Vitro and In Vivo Response to Low-Modulus PMMA-Based Bone Cement

The high stiffness of acrylic bone cements has been hypothesized to contribute to the increased number of fractures encountered after vertebroplasty, which has led to the development of low-modulus cements. However, there is no data available on the in vivo biocompatibility of any low-modulus cement. In this study, the in vitro cytotoxicity and in vivo biocompatibility of two types of low-modulus acrylic cements, one modified with castor oil and one with linoleic acid, were evaluated using human osteoblast-like cells and a rodent model, respectively. While the in vitro cytotoxicity appeared somewhat affected by the castor oil and linoleic acid additions, no difference could be found in the in vivo response to these cements in comparison to the base, commercially available cement, in terms of histology and flow cytometry analysis of the presence of immune cells. Furthermore, the in vivo radiopacity of the cements appeared unaltered. While these results are promising, the mechanical behavior of these cements in vivo remains to be investigated.

Mechanical Properties and Cytocompatibility Improvement of Vertebroplasty PMMA Bone Cements by Incorporating Mineralized Collagen

Polymethyl methacrylate (PMMA) bone cement is a commonly used bone adhesive and filling material in percutaneous vertebroplasty and percutaneous kyphoplasty surgeries. However, PMMA bone cements have been reported to cause some severe complications, such as secondary fracture of adjacent vertebral bodies, and loosening or even dislodgement of the set PMMA bone cement, due to the over-high elastic modulus and poor osteointegration ability of the PMMA. In this study, mineralized collagen (MC) with biomimetic microstructure and good osteogenic activity was added to commercially available PMMA bone cement products, in order to improve both the mechanical properties and the cytocompatibility. As the compressive strength of the modified bone cements remained well, the compressive elastic modulus could be significantly down-regulated by the MC, so as to reduce the pressure on the adjacent vertebral bodies. Meanwhile, the adhesion and proliferation of pre-osteoblasts on the modified bone cements were improved compared with cells on those unmodified, such result is beneficial for a good osteointegration formation between the bone cement and the host bone tissue in clinical applications. Moreover, the modification of the PMMA bone cements by adding MC did not significantly influence the injectability and processing times of the cement.

The effect of unsaturated fatty acid and triglyceride oil addition on the mechanical and antibacterial properties of acrylic bone cements

Acrylic bone cements have an elastic modulus several times higher than the surrounding trabecular bone. This has been hypothesized to contribute to certain clinical complications. There are indications that the addition of specific fatty acids and triglyceride oils may reduce the elastic modulus of these types of cements. Some of these additives also appear to have inherent antibiotic properties, although this has never been evaluated in bone cements. In this study, several types of fatty acids and triglyceride oils were evaluated for use in acrylic bone cements. Their mechanical properties were evaluated under uniaxial compression testing and selected cements were then further characterized in terms of microstructure, handling and antibacterial properties using scanning electron microscopy, polymerization temperature measurements, agar diffusion tests and bactericidal activity assays of cement extracts. It was found that any of the evaluated fatty acids or triglyceride oils could be used to tailor the stiffness of acrylic bone cements, although at varying concentrations, which also depended on the type of commercial base cement used. In particular, the addition of very small amounts of linoleic acid (<2.0 wt%) resulted in Young’s moduli and compressive strengths in the range of human trabecular bone, while maintaining a similar setting time. Further, the addition of 12.6 wt% ricinoleic acid to Osteopal V cement was found to have a significant antibacterial effect, inhibiting growth of Staphylococcus aureus in an agar diffusion test as well as demonstrating 100% bactericidal activity against the same strain.

