Сraig. Dental Materials

crystals are smaller and less well formed than those in dentin. Because of the vascularity of bone, the mineral phase serves as a major reservoir of calcium and phosphate ions for the body's metabolic processes.

The ECM of bone (osteoid) is synthesized by osteoblasts that constitute the innermost layer of the periosteum and endosteum. The osteoblasts also initiate mineralization of this ECM. As bone is formed, osteoblasts are trapped within the ECM and become osteocytes that reside in lacunae, communicate with cells in other lacunae through canaliculi, and maintain the vitality of bone. These cells die if surgical manipulation destroys the vascular supply or heats the bone above 45' C for more than a few minutes. Another bone-cell type, the osteoclast, decalcifies the ECM and resorbs the organic portion of bone. It also responds to both physiological stimuli and injury. The coupling of osteoblastic and osteoclastic activity directs remodeling and occurs almost continuously throughout life.

Bone has an excellent capacity for self-repair. In bony defects caused by tooth extraction or bone fracture, the site initially fills with blood. The fibrin cascade results in a blood clot that fills this site and attaches to the walls of the alveolar bony socket. Subsequently, mesenchymal cells and endothelial cells grow into the blood clot from the surrounding connective tissue of the alveolar bone and create a young, vascular granulation tissue. With succeeding weeks and months, new osteoblasts differentiate from the granulation tissue and elaborate an ECM that gradually mineralizes. Through osteoblastic and osteoclastic influence, the new bone subsequently remodels itself to the general shape and architecture of the surrounding bone. Alveolar bony mass and height are gradually lost, however, because of the lack of tensile forces from teeth and functional periodontal ligament.

Osseointegration and Biointegration

An issue that faces dentists, and particularly implantologists, is the development of materials for implants that are physically and biologically compatible with alveolar bone. Ideally, bone does not

respond to the material as a foreign substance by forming a fibrous tissue capsule around it, but rather should integrate the material, substance, or device into the remodeled bone structure. Under optimum circumstances, bone differentiation should occur directly adjacent to the material (osseointegration). Ideally, osseointegration provides a stable bone-implant connection that can support a dental prosthesis.

Osseointegration is defined as the close approximation of bone to an implant material (Fig. 5-6). To achieve osseointegration, the bone must be viable, the space between the bone and implant must be less than 100 A and contain no fibrous tissue, and the bone-implant interface must be able to survive loading by a dental prosthesis. In current practice, osseointegration is an absolute requirement for the successful implant-supported dental prosthesis. To achieve osseointegration between an implant and bone, a number of factors must be correct. The bone must be prepared in a way that does not cause necrosis or inflammation (see previous discussion, this section). The implant must be allowed to heal for a time without a load. Finally, the proper material must be placed, because not all materials will promote osseointegration. For reasons that are not completely clear, titanium alloys are by far the most successful materials in promoting osseointegration and are therefore widely used as dental implants. The physical properties of titanium alloys are discussed in Chapter 16.

In recent years there has been a trend to coat titanium alloys with a calcium-phosphate ceramic to better promote an implant bone connection. If successful, the ceramic coating becomes completely fused with the surrounding bone. In this case, the interface is called biointegration rather than osseointegration and there is no intervening space between the bone and the implant (see Fig. 5-6). A number of ceramic coatings have been used in this manner, including tricalcium phosphate, hydroxyapatite, and Bioglass. The long-term integrity of the ceramic coating in vivo is not known, but some evidence indicates that these coatings will resorb over time.

The periodontium is a combination of tissues, including the periodontal ligament (PDL), cementum, and alveolar bone. The cementum and alveolar bone are mineralized extracellular matrices with the associated cells that are responsible for generating and maintaining them.

Collagenous fibers of the PDL extend from cementum to fibrous connective tissue above the alveolar crest, to alveolar cortical bone, or to the cementum of adjacent teeth. The ends of these collagenous fibers are anchored in a calcified ECM synthesized by cementoblasts (cementoid) or osteoblasts (osteoid). The orientation of the fibers translates compressive forces of mastication to tensile forces on the cementum and alveolar bone. The tensile stress stimulates low-grade cementogenesis and osteogenesis and maintains fairly constant alveolar bone heights, cementum thicknesses, and PDL widths. The exact mechanisms involved in these processes remain largely unknown and are of great interest in the fields of periodontology, implantology, and orthopedic surgery. In contrast, direct pressure (compression) on alveolar bone and cementum (e.g., in orthodontic movement) results in PDL and alveolar bone necrosis and an active biological resorption that removes portions of alveolar bone, cementum, and dentin from the tooth root. The necrosis is caused by ischemia of PDL and alveolar bone.

