Сraig. Dental Materials

shown in Table 16-1, the nickel-chromium alloys used with porcelain contain significantly less carbon than the alloys used for partial dentures. The presence of 3% to 6% molybdenum contributes to the strength of the alloys.

Aluminum in nickel-containing alloys forms a compound of nickel and aluminum (Ni,Al). This compound increases the ultimate tensile and yield strengths of the alloy considerably. The addition of as little as 1% to 2% beryllium to nickel-based alloys lowers the fusion range by about 100" C. However, recent studies suggest that this concentration of beryllium may adversely affect ductility. Corrosion resistance is also compromised, as corrosion occurs preferentially in the Ni-Be eutectic phase, presumably releasing quantities of beryllium greater than the nominal alloy composition of 1% to 2%. Silicon and manganese are added to increase the fluidity and castability of these alloys. Nitrogen, which cannot be controlled unless the castings are made in a controlled atmosphere, such as in a vacuum or under argon, also contributes to the brittle qualities of these cast alloys. When the nitrogen content of the final alloy is more than 0.1%, the castings lose some of their ductility. Numerous other modifications in composition

are being proposed to develop more ductile and stronger alloys. There is a remarkable similarity in properties of different alloys having a relatively wide variation in composition. However, as discussed later, it is primarily the minor alloying elements of carbon, nitrogen, and oxygen that influence casting and the properties of a final casting.

MICROSTRUCTURE OF CAST BASE-METAL ALLOYS

The microstructure of any substance is the basic factor that controls properties. In other words, a change in the physical properties of a material is a strong indication that there must have been some alteration in its microstructure. Sometimes this variation in microstructure cannot be distinguished by ordinary means. Neither cobaltchromium nor nickel-chromium alloys have simple microstructures, and their microstructures change with slight alterations of manipulative conditions.

The microstructure of cobalt-chromium alloys in the cast condition is inhomogeneous, consisting of an austenitic matrix composed of a solid solution of cobalt and chromium in a cored

dendritic structure. The dendritic regions are cobalt-rich, whereas the interdendritic regions can be a quaternary mixture consisting of a cobalt-rich y-phase; a chromium-rich MZ3C6 phase, where M is Co, Cr, or Mo; an M,C, carbide phase; and a chromiumand molybdenum-rich o-phase. Interdendritic casting porosity is also associated with this structure.

Many elements present in cast base-metal alloys, such as chromium, cobalt, and molybdenum, are carbide-forming elements. Depending on the composition of a cast base-metal alloy and its manipulative condition, it may form many types of carbide. Furthermore, the arrangement of these carbides may also vary depending on the manipulative condition.

The microstructure of a commercial cobaltchromium alloy is illustrated in Fig. 16-1. In Fig. 16-1, A, the carbides are continuous along the grain boundaries. Such a structure is obtained when the metal is cast as soon as it is completely melted. In this condition, the cast alloy possesses low elongation values with a good and clean surface. Carbides that are spherical and discontinuous, like islands, are shown in Fig. 16-1, B. Such a structure can be obtained if the alloy is heated about 100" C above its normal melting temperature; this results in a casting with good elongation values but with a very poor surface because of an increased reaction with the investment. The surface is so poor that the casting cannot be used in dentistry.

Dark eutectoid areas, which are lamellar in nature, are shown in Fig. 16-1, C.Such a structure is responsible for very low elongation values but a good and clean casting. From these three examples, it is clear that microstructure can strongly affect physical and mechanical properties. The microstructure of Ni-Cr alloys is strongly dependent on alloy composition. Alloys containing Be form an interdendritic NiBe phase, as shown in Fig 16-1,D. In fact, during normal metallographic procedures involving acid etching of alloy specimens, the NiBe phase is dissolved; what is seen in the figure is the void area left behind. The susceptibility of the NiBe phase to acid attack has been taken advantage of in developing resinbonded retainers. The retainer may be etched in

selected areas, where a composite-like luting agent can then mechanically adhere to the retainer after curing.

Alloys not containing Be have complicated, multiphase microstructures such as that shown in Figure 16-1, E. The precipitates dispersed within the matrix include complex carbides, and, in alloys where Nb is present, Mo-Nb-Si compounds. All these precipitates are relatively unaffected by the heat treatments the alloys are subjected to during the porcelain firing procedures, although the loss of elements due to oxidation of the alloy surface may be sufficiently great to change the stability of some phases, which then re-dissolve in the alloy matrix.

