Abstract
Objective
Although metal implants have successfully been used for decades, devices made out of metals do not meet all clinical requirements, for example, metal objects may interfere with some new medical imaging systems, while their stiffness also differs from natural bone and may cause stress-shielding and over-loading of bone.
Methods
Peer-review articles and other scientific literature were reviewed for providing up-dated information how fiber-reinforced composites and bioactive glass can be utilized in implantology.
Results
There has been a lot of development in the field of composite material research, which has focused to a large extent on biodegradable composites. However, it has become evident that biostable composites may also have several clinical benefits. Fiber reinforced composites containing bioactive glasses are relatively new types of biomaterials in the field of implantology. Biostable glass fibers are responsible for the load-bearing capacity of the implant, while the dissolution of the bioactive glass particles supports bone bonding and provides antimicrobial properties for the implant. These kinds of combination materials have been used clinically in cranioplasty implants and they have been investigated also as oral and orthopedic implants.
Significance
The present knowledge suggests that by combining glass fiber-reinforced composite with particles of bioactive glass can be used in cranial implants and that the combination of materials may have potential use also as other types of bone replacing and repairing implants.
1
Introduction
Biodegradable and biostable medical and dental composite materials have been developed considerably in recent decades. Currently, they can be used in many applications in reconstructive medicine. Although metal implants have been used successfully for many years, devices made out of metals do not meet all biomechanical requirements, such as isoelasticity of skeleton and bone and may lead to insufficient (stress-shielding) or over-loading situations around the implant . This problem has been recognized specifically when used as metal implants in long bones. Metal implants may also induce cytotoxic reactions arising from the release of corrosion products of metal ions, and nanoparticles . In addition, metallic objects interfere with medical diagnostics when using magnetic resonance imaging and do not allow postoperative radiation therapy to be performed . In contrast, durable and tough non-metallic composites can be made from high-aspect ratio fillers, namely fibers embedded in a polymer matrix. The first studies using fiber-reinforced composites (FRCs) in medicine and dentistry occurred in the early 1960s, but more extensive research started in the early 1990s . Introduction of FRCs as prosthodontic material in dentistry occurred in larger scale at the end of the 1990s. Applications of FRCs cover several fields of dentistry from restorative dentistry to prosthodontics, but fibers are also used in orthodontics and periodontology . Besides the dental applications, FRCs have started to be used clinically as well in implant dentistry, with the first approved clinical applications in cranial surgery . Research to develop oral and orthopedic implants based on FRC is ongoing. Implant applications utilize certain biomechanical properties of FRC, and benefit from the possibility of incorporating additional bioactive components to the implant structure. Particulates of bioactive glass have proved its suitability in this purpose . FRC materials in implantology have been focused initially on cranial implants because nonmetallic implants enable the increasing utilization of magnetic resonance imaging in identifying a large number of infections related to autologous bone flaps and implants . In implant dentistry, radiopaque materials like titanium and zirconia cause severe artifacts in cone beam computer tomography images .
Regardless of the location of the bone replacing or bone anchoring implant (maxillofacial, cranial or long bones), the material of the implant and implant device have to fulfill the requirements of permanently implantable medical devices of the European Directive classes 2B or 3. If the implant has an active role in tissues through its components, such as bioactive glass, the implant belongs to class 3 medical devices. Bioactive glass is considered an active component because of antimicrobial nature, which is a desired property for implants. This review describes the present status of the development and use of non-metallic glass FRC – bioactive glass implants.
2
Structure and material components of FRC
FRC is made of reinforcing fibers embedded in a polymer matrix. The glass fiber properties tensile strength and elongation at break partly control the reinforcing capacity of the fibers in polymers. However, several other factors such as fiber orientation and length, fiber adhesion to the polymer matrix, and volume fraction of fibers in the polymer matrix also contribute to the strength of the composite . Correct material selection, through composite fabrication process, and especially correct design, enables the utilization of FRC in applications where high static and dynamic strength are required. In implant applications, fibers have been used as continuous unidirectional fibers or bidirectional fabric type arrangements according to the loading conditions. Anisotropicity of continuous unidirectional fibers limits their use in applications where direction of load is not known or cannot be predicted, but then continuous unidirectional fibers provide the highest possible reinforcing efficiency (Krenchel’s factor) against the known direction of stress . Oral and orthopedic implants, which are under development are manufactured using continuous unidirectional fibers whereas continuous bidirectional fibers are utilized in calvarial implants ( Figs. 1 and 2 ).
