CHAPTER 15 Impression Materials and Procedures for Removable Partial Dentures
Impression materials used in the various phases of partial denture fabrication may be classified as rigid, thermoplastic, or elastic substances. Rigid impression materials are those that set to a rigid consistency. Thermoplastic impression materials are those that become plastic at higher temperatures and resume their original form when cooled. Elastic impression materials are those that remain in an elastic or flexible state after they have set and have been removed from the mouth.
Although rigid impression materials may be capable of recording tooth and tissue details accurately, they cannot be removed from the mouth without fracture and reassembly. Thermoplastic materials cannot record minute details accurately because they undergo permanent distortion during withdrawal from tooth and tissue undercuts. Elastic materials are the only ones that can be withdrawn from tooth and tissue undercuts without permanent deformation and therefore are used generally for making impressions for removable partial dentures, immediate dentures, crowns, and fixed partial dentures when tooth and tissue undercuts and surface detail must be recorded with accuracy.
One type of rigid impression material is plaster of Paris, which has been used in dentistry for over 200 years. Although all plaster of Paris impression materials are handled in approximately the same manner, the setting and flow characteristics of each manufacturer’s product will vary. Some are pure and finely ground with only an accelerator added to expedite setting within reasonable working limits. Others are modified impression plasters to which binders and plasticizers have been added to permit limited border manipulation while the material is setting. These do not set as hard or fracture as cleanly as pure plaster of Paris and therefore cannot be reassembled with as much accuracy if fracture occurs.
Plaster of Paris was once the only material available for removable partial denture impressions, but now elastic materials have completely replaced the impression plasters in this phase of prosthetic dentistry. It can be used for making accurate transfers of abutment castings or copings in the fabrication of fixed restorations and internal attachment dentures and for making rigid indexes and matrices for various purposes in prosthetic dentistry. Modified impression plasters may be used to record maxillomandibular relationships.
A second type of rigid impression material is metallic oxide paste, which is usually some form of a zinc oxide–eugenol combination. A number of these pastes are available; however, they are not used as primary impression materials and should never be used for impressions that include remaining natural teeth. They also are not to be used in stock impression trays.
Metallic oxide pastes are manufactured with a wide variation of consistencies and setting characteristics. For convenience, most of them are dispensed from two tubes; this enables the dentist to dispense and mix the correct proportion from each tube on a mixing slab. The previously prepared tray for the edentulous ridge segments is loaded and positioned in the mouth with or without any attempt at border molding. Border molding with metallic oxide impression pastes is not advisable because wrinkles will occur if movement is permitted at the time the material reaches its setting state.
As with all impression techniques, the accuracy of the primary impression and of the impression tray has a great influence on the final impression. Some metallic oxide pastes remain fluid for a longer period than do others, and some manufacturers claim that border molding is possible. In general, however, all metallic oxide pastes have one thing in common with plaster of Paris impression materials: they all have a setting time during which they should not be disturbed and after which no further border molding is effective.
Metallic oxide pastes, which are rigid substances, can be used as secondary impression materials for complete dentures and for extension base edentulous ridge areas of a removable partial denture if a custom impression tray has been properly designed and attached to the removable partial denture framework (see Chapter 16).
Metallic oxide pastes can also be used as an impression material for relining distal extension denture bases and may be used successfully for this purpose if the original denture base has been relieved sufficiently to allow the material to flow without displacement of the denture or the underlying tissues.
Like plaster of Paris, modeling plastic is among the oldest impression materials used in prosthetic dentistry. This material is most often used for border correction (border molding) of custom impression trays for Kennedy Class I and Class II removable partial denture bases. It is manufactured in several different colors, each color being an indication of the temperature range at which the material is plastic and workable. A common error in the use of modeling plastic is that it is often subjected to higher temperatures than intended by the manufacturer. It then becomes too soft and loses some of its favorable working characteristics. If a temperature-controlled water bath is not used, a thermometer should be used to maintain the water temperature. If modeling plastic is softened at a temperature above that intended by the manufacturer, the material becomes brittle and unpredictable. Also, there is the ever-present danger of burning the patient when the temperature used in softening the modeling plastic is too high.
The most commonly used modeling plastic for corrected impressions of extension base areas is the red (red-brown) material, in cake form, that softens at about 132° F. It should never be softened at temperatures much above this. Neither it nor any other modeling plastic should be immersed in the water bath for an indefinite period. It should be dipped and kneaded until soft and subjected to no more heat than necessary before the tray is loaded and it is placed in the mouth. Then it may be flamed with an alcohol torch for the purpose of border molding, but it should always be tempered by being dipped back into the water bath before its return to the mouth to avoid burning the patient. The modeling plastic then may be chilled using a water spray before removal from the mouth, although this is not necessary if care is used in removing the impression. During sectional flaming and border molding, the modeling plastic should be chilled in ice water after each removal from the mouth; then it may be trimmed with a sharp knife without danger of fracture or distortion.
Red, gray, and green modeling plastics are obtainable in stick form for use in border molding an impression or an impression tray. The green material is the lowest fusing of the modeling plastics. The red and gray sticks have a higher and broader working range than do the cakes of like color so they may be flamed without harming the material. The gray material in stick form is preferred by some dentists for border molding because of its contrasting lighter color. The choice between the use of green and gray sticks is purely optional and entirely up to the dentist.