HEAT TREATMENT OF BASE-METALALLOYS

The early base-metal alloys used in partialdenture prostheses were primarily cobaltchromium and were relatively simple. Heat treating these alloys up to 1 hour at 1000" C did not change their mechanical properties appreciably. Base-metal alloys available today for partialdenture prostheses, however, are more complex. Presently, complex cobalt-chromium alloys, as well as nickel-chromium and iron-chromium alloys, are used for this purpose.

Studies have shown that many heat treatments of cobalt-based alloys reduce both the yield strength and elongation. If for any reason some soldering or welding must be performed on these partial dentures, the lowest possible temperature should be used with the shortest possible time of heating to the elevated temperature.

The coarse grain size and interdendritic carbide and o-phases present in cast Co-Cr-Mo limit the strength and ductility of the as-cast alloy. Because the interdendritic phases are associated with reduced ductility and reduced corrosion resistance, cast Co-Cr-Mo is typically solutionannealed at approximately 1225" C. If the treatment and chemical composition are well controlled, such a thermal treatment results in the transformation of o to MZ3CGand the partial dissolution of the M2,C6 phase, leading to increased yield strength and ductility. In general, yield and fatigue strengths of this alloy are be-

lieved to be controlled by the ability of the solute atoms C, Cr, and Mo to inhibit dislocation motion.

Slow cooling to temperatures below those at which incipient melting occurs enhances ductility. A reduction in the carbon content also enhances ductility. However, these increases in ductility are at the expense of yield strength. In general, excess grain-boundary carbide decreases ductility, whereas structures with carbide-free grain boundaries are characterized by markedly reduced yield and tensile strengths.

PHYSICAL PROPERTIES

Melting Temperature The melting temperature of base-metal alloys differs significantly from that of dental gold casting alloys. Most base-metal alloys melt at temperatures of 1400" to 1500' C, as compared with cast gold alloy Types I to IV, which have a melting range of 800" to 1050" C. Only one commonly used nickel-chromium alloy (Ticonium) melts below 1300' C, at a temperature of 1275" C. The addition of 1% to 2% beryllium lowers the melting temperature of Ni-Cr alloys about 100" C. The melting temperature is important in the selection of casting equipment and control of the casting technique.

Density The average density of cast basemetal alloys is between 7 and 8 g/cm3, which is

approximately half the density of most dental gold alloys. Density is of some importance in bulky maxillary appliances, in which the force of gravity causes the relative weight of the casting to place additional forces on the supporting teeth. With certain appliances, therefore, the reduction of weight resulting from the lower density of the cast base-metal alloys can be considered an advantage.

MECHANICAL PROPERTIES

Typical mechanical properties of the partialdenture alloys listed in Table 16-1 have been assembled in Table 16-2, together with a representative range of values for Type IV casting gold alloys subjected to a hardening heat treatment.

Yield Strength The yield strength gives an indication when a permanent deformation of a device or part of a device, such as a clasp, will occur. As such, it is one of the important properties of alloys intended for removable partialdenture restorations. It is believed that dental alloys should have yield strengths of at least 415 MPa to withstand permanent deformation when used as partial-denture clasps. It may be seen from Table 16-2 that base-metal dental alloys have yield strengths greater than 600 MPa.

486 Chapter 16 CAST AND WROUGHT BASE-METALALLOYS

enced by variations in specimen preparation and test conditions than are some other properties, such as elongation. Table 16-2 shows that the ultimate tensile strength of cast base-metal dental alloys is greater than 800 MPa. Table 16-2 also demonstrates that hardened partial-denture gold alloys can have ultimate tensile strengths almost equal to those of cast base-metal alloys.

Elongation The percent elongation of an alloy is important as an indication of the relative brittleness or ductility a restoration will exhibit. There are many occasions, therefore, when elongation is an important property for comparison of alloys for removable partial-denture appliances. For example, as described in Chapter 4, the combined effect of elongation and ultimate tensile strength is an indication of toughness of a material. Because of their toughness, partialdenture clasps cast of alloys with a high elongation and tensile strength do not fracture in service as often as do those with low elongation.

The percent elongation is one of the properties critical to accurate testing and to proper control during test preparation. For example, a small amount of microporosity in the test specimen will decrease the elongation considerably, whereas its effect on yield strength, elastic modulus, and tensile strength is rather limited. One can therefore assume that practical castings may exhibit similar variations in elongation from one casting to another. To some degree this is borne out in practice, with some castings from the same product showing a greater tendency toward brittleness than others. This observation indicates that control of the melting and casting variables is of extreme importance if reproducible results are to be obtained.

