Readers' C&B Dilemmas Solved!
Posted Apr 28, 2011 in Technical
We are having a few problems with high bites. What do you recommend? -Catherine Barlette, H&O Dental Laboratory, Manchester, NH
There are two areas to check for the cause of high bites:
Most importantly, be sure to equilibrate your models after they're mounted. Simply place articulating paper between the opposing teeth on the entire model and lightly tap to create marks on the contact points. Check to see that the marks are even on all of the teeth that should be in contact.
Often, there are only a few contact marks, even though more should be evident, especially if there are obvious wear facets that aren't showing a mark. Lightly scrape the marked areas and recheck with the articulating paper. Continue the process until all of the teeth that should be touching are, in fact, doing so.
Unfortunately, even the slightest distortion in the impression, setting of the stone and minor defects in the pour can cause an inaccurate reproduction in the contact areas of the opposing arches. Often, the opposing model is the culprit because it's usually obtained with an alginate impression. When and how it is poured--and how accurately it reproduces the anatomy and detail of the teeth--dictates the accuracy of the restoration's occlusion.
Are high bites occurring on cases in which triple trays are used? The triple tray mounting can result in a high bite because if the patient doesn't bite down through the mesh in the tray, more than likely the articulation will be open and result in a high restoration.
Before pouring the model, hold the impression up to the light and see if there are perforations where the opposing teeth are in contact. Also make sure that any stone that flowed out of the anterior and posterior portion of the tray has been ground off and isn't hitting the opposing model, preventing the teeth from properly coming in contact. Keep in mind that the patient is anaesthetized, and the dentist cannot thoroughly check the patient's occlusion while the triple tray is in place.
Unfortunately, any quadrant impression is going to provide limited occlusal information, due to the fact that there aren't many teeth coming into contact. That's why it's always best to request full mouth impressions and opposing models. (Don't roll your eyes! Even if a few of your clients comply with your request, that's something!) The occlusal contacts will be more accurate (you still have to equilibrate) and excursive interferences can be properly eliminated, which saves the dentist chairtime in the long run.
To what would you attribute open interproximal contacts on two or three out of every 25 cases delivered? The contacts are closed on the solid model, but open upon seating. -Alan Aksomitas, CDT, Owner, Dental Contours, Inc., North Palm Beach, FL
Ah, yes, one of the great mys teries in dental technology, second only to the occasional high bite syndrome! A number of things could be happening here:
- Is this situation occurring with a specific client? If so, it may be related to his provisionals. If the provisional has overly tight contacts, the adjacent teeth may be physically pushed apart just enough to create an open contact in your restoration. Remember, the impression was taken before the temporary was placed, with no pressure on the adjacent teeth.
- Also, I'm willing to bet that the problem usually occurs in the posterior region. If so, it may again be related to the temporary and the patient's occlusion. If the patient is biting into the temporary and not hitting the proper centric contact area, the tooth could be under occlusal forces that are pushing it mesially or distally, which again affects the contacts you've established on the model. This is more likely in patients whose teeth may be slightly mobile due to periodontal conditions, or when the prepared tooth is missing one or more adjacent teeth (a last molar, for example). This is why I always make the mesial contact tighter than usual on those teeth that have no adjacent tooth on the distal.
- Check to see that the impression material is still securely held in the impression tray before pouring your solid contact model. Occasionally, the impression will separate from the tray and even the slightest separation could affect the accuracy of the model, including the relationship of the interproximal contacts.
What can we do to eliminate gassing problems, especially on our occlusal gold PFMs? We follow all the proper procedures. -Darlene Hess, New West Dental, Lake Havasu, AZ
I'm assuming when you say "occlusal gold PFMs," you're using a yellow high-gold-content alloy. This type of alloy is very soft and should not be finished with the same grit of finishing stone, bur or diamond that you would use on a harder alloy. If you do, the metal will actually be pulled or drawn over itself in the finishing process, creating small air pockets that are difficult to see with your eye, and even more difficult to fill with opaque. Eventually, the trapped air works its way to the surface of the porcelain, usually during glazing (just when you think you're done!), resulting in a pit all the way down to the metal.
Avoid this situation by finishing the metal with smooth stones, rather than abrasive ones. We use a coarse, non-contaminating rubber wheel to finish our 87% gold copings for this reason; then we blast the entire casting with aluminous oxide, steam clean and degas. (Just a note: reduce the number of turns on your casting ring by one half turn when casting alloys with high specific gravity. Too much force causes turbulence and possible porosity in the casting, which could also cause gassing issues.)
If you're having these problems with other alloys (not high gold), the problem is more than likely related to micro-porosity (which can also happen with high-gold-content alloys). Since you mention that it occurs more frequently on metal occlusal designs, you should examine your sprue diameter, area of attachment as it relates to wax thickness in your pattern, angulation of the sprue, the amount of alloy used for casting and casting arm speed. Here are some tips:
- Use a 6-8 gauge sprue when casting a heavy pattern, similar to what you would use for a full cast crown.
- Make sure the sprue is attached to the thickest portion of the wax pattern, and at an angle that allows the molten alloy to flow smoothly as it enters the mold.
- Use enough alloy to provide the casting with an adequate button from which to draw during the initial phase of cooling and solidification.
- If the pattern is really large and thick in the metal occlusal area, place a sprue on each lingual cusp to provide a more efficient flow for the alloy.
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