I’ve had several calls in the last few weeks from both laboratory personal and dentists, regarding the fit of contacts on laboratory restorations returned to the Doctor for placement. Most have been loose contacts some have been tight, but either way the situation is frustrating to the laboratory, dentist, clinical staff and the patient.
Generally, the Doctors call in to see how they can get their labs to improve the model work being done, and are sure that it is something with the expansion of the stone. After many years of working with hundreds of dentists, I can tell you from experience that the number one reason for contact issues is temporaries. The second is distorted impressions and third is the laboratory process.
While most Doctors and Assistants feel that their procedures are not incorrect, I also know that most of the labs that call in have only one doctor complaining out of the 12 to 15 they work with. Any scientific methodology would tell you to look where the problem exists to find the answer.
We can all agree that teeth move, otherwise Orthodontia is a scam, and they move differently on different patients. When they do move, it is a variable based on a lot of things, but the one we are talking about here is the pressure or lack of pressure temporaries place on the teeth adjacent to a preparation. The ‘tightness’ of the temp, in the now prepped space, can place enough pressure or lack of pressure on the teeth causing them to move slightly out of the original position captured in the impression sent to the laboratory. The contacts of the crown returned to the Doctor for placement will reflect the position of the teeth as they were captured in the impression two weeks earlier. Thus, the clinical staff placing the restoration may see a difference in how the crown fits versus how the temporary fit. That is if the temporary is still in place at the seat date.
To help the dental lab understand the individual clinical method employed for checking these contacts it would be wise for the Doctor to invite the laboratory to sit with the clinical staff as they place a temporary. In so doing, they can feel the ‘Snap’ of the floss, or the ‘pull’ of the shim stock desired by the clinician and then repeat that in the laboratory. If the temp is too tight they can also help the staff to understand why this is problematic.
This also applies for the occlusal contacts. When the Doctor calls to say a “crown was too high” and the lab knows that they took the crown out of occlusion on the models by two to three occlusal tape thicknesses, we know that either the preparation or the opposing dentition has erupted. The most likely factor to cause that this is again the temporary. I’ve had crowns placed personally where the Extended Function assistant simply asked, “How does that feel”, would mark with paper or film and then wholesale grind away until I said I couldn’t feel interference. Remember, I’m still partially numb. As a Technologist I focus on the touch and feel of the other teeth in the quadrant, but what about the public patient who knows nothing? It is not good enough to just not interfere. Temporaries need to be placed correctly and reflect the occlusion that was there originally to hold the correct space and duplicate what was sent in the impression to the laboratory.
The common errors in model work on the lab side are twofold; first is the addition of water to already mixed gypsum as the gypsum is starting to set. This can cause changes to the expansion of the next models poured and should not be done.
Second and most important, is the relationship of the base stone to the die stone master model. In the process the die stone is fully set and pinned, and then the base stone is poured to secure the pins. If the lab does not wait long enough for the base stone to set and expand before separating the dies, this will allow the base stone to pull apart the now individual pieces of the master model and change the relationship. The best combination is to always use a low expansion basing stone (.08-.10% at 2 hours). Regardless of the expansion of the die stone, wait at least two hours before separation unless the stone has specifically been designed for rapid use and achieves a minimum of 85% expansion in the first hour. If the laboratory is using a plastic base, then the same principle of time can be applied to the die stone prior to separation of the dies.
The issue of tight or loose contacts needs to be examined on both the clinical and the dental lab side and a solution, not an adjustment of the crown, needs to be found so that the patient receives the best dentistry possible.
This article originally appeared on Dentistry IQ. To view it please click here.
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