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BIOMEDICAL APPLICATION OF COMPOSITE MATERIALS: APPLICATION IN ORTHOPEDICS

Posted by Connie R. Aponte on November 8, 2013 in Economy |

ORTHOPEDICS

Orthopedics is the medical area where application of the biomaterials is highly developed both for commercialization and research purpose. The need for new materials in orthopedic surgery arises from the recognition of the stress-shielding effect of bone by high-modulus implants presently made of engineering alloys. A lower modulus implant material will result in the construction of a more biomechanically compatible prosthesis. In this respect, composite materials are gaining importance because they offer the potential for implants with tailor-made stiffness in contrast to metals. Economics

Zirconia toughened alumina (ZTA) composites are considered today as promising materials for orthopedic applications, since they offer a higher crack resistance than alumina and zirconia monoliths. However, despite the presence of zirconia in the material, there is lack in literature concerning the crucial question of ageing. In particular, the role of the quality of the dispersion of zirconia in the alumina matrix has never been discussed. In the work of Gutknecht Dan, et al, the dispersion behavior of alumina and zirconia powder was studied. Using an optimal dispersion at pH 4.5, homogeneous ZTA were obtained. Neither alumina nor zirconia aggregates were present in the final microstructure. Ageing experiments were conducted on ZTA of different compositions for both yttria stabilized and unstabilized zirconia. The results were compared with previous works where aggregates were present in the final materials. The ageing kinetics showed a drastic difference between ZTA with or without aggregates. For ZTA containing unstabilized zirconia, aggregates were transformed during cooling, giving rise to an ageing sensitivity, even for low zirconia content (i.e. 10 vol.%). For ZTA containing yttria stabilized zirconia, aggregates transformation occurs during the first stages of ageing. On the other hand, no transformation at all was observed for materials without aggregates, provided that the zirconia content is kept below the percolation threshold (16 vol.%). Polyetheretherketone-hydroxyapatite (PEEK-HA) composites were developed as alternative materials for load-bearing orthopedic applications. The amount of HA incorporated into the PEEK polymer matrix ranges from 5 to 40 vol% and these materials were successfully fabricated by injection molding. The study of Abu Bakar M. S., et al presented the mechanical and biological behavior of the composite materials developed. It was found that the amount of HA in the composite influenced the tensile properties. Dynamic behavior under tension-tension fatigue revealed that the fatigue-life of PEEK-HA composites were dependent on the HA content as well as the applied load. The biological responses of PEEK-HA composites carried out in vivo verified the biocompatibility and bioactive nature of the composite materials.

Bone graft

Bone is composed of a cellular component and an extracellular matrix. The cellular component is made of osteoblasts, bone-forming cells, osteoclasts, bone-destroying cells, and osteocytes, bone-maintaining cells which are inactive osteoblasts trapped in the extracellular matrix. Natural bone is actually an inorganic/ organic composite mainly made up of nano-structure hydroxyapatite (Cai0(PO4)6(OH)2, HA) and collagen fibers. It is of most importance to synthesize nano-composites of inorganic/organic in order to have good biocompatibility, high bioactivity and great bonding properties. In this work, HA nano-particle and HA/chitosan (CTS) nano-composite with a homogeneous microstructure were prepared and characterized. It is proposed that the nano-structure of HA/CTS composite will have the best biomedical properties in the biomaterials applications.