Although nickel and cobalt are interchangeable in cobalt-nickel-chromium alloys, increasing the nickel content with a corresponding reduction in cobalt generally increases the ductility and elongation. High values of elongation are obtained by casting at the normal melting temperature and by not heating the alloy 100" C above its normal casting temperature. High elongation is achieved without sacrificing strength and is the

result of the precise and proper combination of carbon and molybdenum content.

Elastic Modulus The higher the elastic modulus, the more rigid a structure can be expected, provided the dimensions of the casting are the same in both instances. Some dental professionals recommend the use of a welldesigned, rigid appliance because it properly distributes forces on the supporting tissues when in service. With a greater elastic modulus, one can design the restoration with slightly reduced dimensions. From Table 16-2, it can be seen that the elastic modulus of base-metal alloys is approximately double the modulus of Type IV cast dental gold alloys.

Hardness Differences in composition of the cast base-metal alloys have some effect on their hardness, as indicated by the values given in Table 16-2. In general, cast base-metal alloys have a hardness about one third greater than gold alloys used for the same purpose.

Hardness is an indication of the ease of finishing the structure and its resistance to scratching in service. The higher hardness of the cast base-metal alloys as compared with gold alloys requires the use of special polishing equipment, which may be considered a disadvantage, but the finishing operation can be completed without difficulty by experienced operators. It is a common practice to use electrolytic polishing for a portion of the finishing operation, which reduces the time and effort necessary for mechanical finishing operations. With an electrolytic polishing procedure, cast base-metal restorations are deplated, and only a very small amount of alloy (a few ~ n ~ s t r o mis )removed from the surface. Electrolytic polishing works on the reverse principle of electroplating, with the restoration serving as the anode. The deplating exposes a new surface, which is smoother than the cast surface because the rough areas are deplated more readily than the smooth ones. The cast base-metal appliances retain their polish well in service. Deplating such a small amount of alloy from the tissue-bearing side of the prosthesis produces a

clean, shiny surface, but does not alter the fit. The non-tissue bearing side of the partial denture can be further polished on a high-speed lathe.

Fatigue The fatigue resistance of alloys used for partial denture is important when it is considered that these appliances are placed and removed daily. At these times, the clasps are strained as they slide around the retaining tooth, and the alloy undergoes fatigue. Comparisons among cobalt-chromium, titanium, and gold alloys shows that cobalt-chromium alloys possess superior fatigue resistance, as indicated by a higher number of cycles required to fracture a clasp. Any procedures that result in increasing the porosity or carbide content of the alloy will reduce fatigue resistance. Also, soldered joints, which often contain inclusions or pores, represent weak links in the fatigue resistance of the appliance.

CORROSION

Recent research on dental casting alloys has been dominated by studies on corrosion and potential biological effects of metal ion release. In general, in vitro corrosion tests have evaluated a number of important variables, including effects of electrolytic media and artificial saliva, alloy composition, alloy microstructure, and surface state of the metal. These variables account for 2 to 4 orders of magnitude variation in the amount of species released. The surface state of the metal is an extremely important factor influencing corrosion, because the surface composition is almost always different from that of the bulk alloy. Another important consideration is corrosion coupled with wear. Up to three times the mass of metal ions, such as Ni and Be is released during occlusal rubbing in combination with corrosion than during corrosion alone for Ni-Cr alloys.

CROWN AND BRIDGE CASTING ALLOYS

The nickel-chromium alloys can be divided into those containing or not containing beryllium. Most of the alloys contain 60% to 80% nickel,

10% to 27% chromium, and 2% to 14% molybdenum. As a comparison, cobalt-chromium alloys contain 53% to 67% cobalt, 25% to 32% chromium, and 2% to 6% molybdenum. Those alloys that contain beryllium contain 1.6%to 2.0%of the element. They may also contain small amounts of aluminum, carbon, cobalt, copper, cerium, gallium, iron, manganese, niobium, silicon, tin, titanium, and zirconium. The low atomic weight of about 9 for beryllium, compared with about 59 for nickel and 52 for chromium, results in atomic percentages of beryllium in these alloys of about 11%.

Properties of these alloys are similar to those reported in Table 16-2 for the cobalt-chromium alloys. Crown and bridge casting alloys exhibit a higher hardness and elastic modulus than do noble alloys, but they prove more difficult in casting and soldering. They are also more technique sensitive, and because of their higher solidification shrinkage, producing a restoration with a satisfactory fit is more difficult.

