Surface studies on acrylic bone cement

Analysis of the Effect of Component Ratio Imbalances on Selected Mechanical Properties of Seasoned, Medium Viscosity Bone Cements

The paper presents the results of experimental strength tests of specimens made of two commercially available bone cements subjected to compression, that is a typical variant of load of this material during use in the human body, after it has been used for implantation of prostheses or supplementation of bone defects. One of the factors analysed in detail was the duration of cement seasoning in Ringer's solution that simulates the aggressive environment of the human body and material degradation caused by it. The study also focused on the parameters of quantitative deviation from the recommended proportions of liquid (MMA monomer, accelerator and stabiliser) and powder (PMMA prepolymer and initiator) components, i.e., unintentional inaccuracy of component proportioning at the stage of cement mass preparation. Statistical analysis has shown the influence of these factors on the decrease in compressive strength of the cements studied, which may be of significant importance in operational practice.

The chitosan/tri-calcium phosphate bio-composite bone cement promotes better osteo-integration: an in vitro and in vivo study

Background

Polymethylmethacrylate bone cement has a variety of applications in orthopedic surgery, but it also has some shortcomings such as high heat generation during polymerization and poor integration with bone tissue. In this study, a bio-composite bone cement composed of tri-calcium phosphate and chitosan as additives to acrylic bone cement was developed. Our hypothesis is that this new bio-composite bone cement has a better osteo-integration than pure polymethyl methacrylate cement.

Methods

Physiological composition, i.e., 65 wt% inorganic and 35 wt% organic components, of tri-calcium phosphate and chitosan contents was selected as degradable additives to replace acrylic bone cement. A series of properties such as exothermic temperature changes, setting time, bio-mechanical characteristics, degradation behaviors, and in vitro cytotoxicity were examined. Preliminary in vivo animal study was also performed.

Results

The results showed that the bio-composite bone cement exhibited lower curing temperature, longer setting time, higher weight loss and porosity after degradation, lower compressive Young’s modulus, and ultimate compressive strength as compared with those of pure polymethyl methacrylate cement. Cell proliferation tests demonstrated that the bio-composite bone cement was non-cytotoxic, and the in vivo tests revealed that was more osteo-conductive.

Conclusions

The results indicated that the modified chitosan/tri-calcium phosphate/polymethyl methacrylate bio-composites bone cement could be degraded gradually and create rougher surfaces that would be beneficial to cell adherence and growth. This new bio-composite bone cement has potential in clinical application. Our future studies will focus on long-term implantation to investigate the stability of the bio-composite bone cement in long-term implantation.

Settable polymer/ceramic composite bone grafts stabilize weight-bearing tibial plateau slot defects and integrate with host bone in an ovine model

Bone fractures at weight-bearing sites are challenging to treat due to the difficulty in maintaining articular congruency. An ideal biomaterial for fracture repair near articulating joints sets rapidly after implantation, stabilizes the fracture with minimal rigid implants, stimulates new bone formation, and remodels at a rate that maintains osseous integrity. Consequently, the design of biomaterials that mechanically stabilize fractures while remodeling to form new bone is an unmet challenge in bone tissue engineering. In this study, we investigated remodeling of resorbable bone cements in a stringent model of mechanically loaded tibial plateau defects in sheep. Nanocrystalline hydroxyapatite-poly(ester urethane) (nHA-PEUR) hybrid polymers were augmented with either ceramic granules (85% β-tricalcium phosphate/15% hydroxyapatite, CG) or a blend of CG and bioactive glass (BG) particles to form a settable bone cement. The initial compressive strength and fatigue properties of the cements were comparable to those of non-resorbable poly(methyl methacrylate) bone cement. In animals that tolerated the initial few weeks of early weight-bearing, CG/nHA-PEUR cements mechanically stabilized the tibial plateau defects and remodeled to form new bone at 16 weeks. In contrast, cements incorporating BG particles resorbed with fibrous tissue filling the defect. Furthermore, CG/nHA-PEUR cements remodeled significantly faster at the full weight-bearing tibial plateau site compared to the mechanically protected femoral condyle site in the same animal. These findings are the first to report a settable bone cement that remodels to form new bone while providing mechanical stability in a stringent large animal model of weight-bearing bone defects near an articulating joint.

