Restorative materials may cause reactions in the oral soft tissues such as the gingiva. It is not clear how much of the in vivo cytotoxicity observed is caused by the restorative materials and how much is caused by products of bacterial plaque that accu- mulate on teeth and restorations. In general, con- ditions that promote retention of plaque, such as rough surfaces or open margins, increase inflam- matory reactions around these materials. However, released products from restorative materials also contribute either directly or indirectly to this inflam- mation. This is particularly true in areas where the washing effects of saliva are minimal, such as in interproximal areas, in deep gingival pockets, or under removable appliances. Several studies have documented increased inflammation or recession of gingiva adjacent to restorations where plaque indi- ces are low. In these studies, released products from materials could cause inflammation in the absence of plaque or could inhibit formation of plaque and cause inflammation in gingiva. In vitro research has shown that components from dental materials and plaque may synergize to enhance inflammatory reactions.
Cements exhibit some soft-tissue cytotoxicity in the freshly set state, but this decreases substantially over time. The buffering and protein-binding effects of saliva appear to mitigate these cytotoxic effects.
Resin composites in direct contact with fibroblasts are initially very cytotoxic in vitro. This cytotoxicity most likely results from unpolymerized components in the air-inhibited layer that leach out from the mate- rials. Other in vitro studies, in which the composites were aged in artificial saliva for up to 6 weeks, have shown that toxicity diminishes with some materials but remains high for others. Some composites with non-Bis-GMA and non-UDMA matrices have sig- nificantly lower cytotoxicity in vitro, presumably because of lower amounts of leached components.
Polished composites show markedly less cytotoxic- ity in vitro, although some materials are persistently toxic even in the polished state.
Recently, there has been significant controversy about the ability of bisphenol A and bisphenol A dimethacrylate to cause estrogen-like responses in vitro. These compounds are basic components of many commercial composites. However, there is no evidence that xenoestrogenic effects are a con- cern in vivo from any commercial resin. Relatively little is known about other in vivo effects of released components of composites on soft tissues, although the concerns are similar to those regarding denture base resin and soft liners (see later discussion in this section). There is some evidence that methacrylate- based composite components may cause significant rates of hypersensitivity, although few clinical trials exist.
Amalgams have been extensively used for 150 years.
In spite of its substantial history, however, periodically concern arises about the biocompatibility of amalgam.
Allergic reactions to amalgam restorations are rare, although there are case reports of allergic contact der- matitis, gingivitis, stomatitis, and remote cutaneous reactions. Such responses usually disappear in a few days or, if not, on removal of the amalgam or with use of a cavity liner. Other local or systemic effects from mercury contained in dental amalgam have not been demonstrated. No well-conducted scientific study has conclusively shown that dental amalgam, placed and used correctly, produces any ill effects. Despite this, a global consensus has been reached to phase down the use of mercury in all industries, dentistry included.
Thus amalgam use continues to decline throughout the world, and this in large part is due to environmen- tal concerns over mercury contamination in the air, water, and soil.
In patients with oral lesions near amalgam sites, positive patch tests have been reported. However, the appropriate patch test has still not been determined.
Amalgam restorations carried into the gingival crev- ice may cause inflammation of the gingiva because
of products of corrosion or bacterial plaque. Seven days after placing an amalgam, a few inflammatory cells appear in the gingival connective tissue, and hydropic degeneration of some epithelial cells may be seen. Some proliferation of epithelial cells into the connective tissue may also occur by 30 days, and chronic mononuclear cell infiltration of connective tissue is evident. Increased vascularity persists, with more epithelial cells invaginating into the connec- tive tissue. Some of these changes may be a chronic response of the gingiva to plaque on the margins of the amalgam. Nevertheless, corrosion products from amalgam cannot be ruled out at this time because implanted amalgams produce similar responses in connective tissues in animals. In addition, although copper enhances the physical properties of amalgam and is bactericidal, it is also toxic to host cells and causes severe tissue reactions in implantation tests.
There is literature that shows that amalgam and resin composites release cytotoxic materials that cause tissue responses, at least at sites of implanta- tion. However, in general, implantation tests show that the material is fairly well tolerated in soft and hard tissues. For materials that are placed where they are rinsed in saliva, these cytotoxic agents are prob- ably washed away before they harm the gingiva.
