Сraig. Dental Materials

tion. These solutions, excluding saline, also corrode stainless steel at room temperature. Therefore, irrigants should be rinsed from the instruments as soon as possible after use.

NICKEL-TITANIUM ENDODONTIC

INSTRUMENTS

The alloys used in nickel-titanium root canal instruments contain about 56% Ni and 44% Ti by weight, which calculate to be 50% of each by atoms. In some instances, <2% of cobalt may be substituted for nickel. Of special interest for this application is that these alloys can change their structure from austenitic (body-centered cubic) to martensitic (close-packed hexagonal) as a function of stress during root canal preparation. When the instrument is used in the canal, the rate of stress levels off due to progressive deformation even if strain is increased due to the transformation to martensite. Note the modulus of Ni-Ti austenite is 120 GPa, and that of martensite is 50 GPa. This effect results in what is termed super-elasticity, When the stress decreases, springback occurs without permanent deformation and a return to the austenitic phase.

The super-elasticity of Ni-Ti permits deformations of 8% strain in endodontic files with complete recovery. This value compares to 4 % with stainless steel instruments. In addition, the Ni-Ti alloys have higher strengths and lower moduli of elasticity than stainless steel, advantages in preparing curved root canals. Also, it has been shown that Ni-Ti and stainless steel endodontic files do not differ with respect to corrosion resistance.

These improved properties of Ni-Ti root canal instruments have permitted them to be effectively used as engine-driven, rotary instruments. In spite of these improved properties, Ni-Ti instruments can fracture. Studies of cyclic fatigue have shown that the radius of curvature of the file was the most important factor in fatigue resistance, with decreasing radius of curvature (increasing diameter) resulting in decreased fracture time. The fracture was always of a ductile type, suggesting that cyclic fatigue was the major cause of failure.

One drawback of Ni-Ti endodontic instruments is that because of their super-elastic quality they must be manufactured by machining rather than by twisting of tapered wire blanks.

The instrument design must therefore be ground into the Ni-Ti tapered blanks, thus increasing their cost.

BASE-METAL PREFABRICATED CROWNS

Stainless steel crowns were introduced in 1950 and are recommended for the permanent restoration of primary teeth, particularly in children suffering from rampant caries or in situations where the crowns of the teeth are destroyed. The approximate composition of stainless steel used for crowns and its mechanical properties are listed in Table 16-6. A titaniumstabilized stainless steel may also be used. For purposes of comparison, the properties of a nickel-based alloy used for permanent prefabricated crowns and tin-based and aluminum-based alloys used for temporary prefabricated crowns are also shown. The mechanical properties of stainless steel and nickel-based materials are similar, and the high ductility is important in the clinical adaptation of the crowns. In addition, they have reasonable hardness and strength and, as a result, are classified as permanent restorations. The tin-based and aluminum-based tem-

porary types also have high ductility, but are soft and have lower yield and tensile strengths and thus d o not resist clinical wear as do the stainless steel and nickel-based types.

COMPOSITION

A cobalt-chromium-nickel alloy known as Elgiloy is available in wire and band form for various dental appliances. Elgiloy is nominally composed of 40% cobalt, 20% chromium, 15% nickel, 7% molybdenum, 2% manganese, 0.4% beryllium, 0.15% carbon, 15.4% iron, and 0.05% other. Of particular interest is the fact that beryllium serves to decrease the alloy's melting point, facilitating manufacturing. In composition, this alloy resembles the cast base-metal alloys more than it does stainless steel.

PROCESSING AND MANIPULATION

The orthodontic wire is supplied in various tempers (amounts of cold work): soft, ductile, semi-

spring temper, and spring temper. The wires are typically formed in a ductile condition, allowing them to be easily deformed and shaped into appliances and then heat-treated to maximize strength. The standard heat treatment, similar to the treatment used to relieve stress in a stainless steel wire, is 482' C for 7 minutes. Lowtemperature heat treatment causes a phase change and stress relief. Excessive heat treatment can cause embrittlement.

The fabrication and soldering techniques used with Elgiloy are similar to those used with stainless steel wires. Elgiloy wires should be soldered with a silver solder in the presence of a fluoride flux or joined by spot welding.