Effect of Sonication on the Elution of Antibiotics from Polymethyl Methacrylate (PMMA)

Background: In the setting of prosthetic joint infections treated with a two-stage procedure, spacers can be sonicated after removal. We hypothesize that the sonication process may cause an increased elution of antibiotics from the spacer, leading to elevated concentrations of antibiotics in the sonication fluid inhibiting bacterial growth. We aimed to evaluate in vitro the influence of sonication on the elution of antibiotics from polymethyl methacrylate (PMMA) over time and to determine whether these concentrations are above the minimum inhibitory concentrations (MIC) for microorganisms relevant in prosthetic joint infections. Methods: PMMA blocks impregnated with vancomycin, fosfomycin, gentamicin or daptomycin were incubated in phosphate-buffered saline (PBS) at 37°C for up to 6 weeks. PBS was changed once a week. Concentrations were determined from samples of each antibiotic every week, and after 5 minutes of sonication at 2, 4 and 6 weeks. Results: With sonication there was a trend toward an increase of the elution of antibiotics. This increase was significant for vancomycin at 2 and 4 weeks (p=0.008 and 0.002 respectively) and for fosfomycin at 2 weeks (p=0.01). Conclusion: The effect of sonication could play a role in clinical results, especially for daptomycin and gentamicin for which the MIC is close to the concentration of antibiotics at 4 and 6 weeks. We conclude that elution of antibiotics from PMMA along with the effect of sonication could inhibit bacterial growth from spacers, resulting in false negative results in the setting of two-stage exchange procedures for prosthetic joint infections.

Resorbable Nanocomposites with Bone-Like Strength and Enhanced Cellular Activity

Bone cements for treatment of fractures at weight-bearing sites are subjected to dynamic physiological loading from daily activities. An ideal bone cement rapidly sets after injection, exhibits bone-like strength, stimulates osteogenic differentiation of endogenous cells, and resorbs at a rate aligned with patient biology. However, currently available materials fall short of these targeted properties. Nanocrystalline hydroxyapatite (nHA) enhances osteogenic differentiation, new bone formation, and osteoclast differentiation activity compared to amorphous or micron-scale crystalline hydroxyapatite. However, the brittle mechanical properties of nHA precludes its use in treatment of weight-bearing bone defects. In this study, we report settable nHA-poly(ester urethane) (PEUR) nanocomposites synthesized from nHA, lysine triisocyanate (LTI), and poly(caprolactone) triol via a solvent-free process. The nanocomposites are easily mixed and injected using a double-barrel syringe, exhibit mechanical properties exceeding those of conventional bone cements, enhance mineralization of osteoprogenitor cells in vitro, and undergo osteoclast-mediated degradation in vitro. This combination of properties cannot be achieved using other technologies, which underscores the potential of nHA-PEUR nanocomposites as a new approach for promoting bone healing at weight-bearing sites.

Release of zirconia nanoparticles at the metal stem-bone cement interface in implant loosening of total hip replacements

In a previous failure analysis performed on femoral components of cemented total hip replacements, we determined high volumes of abraded bone cement. Here, we describe the topography of the polished surface of polymethyl methacrylate (PMMA) bone cement containing zirconia radiopacifier, analyzed by scanning electron microscopy and vertical scanning interferometry. Zirconia spikes protruded about 300nm from the PMMA matrix, with pits of former crystal deposition measuring about 400nm in depth. We deduced that the characteristically mulberry-shaped agglomerates of zirconia crystals are ground and truncated into flat surfaces and finally torn out of the PMMA matrix. Additionally, evaluation of in vitro PMMA-on-PMMA articulation confirmed that crystal agglomerations of zirconia were exposed to grain pullout, fatigue, and abrasion. In great quantities, micron-sized PMMA wear and zirconia nanoparticles accumulate in the cement-bone interface and capsular tissues, thereby contributing to osteolysis. Dissemination of nanoparticles to distant lymph nodes and organs of storage has been reported. As sufficient information is lacking, foreign body reactions to accumulated nanosized zirconia in places of long-term storage should be investigated.