However, rough surfaces on these types of resto- rations have been associated with increased inflam- mation in vivo. Usage tests in which restorations were extended into the gingival crevice have shown that finished materials gave a much milder inflam- matory response than unfinished materials. The detrimental effect of surface roughness has been attributed to the increased plaque retention on these surfaces. However, rough surfaces on alloy restora- tions have also been shown to cause increased cyto- toxic effects in vitro, where plaque was absent. This and other in vitro studies would again suggest that the cytotoxic response to alloys may be associated with release of elements from the alloys, and that the increased surface area of a rough surface may enhance release of these elements.
In another series of studies, low- and high-copper amalgam powders and various phases of amalgam were implanted subcutaneously in guinea pigs.
After 1.5 to 3 months, fine secondary particles con- taining silver and tin were distributed throughout the lesions. These gave rise to macroscopic tattooing of the skin. Secondary material and small, degrad- ing, primary particles from both types of amalgam were detected in the submandibular lymph nodes.
Elevated mercury levels were detected in the blood, bile, kidneys, liver, spleen, and lungs, with the highest concentrations found in the renal cortex. In another study, primates received occlusal amalgam fillings or maxillary bone implants of amalgam for 1 year. Amalgam fillings caused deposition of mercury
in the spinal ganglia, anterior pituitary, adrenal, medulla, liver, kidneys, lungs, and intestinal lymph glands. Maxillary amalgam implants released mer- cury into the same organs, except for the liver, lungs, and intestinal lymph glands. Organs from control animals were devoid of precipitate. However, neither of these studies, nor any other, has demonstrated any changes in biochemical function of any of the laden organs.
Note that studies using powdered amalgam likely overestimate the amount of breakdown products, and therefore biological response, because the sur- face area of powders can be 5 to 10 times the surface area of a solid component. It must also be empha- sized that any reaction to amalgam, whether in cell culture, local tissue response, or systemic response, does not necessarily imply a reaction to mercury.
Such reactions could be in response to some other constituent of the amalgam or corrosion product. For example, in vitro cell culture testing that measured fibroblasts affected by various elements and phases of amalgams has shown that pure copper and zinc show greater cytotoxicity than pure silver and mer- cury. Pure tin has not been shown to be cytotoxic (Fig. 6.11). The γ1 phase is moderately cytotoxic.
Cytotoxicity is decreased by the addition of 1.5%
and 5% tin (Fig. 6.12). However, the addition of 1.5%
zinc to γ1 containing 1.5% tin increases cytotoxicity to the same level as that of pure zinc. Whenever zinc is present, higher cytotoxicity is revealed. High-copper amalgams show the same cytotoxicity as a zinc-free, low-copper amalgam. The addition of selenium does not reduce amalgam cytotoxicity, and excessive additions of selenium increase cytotoxicity. The cyto- toxicity of amalgams decreases after 24 hours, pos- sibly from the combined effects of surface oxidation and further amalgamation. The results of this study
300 250 200 150
Ag Affected area (mm2)
100 50
0 Sn Cu Zn Hg
Element
FIG. 6.11 Quantitative representation of the affected areas of fibroblasts reveals the magnitude of cytotoxicity of amalgam elements. Standard deviations are represented by vertical bars. Ag, Silver; Cu, copper; Hg, mercury; Sn, tin;
Zn, zinc. (From Kaga M, Seale NS, Hanawa T, et al. Cytotoxicity of amalgams, alloys, and their elements and phases. Dent Mater.
1991;7(1):68–72.)
suggest that the major contributor to the cytotoxic- ity of amalgam alloy powders is probably copper, whereas that for amalgam is zinc.
Casting alloys have a long history of in vivo use with a generally good record of biocompatibility.