PROPERTIES

The properties of Elgiloy as received are similar to those of stainless steel wires; however, its properties can be modified slightly by the heat treatment (7 minutes at 482" C) used to stress relieve a stainless steel wire. The mechanical properties of spring-temper Elgiloy wire are proportional limit, 1610 MPa; 0.2% yield strength, 1930 MPa; tensile strength, 2540 MPa; and Vickers hardness number, 700 kg/mm2. The number of 90-degree bends until failure for 0.36-mm and 0.46-mm wires is listed in Table 16-7 for asreceived and heat-treated conditions. Note that temper, heat treatment, and wire size all affect this property. Bending moment-angular deflection curves for several Elgiloy wires are shown in Fig. 16-12.The stiffness in bending of the wires is similar; however, the angle at which permanent deformation occurs increases from soft-temperto spring-temper types. The permanent set after a 90-degree bend decreases from soft-temper to spring-temper types. Heat treatment for 7 minutes at 482' C causes an increase in the angle at which permanent deformation occurs but decreases the permanent set.

Awrought nickel-titanium alloy known as Nitinol was introduced as a wire for orthodontic appli-

For periodic updates,

From Craig RG, editor: Dental materials: a problem oriented approach, St Louis, 1978, Mosby.

*Heated at 482' C for 7 minutes.

ances in 1972. Nitinol is characterized by its high resiliency, limited formability, and thermal memory.

COMPOSITION AND SHAPE-MEMORY EFFECT

The industrial alloy is 55% nickel and 45% titanium and possesses a temperature transition range (TTR). At temperatures below the TTR, the alloy can be deformed plastically. When the alloy is then heated from below to above the TTR, a temperature-induced crystallographic transformation from a martensitic to an austenitic microstructure occurs and the alloy will return to its original shape. Hence, nickel-titanium is called a shape-memoly alloy. The orthodontic alloy contains several percent cobalt to lower the TTR. A number of variations of the Ni-Ti alloy have been developed in dentistry. Compositional variations lead to changes in the martensitic and austenitic start and finish temperatures and mechanical properties. Only those wires with austenitic finish temperatures less than 37" C exhibit superelasticity.

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Angular deflection (degrees)

Fig. 16-12 Bending moment-angular deflection curves for 0.46-mm diameter cobalt-chromium-nickel (Elgiloy)wires of the soft (***), ductile (DOO), ductile heat-treated for 7 minutes at 482" C (ooo), and spring temper (al *) types.

(From Craig RG, editor: Dental materials: a problem-oriented approach, St Louis, 1978, Mosby.)

PROPERTIES AND MANIPULATION

Angular deflection (degrees)

Fig. 16-13 Stored energy at a fixed bending moment below the proportional limit for 0.48-mm by 0.64-mm wires of alloys stainless steel (SS), beta titanium (TM), and nickel-titanium (NT). The stored energy is equal

to the shaded areas under the curve for each wire. The spring rate is equal to the slope of each curve.

(From Drake SR, Wayne DM, Powers JM et al: Am J Orthod

82:206, 198%)

be bent over a sharp edge or into a complete loop; thus the wire is more suited for use with pretorqued, preangulated brackets. The alloy is brittle and therefore cannot be soldered or welded, so wires must be joined mechanically.

Mechanical properties of an orthodontic nickeltitanium alloy are compared with those of stainless steel and a beta-titanium alloy in tension, bending, and torsion in Table 16-4. The nickeltitanium alloy has the lowest elastic modulus and yield strength but the highest springback (maximum elastic deflection). As shown in Figs. 16-13 and 16-14, nickel-titanium has the lowest spring rate but the highest resiliency in bending and torsion of the three alloys used for orthodontic wires. Clinically, the low elastic modulus and high resiliency mean that lower and more constant forces can be applied with activations and an increased working range. The high springback is important if large deflections are needed, such as with poorly aligned teeth. Nitinol wire requires special bending techniques and cannot

WROUGHT BETA-TITABJ

COMPOSITION AND MICROSTRUCTURE

A titanium-molybdenum alloy known as betatitanium was introduced in 1979 as a wrought orthodontic wire. As discussed previously, c.p. Ti exists in a hexagonal close-packed crystal lattice at temperatures below 883" C and in a bodycentered cubic crystal lattice at higher temperatures. These structures are referred to as alphatitanium and beta-titanium, respectively. The beta form of Ti can be stabilized at room temperature by alloying with certain elements. Beta-titanium alloy for dental use has the composition 78% titanium, 11.5% molybdenum, 6% zirconium, and 4.5% tin and is supplied as wrought wire.