Precautions should be taken to avoid exposure to metallic vapor, dust, or grindings containing beryllium and nickel. The safety standard for beryllium dust is 2 pg/m3 of air for a timeweighted, 8-hour day. A higher limit of 25 pg/m3 is allowed for a minimum exposure time of less than 30 minutes. Physiological responses may range from contact dermatitis to severe chemical pneumonitis. Therefore efficient local exhaust and filtration systems should be used when casting, finishing, and polishing these berylliumcontaining alloys.

The presence of nickel is of greater importance because it is a known allergen. The incidence of allergic sensitivity to nickel has been reported to be from 5 to 10 times higher for females than for males, with 5% to 8% of females showing sensitivity. However, no correlation has been found between the presence of intraoral nickel-based restorations and sensitivity. A cobalt-chromium alloy without nickel or other non-nickel containing alloy should be used on patients with a medical history indicating an allergic response to nickel. The safety standard for nickel is 15 pg/m3 of air for a 40-hour week.

To minimize exposure of patients to metallic dust containing nickel or beryllium, intraoral finishing should be done with a high-speed evacuation system.

OTHER APPLICATIONS OF CAST

BASE-METALALLOYS

Cast cobalt-chromium alloys serve a useful purpose in appliances other than removable partialdenture restorations. In the surgical repair of bone fractures, alloys of this type have been used for bone plates, screws, various fracture appliances, and splints. Metallic obturators and implants for various purposes are formed from cast base-metal alloys. The use of cobalt-chromium alloys for surgical purposes is well established, and these alloys have numerous oral surgical uses. They can be implanted directly into the bone structure for long periods without harmful reactions. This favorable response of the tissue is probably attributable to the low solubility and electrogalvanic action of the alloy; the metal is inert and produces no inflammatory response. The product known as surgical Vitallium is used extensively for this purpose. The primary metal used in oral implantology today is titanium.

TITANIUM AND TITANIUM AL

Titanium's resistance to electrochemical degradation; benign biological response elicited; relatively light weight; and low density, low modulus, and high strength make titanium-based materials attractive for use in dentistry. Titanium forms a very stable oxide layer with a thickness on the order of angstroms, and it repassivates in a time on the order of nanoseconds seconds). This oxide formation is the basis for the corrosion resistance and biocompatibility of titanium. Titanium has therefore been called the "material of choice" in dentistry.

Commercially pure titanium (c.p. Ti) is used for dental implants, surface coatings, and, more recently, for crowns, partial and complete dentures, and orthodontic wires. Several titanium alloys are also used. Of these alloys, Ti-6A1-4V is

the most widely used. Wrought alloys of titanium and nickel and titanium and molybdenum are used for orthodontic wires. The term titanium is often used to include all types of pure and alloyed titanium. However, it should be noted that the processing, composition, structure, and properties of the various titanium alloys are quite different, and also that differences exist between the wrought and cast forms of a given type of titanium.

COMMERCIALLY PURE TITANIUM

Commercially pure Ti is available in four grades, which vary according to the oxygen (0.18 to 0.40 wt%) and iron (0.20 to 0.50 wt%) content. These apparently slight concentration differences have a substantial effect on the physical and mechanical properties.

At room temperature, c.p. Ti has an HCP crystal lattice, which is denoted as the alpha (a) phase. On heating, an allotropic phase transformation occurs. At 883' C, a body-centered cubic (BCC) phase, which is denoted as the beta (P) phase, forms. A component with a predominantly P phase is stronger but more brittle than a component with an a-phase microstructure. As with other metals, the temperature and time of processing and heat treatment dictate the amount, ratio, and distribution of phases, overall composition and microstructure, and resultant properties. As a result, casting temperature and cooling procedure are critical factors in ensuring a successful casting.

The density of c.p.Ti (4.5 g/cm3) is about half the value of many of the other base metals. The modulus (100 GPa) is also about half the value of the other base metals. The yield and ultimate strengths vary, respectively, from 170 to 480 MPa and 240 to 550 MPa, depending on the grade of titanium.

TITANIUM ALLOYS: GENERAL

Alloying elements are added to stabilize either the a or the p phase, by changing the P transformation temperature. For example, in Ti-6Al- 4V, aluminum is an a stabilizer, which expands

the a-phase field by increasing the ( a + P) to P transformation temperature, whereas vanadium, as well as copper and palladium, are P stabilizers, which expand the P-phase field by decreasing the (a + p) to P transformation temperature.