Design and evaluation of a nanoenhanced anti-infective calcium phosphate bone cements

Post-operative complications due to infections are the most common problems that occur following dental and orthopedic implant surgeries and bone repair procedures. Preventing post-surgical infections is therefore a critical need that current polymethylmethacrylate (PMMA) bone cement fail to address. Calcium phosphate cements (CPCs) are unique in their ability to crystallize calcium and phosphate salts into hydroxyapatite (HA) and hence is naturally osteoconductive. Due to its low mechanical strength its use in implant fixation and bone repair is limited to non-load bearing applications. The present work describes a new and novel antibiotic-doped clay nanotube CPC composite with enhanced mechanical properties as well as sustained release properties.

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.

Surface functionalized barium sulfate nanoparticles: controlled in situ synthesis and application in bone cement

Controlled in situ synthesis of MSAH-coated BaSO₄ nanoparticles improves the mechanical properties and in vitro biocompatibility of the bone cements.

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.

Mechanical and cytotoxicity testing of acrylic bone cement embedded with microencapsulated 2-octyl cyanoacrylate

The water-reactive tissue adhesive 2-octyl cyanoacrylate (OCA) was microencapsulated in polyurethane shells and incorporated into Palacos R bone cement. The tensile and compressive properties of the composite material were investigated in accordance with commercial standards, and fracture toughness of the capsule-embedded bone cement was measured using the tapered double-cantilever beam geometry. Viability and proliferation of MG63 human osteosarcoma cells after culture with extracts from Palacos R bone cement, capsule-embedded Palacos R bone cement, and OCA were also analyzed. Incorporating up to 5 wt % capsules had little effect on the compressive and tensile properties of the composite, but greater than 5 wt % capsules reduced these values below commercial standards. Fracture toughness was increased by 13% through the incorporation of 3 wt % capsules and eventually decreased below the toughness of the capsule-free controls at capsule contents of 15 wt % and higher. The effect on cell proliferation and viability in response to extracts prepared from capsule-embedded and commercial bone cements were not significantly different from each other, whereas extracts from OCA were moderately toxic to cells. Overall, the addition of lower wt % of OCA-containing microcapsules to commercial bone cement was found to moderately increase static mechanical properties without increasing the toxicity of the material.

Cement-implant interface contamination: possible reason of inferior clinical outcomes for rough surface cemented stems

BACKGROUND

Shape-closed cemented implants rely on a stronger bond and have displayed inferior clinical outcomes when compared to force-closed designs. Implant contamination such as saline, bone marrow and blood prior to cement application has the potential to affect the cement-implant bond. The consequences of implant contamination were investigated in this study.

METHODS

Fifty Titanium alloy (Ti-6Al-4V) dowels were separated into ten groups based on surface roughness and contaminant, and then cemented in polyvinyl chloride tubes. Push-out testing was performed at 1mm per minute. The roughness of the dowel surface was measured before and after the testing. The dowel surface and cement mantel were analyzed using a Scanning Electron Microscopy (SEM) to determine the distribution and characteristics of any debris and contaminants on the surface.

RESULTS

Contaminants largely decreased stem-cement interfacial shear strength, especially for rough surfaces. Saline produced the greatest decrease, followed by blood. The effect of bone marrow was less pronounced and similar to that of oil. Increasing surface roughness increased the interfacial bonding strength, even with contaminants. There was a non-significant increase in mean bonding strength for smooth surfaces with bone marrow and oil contamination. SEM showed that contaminants influence the interfacial bond by different mechanisms. More debris was found on rough samples following testing.

CONCLUSIONS

The results of this study underscore the importance of keeping an implant free from contamination, and suggest if contamination does occur, a saline rinse may further decrease the stability of an implant. The deleterious effects of contamination on rough surface cement bonding were considerable, and indicate that contamination at the time of surgery may, in part, contribute to inferior clinical outcomes for rough surfaced cemented stems.