Torque angle (degrees)

Fig. 16-14 Stored energy at a fixed torsional moment below the proportional limit for 0.48-mm by 0.64-mm wires of alloys stainless steel (SS), beta titanium (TM), and nickel-titanium (NT). The stored energy is equal to the shaded area under the curve for each wire. The spring rate is equal to the slope of each curve.

(From Drake SR, Wayne DM, Powers JM, Asgar K: Am J Orthod 82:206, 1982.)

MANIPULATION

Beta-titanium nrirescan be shaped easily. and the wires can he soldered and m-elded.Joints can be made by electrical resistance welding. Under proper welding conditions, minimum distortion of the cold-worked microstructure occurs.

PROPERTIES

Compared with stainless steel and Elgiloy wires, beta-titanium wire has lower force magnitudes, a

lower elastic modulus, higher springback (maximum elastic deflection), a lower yield strength, and good ductility, weldability, and corrosion resistance. The mechanical properties of betatitanium alloy in tension, bending, and torsion are compared with stainless steel and nickeltitanium alloys in Table 16-4 and Figs. 16-13 and 16-14. Beta-titanium alloy has values of yield strength, modulus of elasticity, and springback intermediate to those of stainless steel and Nitinol. Its formability and weldability are advantages over Nitinol, and it has a larger working range than do stainless steel or Elgiloy wires.

Recent developments in orthodontic wires include a titanium-based alloy (Ti-15V-3Cr-3A1- 3Sn) which is reported to offer a yield strength/ modulus ratio slightly greater than that of betatitanium. Fiber-reinforced thermoplastic wires have also been studied. Candidate fibers include fiberglass and aramid. Candidate resins include polycarbonate and polyethylene terephthalate glycol. For each residfiber system, there is a heating or working range where the material can be formed without property degradation. This temperature range is primarily related to the glass transition temperature (Tg) of the resin matrix. A temperature above the T, is necessary to allow sufficient softening. However higher temperatures will lead to structural changes and reduction in flexural modulus.

SUMMARY OF ORW

Properties of stainless steel, nickel-titanium, and beta-titanium wires in tension, bending, and torsion are compared in Table 16-4. Of the three types of wires, stainless steel wire has the highest values of yield strength, elastic modulus, and spring rate and the lowest springback (elastic deflection or yield strength/elastic modulus). Elgiloy is easily deformed and shaped.

Nickel-titanium alloy has the lowest elastic modulus and yield strength but the highest springback. Nickel-titanium has the lowest spring rate but highest resiliency in bending and torsion of the three alloys used for orthodontic wires. The disadvantages are that it is hard to bend and cannot be soldered, welded, or heattreated. Beta-titanium offers an intermediate force delivery system and greater formability and weldability.

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Certain alloys have considerable value for dental instruments and equipment because of their special properties. For example, Monel metal is an alloy of copper and nickel used for equipment parts because of its good physical properties and resistance to tarnish or corrosion. It has not been popular for constructing appliances placed in the mouth because of difficulties in manipulation. Such an alloy has a composition of approximately 28% copper, 68% nickel, 2% iron, 1.5% manganese, and 0.2% silicon.

Other stainless steels are also used, to lesser extents. A steel with 3% A1 may be heat-treated and can be used in wrought condition. Following forming, heat treatment at 900" C for 1hour leads to an increase in properties. The heat treatment forms coherent precipitate Ni,Al, which strainhardens the lattice.

An experimental Co-Cr alloy with addition of 4% to 6% Ti has been developed and reported to

have better fatigue resistance than the Co-Cr alloy alone.

Another recently developed alloy is Zr-I'd-Ru with an intermetallic compound structure. This alloy undergoes transformation toughening and has good fracture toughness. The stress-induced microstructural changes are also hypothesized to provide good wear resistance. The 30Ni-30Cu- 40Mn alloy is an experimental base-metal casting alloy. The hypothesized advantages of this alloy include a melting point of less than 1000" C and a resultant reduced tendency to absorb oxygen. The secondary alloying elements Al, In, and Sn further reduce the melting point, refine and reduce the dendritic microstructure, and increase hardness and casting accuracy. The secondary alloying elements also segregate to the intermetallic regions, leading to an increase in corrosion resistance.