Bone graft is essential when injury is too severe and loss of volume is too high, healing is incomplete, fibrous tissue forms, mechanical functionality is not restored. There are always problem associated with autograft and allograft. Autograft is considered ideal for grafting procedures, providing osteoinductive growth factors, osteogenic cells, and an osteoconductive scaffold. Limitations, however, exist regarding donor site morbidity and graft availability. Allograft on the other hand, possesses the risk of disease transmission. Synthetic graft substitutes lack osteoinductive or osteogenic properties. Composite grafts combine scaffolding properties with biological elements to stimulate cell proliferation and differentiation and eventually osteogenesis. Bioresorbable bone graft substitutes may significantly reduce the disadvantages associated with autografts, allografts and other synthetic materials currently used in bone graft procedures. The biocompatibility and osseointegration of a bioresorbable bone graft substitute made from the unsaturated polyester poly(propylene-glycol-co-fumaric acid), or simply poly(propylene fumarate), PPF, which is cross linked in the presence of soluble and insoluble calcium filler salts was investigated by Lewandrowski, et al. Histological analysis revealed that in vivo biocompatibility and osseointegration of bone graft substitutes was optimal when calcium acetate was employed as soluble salt filler. Other formulations demonstrated implant surface erosion and disintegration which was ultimately accompanied by an inflammatory response. This study suggested that PPF-based bone graft substitutes can be designed to provide an osteoconductive pathway by which bone will grow in faster because of its capacity to develop controlled porosities in vivo. Immediate applicability of this bone graft substitute, the porosity of which can be tailored for the reconstruction of defects of varying size and quality of the recipient bed, is to defects caused by surgical debridement of infections, previous surgery, tumor removal, trauma, implant revisions and joint fusion.

In study of Simon, composite bone graft consisting of calcium phosphate cement (CPC) and poly(lactide-co-glycolide) (PLGA) microspheres (approximate diameter of 0.18-0.36 mm) using cell culture techniques have been made. CPC powder is mixed with PLGA microspheres and water to yield a workable paste that could be sculpted to fit the contours of a wound. The cement then hardens into a matrix of HA microcrystals containing PLGA microspheres. The rationale for this design is that the microspheres will initially stabilize the graft but can then degrade to leave behind macropores for colonization by osteoblasts. The CPC matrix could then be resorbed and replaced with new bone. In this study, osteoblasts-like cells (MC3T3-E1 cells) were seeded onto graft specimens and evaluated with fluorescence microscopy, environmental scanning electron microscopy and the Wst-1 assay (an enzymatic assay for mitochondrial dehydrogenase activity). Cells were able to adhere, attain a normal morphology, proliferate and remain viable when cultured on the new composite graft (CPC-PLGA) or on a control graft (CPC alone). These results suggest that cement consisting of CPC and PLGA microspheres is biocompatible.

In the study of Xiaoyan Lin, et al, nano-composite of bone-like apatite/collagen was prepared by a new method—low-temperature in situ synthesis using calcium nitrate, diammonium hydrogen phosphate and cow hide collagen as starting materials. The composite was investigated via X-ray diffraction (XRD), Fourier transform infrared (FTIR), SEM and transmission electron microscopy (TEM). It was found that bone-like nano apatite particles were distributed uniformly in collagen fibrils in the composite. The composite with homogeneous microstructure was similar to natural bone in crystallite phase composition and crystal size. In one more study of Khaled R. Mohamed, et al, nano-sized particles of calcia stabilized zirconia (CSZ) was used as filler in poly(hydroxyethylmethacrylate-methylmethacrylate) grafted onto chitosan copolymer to produce a bioactive composites analogous to bone. Results confirmed that the grafting percentage of CSZ-copolymer composite was enhanced compared to the copolymer as a result of nano-sized filler. Thermo-gravimetric analysis (TGA) proved the presence of attached copolymer layer onto the filler particles for CSZ-copolymer composite. Swelling properties was reduced for CSZ-copolymer composite proving the stability and lower affinity of this composite to water molecules. In vitro tests indicate that the adsorption of calcium ions (Ca2+) and phosphate ions (PO43-) on the surface of the composites are enhanced. That was confirmed by the formation of a bone-like apatite layer. FTIR spectrophotometer post-immersion confirmed the formation of carbonate-apatite layer onto the surface of the copolymer and CSZ-copolymer composite at 3 and 21 days, respectively. SEM post-immersion showed enhanced bone-like apatite layer (calcium-phosphate layer) onto the CSZ-polymer composite compared to the copolymer. Subsequently, the formation of a bone-like apatite layer is found to be controlled by the change in content of CSZ filler.