STATEMENT OF SIGNIFICANCE

The production of wear particles of PMMA bone cement in the interface to joint replacement devices, presents a local challenge. The presence of zirconia particles results in frustrated digestion attempts by macrophages, liberation of inflammatory mediators, and necrosis leading to aseptic inflammation and osteolyses. Attempts to minimize wear of articulating joints reduced the attention to the deterioration of cement cuffs. We therefore investigated polished surfaces of retrieved cuffs to demonstrate their morphology and to measure surface roughness. Industrially admixed agglomerates of the radiopacifier are abraded to micron and nano-meter sized particles. The dissemination of zirconia particles in the reticulo-endothelial system to storage organs is a possible burden. Research to replace the actual contrast media by non-particulate material deserves more attention.

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.

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

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

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

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

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

CHITOSAN IN APPLICATIONS OF BIOMEDICAL DEVICES

Chitosan is a natural polysaccharide with great potential for biomedical applications due to its biocompatibility, biodegradable capability, and nontoxicity. Various techniques used for preparing chitosan microspheres/membranes and evaluations of these fabrications have also been reviewed. The hydrophilicity of chitosan provides unique characteristics of hydrogel formation with the acidic media and may entrap the drug content inside of the matrix for controlled release. In order to improve upon the scope of preparation of chitosan microspheres, we had successfully employed and incorporated a high-voltage system into the direct pumping injection process. The wide range of drug release profiles could be attributed to the surface characteristics, porosities, and various structures of chitosan microspheres upon treatment with Na5P3O10/NaOH solutions of various volume ratios. We also demonstrated that with the addition of chitosan/β-TCP microspheres as a constituent into the PMMA cement significantly decreases the curing peak temperature and increases the setting time. The excellent gelforming property of chitosan offers another biomedical application in membrane separation fields. Chitosan membranes were prepared by a thermal induced phase separation method, following treatment with nontoxic NaOH gelating and Na5P3O10, Na2SO3 crosslinking agents. In order to further improve the mechanical strength and biocompatibility and to expand the potential of chitosan GTR membranes in periodontal applications, various chitosan membranes incorporating with negatively charged alginate, bioactive tricalcium phosphate, and platelet rich plasma, respectively, were also prepared and characterized. Moreover, we had also utilized chitosan, which with good blood-clotting, cheap, and easy preparation characteristics, as the raw material to prepare rapid clotting wound dressing and tooth plug.

Biocompatibility and bone formation with porous modified PMMA in normal and irradiated mandibular tissue

A cemented mandibular endoprosthesis is a potentially viable option for mandibular reconstruction after ablative surgery. The commonly used PMMA cement has the inherent weakness of a lack of bioactivity. Improvement by the addition of porosities and bioactive compounds like calcium phosphates may resolve this issue.

OBJECTIVE

The objective of this study was to assess the bone and tissue response to two modified PMMA cements with post-operative radiation as an additional influencing factor.

MATERIALS & METHODS

An in vivo animal study was performed using a mandibular rabbit model. A porous PMMA cement (A) and a porous cement incorporated with Beta-tricalcium phosphate particles (b-TCP) (B) were placed in bilateral mandibular defects with exposed roots and mandibular nerve of 20 animals. Half of the animals underwent additional post-operative radiation.

RESULTS

The animals were healthy with only a minor complication in one rabbit. Temperature analysis showed no significant risk of thermal necrosis with the maximal in vivo cement temperature at 37.8°C. Histology demonstrated: (1) good bone ingrowth around the defect as well as within the pores of the cement and defect bridging was achieved in 70% of the specimens after 12-15 weeks of implantation, (2) no pulpal injury with minor secondary cementum response, (3) an intact mandibular nerve with no inflammation, (4) extensive degradation and resorption of the b-TCP particles by 12-15 weeks, and (5) presence of an intervening thin fibrous tissue at the bone-to-cement interface. Histomorphometrical analysis revealed that there was no difference between the different cements and the presence or absence of post-operative radiation. The 12-15 weeks specimens showed significantly more bone ingrowth and bone maturity than the 4-7 weeks specimens.

CONCLUSION

Both modified PMMA cements have good biocompatibility, bioactivity and support bone ingrowth and additional post-operative radiation did not show any negative effects.

Bone cements and their potential use in a mandibular endoprosthesis

Bone cement was first used in the 1950s. Since then many modifications have been made and alternatives developed to the original polymethylmethacrylate (PMMA) cement. In view of the use of bone cement in a novel mandibular endoprosthetic system, we performed a review of the current literature on this material. Different cements are described and their potential use in a mandibular endoprosthetic system discussed. The PMMA-based cements are currently the most suitable choice. Plain PMMA has the longest track record and is the default choice for the initial development phase of this system. If there is a significant risk of infection, then an antibiotic-loaded PMMA cement can be selected. However, modified PMMA cements, composite resin cements, osteoinductive calcium phosphate compounds, and cementless fixation are options that offer advantages over PMMA cements, and further research should be conducted to study their suitability.

