Achieving a Secure Bond Between Denture Teeth and Base

Modern teeth and acrylics need to be extremely robust to stand up to the functional demands required both inside and outside the mouth, if they are removable. While the importance of dentures to patients’ quality of life in terms of their appearance and ability to speak clearly cannot be understated, of greatest importance to patient health is the restoration of the chewing function required for eating a varied diet. Dentures are exposed to very high loads during mastication, especially when chewing meat and hard foods, such as raw carrots. They may also face additional stresses imposed by patients’ oral para-functional habits including tooth clenching and/or grinding or chewing items unrelated to eating, such as pencils or fingernails. However, the strength of the material alone is not the only factor that impacts the bond between the denture teeth and the denture base, which must reliably ensure that partial, removable, and hybrid dentures—some of which contain metal—remain stable and functional over extended time periods. Various factors related to achieving this bond may concern the respective materials and the methods used to maximize the bond.^1,2

The Importance of the Bond

Because the connection between acrylic denture teeth and the denture base is essential for the integrity of various types of dentures, the quality of bonding of teeth is subject to regulation by the International Organization for Standardization (ISO). EN ISO 20795-1:20081 sets out minimum requirements for denture acrylics, and EN ISO 22112:20062 sets out minimum requirements for artificial teeth, including that they be securely bonded to the base; therefore both EN ISO 22112 and EN ISO 20795-1 require the quality of bonding to be tested. According to their guidelines, the quality of bonding is sufficient if the tooth or denture base suffer a fracture at the defined tensile load while the bond between them survives. Alternatively, the quality of bonding can be tested by applying increasing forces to a tooth attached to the base acrylic at an angle of 45 degrees.^3,4

Denture Base and Teeth Materials

Polymethyl methacrylate (PMMA), or acrylic glass, has long been the base material of choice for manufacturing denture bases and teeth.⁵

PMMAs, which are made up of a separate powder and liquid formula, have undergone vast improvements in recent years, due mainly to cross-linking and the addition of new ingredients designed to compensate for problems associated with pure PMMA such as abrasion, fracture resistance, color stability, and plaque formation.

As a denture base material in particular, modern formulations of cured PMMA offer many advantages including the ability to bond well with older materials to form a single unit and to be modified with extensions and relinings if a patient’s oral cavity changes in any way, including through the loss of additional teeth or alveolar ridge atrophy.

Denture teeth made of this material benefit similarly in many ways. However, the additional strength provided by these densely cross-linked dental acrylics diminishes the ability of the tooth to bond to the acrylic denture base, which requires that the cervix of the tooth be able to partially dissolve with the pink base acrylic.

Supporting the Bond

Monolithic Teeth

To accommodate the need for different material requirements in different regions of acrylic teeth, there are tooth lines with a multi-layered structure that can allow for adjustments in keeping with the requirements of each layer—eg, the incisal/enamel layer, which is exposed to high chewing loads, must be strongly resistant to abrasion, while the cervix, which forms the base of the tooth, should be less densely cross-linked and easier to partially dissolve in order to ensure an optimum bond with denture acrylics. These teeth are fabricated using separate moulds used for each layer, which is individually heat-polymerized, starting with the incisal layer and followed by the dentin core and cervix of the tooth. A study comparing the bond strengths of two brands of acrylic and one brand of multi-lithic denture teeth to acrylic denture base material found the bonding strength of multi-lithic denture teeth to denture base resins to be slightly higher than conventional acrylic denture teeth.⁶

Surface Treatments

Jain and others also observed that cross linking at the ridge lap region ‘acts as a double edge sword’ as it weakens the bond between denture base and tooth. They therefore tested the application of monomethyl methacrylate monomer and dichloromethane with no surface treatment acting as control on the ridge lap surface of the resin teeth before packing of the dough into the mould. They concluded that the application of dichloromethane significantly increased the bond strength between cross-linked acrylic resin teeth and heat-cured denture base resin.⁷

Also to strengthen the bond, there is support for roughening the teeth with a coarse diamond or sandpaper in addition to applying a bonding agent. A study by Sadar et al studied the effect of change in the surface treatment and surface configuration of the ridge lap surface of the teeth on retention of cross-linked acrylic teeth on cross-linked acrylic resin denture base.⁸ They found that sand papering the ridge lap surfaces improves the shear compressive bond and that maximum shear compressive bond strength can be increased by application of monomer.⁸ Similarly, a study that evaluated the effect of surface treatments on shear bond strength of denture teeth to heat-polymerized and auto-polymerized denture base resins concluded that monomer application and airborne particle abrasion of the ridge lap area of the denture teeth improved their shear bond strengths to the denture base resins regardless of the type of polymerization.⁹

Further evidence that acrylic tooth pretreatment should be considered to enhance bonding between tooth and base was a study that examined the role of conditioning liquid in improving bonding of acrylic denture teeth to MMA/PMMA and light-curing denture base materials. The authors of that study called pre-treatment of denture teeth “critical regarding their bond-strength to denture base materials and in turn for the integrity of removable full and partial dentures,” and determined that light-curing denture base resins are more sensitive to the correct tooth pre-treatment compared to conventional MMA/PMMA materials, requiring specific conditioning liquids.¹

Another ridge lap area pre-treatment approach studied was using laser irradiation to enhance the bond strength of acrylic resin denture teeth to heat-polymerized PMMA denture base resin, possibly as an alternative to wetting with MMA monomer. Those authors suggested that such surface treatment of the ridge lap area should be performed as part of denture fabrication.¹⁰

Avoiding Bonding Failure

As poor bonding may occur with even the highest quality materials, practitioners should generally be aware of potential pitfalls that lead to bonding failure.