Some dentists still prefer to use modeling plastic as a secondary impression material to record edentulous ridges in removable partial denture fabrication. When this is done, it is generally used only as a means of building up the underside of the denture before the tissues are recorded with some secondary impression material (see Chapter 16).
A second group of thermoplastic impression materials consists of those impression waxes and resins commonly spoken of as mouth-temperature waxes. The most familiar of these have been the Iowa wax (Kerr Co., Romulus, MI) and the Korecta waxes (D-R Miner Dental, Concord, CA), all of which were developed for specific techniques.
The Iowa wax was developed for use in recording the functional or supporting form of an edentulous ridge. It may be used as a secondary impression material or as an impression material for relining the finished removable partial denture to obtain support from the underlying tissues. The mouth-temperature waxes lend themselves well to all relining techniques as they will flow sufficiently in the mouth to avoid displacement of tissues. As with any relining technique, it is necessary that sufficient relief and venting be provided to give the material the opportunity to flow.
The difference between impression wax and modeling plastic is that impression waxes have the ability to flow as long as they are in the mouth and thereby permit equalization of pressure and prevent displacement. The modeling plastics flow only in proportion to the amount of flaming and tempering that can be done outside of the mouth; this does not continue after the plastic has approached mouth temperature. The principal advantage of mouth-temperature waxes is that, given sufficient time, they permit a rebound of those tissues that may have been forcibly displaced.
The impression waxes also may be used to correct the borders of impressions made of more rigid materials, thereby establishing optimum contact at the border of the denture. All mouth-temperature wax impressions have the ability to record border detail accurately and include the correct width of the denture border. They also have the advantage of being correctable.
Mouth-temperature waxes vary in their working characteristics. They are designed primarily for impression techniques that attempt to record the tissues under an occlusal load. In such techniques, the occlusion rim or the arrangement of artificial teeth is completed first. Mouth-temperature wax is then applied to the tissue side of the denture base, and the final impression is made under functional loading by using various movements to simulate functional activity. These mouth-temperature materials also may be used successfully in open-mouth impression techniques. Iowa wax will not distort after removal from the mouth at ordinary room temperatures, but the more resinous waxes must be stored at much lower temperatures to avoid flow when they are out of the mouth. Resinous waxes are not ordinarily used in removable partial denture impression techniques except for secondary impressions.
Reversible (agar-agar) hydrocolloids, which are fluid at higher temperatures and gel on reduction in temperature, are used primarily as impression materials for fixed restorations. They demonstrate acceptable accuracy when properly used; however, the reversible hydrocolloid impression materials offer few advantages over the irreversible (alginate) hydrocolloids when used as a removable partial denture impression material. Present-day irreversible hydrocolloids are sufficiently accurate for making master casts for removable partial dentures. However, border control of impressions made with these materials is difficult.
Irreversible hydrocolloids are used for making diagnostic casts, orthodontic treatment casts, and master casts for removable partial denture procedures. Because they are made of colloid materials, neither reversible nor irreversible hydrocolloid impressions can be stored for any length of time, but must be poured immediately.
These materials have low tear strength, provide less surface detail than other materials (e.g., mercaptan rubber base), and are not as dimensionally stable as other materials. They can, however, be used in the presence of moisture (saliva); are hydrophilic; pour well with stone; have a pleasant taste and odor; and are nontoxic, nonstaining, and inexpensive. The combination reversible-irreversible hydrocolloids have demonstrated a tendency to separate and should be used with that understanding. The hydrocolloids can be acceptably disinfected with a spray solution of 2% acid glutaraldehyde, stored in 100% humidity, and poured within 1 hour.
The mercaptan rubber–base (Thiokol) impression materials can also be used for removable partial denture impressions and especially for secondary corrected or altered cast impressions. To be accurate, the impression must have a uniform thickness that does not exceed 3 mm ( inch). This necessitates the use of a carefully made individual impression tray of acrylic-resin or some other material possessing adequate rigidity and stability. Those materials that are highly cross-linked (medium and heavy body) do not recover well from deformation and should not be used when large or multiple undercuts are present. For example, when large numbers of teeth with natural tooth contours that display multiple undercuts remain, these materials will be subjected to clinically significant distortion upon withdrawal. The long-term dimensional stability of these materials is poor because of water loss after setting. The material must be held still during the impression-making procedure because it does not have a snap set; it should be allowed to rebound for 7 to 15 minutes after it is removed from the mouth and should then be poured immediately. Many of these materials have an unpleasant odor and can stain clothes. These materials are moderately inexpensive, have high tear strength and long working and setting times (8 to 10 minutes), and can be disinfected in liquid, cold-sterilizing solutions. The accuracy of mercaptan rubber base is acceptable for making impressions for removable partial dentures; however, as with hydrocolloid impression materials, certain precautions must be taken to avoid distortion of the impression. Mercaptan rubber–base impression materials do have an advantage over hydrocolloid materials in that the surface of an artificial stone poured against them is of a smoother texture and therefore appears to be smoother and harder than one poured against a hydrocolloid material. This is probably so because the rubber material does not have the ability to retard or etch the surface of the setting stone. Despite their accuracy, this has always been a disadvantage of all hydrocolloid impression materials. The fact that a smoother surface results does not, however, preclude the possibility of a grossly inaccurate impression and stone cast resulting from other causes. Rubber-base impression materials possess a longer setting time than the irreversible hydrocolloid materials and lend themselves better to border molding in adequate supporting trays.