2.1
Resin matrix
Thermoplastic and thermoset resins are used in FRC implants. Examples of thermoplastics with biomedical applications are polyethylene (PE), polyetheretherketone (PEEK), polyacetal (PA), and polyurethane (PU). Examples of thermosets which are utilized as biomaterials are epoxy polymer, bis-glycidyl-A-dimethacylate (Bis-GMA), and triethyleneglycoldimethacryalate (TEGDMA) copolymer. Methacrylated dendritic polyesters have been tested also as resin matrix for biomedical FRCs with good success . Thermosets exhibit high cross-linking density, which makes them stiffer and more fragile compared to thermoplastics. Mechanical properties of materials used in implants and properties of bone are presented in Table 1 .
| Material | Modulus (GPa) | Tensile strength (MPa) |
|---|---|---|
| Polymers | ||
| Polyethylene (PE) | 0.9 | 35 |
| Polyetheretherketone (PEEK) a | 8.3 | 139 |
| Polyacetal (PA) | 2.1 | 67 |
| Polyurethane (PU) | 0.02 | 35 |
| Polymethylmethacarylate (PMMA) a | 2.5 | 59 |
| Polytetrafluoroethylene (PTFE) | 0.5 | 28 |
| Polyethylene terepthalate (PET) | 2.8 | 61 |
| Bis-GMA-TEGDMA copolymer a | 8.0 | 52 |
| Fiber-reinforced composites | ||
| PEEK-carbon/graphite fiber | ||
| Glass fiber/Bis-GMA-TEGDMA | ||
| Unidirectional (60 vol%) | 20 | 1200 b |
| Bidirectional (45°/45°) (60 vol%) | 8 | 400 b |
| Metals | ||
| Stainless steel | 190 | 586 |
| Cobalt-chromium alloy | 210 | 1085 |
| Titanium alloy | 116 | 965 |
| Hard tissues | ||
| Cortical bone (longitudinal) | 17.7 | 133 |
| Cortical bone (transversal) | 12.8 | 52 |
| Cancellous bone | 0.4 | 7.4 |
| Enamel | 84.3 | 10 |
| Dentin | 11.0 | 39.3 |
a Polymers have been used as matrix polymer for FRC implants.
Resin matrix for the reinforcing fibers is cured in contact to the reinforcing glass fibers (thermosets), or melted or dissolved for impregnation of fibers (thermoplastics). By sizing the hydroxyl group covered surface of glass fibers with silane coupling agent and by using monomer resin system of thermoset resin, the chemical adhesion of polymer matrix to fibers can be obtained. In melted thermoplastics, however, reinforcing fibers are attached only physically to the surface .
Polymerization reaction of monomer systems of thermoset resins is based on free radical (vinyl) polymerization. Initiation of the polymerization is made by radiation of blue light or by increasing temperature, resulting to the monomer conversion degree of 60–65% by light activation only, while 85–90% can be achieved in post-polymerization by heat . Higher monomer conversion reduces the quantity of leachable residual monomers and thus improves biocompatibility of the polymer . Optimal post-curing temperature is close to the glass transition temperature of the polymer, in which enough thermal energy is available to create free volume, enabling unreacted carbon–carbon double bonds to form free radicals and react with each other .
2.2
Reinforcing fibers
Reinforcing fibers in biomedical FRCs are made out of nonresorbable glass (E-glass, electrical glass; S-glass, high strength glass) and carbon/graphite. Chemically durable alumino borosilicate glass fibers are commonly known as E-glass fibers. E-glass fibers are used mainly to reinforce polymer matrix composites and are considered inert in body environments. E-glass composition differs from melt-derived bioactive glass compositions by the low alkali oxide content (<2 wt%) and high Al 2 O 3 (14 wt%) and B 2 O 3 contents (10 wt%). Changes in the fiber composition, silanation, and modifications of the fiber surface composition have facilitated better adhesion of fibers to the polymer matrix. Glass fibers presently used in bone implants are of type E- or S-glass, with fiber diameter of 14–20 μm. Fibers are used as unidirectional rovings and bidirectional woven fabrics. Unidirectional fibers have been tested in long bone applications, such as treatments of segmental defects and in oral implants, whereas woven fabrics are used in cranial implants replacing lamellar bone. Although fiber loading in the composites is between 60 and 65 vol%, allowing high anisotropic flexural strength (up to 1200 MPa) for the material, the final durability of the implant device is related to the fiber geometry and design of the FRC implant. For this reason, presently used cranial implants combine properties of woven fiber fabrics and unidirectional bars in the implant device.