It is important that they use best practices and follow manufacture’s instructions for the specific products used and be aware of recommended techniques for achieving a secure bond between acrylic resin denture teeth and the denture base, with care taken during each step of the process, as each may have implications for potential bonding failure; these steps include: (1) investment of wax-up and/or preparation of matrix or full cast mould (pouring technique); (2) wax extraction (boil out); (3) application of plaster separator; (4) tooth treatment: roughening and wetting with bonding agent; acrylic preparation; (6) introducing acrylic into the mould; (7) polymerization; (8) deflasking or removal of matrix or cast mould.

For example, de-bonding may occur when a release of silicone oil causes a silicone coat to form around the denture teeth, when there is wax or separator residue on the ridge laps of denture teeth, or when there is an excessive delay between the application of the bonding agent and further processing. Therefore, a tooth manufacturer, which also recommends the use of a pure plaster investment, has these suggestions for users of its multi-lithic teeth: allow the flask to cool slowly and rest before deflasking, and to remove the glaze from the ridge laps of the denture teeth, use clear boiling water to boil out that wax residue, then rinse the separator residue off denture teeth with bonding agent.¹¹

Radford and others stress the importance of adhering to tooth manufacturer’s instructions—including for liquid/powder ratio and curing cycle, and using its base acrylic resin.¹² Findings from a study on whether a strong bond could best be achieved using denture base and teeth from the same manufacturer noted “a trend that the bond strength achieved between the teeth and base material from the same manufacturer was higher than the unmatched pairs” but statistical significance was not achieved.¹³

Conclusion

Adhesive de-bonding of acrylic denture teeth from the denture base in the highly cross-linked cervix of the teeth is the most common failure in removable dentures. Approaches to improving this bond include chemical and abrasive treatments, using multi-layered teeth whose cervix layer is less highly crossly linked, and taking care to follow best practices and manufacturer’s instructions for use of their products.

References

1.Palitsch A, Hannig M, Ferger P, Balkenhol M. Bonding of acrylic denture teeth to MMA/PMMA and light-curing denture base materials: The role of conditioning liquids. J Dent. 2012;40(3):210–221.]

2.Shimizu H, Takahashi Y. Review of adhesive techniques used in removable prosthodontic practice. J Oral Sci. 2012;54(3):205-11

3 DIN Deutsches Institut für Normung e.V.: Dentistry – Base polymers – Part 1:Denture base polymers (ISO 20795-1:2008); German version EN ISO 20795-1:2008.

4 DIN Deutsches Institut für Normung e.V.: Dentistry – Artificial teeth for dental prostheses (ISO 22112:2005); German version EN ISO 22112:2006.

5. Bonsor SJ, Pearson G. A Clinical Guide to Applied Dental Materials. Amsterdam; Boston: Elsevier/Churchill Livingstone, 2013.

6. Mosharraf R, Abed-Haghighi M. A comparison of acrylic and multi-lithic teeth bond strengths to acrylic denture base material. J Contemp Dent Pract. 2009;10(5):E017-24.

7. Jain G, Palekar U, Awinashe V, et al. The effect of different chemical surface treatments of denture teeth on shear bond strength: a comparative study. J Clin Diagn Res. 2014;8(6):ZC15-8.

8. Sadar L, Dhume S, Maniar N, et al. Comparative evaluation of shear compressive bond strength between cross-linked acrylic resin denture base and cross-linked acrylic resin teeth with different modifcations of their ridge lap surfaces. J Contemp Dent Pract. 2013;14(5):898-903.

9. Bahrani F, Khaledi AA. Effect of surface treatments on shear bond strength of denture teeth to denture base resins. Dent Res J (Isfahan). 2014;11(1):114-8.

10. Akin H, Kirmali O, Tugut F, Coskun ME. Effects of different surface treatments on the bond strength of acrylic denture teeth to polymethylmethacrylate denture base material. Photomed Laser Surg. 2014 Sep;32(9):512-6.

11. Kulzer. Denture Teeth and Acrylics: Materials, Processing, Tips And Tricks. 1st edition. Hanau, Germany: Kulzer GmbH; 2113.

12. Radford DR, Juszczyk AS, Clark RK.The bond between acrylic resin denture teeth and the denture base: recommendations for best practice. Br Dent J. 2014;216(4):165-167.[abstract].

13. Patil R, Juszczyk AS, Radford DR, Clark RK. Is the bond between acrylic resin denture teeth and denture base resin stronger if they are both made by the same manufacturer? Eur J Prosthodont Restor Dent. 2010;18(1):39-41.