Effect of surface modification of polymer beads on the mechanical properties of acrylic bone cement

The effect of surface modification of polymer filler on the static mechanical properties of acrylic bone cement was studied. The surface of polymer beads was modified with carboxylic and amino groups by photochemical reaction with azide compounds. Monomer modifiers (maleic anhydride, methacrylic acid and p-aminostyrene) are attached to the functionalized surface of polymer beads. Functional allyl groups, which are capable of the graft polymerisation reaction, are attached to the surface via photochemical reaction with N-(2-nitro-4-azidophenyl)-N-(-propen) amine. This approach to bone cement provides the additional covalent bonds between the polymer beads and the inter-bead matrix. The static mechanical properties of bone cements containing modified polymer beads were investigated and compared with the static mechanical properties of unmodified cements. The absolute values of compressive strength for the modified and unmodified cements were found to be similar. An increase in flexural strength for the modified cements (dry and after water storage) was observed. The structure of the surface functional groups affects the methyl methacrylate grafting resulting in a higher value of flexural strength for the maleic anhydride- and p-aminostyrene-modified cements. The scanning electron microscopy examination of the fracture surface of the cement samples showed an improvement of the adhesion between the beads and the matrix after modification.

Histopathologic findings of retrieved specimens of vertebroplasty with polymethylmethacrylate cement: case control study

STUDY DESIGN

Case control study.

OBJECTIVE

To investigate the histopathologic findings of 2 retrieved specimens from failed vertebroplasty with polymethylmethacrylate (PMMA) cement.

SUMMARY OF BACKGROUND DATA

Vertebroplasty using PMMA cement has been commonly used to treat debilitating back pain from compression fracture, angiomas, and metastatic cancer. However, there was concern about the unpredictable future results with PMMA cement. The histopathologic changes were rarely reported.

METHODS

There were 2 PMMA augmented and 3 nonaugmented fractured vertebral bodies retrieved for histopathologic study. Between the 2 groups, we compared the findings of bone necrosis, foreign body reaction, fibrotic wall formation, and neovascularization.

RESULTS

Bone necrosis was noted in the periphery of PMMA cement, which was surrounded by fibrotic tissues. In contrast, no fibrotic wall formation could be found in the nonaugmented control group. Foreign body reaction was only noted in PMMA augmented cases, and neovascularization was only noted in the control cases.

CONCLUSION

PMMA cement might not be as bioinert as we considered. Therefore, the long-term safety of vertebroplasty should be further evaluated.

Evaluation of chitosan/beta-tricalcium phosphate microspheres as a constituent to PMMA cement

Two methods, a traditional emulsion technique and a high voltage electrostatically modified encapsulation system, were used to fabricate degradable chitosan/beta -tricalcium phosphate (beta-TCP) microspheres. The two distinct kinds of microspheres both exhibited good sphericity and the beta-TCP was trapped well inside the chitosan gel. The microspheres prepared by high voltage electrostatic system exhibited a rougher outer surface and narrower size distribution. These microspheres were then used as an added constituent to commercially available PMMA bone cement. Four modified cement composites that were prepared with different composition ratios of the two kinds of chitosan/beta-TCP microspheres that were made from emulsion technique (C1P1 and C2P1) and from a process by a high voltage electrostatic system (EC1P1 and EC2P1) were compared with the PMMA cement (Pure P). The characteristics of these materials indicate that with the addition of chitosan/beta-TCP microspheres as a constituent into the PMMA cement significantly decreases the curing peak temperature. Furthermore, the setting time increases from 3.5 min to 9 min, as compared to the PMMA cement. These changes could be beneficial for the handling of the bone cement paste and causing less damage to the surrounding tissues. Understandably, the presence of chitosan/beta-TCP microspheres in the prepared composites reduced the ultimate compressive strength and bending strength. From the degradation test and SEM observations, the modified chitosan/beta -TCP/PMMA composites could be degraded gradually and create rougher surfaces that would be beneficial to cell adherence and growth.