The PDL and its attachments to alveolar bone and teeth are maintained by cellular synthetic processes. At least in some animal species, there appears to be some compartmentalization of differentiated cells within the PDL, with fibroblasts in the tooth half of the PDL moving mesially with the constantly erupting incisor. Thus specific populations of differentiated, predifferentiated, or precursor cells probably give rise to and maintain the matrices of cementum, alveolar bone, and PDL. When cells that maintain the PDL are destroyed during injury and have no source of progenitor cells, ankylosis may result between tooth and bone (e.g., after tooth transplantation or placement of dental implant).

Regeneration of PDL, epithelial attachment, and alveolar bone around periodontally diseased

teeth is an important issue in dentistry. In an attempt at regeneration, gingival epithelium replaces crevicular epithelium, which was originally responsible for the epithelial attachment of the tooth. Following scaling and curettage of periodontal pockets, this gingival epithelium proliferates faster than the PDL fibers can reattach in newly formed cementum. Although fibrous reattachment to alveolar bone appears to occur quite readily, the original orientation of fiber to the tooth surface seems quite difficult to achieve. This results in epithelial-lined subcrestal pockets in the PDL space between alveolar bone and tooth surface. When this process advances, it has the effect of exfoliating the tooth. Investigators have made efforts to limit epithelial downgrowth of the gingiva, to enhance PDL reattachment to tooth and bone surfaces by chemical and surgical means, and to use implant materials that maximize epithelial and connective tissue cell attachment and limit the apical migration of epithelial cells.

GINGIVA AND MUCOSA

The linings of the oral cavity are composed of gingiva and oral mucosa. The gingiva is a connective tissue with an epithelial surface that covers the alveolar ridge, surrounds the cervices of the teeth, and fills interproximal spaces between teeth. Gingiva is divided into attached and free gingiva. The attached gingiva forms a junction with the alveolar oral mucosa toward the vestibule of the mouth and with the free gingival margin toward the crowns of the teeth. The free gingiva fuses with the attachment epithelium, which surrounds the tooth at its cervix in the young, healthy tooth. Some of the crevicular epithelium and all the attachment epithelium, at least in the young individual, are derived embryologically from reduced enamel epithelium. The oral mucosa is composed of a loose fibroelastic connective tissue with a well-vascularized and innervated lamina propria and submucosa and is covered mainly by a parakeratinized stratified squamous epithelium.

The oral mucous membrane can be injured chemically or physically by dental materials. If

134 Chapter 5 BlOCOMPATlBlLlTY OF DENTAL MATERIALS

the injury is short-term (acute) and leads to loss of tissue but does not involve infection by pathogenic microorganisms, the connective tissue defect is filled in with granulation tissue within 3 to 4 days and epithelium regenerates over the surface within a week. The tissue is remodeled to nearly normal by the end of 2 to 3 weeks. As with other body tissues, the ability to heal depends on the metabolic status of the patient and the removal of external irritating factors. Occasionally, immune hypersensitivity to materials or pharmaceutical agents may delay the healing response. The presence of microorganisms or immune hypersensitivity reactions results in an infiltration of acute or chronic inflammatory cells and delay of healing.

The gingival response to injury may be complicated by its association with the tooth. Calculus deposition on the tooth, malocclusion, and faulty restorations may enhance the destructive effects of microorganisms. The crevicular epithelium then becomes vulnerable to endotoxin and various exogenous and endogenous chemicals. The resulting breakdown of tissue leads to an acute inflammatory response by the gingival connective tissue called acute gingivitis. This condition is often reversible if the injurious agent is removed and the reaction is limited to the connective tissue above the alveolar bony crest. If the insult continues, the inflammatory infiltrate becomes mixed and then predominantly mononuclear. Inflamed epithelial-lined granulation tissue gradually spreads apically to and below the alveolar bony crest. At this point the condition is described as chronic periodontal disease, a progressive disease process that is probably reinforced by immune mechanisms. The relationship between periodontal disease and dental materials that are in close contact with the gingiva is not known but is an active area of research.