Joint prosthesis

The use of composite materials in artificial joints is a promising research field. The implant properties can be varied and tailored to suit the correct mechanical and physiological response.

Hips

Polymer composites of Al-Cu-Fe quasicrystals and ultra-high molecular weight polyethylene (UHMWPE) are used in acetabular cup prosthetics. The wear properties of the Al-Cu-Fe/UHMWPE samples and a 440 steel ball counterface were measured in study carried by Brian C. Anderson, et al. The mechanical strength of the Al-Cu-Fe/ UHMWPE composites was compared to UHMWPE and alumina/UHMWPE. The biocompatibility of the composite material was tested using a direct contact cytotoxicity assay. Al-Cu-Fe/UHMWPE demonstrated lower volume loss after wear and higher mechanical strength than UHMWPE. This composite material also showed no increase in counterface wear or cytotoxicity relative to UHMWPE. These combined results demonstrate that Al-Cu-Fe/UHMWPE composites are promising candidate materials for acetabular cup prosthetics.

In study of Qingliang Wang, et al, UHMWPE composites reinforced with Bovine Bone HA in different contents were prepared by heat pressing formation method. A hip joint wear simulator was used to investigate the biotribological behavior of UHMWPE/ BHA composite acetabular cups against CoCrMo alloy femoral heads in bovine synovia lubrication at 37 ±1 °C. It was found that the addition of BHA powder to UHMWPE can improve the hardness and creep modulus of UHMWPE/BHA composites, and decrease their wear rates under bovine synovia lubrication. When the content of BHA filler particles was up to 30 wt%, UHMWPE/ BHA composites demonstrated the well design performances of the surface and biotribological properties. Fatigue, ploughing and slight adhesive wear were the main wear mechanisms for UHMWPE and its composites. In addition, the sizes of wear particles became larger with an increase in BHA powder addition. These results suggest that BHA filler is a desirable component to increase the wear resistance of UHMWPE/BHA composites for biomedical applications.

Polymer composites are potentially useful materials for orthopedic applications as they can be tailored to closely match the various moduli of bone. The epiphyseal cup has been designed using one such material, carbon-fibre-reinforced polybutyleneterephthalate (CFRPBT), so that more of the load can be transferred to bone in order to reduce bone loss around the implant. CFRPBT showed no toxicity in bulk form and only minimal toxicity as a particulate. It was non-hemolytic, non-immunogenic and showed no genotoxicity. No adverse responses were seen after in vivo implantation, and the results obtained following 9 months of clinical evaluation were very encouraging with a good clinical outcome and good bone bonding to the hydroxyapatite-coated CFRPBT. In total hip arthroplasty, concerns such as corrosion and stress shielding associated with stiff metallic femoral components have led to the development of low stiffness advanced fibre-reinforced polymer (FRP) composite femoral components. Carbon fibre-reinforced Polyetheretherketone (CF/PEEK) composite material is now one of the primary material systems being considered for composite hip stem development. As a hip stem, a composite material must be able to support a complex state of stress in the in vivo environment without failure. Considering the loading conditions of a hip stem (superimposed compression and bending), and the fact that FRP composites typically possess lower compressive than tensile strength, the compressive behavior of FRP composite becomes very important for femoral component design. Extensive hip joint simulator tests were conducted to optimize the microstructure of the composite and the counterface material. A softer and more graphitic carbon fiber is preferred to a harder and more abrasive fiber. A ceramic counterface is preferred to a metal counterface. When tested on a hip simulator run for 10 million cycles, a reduction in the wear rate of almost two orders of magnitude was achieved with this wear couple in comparison to a conventional UHMWPE/metal or UHMWPE/ceramic couple.