The possibility of developing satisfactory substitutes for gold in dental appliances is far from being exhausted. Numerous new alloys are being developed for use by the engineering profession, some of which eventually may be found to be satisfactoiy for dental purposes. Many metals, such as tantalum, molybdenum, columbium, vanadium, and gallium, are becoming available in increasing quantities. These metals and their alloys, along with chromium, nickel, cobalt, titanium, stainless steel, and various copper, aluminum, or magnesium alloys, may be developed to possess physical and chemical qualities that satisfy the requirements of various dental applications.

I SELECTED PROBLEMS

Problem 1

The elastic modulus values of nickelchromium and cobalt-chromium alloys are about twice those of gold alloys, thus the thickness of a restoration in the direction of a bending force can be about halved and still

have the same deflection. Is this a justifiable conclusion? (Defend your answer.)

Solution

No. Although a stress-strain curve in tension might lead you to respond affirmatively, it

should be remembered that although the deflection of a beam is directly proportional to the modulus, it is inversely proportional to the cube of the thickness. As a result, halving the thickness of the nickel-chromium beam will dramatically increase the deflection as compared with that of the gold beam, even though it has twice the elastic modulus. It can be shown that only small decreases in thickness of the nickel-chromium beam can be made and yet maintain the same stiffness as the gold alloy beam.

Problem 2

During the soldering of a cobalt-chromium partial denture, the temperature of the torch became very high. What might be the expected effect on the properties of the cast framework?

Solution

Excessive heating of the framework will result in a decrease in the yield strength and the percent elongation. This can result from migration of atoms and formation of carbides, resulting in chromium depletion in the grains, which can cause increased corrosion.

Problem 3

Nickel-chromium alloys that can be cast into gypsum-bonded investments contain up to 2% beryllium to reduce the melting temperature. An alloy labeled as containing 77% nickel, 21% chromium, and 2% beryllium will contain 2 at% of Be. Is that statement correct? Defend your response.

Solution

No. The values listed are weight percentages. To obtain the atomic percent, you must first divide the weight percent of each element by the respective atomic number. These quotients are then added, and the atomic percentage calculated by dividing each quotient by the sum times 100. For example, 77 wt% Ni + 59 = 1.305, 21 wt% Cr + 52 = 0.404, and

2 wt% Be + 9 = 0.222. The sum of the quotients is 1.931, and 0.222 + 1.931 x 100 = 11.5 atomic % Be.

Problem 4

An austenitic 18-8 stainless steel orthodontic wire was bent and then heat-treated for 3 minutes, but was inadvertently heated to 816" C rather than the recommended 482" C. What effect would this have on the wire?

Solution

Substantial recrystallization and precipitation of trapped carbon atoms in the iron lattice of the wrought wire would occur, which would result in reduced stiffness and reduced resistance to permanent bending. The recommended heating to 482' C is to relieve stresses introduced during bending of the wire and is sufficiently low so that changes in stiffness and resistance to bending do not occur.

Problem 5

If you wished to select an orthodontic wire with the most constant force without reactivation as the tooth moved, would you select an 18-8 stainless steel, beta titanium, or nickeltitanium wire? Why?

Solution

Based on the bending and torsional moment versus deflection curves, a nickel-titanium wire should be chosen. This wire has the lowest spring rate for a given size but has the highest resiliency and the smallest decrease in force with movement of the tooth. Therefore longer times between activations and a more constant force result.

Problem 6

Endodontic stainless steel K files are used with a 90-degree clockwise rotation and withdrawal. If the tip of a file becomes bound in a root canal, why should care be taken not to rotate it much in the counterclockwise direction?

Solution

The files are twisted during construction so that if the tip is bound in a root canal, continued clockwise rotation will untwist the file, but counterclockwise rotation will twist the file tighter and possibly result in brittle rather than ductile fracture. As a result, these instruments fracture more readily in counterclockwise rotation, frequently in as little as a onequarter turn.

Problem 7

Why can the carbon content of berylliumor boron-containing nickel-chromium alloys be such a low value as 0.02% when 0.1% is needed in Ticonium and 0.5% in Vitallium?

Solution

Ticonium and Vitallium are used for partialdenture frameworks and require the high yield strength provided by the carbon content, while the berylliumand boron-containing nickel-chromium alloys are used for the copings for porcelain-fused-to-metal restorations, where high yield strength is not as critical but some ductility is advantageous.

Cast Base-Metal Alloys

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Wrought Base-Metal Alloys

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Titanium

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