The reaction of gingival tissues to oral implants is also an important area of research. Permucosal implants present special problems, including epithelial ingrowth, encystification, and exfoliation of the implant. There is also the problem of maintaining a close epithelial attachment between the implant and soft tissue to exclude bacteria. Thus the implant material should

ideally encourage firm attachment of epithelial cells on its surface, but only limited growth and migration of these cells. An inflammatory disease around implants, called peri-implantitis, has been described and is probably similar in etiology and progression to periodontal disease. Periimplantitis occurs in response to bacteria that attach to implants and exist near the gingiva. The role of materials in altering the peri-implantitis process is not currently known.

Dental materials that are antigenic can cause immune hypersensitivity reactions in oral mucosa and gingiva. Local binding of antigens to membranes of white blood cells (i.e., lymphocytes, macrophages, basophils, mast cells) or Langerhans' cells of skin and oral mucosal epithelium plays a role in activating these various reactions. Although a few of the mucosal reactions are documented as Type I reactions (wherein vasoactive substances are released from mast cells because of antigen-IgE reactions), most reactions to dental materials are classified as Type IV (T-cell-mediated) reactions. This type of reaction is sometimes called contact mucositis. Skin testing can be used to help document Type I and Type IV reactions to environmental antigens, metallic elements used in alloys, and byproducts from polymers. In vitro tests for cell-mediated hyperimmunity are occasionally performed and include transformation of the patient's lymphocytes and production of a migration-inhibition factor by these cells in response to the antigenic stimulus.

MEASURING BlOCOMPATt

Measuring the biocompatibility of a material is not simple, and the methods of measurement are evolving rapidly as more is known about the interactions between dental materials and oral tissues and as technologies for testing improve. Historically, new materials were simply tried in humans to see if they were biocompatible. However, this practice has not been acceptable for many years, and current materials must be extensively screened for biocompatibility before they are ever used in humans. Several varieties of

tests are currently used to try to ensure that new materials are biologically acceptable. These tests are classified as in vitro, animal, and usage tests. These three testing types include the clinical trial, which is really a special case of a usage test in humans. The remainder of this section will discuss several of each type of test, their advantages and disadvantages, how the tests are used together, and standards that rely on these tests to regulate the use of materials in dentistry.

IN VITRO TESTS

In vitro tests for biocompatibility are done in a test tube, cell-culture dish, or otherwise outside of a living organism. These tests require placement of a material or a component of a material in contact with a cell, enzyme, or some other isolated biological system. The contact can be either direct, where the material contacts the cell system without barriers, or indirect, where there is a barrier of some sort between the material and the cell system. Direct tests can be further subdivided into those in which the material is physically present with the cells and those in which

some extract from the material contacts the cell system. In vitro tests can be roughly subdivided into those that measure cytotoxicity or cell growth, those that measure some metabolic or other cell function, and those that measure the effect on the genetic material in a cell (mutagenesis assays). Often there is some overlap in what a test measures. In vitro tests have a number of significant advantages over other types of biocompatibility tests (Table 5-1). They are relatively quick to perform, generally cost much less than animal or usage tests, can be standardized, are well suited to large scale screening, and can be tightly controlled to address specific scientific questions. The overriding disadvantage of in vitro tests is their questionable relevance to the final in vivo use of the material (see later section on correlation between tests). Other disadvantages include the lack of inflammatory and other tissue-protective mechanisms in the in vitro environment. It should be emphasized that in vitro tests alone cannot usually predict the overall biocompatibility of a material.

Standardization of in vitro tests is a primary concern of those trying to evaluate materials.

Fig. 5-7 Light microscopic view of a noncytotoxic interaction between a material (dark image at bottom of the picture) and periodontal ligament fibroblasts in a cell-culture(in vitro) test. The morphology of the fibroblasts indicates that they are alive and are not suffering from a toxic response (see Fig. 5-8 for contrast). The material in this case was a calcium hydroxide pulp-capping agent.

Two types of cells can be used for in vitro assays. Primary cells are cells taken directly from an animal into culture. These cells will grow for only a limited time in culture but may retain many of the characteristics of cells in vivo. Continuous cells are primary cells that have been transformed to allow them to grow more or less indefinitely in culture. Because of their transformation, these cells may not retain all in vivo characteristics, but they consistently exhibit any features that they do retain. Primary cell cultures would seemingly be more relevant than continuous cell lines for measuring cytotoxicity of materials. However, primary cells have the problems of being from a single individual, possibly harboring viral or bacterial agents that alter their behavior, and often rapidly losing their in vivo functionality once placed in a cell culture. Furthermore, the genetic and metabolic stability of continuous cell lines contributes significantly toward standardizing assay methods. In the end, both primary and continuous cells play an important role in in vitro testing; both should be used to assess a material.