FINITE ELEMENT ANALYSIS OF A MODEL OF SIMULATED VERTEBRAL CEMENT AUGMENTATION: INFLUENCE OF THE REPRESENTATION OF THE SHAPE OF THE CEMENT DOMAIN ON BIOMECHANICAL PARAMETERS

Vertebral cement augmentation is rapidly becoming the modality of choice for treating patients who are experiencing severe and persistent pain because of osteoporosis-induced vertebral compression fracture(s). The resulting cement domain (the part of the vertebral body (VB) filled with the cement) has an irregular or complicated shape. In literature reports of finite element analysis (FEA) of models of simulated vertebral cement augmentation, a variety of representations of the shape of the cement domain have been used. In the literature, only very limited attention has been given to the issue of the influence of cement domain shape representation on biomechanical parameters for a given combination of model and loading. This issue is the subject of the present work, with the model being of the L1-L3 motion segments. Augmentation of an unfractured L2 (prophylactic augmentation) was simulated, three cement domain shapes were considered — namely, solid cylinder, with rounded edges; two prolate spheroids; and oblate spheroid — and the applied loading comprised a simultaneous application of a uniform compressive pressure of 0.53 MPa (equivalent to an 800-N compression load) and a counter-clockwise-acting axial rotation moment of 1 Nm to the superior surface of L1. It was found that (1) while the cement domain shape representation has a marked influence on the mean von Mises stress (σAVM), the maximum von Mises stress (σMVM), and the strain energy density (MSED) distribution in the cement domain, its influence on each of these parameters in each of the biological tissues in the model as well as on the total segmental range of motion is minimal and (2) for σAVM and σMVM, the lowest value of each of these parameters was obtained when the oblate spheroid model was used. From both clinical and computational perspectives, these findings are significant. For example, the latter finding suggests that there is scope for researching the combination of key process variables used, such as the cement chemistry, the cement delivery system, and the augmentation technique/approach, that would ensure that the final cement domain shape in cement-augmented VBs of patients be oblate spheroid on a consistent and predictable basis.

Microbiological and pharmacological tests on new antibiotic-loaded PMMA-based composites for the treatment of osteomyelitis

Local antibiotic diffusion in rabbit femurs from two new PMMA-based and nail-shaped composites, enriched with β-tricalcium phosphate (P-TCP) and BaSO(4) or only with BaSO(4) (P-BaSO(4) ), and soaked in a solution of gentamicin (G) and vancomycin (V) was studied. Nails were implanted into the intramedullary cavity of healthy and osteomyelitic femurs to study the resolution of infection and to quantify the antibiotic penetration into bone by microbiological, pharmacological, and histological tests. A significant progression of osteomyelitis was recorded 7 weeks after MRSA inoculation, whereas no bacteria were found in animals treated with antibiotic-loaded nails as confirmed by microbiology and histology (Smeltzer score). The release of both antibiotics from composites was high and prompt both in healthy and infected bone; the amount of V was higher than that of G in all bone samples. Antibiotics of both composites were still present in bone 3 weeks after nail implantation. The P-BaSO4 composite released a lower amount of antibiotics than did P-TCP. The G-V combination in vivo exerted a synergistic bactericidal effect, which was confirmed by microbiological, histological, and clinical results (no infection). These new porous PMMA composites, soaked in G-V solution in the operating room, might be an effective and useful drug delivery system for osteomyelitis treatment.

Corrosion fatigue behaviors of two biomedical Mg alloys - AZ91D and WE43 - In simulated body fluid

Magnesium alloys have been recently developed as biodegradable implant materials, yet there has been no study concerning their corrosion fatigue properties under cyclic loading. In this study the die-cast AZ91D (A for aluminum 9%, Z for zinc 1% and D for a fourth phase) and extruded WE43 (W for yttrium 4%, E for rare earth mischmetal 3%) alloys were chosen to evaluate their fatigue and corrosion fatigue behaviors in simulated body fluid (SBF). The die-cast AZ91D alloy indicated a fatigue limit of 50MPa at 10⁷ cycles in air compared to 20MPa at 10⁶ cycles tested in SBF at 37°C. A fatigue limit of 110MPa at 10⁷ cycles in air was observed for extruded WE43 alloy compared to 40MPa at 10⁷ cycles tested in SBF at 37°C. The fatigue cracks initiated from the micropores when tested in air and from corrosion pits when tested in SBF, respectively. The overload zone of the extruded WE43 alloy exhibited a ductile fracture mode with deep dimples, in comparison to a brittle fracture mode for the die-cast AZ91D. The corrosion rate of the two experimental alloys increased under cyclic loading compared to that in the static immersion test.