The composition of glass plays a decisive role in the manufacturing of fibers and in chemical stability of fibers against effects of moisture. Chemical resistance of glasses toward glass corrosion is related to the bulk and surface composition, the state of the glass surface, the amount, flow rate, and type of attacking solvent medium, as well as the temperature and time. In the presence of water, the strength of the fiber may be reduced, especially with glass fibers of high alkali metal oxide content . In general, continuous fibers can be drawn from several glass compositions. The biostable fibers, most suitable to be used in dental and medical applications, are drawn from so-called alkali-free glasses ( Table 2 ). Certain glass forming agents such as B 2 O 3 are easily leached from the fiber surface. This may lead to local breakage of the glass network and fibers. In the fiber processing, easily leaching species may be eliminated in order to increase the stability of the final fibers. In an acidic environment, incongruent dissolution from the fiber surface leads to differences in the surface and interior composition of the fiber. Continuous and short glass fibers in dental FRC products are usually made of alkali-free glass (up to 1% Na 2 O + K 2 O), known as E-glass. E-glass is based on the system SiO 2 –Al 2 O 3 –CaO–MgO, which has good glass forming ability. Because of the high calcium oxide content, glasses in this system show poor chemical resistance in acidic solutions. For this reason, the composition of E-glass has been modified by introducing boron oxide (B 2 O 3 ) and by decreasing the CaO content. Thus, the E-glass composition is described often as aluminum borosilicate glass. The glass composition typically used in dental FRCs is S-glass, which provides slightly higher tensile strength than E-glass. S-glass composition belongs to the system magnesium aluminosilicates. Long-term success of the reconstructive medical composite materials in biological environment depends on hydrolytic stability of the composite. Hydrolytic stability is dependent on the stability of polymer matrix, stability of fillers, and stability of the interface between fillers and polymer matrix. It is known that good quality and surface purified E-glass fiber itself exhibits good stability in solutions with pH between 4 and 10 . A high quality glass FRC has flexural strength of above 1200 MPa and the only known reduction in the mechanical properties in the time span of ten years occurs during the first 30 days immersion in water by plasticizing action of water molecules in polymer matrix . There is a positive correlation between the water sorption of polymer matrix and reduction of flexural properties. For instance, high water sorption of polyamide (nylon) matrix of FRC causes reduction of 50% in the flexural properties . Reduction in the flexural properties by presence of plasticizing water is, however, reversible.
| Component | Type of glass | ||||
|---|---|---|---|---|---|
| E | S | 45S5 | S53P4 | 13-93 | |
| SiO 2 | 53–55 | 62–65 | 45 | 53 | 53 |
| Al 2 O 3 | 14–16 | 20–25 | – | – | – |
| CaO | 20–24 | – | 24.5 | 20 | 20 |
| MgO | 20–24 | 10–15 | – | – | 5 |
| B 2 O 3 | 6–9 | 0–1.2 | – | – | – |
| K 2 O | <1 | 0–1.1 | – | – | 12 |
| Na 2 O | <1 | 0–1.1 | 24.5 | 23 | 6 |
| Fe 2 O 3 | <1 | 0.2 | – | – | – |
| P 2 O 5 | 6 | 4 | 4 | ||
2.3
Biological considerations
Biocompatibility of biostable composites correlates with biocompatibility of the polymer matrix, reinforcing fibers and their combination with surface sizing of fibers, as well as the additional bioactive modifiers. In general, biocompatibility indicates the biological performance of the material in terms of structural and surface compatibility. Structural compatibility again refers to the biomechanical properties of the implant material, such as modulus of elasticity and strength, and implant design. These give the stiffness, which is the product of the elastic modulus and the second moment of area of the structure, and optimal load transmission at the implant tissue interface (minimum interfacial strain mismatch and load transfer). Metals and ceramics have higher elastic modulus than dentin and bone, providing an example of one of the problems in orthopedic surgery: the mismatch of stiffness between the bone and metallic implant. Thus, in the presence of an implant with a higher stiffness, the bone will be insufficiently loaded, and undergoes so-called stress-shielding . Stress-shielding affects bone remodeling and healing, leading to increased bone atrophy. Adequate design of FRC structures enables implants with bone-like modulus of elasticity and high strength. Thus, by controlling the volume fraction and arrangement of the fibers, the properties and designs of FRCs can be varied and tailored to suit the mechanical and physiological conditions of the host tissues. Thus, FRCs can transfer interfacial stresses and strains between the implant and bone more evenly than, for example, titanium implants. Further, using FRCs instead of metal implants eliminates formation of corrosion products and release of metal ions and metal nanoparticles, which may predispose aseptic loosening of the implant and unwanted tissue reactions. When used in load bearing areas, tribological properties of the composites and fixation devices must be optimized to eliminate any potential risk of debris formation from the implant.