Cytotoxicity Tests Cytotoxicity tests assess the cytotoxicity of a material by measuring

Fig. 5-8 Light microscopic view of a cytotoxic interaction between a material (dark image at the bottom of the picture) and periodontal ligament fibroblasts in a cell-culture test. The fibroblasts are rounded and detached (see Fig 5-7 for contrast), indicating that they are either dead or dying. The material is a type of calcium hydroxide pulp-capping agent, different from the one shown in Fig. 5-7.

cell number or growth after exposure to a material. Cells are plated in a well of a cell-culture dish where they attach. The material is then placed in the test system. If the material is not cytotoxic, the cells will remain attached to the well and will proliferate with time. If the material is cytotoxic, the cells may stop growing, exhibit cytopathic features (Figs. 5-7 and 549,or detach from the well. If the material is a solid, then the density (number of cells per unit area) of cells may be assessed at different distances from the material, and a "zone" of inhibited cell growth may be described (Fig. 5-9). Cell density can be assessed either qualitatively, semi-quantitatively, or quantitatively. Substances such as Teflon can be used as negative (non-cytotoxic) controls, whereas materials such as plasticized polyvinyl chloride can be used as positive (cytotoxic) controls. Control materials should be well defined and commercially available to facilitate comparisons among testing laboratories.

Another group of tests is used to measure cytotoxicity by a change in membrane permeability (Fig. 5-10). Membranepermeability is the ease with which a dye can pass through a cell membrane. This test is used on the basis that a

Time

Fig. 5-9 A material sample (S) is placed in the center of a cell-culturewell, and cells and medium are added. After 1 to 3 days, the cells have multiplied in areas where the material has not inhibited their growth. The area devoid of growth is often referred to as a

ring of inhibition. Several methods are available to assess the amount of cellular growth around the sam~les.

Fig. 5-10 The selective permeability of cell membranes is the basis for several cytotoxicity tests. Compounds such as NaslCrO, (57Cr), and neutral red (NR) are actively sequestered by healthy cells. These compounds will leach out of the cell if the cell is injured and cannot maintain its membrane integrity. Other compounds such as trypan blue (TB) are excluded by a healthy cell, but can diffuse through the membrane of an injured cell.

loss in membrane permeability is equivalent to or very nearly equivalent to cell death. The advantage of the membrane permeability test is that it identifies cells that are alive (or dead) under the microscope. This feature is important because it is possible for cells to be physically present, but dead (when materials fix the cells). There are two basic types of dyes used. Vital dyes are actively transported into viable cells, where they are retained unless cytotoxic effects increase the permeability of the membrane. It is important to establish that the dye itself does not exhibit

MTT (yellow, soluble) MTT-formazan (blue,insoluble)

Fig. 5-11 MTT is a yellow, soluble molecule that can be used to assess cellular enzymatic activity. If the cell is able to reduce the MTT, the resulting formazan is blue and insoluble and deposits in the cell. The amount of formazan formed is proportional to the enzymatic activity. The activity of a number of cellular enzymes can be assessed in this manner.

cytotoxicity during the time frame of the test. Nonvital dyes are not actively transported, and are only taken up if membrane permeability has been compromised by cytotoxicity. Many types

of vital dyes have been used, including neutral red and N~,"c~o,The. use of neutral red and

N ~ , ~ ' c are~ oparticularly, advantageous because they are neither synthesized nor metabolized by the cell. Examples of nonvital dyes include trypan blue and propidium iodide.

Tests f o r Cell Metabolism or Cell Func- tion Some in vitro tests for biocompatibility use the biosynthetic or enzymatic activity of cells to assess cytotoxic response. Tests that measure deoxyribonucleic acid (DNA) synthesis or protein synthesis are common examples of this type of test. The synthesis of DNA or protein by cells is usually analyzed by adding radioisotopelabeled precursors to the medium and quantify-

ing the radioisotope (e.g., 3~-thymidineor 3 ~ - leucine) incorporated into DNA or protein.