Carbon-fiber-reinforced-carbon composite material is an attractive implant material because its modulus of elasticity can be made similar to that of cortical bone. Distal, as well as proximal, relative micro movements between implant and bone were measured in two testing protocols (axial-load and torsional-load), comparing identically shaped carbon composite (modulus of elasticity=18.6 GPa), Ti6Al4V (100 GPa), and 630 stainless steel (200 GPa) prostheses. In the axial-load test, proximal mediolateral micro motions were significantly larger in the flexible composite stem than in the two metals. In the torsional-load test, rotational micro motions and “slop” displacements in the flexible stem were significantly larger proximally and significantly smaller distally than in the two metals. While these results suggest that proximal stress transfer may be improved by a flexible stem, they raise the possibility of increased proximal micro motion, and suggest that improved proximal fixation may be necessary to achieve clinical success with flexible composite femoral components. Both High-density polyethylene (HDPE) and UHMWPE have long been used successfully as socket materials in hip-joint replacements. Recently, however there are concerns over the adverse biological responses due to the wear debris of these polymers. HA-collagen composites have been prepared by precipitation of calcium phosphate on collagen in the past but very few of these attempts considered the mechanical strength of the composites that suits their realistic uses as implant material. Hybrid composites of HA-collagen-hyaluronic acid or gelatin have been developed with sufficient adherence to both hard and soft tissues and also with good cohesive strength leading to improved mechanical and biological properties. It was possible to prepare acetabular cups of the newly developed composites by compression molding for tests on a hip-joint simulator. Pin specimens for tests on a pin-on-disc apparatus were also molded with these composites. Tests with the acetabular cups and pin specimens indicate that some of the newly developed materials offer wear resistance comparable to those of the presently used socket materials. Biocompatibility tests with these materials show that their haemolysis counts are well below the acceptable range. HA-collagen composites with 10% hyaluronic acid offer suitable mechanical strengths, good friction and wear characteristics and acceptable level of haemolysis and therefore the composite may be considered to be a potential socket material of future generation.

Composite materials have been investigated also as coatings or as the main constituent for femoral stems. The fabrication process for a novel carbon fiber-reinforced polymer (polyamide 12) composite femoral stem using inflatable bladder molding was studied. Effect of processing temperature, holding time and applied internal pressure on the consolidation quality of the composite was investigated. Consolidation quality was evaluated by density and void content measurements and scanning electron microscope analysis. As expected, void content (porosities) and presence of large resin pockets were found to increase for lower processing temperature, holding time and applied pressure. Crystallinity as well as melting temperatures measured using differential scanning calorimetry could be related to molding conditions. A progressive reduction of the previous thermal history (crystalline peak of neat composite) and an increase in crystallinity were obtained for higher molding temperature. Static compression testing with void content analysis of molded specimens was used to determine optimal molding conditions. The composite structure molded showed compressive modulus close to cortical bones. Compression load at failure of composites molded in optimal conditions were found to be three times higher than those of femoral bone for jumping on one leg or 10 times those for normal gait. The molded composite structure appears to be an excellent candidate for femoral stems used in total hip arthroplasty.

Composite material can also be used as femoral head material. Dense composite laminates of alumina (Al2O3) and tantalum (Ta) were fabricated by hot pressing and tested in vitro for potential use as a femoral head material in total hip arthroplasty (THA). Al2O3-Ta composite laminates hot pressed at 1450 °C and 1650 °C had flexural strengths of 940 ± 180 MPa and 1090 ± 340 MPa, respectively, which were far larger than the values of 420 ± 140 MPa and 400 ± 130 MPa for Al2O3 alone hot pressed at 1450 °C and 1650 °C, respectively. The interfacial shear strength, determined by a double-notched specimen test, was 310 ± 80 MPa for the composite laminate hot pressed at 1650 °C, indicating strong interfacial bonding between Al2O3 and Ta. SEM, energy dispersive X-ray (EDS) analysis, and X-ray mapping of polished sections of the hot-pressed laminates showed the presence of an interfacial region formed presumably by diffusion of O (at 1450 °C) or O and Al (1650 °C) from Al2O3 into Ta. Composite femoral heads of Al2O3 and Ta could combine the low wear of an Al2O3 articulating surface with the safety of a ductile metal femoral head.