Thermoset polymer FRC, made of epoxy polymers and of dimethacrylate polymers, has been criticized regarding the potential toxic and sensitizing (allergic) effects of its monomers, which are present as residuals in the FRC . In contrast, thermoset polymers made of dimethacrylate monomer systems of Bis-GMA and TEGDMA have shown good biocompatibility after polymerization at elevated temperatures without presence of oxygen, although potential risk of residual monomer effects have been discussed. It has been demonstrated that hydroxylated metabolites of residual monomers of Bis-GMA were non-mutagenic, non-estrogenic and less cytotoxic than their parent monomers .
Biological testing of E-glass containing FRC with dimethacrylate thermoset polymer matric by cell cultures and animal testing have shown acceptable biocompatibility. E-glass fibers having silane containing surface sizing without resin matrix have shown no signs of cytotoxity in fibroblast or osteoblast cell culture studies . In another study, rat bone-marrow derived osteoblast-like cells were harvested and cultured on experimental FRC plates and on commercially pure titanium plates as control . Cell growth and differentiation kinetics were subsequently investigated by evaluating proliferation, alkaline phosphatase activity, osteocalcin, and bone sialoprotein production. The maximal alkaline phospatase activities on FRC and titanium were observed after three weeks. Expression of osteoblastic markers (osteocalsin and sialoprotein production) indicated that the fastest osteogenic differentiation took place on FRC after seven days. In contrast, a slower differentiation process was observed on titanium than on FRC, as was confirmed by increased mRNA expression of ostecalsin and sialoprotein production. It was concluded that the proliferation and maturation of osteoblast-like cells on FRC appear to be comparable to titanium. In addition, presence of bioactive glass enhanced cell maturation.
2
Structure and material components of FRC
FRC is made of reinforcing fibers embedded in a polymer matrix. The glass fiber properties tensile strength and elongation at break partly control the reinforcing capacity of the fibers in polymers. However, several other factors such as fiber orientation and length, fiber adhesion to the polymer matrix, and volume fraction of fibers in the polymer matrix also contribute to the strength of the composite . Correct material selection, through composite fabrication process, and especially correct design, enables the utilization of FRC in applications where high static and dynamic strength are required. In implant applications, fibers have been used as continuous unidirectional fibers or bidirectional fabric type arrangements according to the loading conditions. Anisotropicity of continuous unidirectional fibers limits their use in applications where direction of load is not known or cannot be predicted, but then continuous unidirectional fibers provide the highest possible reinforcing efficiency (Krenchel’s factor) against the known direction of stress . Oral and orthopedic implants, which are under development are manufactured using continuous unidirectional fibers whereas continuous bidirectional fibers are utilized in calvarial implants ( Figs. 1 and 2 ).
2.1
Resin matrix
Thermoplastic and thermoset resins are used in FRC implants. Examples of thermoplastics with biomedical applications are polyethylene (PE), polyetheretherketone (PEEK), polyacetal (PA), and polyurethane (PU). Examples of thermosets which are utilized as biomaterials are epoxy polymer, bis-glycidyl-A-dimethacylate (Bis-GMA), and triethyleneglycoldimethacryalate (TEGDMA) copolymer. Methacrylated dendritic polyesters have been tested also as resin matrix for biomedical FRCs with good success . Thermosets exhibit high cross-linking density, which makes them stiffer and more fragile compared to thermoplastics. Mechanical properties of materials used in implants and properties of bone are presented in Table 1 .
| Material | Modulus (GPa) | Tensile strength (MPa) |
|---|---|---|
| Polymers | ||
| Polyethylene (PE) | 0.9 | 35 |
| Polyetheretherketone (PEEK) a | 8.3 | 139 |
| Polyacetal (PA) | 2.1 | 67 |
| Polyurethane (PU) | 0.02 | 35 |
| Polymethylmethacarylate (PMMA) a | 2.5 | 59 |
| Polytetrafluoroethylene (PTFE) | 0.5 | 28 |
| Polyethylene terepthalate (PET) | 2.8 | 61 |
| Bis-GMA-TEGDMA copolymer a | 8.0 | 52 |
| Fiber-reinforced composites | ||
| PEEK-carbon/graphite fiber | ||
| Glass fiber/Bis-GMA-TEGDMA | ||
| Unidirectional (60 vol%) | 20 | 1200 b |
| Bidirectional (45°/45°) (60 vol%) | 8 | 400 b |
| Metals | ||
| Stainless steel | 190 | 586 |
| Cobalt-chromium alloy | 210 | 1085 |
| Titanium alloy | 116 | 965 |
| Hard tissues | ||
| Cortical bone (longitudinal) | 17.7 | 133 |
| Cortical bone (transversal) | 12.8 | 52 |
| Cancellous bone | 0.4 | 7.4 |
| Enamel | 84.3 | 10 |
| Dentin | 11.0 | 39.3 |
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