A commonly used enzymatic test for cytotoxicity is the MTT test. This test measures the activity of cellular dehydrogenases, which convert a chemical called MTT, via several cellular reducing agents, to a blue, insoluble formazan compound (Fig. 5-11). If the dehydrogenases are not active because of cytotoxic effects, the formazan will not form. The production of formazan can be

quantified by dissolving it and measuring the optical density of the resulting solution. Alternatively, the formazan can be localized around the test sample by light or electron microscopy. Other formazan-generating chemicals have been used, including NBT, XTT, and WST. Furthermore, many other activities of cells can be followed qualitatively or quantitatively in vitro. Recently, in vitro tests to measure gene activation, gene expression, cellular oxidative stress, and other specific cell functions have been proposed. However, these types of tests are not yet routinely used to assess the biocompatibility of materials.

Tests That Use Barriers (Indirect Tests)

Most of the cytotoxicity tests presented thus far are performed with the material in direct contact with the cell culture. Researchers have long recognized that in vivo, direct contact often does not exist between cells and the materials. Separation of cells and materials may occur from keratinized epithelium, dentin, or extracellular matrix. Thus several in vitro barrier tests have been developed to mimic in vivo conditions. One such test is the agar overlay method (Fig. 5-12) in which a monolayer of cultured cells is established before adding 1%agar or agarose (low melting temperature) plus a vital stain, such as neutral red, to fresh culture media. The agar forms a barrier between the cells and the material, which is placed on top of the agar. Nutrients, gas, and soluble toxic substances can diffuse through the agar. Solid test samples or liquid samples adsorbed onto filter paper can be tested with this assay for up to 24 hours. This assay correlates positively with the direct-contact assays described above and the intramuscular implantation test in rabbits. However, the agar may not adequately represent barriers that occur in vivo. Furthermore, because of variability of the agar's diffusion properties, it is difficult to correlate the intensity of color or width of the zone around a material with the concentration of leachable toxic products.

A second barrier assay is the Millipore filter assay. This technique establishes a monolayer of cells on filters made of cellulose esters. The culture medium is then replaced with medium

ell layer

Fig. 5-12 The agar overlay method has been used to evaluate the cytotoxicity of dental materials. The cell layer, which has been previously stained with neutral red (NR), is covered with a thin layer of agar (A). Samples are placed on top of the agar for a time. If the material is cytotoxic, it will injure the cells and the neutral red will be released, leaving a zone of inhibition.

containing about 1% agar, and this mixture is allowed to gel over the cells. Finally, the filter- monolayer-gel is detached and turned over so that the filter is on top for placement of solid or soluble test samples for 2 or more hours. After exposure to the test samples, the filter is removed and an assay is used to determine the effect of the sample on a cellular metabolic activity. The succinyl dehydrogenase assay described previously can be used with this test. Like the agar overlay test and the cell contact tests, toxicity in the Millipore filter test is assessed by the width of the cytotoxic zone around each test sample. This test also has the drawback of arbitrarily influencing the diffusion of leachable products from the test material. The agar diffusion and Millipore filter tests can provide, at best, a cytotoxic ranking among materials.

Dentin barrier tests have shown improved correlation with the cytotoxicity of dental materials in usage tests in teeth, and are gradually being developed for screening purposes (Fig. 5-13). A number of studies have shown that dentin forms a barrier through which toxic materials must diffuse to reach pulpal tissue. Thus pulpal reaction to zinc oxide-eugenol is relatively mild as compared with the more severe reactions to the same material in direct contact with cells in in vitro assays and tissue in implantation tests. The thickness of the dentin correlates directly with the protection offered to the pulp. Thus

Out

Fig. 5-13A dentin disk used as a barrier in cytotoxicity tests that attempt to predict the toxicity of materials placed on dentin in vivo. The material is placed on one side (A) of the dentin disk ( 6 ) in the device used to hold the dentin disk. Collection fluid (cell culture medium or saline) is on the other side of the disk (C). Cells can also be grown in the collection side. Components of the material may diffuse through the dentin and the effect of the medium on cell metabolism can then be measured. To assess the rate of diffusion, the collection fluid can be circulated into and out of the collection chamber (C).

assays have been developed that incorporate dentin disks between the test sample and the cell assay system. The use of dentin disks offers the added advantage of directional diffusion between the restorative material and the culture medium.