Composite of Al2O3 and niobium (Nb) was tested in vitro for potential use as an alternative femoral head material in vivo. Dense composite laminates of Al2O3 and Nb were fabricated by hot pressing, and their microstructure and mechanical properties were evaluated. The flexural strength of Al2O3-Nb laminates in four-point loading was 720 ± 40 MPa, compared with a value of 460 ± 110 MPa for Al2O3 alone. SEM and XRD showed a well-bonded interface between the Al2O3 and Nb without measurable formation of an interfacial reaction phase. The interfacial shear strength between Al2O3 and Nb, measured by a double-notched specimen test, was 290 ± 15 MPa. The feasibility of fabricating prototype femoral heads (32 mm in diameter), consisting of an Al2O3 surface layer (2-3 mm thick) and an Nb core, by hot pressing was shown. The composite femoral head combined the low wear of an Al2O3 articulating surface with the safety of a ductile metal femoral head.

Knee

The knee joint is the most exposed to traumatic and degenerative injuries. Knee geometry and kinematics are more complex than those of hip, even if with lower loads. This is why different geometries and types of knee joint replacements have been developed and the long-term success of the implants is lower than for hip replacements. Wear of the UHMWPE components in total knee arthroplasty is a potential long-term problem. Ninety total knees of various designs with implant times up to 10 years were retrieved. The wear noted in the majority of components was much greater than that noted in wear studies of acetabular components in total hip prostheses. Abrasions from cement or bone and delamination wear were particularly pronounced in the knee. Delamination, consisting of complete breakup of material in flakes and particles, appeared to be initiated by intergranular material defects and propagated by the excessive subsurface stresses beneath the contact zone. Material that was free of defects did not show delamination wear even after long time periods in a highly stressed, low-conformity design. Wear particles of UHMWPE can result in adverse tissue reaction with cellulites, giant cell reaction, and necrotic tissue and these effects could be cumulative with time. There is some evidence that particles can lead to bone resorption, including at the implant-bone interface, which could accelerate loosening. There is cause for concern as to the long-term effects of UHMWPE in total knee arthroplasty. This suggests the need for improved processing methods or more wear-resistant materials.

Ceramic composites can also be used. Whereas the use of metal components in total knee replacement (TKR), is well established, mechanical loosening in recently introduced ceramic components are a cause of concern. Composite femurs were implanted with commercially available TKR metal components, and with ceramic components having identical shape to the metal ones. Implanted femurs were tested on a knee simulator for up to 5 x 10-6 cycles. Inducible micro motions and permanent migrations were recorded throughout the test. The cement layers were inspected for signs of damage or fracture. Micro motions and migrations were similar for metal and ceramic components: their magnitude and trend over time indicated that no implant was becoming loose. When there were statistically significant differences, the ceramic components were more stable than the metal ones. When the cement layers were inspected, a few short cracks were observed; most such cracks appeared during the first cycles, while no further damage occurred in the rest of the test. The type of damage found for both the metal and the ceramic components is compatible with well-fixed implants after long-term cycling. Altogether, no remarkable difference was found between the metal and ceramic components. Therefore, the study of Cristofolini, et al rejects the hypothesis that ceramic TKR femoral components are more prone to mechanical loosening.

Bone cement

Bone cement chemically is nothing more than Plexiglas (i.e. polymethyl methacrylate or PMMA). Bone cements have been used very successfully to anchor artificial joints (hip joints, knee joints, shoulder and elbow joints) for more than half a century. The bone cement fills the free space between the prosthesis and the bone and plays the important role of an elastic zone. This is necessary because the human hip is acted on by approximately 10-12 times the body weight and therefore the bone cement must absorb the forces acting on the hips to ensure that the artificial implant remains in place over the long term. The water absorption behavior of bone-cements of PMMA reinforced with 1.5 wt.%, 2.5 wt.%, and 3.5 wt.% silane-treated powders of HA (synthetic or derived from bovine bones, or teeth enamel), P-tricalcium phosphate (TCP), bioactive glass (45S5), and zirconia, was investigated by Daglilar S., et al. The experimental results showed that addition of calcium phosphate in the polymeric matrix favors water absorption except in the case of enamel HA. Water uptake is also suppressed when 45S5-bioglass or zirconia is added. The solubility is not affected by addition of HA or 45S5-bioglass but it increases when P-TCP or zirconia is added. It suggests the superior behavior of bone-cements reinforced with bioglass-45S5 and enamel HA.

