Making A Connection
Various techniques employing DENTSPLY products can enhance denture tooth adhesion to acrylic bases.
By George McIntosh, AAS, CDT
The number of denture wearers in North America is expected to increase over the next decade.1 The use of hardened plastic teeth has become an industry standard—and while the adhesion of these teeth to denture bases is generally very good, the de-bonding of plastic teeth from acrylic denture bases can be a frustration for the patient, dentist, and laboratory.2-4 Multiple techniques, backed by research, have been recommended to enhance this bond.5
Based on the literature and the author’s experience, denture tooth “pop off” is mainly related to fabrication technique.6 It is essential to properly understand and follow industry-standard techniques as defined in directions for use and educational materials. Perfect technique is even more important with screw-retained implant dentures, due to the often-limited amount of tooth structure and denture base material.7
There are many causes for tooth adhesion failure, such as tooth breakage (Figure 1) and lack of acrylic adhesion, most of which can be traced back to technical errors or oversights during the denture-processing phase. This article will detail some key techniques that the dental technician can employ to enhance tooth adhesion to acrylic bases.
Waxing with Attention
The necks of the denture teeth should be covered with a sufficient gingival roll of wax to provide bulk and strength after the wax is converted to acrylic. The wax must extend to the collar of the denture teeth. A fully contoured denture wax-up with adequate bulk at the gingival crest provides strength to help retain the tooth (Figure 2).
Cleaning the Boil Out Tank
In the author’s experience, the most likely cause of tooth pop off is contamination in boil out tanks. The residue is more than likely residual wax from the denture boil out (Figure 3). The boil out tanks should be cleaned thoroughly every day prior to filling with water (Figure 4). With busier facilities, this cleaning may be needed at midday as well, or perhaps even more often.
One method that has proven successful is to allow the previous day’s wax residue to remain in the tank overnight, where it will harden as it cools, forming a sheet of wax on the surface of the water. In the morning, this sheet is simply lifted out of the tank and discarded. The remaining dirty water is heated to sufficiently soften the wax remaining in the tanks, and this is scrubbed off using an appropriate wax solvent with heavy-gauge steel wool scrubbing pads. All of these materials are disposed of according to local requirements.
Filling the tank with distilled vinegar and allowing it to stand overnight or over the weekend usually dissolves or removes any hard-scale formations on the tank’s walls and bottom. Depending on the amount of build-up, more than one soaking might be required.
Two tanks of rapidly boiling water should always be used. One tank is kept free of wax. In facilities that have boil out tanks that re-circulate the water in the tank, this is especially problematic. This configuration requires that two tanks be used—one containing the dirty water to soften and remove the bulk wax, the other containing clean water. After the bulk wax is removed, a solvent is scrubbed in the molds and then rinsed into the dirty water, using water from the clean tank. Ivory laundry detergent powder is an excellent solvent because it is a pure soap with no additives. It also provides an extra scouring action to help remove the wax. Simply dip a wet stiff-bristle brush into the soap, and then scrub the molds. If using commercially available soaps, make sure they do not contain additives, such as fabric softeners, bleaches, etc.
Ensure that all residual wax is removed from the surfaces of the denture teeth. Even the cleanest boil out tanks can contain enough wax to leave a micro-layer on the tooth surfaces. A wax remover, such as Eclipse®, is useful in removing wax contamination when applied to the processed base plate.
Applying Tinfoil Substitute
When applying tinfoil substitute, such as Alcote®, do not allow it to come in contact with the ridge lap areas of the teeth. Close examination with magnifying loupes can be used to check this procedure. If a layer is on the teeth, it must be removed. Some of these products will form a thin layer on the ridge lap, preventing the acrylic from bonding. Take care not to allow the tinfoil separator to puddle around the necks of the teeth, as this can cause a micro space, resulting in no bonding and a space that will eventually collect stains in the finished product. A #3 camel-hair brush is effective for applying Alcote around the denture teeth because of its smaller size (Figure 5). Silicone putty, such as Trixa®, can also be placed around the denture teeth during flasking procedures, which eliminates the need to paint tinfoil substitute around the denture teeth.
Bench Curing Acrylics
Most acrylics require “bench curing,” allowing the closed, packed flask to remain on the bench prior to commencing curing processes. Depending on the type of tooth being used, this step can take 15 minutes for conventional plastic teeth and up to 30 minutes for hardened teeth (Figure 6). This allows time for the free monomer to impregnate the teeth, thus providing a better bond. Follow the manufacturer’s directions to determine the appropriate liquid-powder ratios for the acrylic powder and liquid (Figure 7).