In general, a-titanium is weldable, but difficult to form or work with at room temperature. Betatitanium, however, is malleable at room temperature and is thus used in orthodontics. The (a + p) alloys are strong and formable but difficult to weld. Thermal and thermochemical treatments can refine the postcast microstructures and improve properties.

At room temperature, Ti-6A1-4V is a two-phase (a + p) alloy. At approximately 975' C, an allotropic phase transformation takes place, transforming the microstructure to a single-phase BCC p alloy. Thermal treatments dictate the relative amounts of the a and p phases and the phase morphologies and yield a variety of microstructures and a range of mechanical properties. Microstructural variations depend on whether working and heat treatments were performed above or below the p-transition temperature and on the cooling rate.

Following forging at temperatures in the range of 700" to 950" C, thermal treatments below the p-transition temperature (typically performed at approximately 700" C) produce recrystallized microstructures having fine equiaxed a grains (Fig. 16-21, Equiaxed microstructures are characterized by small (3 to 10 pm), rounded grains that have aspect ratios near unity. This class of microstructure is recommended for Ti-6A1-4V surgical implants.

The mechanical properties of (a + P) titanium alloys are dictated by the amount, size, shape, and morphology of the a phase and the density of alp interfaces. The tensile and fatigue properties of Ti-6A1-4V have been studied extensively. Microstructures with a small (<20 pm) a-grain size, a well-dispersed P phase, and a small alp interface area, such as in equiaxed microstructures, resist fatigue crack initiation best and have the best high-cycle fatigue strength (approxi-

Fig. 16-2 Microstructure of equiaxed Ti-6AI-4V

(x 200).Equiaxed microstructures are characterized by small, rounded a-grains, with aspect ratios near unity.

mately 500 to 700 MPa). Lamellar microstructures, which have a greater a/p surface area and more oriented colonies, have lower fatigue strengths (approximately 300 to 500 MPa) than do equiaxed microstructures.

CAST TITANIUM

Based on the attributes, extensive knowledge, and clinical success of wrought titanium implants, interest has developed in cast titanium for dental applications. Although titanium has been cast for more than 50 years, it has only been recently that nearly precision castings have been attainable. For aerospace and medical components, hot isostatic pressing and specific finishing techniques are routinely practiced. However, these techniques are beyond the capabilities and affordability of most dental laboratories.

The two most important factors in casting titanium-based materials are its high melting point (= 1700' C for c.p. Ti) and chemical reactivity. Because of the high melting point, special melting procedures, cooling cycles, mold material, and casting equipment to prevent metal contamination are required. Titanium readily reacts with gaseous elements such as hydrogen, oxygen, and nitrogen, particularly at high temperatures (>600•‹C). As a result, any manipulation of titanium at elevated temperatures must be

490 Chapter 16 CAST AND WROUGHT BASE-METAL ALLOYS

performed in a well-controlled vacuum. Without a well-controlled vacuum, titanium surfaces will be contaminated with a case, an oxygenenriched and hardened surface layer, which can be as thick as 100 pm. Surface layers of this thickness reduce strength and ductility and promote cracking because of the embrittling effect of the oxygen. The technology required to overcome these factors is what makes casting titanium so expensive.

Because of the high affinity titanium has for hydrogen, oxygen, and nitrogen, standard crucibles and investment materials cannot be used. Investment materials must have oxides that are more stable than the very stable titanium oxide, and must also be able to withstand a temperature sufficient to melt titanium. If this is not the case, oxygen is likely to diffuse into the molten metal. Investment materials such phosphate-bonded silica and phosphate investment materials with added trace elements achieve this goal. It has been shown that with magnesium oxide-based investments, internal porosity results.

Because of the low density of titanium, it is difficult to cast in conventional, centrifugal-force casting machines. In the last 10 to 15 years, advanced casting techniques, which combine centrifugal, vacuum, pressure, and gravity casting, new investment materials, and advanced melting techniques (e.g., electric arc melting) have been developed. These advances have improved the feasibility of casting titanium-based materials in the dental laboratory.

Pure titanium has been cast into crowns, partial dentures, and complete denture bases. Titanium alloys have a lower melting point than pure titanium. By alloying titanium, the melting temperature can be lowered to the same temperature as that of nickel-chromium and cobalt-chromium alloys. For example, the Ti-Pd and Ti-Cu alloys have melting points of 1350" C. Lower casting temperatures may also reduce the reactivity of titanium with oxygen and other gases. Binary and ternary titanium-based alloys have been cast. Ti-13Cu-4.5Ni has been cast into crowns and partial dentures using vacuum investmentcasting technology. Other titanium alloys, such as Ti-6A1-4V,Ti-15V, Ti-20Cu, Ti-30Pd,Ti-Co, and

Fig. 16-3Microstructure of as-cast Ti-6AI-4V.