Some questions about the biological liability of ele- mental release from many of the formulations devel- oped in the past 10 years have arisen, but there is no clinical evidence that elemental release is a problem, aside from hypersensitivity. Nickel allergy is a rela- tively common problem, occurring in 10% to 20%
of females. It is a significant risk from nickel-based alloys, because release of nickel ions from these alloys is generally higher than for noble or high- noble alloys. Stainless steels, commonly used in pre- formed pediatric crowns and orthodontic appliances, also contain a significant concentration of Ni in their composition. Palladium sensitivity has also been a concern in some countries, although the incidence of true palladium allergy is one-third that of nickel allergy. While it has been clinically documented that patients with palladium allergy are virtually always allergic to nickel, the converse is not true.
Numerous in vitro studies have examined the effects of metal ions on cells in the gingival tissues, such as epithelial cells, fibroblasts, and macrophages.
For the most part, the concentrations of metal ions required to cause problems with these cells in vitro are greater than those released from most casting alloys. However, some recent research has shown that extended exposures to low doses of metal ions may also have biological liabilities. This is notewor- thy because the low-dose concentrations approach those known to be released from some alloys. The clinical significance of this research, however, is not known.
Denture base materials, especially methacrylates, have been associated with immune hypersensitivity
reactions of gingiva and mucosa more than any other dental material. The greatest potential for hypersen- sitization is for dental and laboratory personnel who are exposed repeatedly to a variety of unreacted components. Hypersensitivity has been documented to the acrylic and diacrylic monomers, certain cur- ing agents, antioxidants, amines, and formaldehyde.
For patients, however, most of these materials have undergone the polymerization reaction, and the inci- dence of hypersensitization is quite low. Screening tests for sensitization potential include testing the unreacted ingredients, the polymeric substance after reaction, and oil, saline, or aqueous extracts of the polymer using the in vitro tests previously described.
In addition to hypersensitivity, visible light-cured denture base resins and denture base resin sealants have been shown to be cytotoxic to epithelial cells in culture.
Soft-tissue responses to soft denture liners and denture adhesives are of concern because these mate- rials are used in intimate contact with the gingiva.
Plasticizers, incorporated into some materials to make them soft and flexible, are released in vivo and in vitro. Cell culture tests have shown that some of these materials are extremely cytotoxic and affect a number of cellular metabolic reactions. In animal tests, several of these materials have caused signifi- cant epithelial changes, presumably from the released plasticizers. In usage, the effects of the released plas- ticizers are probably often masked by the inflamma- tion already present in the tissues onto which these materials are placed. Denture adhesives have been evaluated in vitro and show severe cytotoxic reac- tions. Several had substantial formaldehyde content.
The adhesives also allowed significant microbial growth. Newer formulations that add antifungal or antibacterial agents have not yet been shown to be clinically effective.
Reaction of Bone and Soft Tissues to Implant Materials
Interest in the biocompatibility of implant materi- als has grown because the use of implants in clini- cal practice has increased dramatically. Successful dental implant materials either promote osseointe- gration or biointegration (see Dental and Orofacial Implants, Chapter 15).
Reactions to Ceramic Implant Materials
Ceramic materials may be conveniently divided into two groups: bioactive materials and nonbioactive ceramics. Most ceramic implant materials have very low toxic effects on tissues, because they are already in an oxidized state and are highly corrosion resis- tant. As a group, not only are they minimally toxic but they also are nonimmunogenic and noncarci- nogenic. Nonbioactive ceramic materials generally 300
250 A: 1
B: 1 1.5%Sn C: 1 5%Sn D: 1 1.5%Sn 1.5%Zn E: 2 F: Y 200
150
Affected area (mm2) 100
50
0 A B C D E F
Amalgam phase
FIG. 6.12 Quantitative representation of the affected areas of fibroblasts reveals the magnitude of cytotoxicity of amalgam phases. Standard deviations are represented by vertical bars. (From Kaga M, Seale NS, Hanawa T, et al.
Cytotoxicity of amalgams, alloys, and their elements and phases.
Dent Mater. 1991;7(1):68–72.)
invoke fibrous encapsulation when implanted, as mentioned earlier.
Reactions to Implant Metals and Alloys
Pure metals and alloys are the oldest type of oral implant materials. Initially, metallic implant materi- als were selected based on strength and ease of fab- rication. Over time, however, biocompatibility with bone and soft tissue and the longevity of the implant have become more important. Although a variety of implant materials have previously been used (includ- ing stainless steel and chromium-cobalt-molybde- num), the only metallic dental implant materials in common use today are titanium-based alloys.