HA composite resin (CAP) is a new type of bioactive bone cement. CAP is composed of 80% w/w HA granules (mean particle size: 2^m) and bis-phenol-A glycidyl methacrylate-based resin. The setting time is 5 min and the peak curing temperature during polymerization is 46°C. In the study of Masanobu Saito, et al, the mechanical strength of CAP and the biological behavior of the CAP-bone interface were examined. The compressive strength of CAP was 260 MPa and this was about three times greater than that of commercial PMMA bone cement. The tensile strength and fracture toughness of CAP also exceeded those of PMMA cement. CAP was implanted into the femoral condyles of rabbits. Two weeks later, new bone formation was already seen on the surface of the CAP implants. At 8 wk, bone was growing directly onto the surface of the CAP implants and no intervening fibrous tissue could be observed at the CAP-bone interface. These results show that CAP is a promising material which possesses superior mechanical strength and the biological property of achieving direct contact with bone.

Bioactive composite bone cements were obtained by incorporation of tricalcium silicate (Ca3SiO5, C3S) into a brushite bone cement composed of P-tricalcium phosphate [P-Ca3(PO4)2, P-TCP] and monocalcium phosphate monohydrate [Ca(H2PO4)2H2O, MCPM], and the properties of the new cements were studied and compared with pure brushite cement. The results indicated that the injectability, setting time and short and long-term mechanical strength of the material are higher than those of pure brushite cement, and the compressive strength of the TCP/MCPM/C3S composite paste increased with increasing aging time. Moreover, the TCP/MCPM/C3S specimens showed significantly improved in vitro bioactivity in simulated body fluid and similar degradability in phosphate-buffered saline as compared with brushite cement. Additionally, the reacted TCP/MCPM/C3S paste possesses the ability to stimulate osteoblasts proliferation and promote osteoblastic differentiation of the bone marrow stromal cells. The results indicated that the TCP/MCPM/C3S cements may be used as a bioactive material for bone regeneration, and might have significant clinical advantage over the traditional PTCP/MCPM brushite cement. Self-curing two-paste bone cements have been developed using methacrylate monomers with a view to formulate cements with low polymerization exotherm, low shrinkage, better mechanical properties, and improved adhesion to bone and implant surfaces. The monomers include bis-phenol. A glycidyl dimethacrylate (bis-GMA), urethane dimethacrylate (UDMA) and triethylene glycol dimethacrylate (TEGDMA) as a viscosity modifier. Two-paste systems were formulated containing 60% by weight of a bioactive ceramic, HA. A methacroyloxy silane (A174) was used as a coupling agent due to its higher water stability in comparison to other aminosilanes to silanate the HA particles prior to composite formulation. A comparison of the FT-infrared spectrum of HA and silanated HA showed the presence of the carbonyl groups ( 1720 cm4), -C=C-( 1630 cm4) and Si-O- (1300-1250 cm-1) which indicated the availability of silane groups on the filler surface. Two methods of mixing were affected to form the bone cement: firstly by mixing in an open bowl and secondly by extruding the two pastes by an auto-mixing tip using a gun to dispense the pastes. Both types of cements yielded low polymerization exotherm with good mechanical properties; however, the lower viscosity of UDMA allowed better extrusion and handling properties. A biologically active apatite layer formed on the bone cement surface within a short period after its immersion in simulated body fluid, demonstrating in vitro bioactivity of the composite. This preliminary data thus suggests that UDMA is a viable alternative to bis-GMA as a polymerizable matrix in the formation of bone cements.

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