From the Benchtop Editor: Building better bonds

Any time removable prosthetic technicians congregate, the conversation eventually turns to dilemmas faced in the lab. There’s no shortage of common problem situations for removable technicians to discuss, but many times questions arise about tooth de-bonding or delamination between the denture tooth and acrylic base.

Since the introduction of tightly cross-linked and nano-composite denture teeth to the market, this situation occurs more frequently than most technicians would like to publicly admit. Because there can be numerous causes for poor bonding, I thought it would be beneficial to discuss various reasons de-bonding occurs and preventative measures to avoid the problem. This will allow you to decide the applicable cause of the next de-bonding situation you face, and provide you with a possible remedy to investigate.

Technique, not material

In many cases when this problem cannot be readily resolved, the acrylic is usually wrongly blamed. If your powder and liquid mix has become doughy and packable, however, your acrylic is chemically sound as the solvent action of the monomer is present. At that point look to your technique and subsequent steps to find the solution to your de-bonding problem.

Causes of tooth de-bonding

Packing state: Once acrylic reaches a snappy rather than doughy stage, the solvent action between base acrylic and tooth acrylic will be diminished.

Inadequate time: If there is not enough time with tooth contact to the doughy stage of the acrylic mix it allows exposure to monomer contained in that stage of the cure (doughy mass). In this situation the polymer dissolves and becomes a plastic/doughy material because of the solvent action with the monomer.

Physical blockage: A film or layer of contaminant can block the ability of the monomer-in the doughy stage-to react with the acrylic contained in denture teeth.

Causes of this blocking can include sloppy application of mold separation agents; or mold washing detergents containing phosphates, lanolin, fragrance and ingredients that leave a transparent film.

Denture waxes contain oils, and residual oils left in unmaintained washout tanks can not only block tooth-to-acrylic bonds, but also can block separators from penetrating stone to provide an isolation barrier. 

Material mismatch: Materials such as composites and nano-composites used in the manufacture of today’s denture teeth to produce natural effects such as translucence and dentin opacity can be incompatible with the acrylic as they do not always bond well to PMMA. Removal of acrylic manufactured into the lingual/cingulum of these teeth when tooth-to-arch space is limited, negates base acrylic exposure to bond friendly material.

Preventative Measures

Proper packing: Pack your cases in the doughy but not sticky stage. Once the acrylic enters the snap stage it becomes more difficult to attain a good bond between a tooth and the base. When the acrylic enters this stage it also indicates the resin has reached its tertiary stage, which means it is dry and will not be as formable and easy flowing.

Ample setting time: Bench setting a case for 20-25 minutes before heat introduction allows prolonged contact between tooth and base before the outside heat induced reaction begins. This also helps prevent porosity by ensuring a slow consistent climb rate once heat is introduced.

Clean tanks: Maintain a clean washout tank using a dental specific tank boil out solution that cuts oil and wax. For direct mold wash out, also lean toward a dental specific product or at the very least, a product with limited fragrance, hand softening agents, and in the case of laundry detergents, optical brightening agents that can leave a micro-layer film on teeth and model.

Important note: Residual optical brighteners, if incorporated into an intraoral appliance, may pose a health concern for the patient.

Get mechanical: For non-compatible tooth material issues, mechanical retention is always indicated. The translucency effects of natural looking teeth also promote value change in shade when highly modified ridge laps, severe hollow grinding or diatorics are used.

I recommend a cingulum style lock or similar undercut mechanism, which involves the outline of the tooth and does not invade too deeply in remaining body of the tooth.

Assist the chemistry: Chemical preparation via a light rubbing of the ridge lap and acrylic contacting surfaces of the tooth with a Q-Tip moistened with heat cure monomer followed by an application of a methacrylate acid or ester based bonding liquid will eliminate the chance of residual emollient or oil film from blocking the bond and enhance chemical bonding properties.

Steps to build bonds

The materials available to today’s removable prosthetic technicians are reliable and can provide more than adequate bonding between denture teeth and denture bases. But following instructions and careful technique are keys to achieving a bond strong enough to withstand anything that might be thrown at a set of dentures (Fig. A). 

01 Gross adjustment of a denture tooth-which is common-often removes the built in bonding layer on the cingulum area or multilayered nano-composite and hybrid composite teeth (Fig. B). When these teeth are modified, it becomes necessary to find creative ways to obtain tooth to base retention.

02 I recommend having the following rotary instruments available for creating retentive undercuts on denture teeth: diamond cutting disk, miniature inverted cone, miniature flame carbide and miniature round burs in various sizes (Fig. C).

03 When looking to create retention for grossly reduced teeth, use a thin diamond cutting disc. This disc allows for depth control and helps prevent intruding any further into the tooth than necessary (Figs. D, E).

04 Because of their shape, miniature inverted cones provide an ideal mechanism for creating retentive undercuts (Fig. F).

05 A miniature flame carbide provides a high degree of carving control when modifications are less severe (Fig. G).

06 By using an inverted cone in a vertical to the tooth fashion, it creates a lip around the periphery and avoids the body of the tooth, so there is less chance of lowering the overall value of the tooth (Fig. H).

07 A final application of bonding agent followed by the manufacturer’s recommended setting time before packing enhances the chemical adhesion of remaining bondable acrylic on tooth necks (Fig. I).

Conclusion

There are many reasons a tooth can become loose and de-bond from the denture base. However, if the problem is occurring frequently it is important to review every step of your procedures to find where things have gone wrong.

Hopefully this article provides a few places for you to start looking so you can quickly identify and remedy the issue. 

About the author

Tom Zaleske is Benchtop Editor for Dental Lab Products. The owner of Matrix Dental Laboratory in Crown Point, Ind., he has more than 25 years of experience in removable prosthodontics and regularly lectures on providing high quality service to dentists and most importantly to their patients. He can be reached at matrixdental@comcast.net.