The Preparation of Acrylic Teeth
The Preparation of Acrylic Teeth
As we know, it is always recommended (if not required) to condition the base of the acrylic teeth that will contact the denture base, prior to bonding them to the acrylic base. Without doing so, the longevity of the bond that is required in this area will be jeopardized. However, even after the bases of the teeth are conditioned, there are many that experience de-bonding of the teeth.
It is not only important when this conditioning procedure is done, but also what is used to properly condition these teeth. There are two important facts that one should consider regarding the proper bonding of acrylic teeth.
1. At what point is it best to prepare the base of the teeth?
The teeth should be conditioned after all the wax has been boiled out of the flask. I’ve heard that some make the mistake of grinding the base of the teeth even prior to setting them up in the wax-up, relying on the boil-out stage to completely remove it.
The water used to boil-out the invested waxed denture always contains some wax residue floating on its surface. This wax residue will then be transferred to the base of the pre-conditioned teeth. This, in turn, forms a thin wax coating which will then isolate and prevent the teeth to properly bond to the denture base, therefore, resulting if future de-bonding.
Conditioning the teeth post boil-out will insure that there is no residue existing, allowing a proper bond that will secure the acrylic teeth to the denture base.
2. What instrument is best to condition the base of the teeth?
When it comes to removing the glaze from the tooth base, most of us use a carbide burr of one type or another. It may be either a round, or pair shaped burr, or a burr specially designed to cut retention tracks. The retention tracks do little to add to the bond strength. If anything, these tracks create more of a surface for the denture base to grab onto the tooth. I want to state that if these instruments are working for you and you have no de-bonding issues, by all means, keep using them. However, if you have, or are experienced de-bonding issues, the following recommendations will be helpful to you.
When we use a carbide burr to break that glaze surface on acrylic teeth, the carbide does little to properly condition and create a retentive surface. It almost reforms secondary glazed surface that may not be adequate to achieve a proper bond.
While a technical advisor for a dental manufacturer, we researched the following:
Rather than use carbide burr, it is best to use either a course diamond burr, at a rate of no more than 10,000 RPM, or between 110um and 125um aluminous oxide at no more than 20lbs air pressure. Sandblasting is adequate, but may prove to be a bit messy, so I prefer a burr. The surface created by either of these methods is an etched surface that will create a chemical bond between the acrylic teeth and the denture base acrylic. Use a blast of air to remove any dust created by the grinding. By all means do not wash with water at this time.
I did a visual test in the following manner and you can do this yourself if you wish. I took six standard denture teeth. Three of them I ground with carbide burrs that one would normally utilize to condition teeth. The other three were conditioned with a coarse diamond burr and one was sandblasted with the method described above. All six teeth were then bonded to denture acrylic in the traditional manner. After processing, I then attempted to challenge the bond by securing the teeth in a vise grip and utilizing pliers and applying pressure until they fractured. There was certainly a strong bond in all six samples. When testing the carbide treated teeth, some of the fracture occurred between the junction of the denture base acrylic and the acrylic teeth. The fracture that occurred with the acrylic teeth that were etched did not occur at the junction, but rather away from the junction of the teeth and the denture.
I then sectioned the teeth, and observed the bond between the acrylic teeth, and the denture base acrylic under a high powered microscope. Interestingly enough, with the teeth conditioned with the carbides, I was able to see the junction line between the denture and the teeth which indicated that only a mechanical bond existed between the two. However, I was able to see that the denture acrylic was absorbed slightly in the diamond and sandblasted surfaces teeth forming a chemical bond between the two.