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.

Bismuth-doped injectable calcium phosphate cement with improved radiopacity and potent antimicrobial activity for root canal filling

A bismuth-doped injectable calcium phosphate cement (BD-ICPC) with improved radiopacity, potent antimicrobial activity and sealability was developed by introducing bismuth salicylate basic (BSB) into the powder phase of the calcium phosphate cement (CPC). The results showed that the radiopacity and sealability of BD-ICPC were improved compared with pure ICPC. Although BSB had a retarding effect on the setting rate of the cement, the addition of BSB reduced the viscosity and yield stress of BD-ICPC, thus enhanced its injectability. It was noteworthy that BD-ICPC had a potent antimicrobial activity with improved sealability. In addition, BD-ICPC afforded a uniform and tight adaptation to the root canal wall. These results indicate that BD-ICPC possesses a combination of good in vitro radiopacity, high injectability, potent antimicrobial activity, improved sealability and tight adaptation to the root canal. It is expected to be used as a novel root canal filling material.

Combined influence of barium sulfate content and co-monomer concentration on properties of PMMA bone cements for vertebroplasty

In this work, the combined influence of barium sulfate content and co-monomer concentration on the properties of acrylic bone cement for percutaneous vertebroplasty (PVP) was investigated using a response surface methodology. Cements were prepared with methyl methacrylate (MMA) and either diethyl amino ethyl methacrylate (DEAEM) or dimethyl amino ethyl methacrylate (DMAEM) as co-monomer in the liquid phase, while variable amounts of barium sulfate were incorporated to the solid phase in order to improve the radiopacity of cements. It was found that various properties such as peak temperature, setting time, residual monomer content, mechanical properties and injectability, had an effect on the occurrence of interactions (combined effect) between the barium sulfate and DEAEM in bone cements formulations when independent variables were at their maximum.

Acrylic bone cements: influence of time and environment on physical properties

Acrylic bone cements are in extensive use in joint replacement surgery. They are weight bearing and load transferring in the bone-cement-prosthesis complex and therefore, inter alia, their mechanical properties are deemed to be crucial for the overall outcome. In spite of adequate preclinical test results according to the current specifications (ISO, ASTM), cements with inferior clinical results have appeared on the market. The aim of this study was to investigate whether it is possible to predict the long term clinical performance of acrylic bone cement on the basis of mechanical in vitro testing. We performed in vitro quasistatic testing of cement after aging in different media and at different temperatures for up to 5 years. Dynamic creep testing and testing of retrieved cement were also performed. 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. We could demonstrate a continuous increase in mechanical properties when testing in air, while testing in water resulted in a slight decrease in mechanical properties after 1 week and then levelled out. Palacos bone cement showed a higher creep than CMW3G and the retrieved Boneloc specimens showed a higher creep than retrieved Palacos. The strength of a bone cement develops more slowly than the apparent high initial setting rate indicates and there are changes in mechanical properties over a period of five years. The effect of water absorption is important for the physical properties but the mechanical changes caused by physical aging are still present after immersion in water. The established standards are in need of more clinically relevant test methods and their associated requirements need better definition. We recommend that testing of bone cements should be performed after extended aging under simulated physiological conditions. Simple quasistatic and dynamic creep tests seem unable to predict clinical performance of acrylic bone cements when the products under test are chemically very similar. However, such testing might be clinically relevant if the cements exhibit substantial differences.