Other Assays for Cell Function In vitro assays to measure immune function or other tissue reactions have also been used. The in vivo significance of these assays is yet to be ascertained, but many show promise for being able to reduce the number of animal tests required to assess the biocompatibility of a material. These assays measure cytokine production by lymphocytes and macrophages, lymphocyte proliferation, chemotaxis, or T-cell rosetting to sheep red blood cells. Other tests measure the ability of a material to alter the cell cycle or activate complement. The activation of complement is of particular concern to researchers working on artificial or "engineered" blood vessels and other tissues in direct contact with blood. Materials that activate complement may generate inflammation or thrombi, and may propagate a chronic inflammatory response. Whereas concerns about complement activation by dental materials are fewer, it is possible that activation of complement

by resins or metals or their corrosion products may prolong inflammation in the gingiva or pulp.

Mutagenesis Assays Mutagenesis assays assess the effect of materials on a cell's genetic material. There is a wide range of mechanisms by which materials can affect the genetic material of the cell. Genotoxic mutagens directly alter the DNA of the cell through various types of mutations. Each chemical may be associated with a specific type of DNA mutation. Genotoxic chemicals may be mutagens in their native states, or may require activation or biotransformation to be mutagens, in which case they are called promutagens. Epigenetic mutagens do not alter the DNA themselves, but support tumor growth by altering the cell's biochemistry, altering the immune system, acting as hormones, or other mechanisms. Carcinogenesis is the ability to cause cancer in vivo. Mutagens may or may not be carcinogens, and carcinogens may or may not be mutagens. Thus the quantification and relevance of tests that attempt to measure mutagenesis and carcinogenesis are extremely complex. A number of government-sponsored programs evaluate the ability of in vitro mutagenesis assays to predict carcinogenicity.

The Ames' test is the most widely used shortterm mutagenesis test and the only short-term test that is considered thoroughly validated. It uses mutant stocks of Salmonella typhimuWm that require exogenous histidine. Native stocks of bacteria do not require exogenous histidine. Exclusion of histidine from the culture medium allows a chemical to be tested for its ability to convert the mutant strain to a native strain. Chemicals that significantly increase the frequency of reversion back to the native state have a reportedly high probability of being carcinogenic in mammals because they significantly alter genetic material. Performance of this test requires experience in the field and special strains of Salmonella to produce meaningful results. Several stains of Salmonella are used, each to detect a different type of mutation transformation. Furthermore, chemicals can be "metabolized" in vitro using homogenates of liver enzymes to simu-

late the body's action on chemicals before testing for mutagenicity.

A second test for mutagenesis is the Styles' Cell Transformation test. This test on mammalian cells was developed to offer an alternative to bacterial tests (Ames test), which may not be relevant to mammalian systems. This assay quantifies the ability of potential carcinogens to transform standardized cell lines so they will grow in soft agar. Untransformed fibroblasts normally will not grow within an agar gel, whereas genetically transformed cells will grow below the gel surface. This characteristic of transformed fibroblasts is the only characteristic that correlates with the ability of cells to produce tumors in vivo. At least four different continuous cell lines (Chang, BHK, HeLa, WI-38) have been used. In 1978, Styles claimed 94% "accuracy in determining carcinogenic or noncarcinogenic activity" when testing 120 compounds in two cell lines. However, there has been some difficulty in reproducing these results.

In a recent report, four short-term tests (STTs) for gene toxicity were compared (Table 5-2). The Ames test was the most specific (86% of non-

carcinogens yielding a negative result). The Ames test also had the highest positive predictability (83% of positives were actually carcinogens) and displayed negative predictability equal to that of other STTs (i.e., 51% of all Ames test negatives were noncarcinogenic). However, the results were in agreement (concordance) with rodent carcinogenicity tests for only 62% of the chemicals. Also, the Ames test was sensitive to only 45% of the carcinogens; that is, it missed over half of the known carcinogens. The other three STTs were assays for chromosomal aberration, sister chromatid exchange in CHO cells, and the mouse lymphoma L5178Y cell mutagenesis assay. The sister chromatid exchange method, the mouse lymphoma mutagenesis assay, and the Ames test had 73%, 70%, and 45% sensitivity, respectively. However, because the Ames test is widely used, extensively described in the literature, and technically easier to conduct in a testing laboratory than the other tests, it is most often conducted in a screening program. These studies suggest that not all carcinogens are genotoxic (mutagenic) and not all mutagens are carcinogenic. Thus, although SlTs for mutagenesis are

Test

Parameter

Specificity

Sensitivity

1 I

test result