Titanium is a pure metal when initially cast.
However, in less than a second the surface forms a thin conformal layer of various titanium oxides. This oxide layer is corrosion resistant and allows bone to osseointegrate. A major disadvantage of this metal is that it is difficult to cast. It has been wrought into various forms, but this process introduces metallic impurities into the surface that may adversely affect bone cell response unless extreme care is taken dur- ing manufacturing. Titanium implants have been used with success as root forms to support a pros- thesis. With frequent recall and good oral hygiene, implants have been maintained in healthy tissue for longer than three decades. Titanium-aluminum- vanadium alloys (Ti-6Al-4V) have been used success- fully in this regard as well. This alloy is significantly stronger than commercially pure titanium, and has better fatigue resistance, but has the same desirable stiffness and thermal properties as the commercially pure (CP) metal. Although titanium and titanium alloy implants have corrosion rates that are markedly less than other metallic implants, they do release tita- nium into the body. Currently, there is no evidence that released titanium ions are a problem locally or systemically. However, questions remain about the liability of released aluminum and vanadium from alloys.
In soft tissue, the bond epithelium forms with titanium is morphologically similar to that formed with the tooth, but this interface has not been fully characterized. Connective tissue apparently does not bond to the titanium, but does form a tight seal that seems to limit ingress of bacteria and bacterial products. Techniques are being developed to limit down-growth of the epithelium and loss of bone height around the implant, because this will ulti- mately cause implant failure. Periimplantitis is now a documented disease around implants and involves many of the same bacteria as periodontitis. The role of the implant material or its released components in the progression of periimplantitis is not known, but this disease is considered to be a major contributor to implant failure and the subject of much investigation.
Reactions to Resorbable Materials
With the survival of implanted materials for decades or more, the predominant thought began to shift in emphasis from achieving a benign (or tolerated) tis- sue response to instead producing bioactive materi- als that could elicit a controlled action and reaction in the physiological environment. Continuing in this vein was the development of resorbable bio- materials that exhibit clinically appropriate, con- trolled chemical breakdown and resorption. In these materials, the problem of a tissue-material interface is resolved, because the materials provoke a physiologic response to replace the material with regenerated tissues. One of the earliest examples of these materials was the development of resorbable sutures. These materials were composed of a copo- lymer of polylactic acid (PLA) and polyglycolic acid (PGA). When implanted in the body, they undergo a hydrolytic decomposition into CO2 and H2O. By the mid-1980s, clinical use of resorbable polymeric sutures was commonplace. Resorbable fracture fixa- tion plates and screws, guided tissue membranes, and controlled drug-release systems have rapidly followed. Although generally well tolerated by tis- sues in vivo, the resorbability of these materials depends on the volume of material implanted and because these materials degrade into acidic by-prod- ucts, the subsequent drop in pH in the surrounding tissues may invoke an inflammatory response. Other polymeric materials such as polycaprolactone, and hyaluronan derivatives, as well as natural polymers such as cross-linked collagen, starch, and cellulose are currently being investigated for their ability to resorb in vivo after serving their function as an implant.
SUMMARY
Biocompatibility of a dental material depends on its composition, location, and interactions with the oral cavity. Metal, ceramic, and polymer materials elicit different biological responses because of differences in composition. Furthermore, diverse biological responses to these materials depend on whether they release their components and whether those compo- nents are toxic, immunogenic, or mutagenic at the released concentrations. The location of a material in the oral cavity partially determines its biocompat- ibility. Materials that are biocompatible in contact with the oral mucosal surface may cause adverse reactions if they are implanted beneath it. Materials that are toxic in direct contact with the pulp may be essentially innocuous if placed on dentin or enamel.
Finally, interactions between the material and the body influence the biocompatibility of the material.
A material’s response to changes in pH, application
of force, or the effect of biological fluids can alter its biocompatibility. Surface features, such as roughness of a material, may promote or discourage attach- ment of bacteria, host cells, or biological molecules.
These effects also determine whether the material will promote plaque retention, integrate with bone, or adhere to dentin.
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