Using Diatorics and/or Bonding Agents
Mechanical retention (diatorics) can also be placed in the teeth. When this technique is used, make sure not to place the diatorics where the tooth can possibly be weakened, such as under the central fossae of posterior teeth. A better diatoric location for posterior teeth would be under two cusps at slightly diverging angles. The diatorics can also be slightly belled out inside to improve retention. A No. 4 round bur is suitable for this procedure.
The tooth ridge lap area can be lightly roughened using a bur. This procedure should routinely be carried out with hardened plastic teeth. Some technicians use a cotton swab slightly moistened with monomer to “prime” the tooth surface. They must make sure the monomer does not puddle around the necks of the denture teeth.
When using highly cross-linked dense teeth, such as Portrait® IPN®, a proven bonding agent like Eclipse (Figure 8) has a positive effect on denture tooth adhesion, when compared to the use of a ground surface with monomer or even diatorics. A recent study compared the bond strength of IPN teeth with conventional acrylics (Lucitone 199®) and VLC denture bases (Eclipse) after a rigorous thermo-cycling regimen (Figure 9).8 The use of the bonding agent enhanced bond strength measured. Elimination of diatoric placement also reduces technique sensitivity and process time.
Manufacturer’s curing techniques (times and temperatures) must be followed exactly. When the directions call for boiling, a rolling boil is intended. Using improper recovery techniques after the denture is processed can also cause tooth pop off. The industry-standard practice is to use a hammer or power-driven chisel to break away the stone mold from the denture (Figure 10). When this method is used, the impact may possibly cause micro-fractures or total fractures of the acrylic around the tooth. If a micro-fracture occurs, then the total failure can occur long after the appliance leaves the laboratory and is delivered to the patient. Most flask manufacturers provide a specialized tool for de-flasking (Figure 11), which should be used, if available.
If none are available, a careful technique must be employed. One method is to use a heavy cut-off disk and score the stone to a sufficient depth at several locations around the mold, then split them by twisting a knife in the splits, breaking the mold into smaller sections. In conjunction with this technique, a heavy-body silicone putty, such as Trixa, can be adapted around the teeth during flasking procedures. Tools like plaster nippers can also be used.
Finishing and Polishing
During finishing and polishing procedures, take care to maintain control of the appliance, not allowing it to be pulled abruptly out of the hands and striking the sides of the polishing bins or the floor. The ultimate goal of employing these various techniques through the denture fabrication process is to achieve a glossy finish that meets the satisfaction of the dentist and the patient (Figure 12).
As the number of denture wearers increases, it is critical to pay special attention to removable appliance fabrication in order to ensure patient satisfaction and to improve esthetics, phonetics, and function. Enhancing denture tooth bond to an acrylic base involves many process improvements and technique refinements. When the laboratory follows the manufacturer’s directions and employs the procedures described above, the result will be improved adhesion and enhanced patient satisfaction.
George McIntosh, AAS, CDT, is a technical specialist at DENTSPLY Prosthetics, with more than 40 years of diverse experience in the dental industry.
1. Douglass CW, Shih A, Ostry L. Will there be a need for complete dentures in the United States in 2020? J Prosthet Dent. 2002; 87(1):5-8.
2. Suzuki S, Sakoh M, Shiba A. Impact resistance of highly crosslinked plastic denture teeth. J Biomed Mater Res. 1990; 24(12):1661-1671.
3. Craig RG. Denture materials and acrylic base materials. Curr Opin Dent. 1991; 1(2):235-243.
4. Patil SB, Naveen BH, Patil NP. Bonding acrylic teeth to acrylic resin denture bases: a review. Gerodontology. 2006; 23(3):131-139.
5. Saavedra G, Valandro LF, Leite FP, et al. Bond strength of acrylic teeth to denture base resin after various surface conditioning methods before and after thermocycling. Int J Prosthodont. 2007; 20(2):199-201.
6. Cunningham JL, Benington IC. An investigation of the variables which may affect the bond between plastic teeth and denture base resin. J Dent. 1999; 27(2):129-135
7. Williamson RA. Evidence-based treatment for the edentulous patient. Dent Today. 2010; 29(10):140, 142-145.
8. Fletcher-Stark ML, Chung KH, Rubenstein JE, et al. Shear bond strength of denture teeth to heat- and light-polymerized denture base resin. J Prosthodont. 2011; 20(1):52-59.
The manufacturer provided the preceding material. The statements and opinions contained therein are solely those of the manufacturer and not of the editors, publisher, or the Editorial Board of Inside Dental Technology.