682 Chapter 21 PROSTHETIC APPLICATIONS OF POLYMERS

be boiled for 5 minutes and then cooled to room temperature before use.

Vacuum-formed polystyrene is also used to make impression trays and baseplates. This material is popular with commercial laboratories because the trays and record bases can be made rapidly. These trays must be handled carefully because they are more flexible than acrylic trays and can be deformed easily by the application of heat. Prefabricated impression trays are very popular with dentists. These stock trays vary considerably between manufacturers. Stock trays

are not as rigid as most custom trays and must be used with care to avoid distortion.

Light-activated tray materials have become quite popular and have many advantages over chemically accelerated acrylic. They are similar to light-activated denture base materials but are of a different color. The trays are strong, easy to make, contain no methyl methacrylate, and have negligible polymerization shrinkage in the light chamber. They can be used soon after processing because there is no clinically significant dimensional change after polymerization.

Problem 1

A denture is to be made for a patient who has previously fractured a denture several times. How will this influence your choice of a denture base material?

Solution

A rubber-reinforced acrylic is the material of choice because this type of denture base material exhibits superior impact resistance and flexural fatigue strength. A metal base also might be used in cases where maximum strength is desired.

Problem 2

A denture is delivered from a dental laboratory with obvious porosity in the denture base. Why might this have occurred? Where is the porosity most likely to occur, and why?

Solution

Several conditions may have caused the porosity. Most commonly, the processing temperature exceeded 74" C. Other causes may be insufficient flask pressure, excessive monomer in the mix, and packing of the material before it reached the doughy consistency. Porosity almost always will occur in the thicker

sections of acrylic because the polymerization reaction is exothermic and the heat cannot be dissipated as quickly from the larger mass.

Problem 3

A patient is having a maxillary denture made that opposes a natural dentition in the mandible. Should acrylic or porcelain teeth be used, and why?

Solution

Acrylic teeth should always be used when opposing natural teeth or restored teeth. Porcelain teeth are abrasive against these surfaces, and excessive wear will quickly occur.

Problem 4

A patient has a history of sensitivity to denture base materials. In making a new denture for this patient, what might decrease the potential for an allergic reaction?

Solution

Residual monomer is almost always the cause of an allergic or toxic reaction to a denture base material. To reduce the residual monomer content to a very low level, the denture should be boiled for at least 1hour at the end

of the processing cycle. Another option is to select a vinyl acrylic or a light-activated denture base material.

Problem 5

A new set of dentures is delivered to a patient, and there is no retention on the maxillary denture. At the trial-denture stage, there was good retention of the baseplate. What are the probable causes?

Solution

Assuming that a denture base material had passed the ANSI/ADA Specification test, the most probable causes are improper processing variables, such as processing too rapidly and at elevated temperatures during the early stages, deflasking before the flask has benchcooled to near room temperature, quenching the hot flask in cold water, or overheating the denture during trimming and polishing. These processing variables can affect the accuracy of even the best denture base materials.

Problem 6

A finished denture is returned from the laboratory, and in several areas there is insufficient material in the interproximal areas. What is the probable cause?

Solution

The probable cause is packing the denture acrylic when it is not in the doughy consistency. Because of the lower viscosity, even with adequate packing pressure the material will squeeze out of the flask before it is forced into all areas of the mold cavity. Conversely, if the acrylic dough has reached the rubbery consistency, it may be too viscous to fill all areas of the mold.

Problem 7

A patient has fractured a lingual cusp and comes to the office for emergency care. You would like to prepare the tooth for a gold crown, and have your dental assistant make a custom tray for the final impression while you are preparing the tooth. In terms of the di-

mensional accuracy of the tray, what must be considered?

Solution

Autopolymerizing tray materials continue to change dimensionally during the first few hours after they are made. Use of a tray before this polymerization is complete can affect the accuracy of the impression. Two things can be done to eliminate this problem: (1) have the dental assistant boil the tray for a short time to force the polymerization reaction to completion, or (2) use a light-activated tray material that experiences no significant dimensional change after processing.

Problem 8

A patient has excessive wear on the anatomical areas of a denture caused by aggressive cleaning with an abrasive cleaner and a toothbrush. What instructions might you give the patient for cleaning the new denture that you will be making?

Solution

Use a soft toothbrush and soap and water (a nonabrasive hand soap) to regularly clean the denture. The patient may also soak the denture occasionally in a commercial cleanser or a solution of 1 tsp of a hypochlorite, such as Clorox, and 2 tsp of Calgon in half a glass of water. Some immersion cleansers have also been demonstrated effective as sanitizing agents.

Problem 9

A patient needs a soft denture liner for an extended period. (a) What material would you choose, and what would you tell the patient?

(b) What problems are associated with these materials?

Solution a

Silicone and plasticized acrylics will both work, although silicones are generally softer with a lower Shore A hardness value. The patient should be told that any soft liner will have to be replaced occasionally because they

684 Chapter 21 PROSTHETIC APPLICATIONS OF POLYMERS

are not as long-lasting as the denture base material.

Solution b

Growth of microorganisms, Candida albicans in particular; poor adhesion to the denture base; and poor tear strength are deficiencies normally associated with these materials.

Problem 10

A patient requires a maxillofacial prosthesis that includes an area of movable tissue near the mouth. How might a silicone maxillofacial material be modified to be more elastic in this instance?

Solution

Medical-grade silicone adhesive can be combined with an RTV silicone base at various ratios to control the elastic properties. By selecting the proper ratio, the stress-strain profiles of the silicone elastomer can be matched to that of human facial tissues. This is most important when the prosthesis rests on movable tissue.

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J Prosthet Dent 57:639, 1987.

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Denture Liners

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Duran RL, Powers JM, Craig RG: Visco-

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Graham BS, Jones DW, Sutow EJ: Clinical implications of resilient denture lining material research. 11. Gelation and flow properties

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Kawano F, Dootz ER, Koran A et al: Sorption and solubility of 12 soft denture liners,

J Prosthet Dent 72:393, 1994.

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Denture Teeth

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688 Chapter 21 PROSTHETICAPPLICATIONSOF POLYMERS

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Maxillofacial Materials

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Kouyoumdjian J, Chalian VA, Moore BK:

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Temporary Fixed Partial Denture Materials

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Tray Materials

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J Prosthet Dent 47:279, 1982.