Ti-Cu are still in the experimental stages and have not yet been implemented in any large clinical studies.

Microstructures of cast titanium materials are similar to those described previously, namely coarse lamellar grains, a result of slow cooling through the P to a or 3j to (a + P) transformation temperature (Fig. 16-3).

The mechanical properties of cast c.p. Ti are similar to those of Types I11 and IV gold alloy, whereas cast Ti-6A1-4V and Ti-15V exhibit properties, except for modulus, similar to those of nickel-chromium and cobalt-chromium alloys. Because of the coarse and heterogeneous microstructure, the properties of cast titanium can be nonuniform.

Recently, cast Ti-6A1-4V microstructures have been refined by temporary alloying with hydrogen. The resulting microstructures (Fig. 16-4) can have a-grain sizes less than 1 ym, aspect ratios near unity, and discontinuous grain-boundary a, microstructural attributes that increase tensile and fatigue strength. These changes in microstructural form and structure result in significant increases in yield strength (974 to 1119 MPa), ultimate strength (1025 to 1152 MPa), and fatigue strength (643 to 669 MPa) as compared with respective values for lamellar (902, 994, and 497 MPa) and equiaxed microstructures (914, 1000, and 590 MPa).

Pure titanium has been cast with a pressure-

Fig, 16-4 Microstructure of hydrogen-alloy-treated Ti-6AI-4V(x 200).

vacuum casting machine. Other researchers have developed a casting machine that uses an electric arc that melts the titanium in an argon atmosphere. Melting is followed by pressurized casting in a copper crucible and investment in phosphate-bonded silica investment. Such a machine provides a relatively oxygen-free environment and, with the use of a tungsten arc, can reach temperatures of 2000" C. This latter casting regime has been used to cast c.p. Ti crowns and full-denture bases. Crowns cast in this manner have been evaluated clinically, and results revealed that, although fitness was inferior to that of silver-palladium alloy, it was superior to that of nickel-chromium. Occlusal adjustment was no more difficult than with conventional crowns, and discoloration, occlusal wear, and plaque retention were similar to other metals. The clinical results of full-denture bases cast in this manner have not been as good.

Observations of randomly chosen cast crowns have revealed gross surface porosities, to a depth of 75 ym, on both the inside and outside of the surfaces, Mechanical polishing is insufficient to remove this porosity. Internal porosities, sometimes measuring up to 30% of the cross-sectional area, are also readily observed. Surfaces of castings can also be contaminated with a case. The cause of the a case is probably poor vacuum control or mold material contamination. For op-

timum functionality of the final casting, the surface layer must be removed during finishing. However, even after the a case is removed, internal oxidation can remain and con~promise the mechanical properties of the final appliance. Further examination of such castings has also revealed multiple microcracks at the edges of the margins. Some cracks are as long as 100 ym. Cracks of this length are catastrophic to a notchsensitive material such as titanium.

As outlined, the difficulties with cast titanium for dental purposes include high melting point and high reactivity, low casting efficiency, inadequate expansion of investment, casting porosity, and difficulty in finishing this metal. From a technical standpoint, titanium is difficult to weld, solder, machine, finish, and adjust. Casting titanium requires expensive equipment. As with any new material or technology, specific casting techniques must be developed, which take time, effort, and money.

DENTAL IMPLANTS

The objectives and intended function of dental implants are to restore function to the oral cavity. To fulfill this requirement over an extended period, several other objectives must be met. The implant must be capable of carrying occlusal stresses. In addition, stresses must be transferred to the adjacent bone. Not only must stresses be transferred, they must also be of a "correct" orientation and magnitude so tissue viability is maintained in as near a physiological state as possible. The ability to transmit stress largely depends on attaining interfacial fixation. Thus two further requirements are that (1) the interface stabilize in as short a time as possible postoperatively, and (2) once stable, the interface remain stable for as long a time as possible. Designing an "optimal" implant that meets all of the above objectives requires the integration of material, physical, chemical, mechanical, biological, and economic factors.

In analyzing an implant/tissue system, three aspects are important: (1) the individual constituents, namely the implant materials and tissues;

(2) the effect of the implant and its breakdown