Two-Way Communication Important in Lab Satisfaction By: Michael C. DiTolla,DDS,FAGD Director of Clinical Research and Education Glidewell Laboratories

The dentist-laboratory partnership is well over 100 years old, and continues to flourish because of the drive for constant improvement from both parties. At the laboratory, we send out a doctor feedback card in every case so that clients can tell us specifically how we can improve. I am always impressed how many dentists call our laboratory and ask for constructive criticism regarding their preps and impressions. It is in this spirit of continuous quality improvement that I present a summary of the most common obstacles seen in our lab that stand in the way of ideal restorations.

Occlusal Reduction The most common error seen on the lab floor is insufficient occlusal reduction. The most common way this occurs is when the occlusal portion of a posterior crown preparation is prepared flat, or parallel to the floor. While there may be enough reduction at the cusp tips, the flat occlusal table leaves no room for properly developed occlusal anatomy. As a result, many laboratories will call the doctor and ask to place a metal occlusal in an attempt to solve this problem, which may be an unacceptable option to the patient. The best way to gauge this reduction is to keep in mind that the proper amount of reduction in the central groove area is approximately the same depth as the pulpal floor of an average Class 1 amalgam preparation. I often find it helpful to remove the old occlusal amalgam first and then reduce the occlusal surface to match that depth, so that the slope of my reductions match the slope of the pre-operative cusps. A use of a football shaped diamond such as a 379-023 bur, will also help prevent flat occlusal tables.

Photo 1 - This often happens as the result of the Curve of Wilson. The maxillary lingual cusps can encroach on the central groove of the preparation if the central groove is not prepared deep enough.

Insufficient occlusal reduction also occurs in the area of the cusp tips, usually on the lingual cusps. It is difficult at times to look into a wet mouth and determine whether or not the reduction is sufficient, whereas once the models are poured it becomes crystal clear. To make reduction easier to judge intraorally, I rely on a product called Flexible Clearance Tabs from Belle de St. Clair, available through dental dealers. These flexible tabs are placed between the prepared tooth and the opposing tooth as the patient closes into centric occlusion. If the tab slides out easily from between the teeth, the proper reduction has been achieved. The tabs are available in 1.0, 1.5, and 2.0mm thicknesses.

Insufficient facial reduction is also seen in the laboratory, although it does not occur at nearly the rate that insufficient occlusal reduction does. Facial reduction can be just as difficult to gauge as occlusal reduction, and facial depth cuts provide a great way of accurately measuring reduction. A bur of known diameter, such as an 847-016 tapered diamond, can be used to place depth cuts. It will also give a uniform facial reduction of 1mm in the gingival third and 1.5mm in the middle and occlusal thirds.

Inadequate Dental Impressions
I recently heard Dr. Gordon Christensen comment at one of his courses, that in his communications with dental laboratories he estimates that nearly 90% of all impressions lack clear, discernable margins around the entire periphery of the prep(s). In the realm of fixed prosthodontics, nothing can doom a case as quickly as a poor impression. Even with inadequate occlusal reduction there are solutions such as reduction copings, but with a bad impression the only real solution is a new impression. From studying impressions in the laboratory, most of the problems appear to be related to insufficient retraction around the complete periphery of the prep(s). There are several different techniques available to achieve the desired retraction, and it behooves the clinician to become proficient at performing at least two of the methods, since no particular technique will work in all cases. As important as fixed prosthodontics impressions are, it seems to be something taken almost for granted by the profession, as evidenced by the nearly complete absence of CE courses dealing with the topic.

Another part of the impression technique that is problematic is the misuse of double-arch impression trays. While these trays function quite well for one or two adjacent preparations, using them for bridge preparations is contraindicated. A laboratory has their hands tied from the start when they are asked to construct a bridge on quadrant models. In larger cases like these, a perforated stock metal tray with a putty/wash combo, or a custom tray with a heavy/light body combo is ideal. You stand a much better chance of delivering a bridge that requires no occlusal adjustments just by using a full-arch tray instead of a double arch tray.

Suggestions For Great Impressions
Photo 2 - Using double-arch impressions for any bridge impressions is contraindicated due to inadequate tray rigidity and insufficient replication of enough teeth for a technician to provide proper lateral excursions.
Photo 3 - This impression not only shows accurate marginal detail, but has material that extends beyond the margin of the preparation. This makes the margin easier to identify for the technician and helps them to develop proper emergence profile.
Photo 4 - The entire buccal margin is missing from this impression and the double-arch tray is impinging on the maxillary tuberosity. It will be impossible to construct an acceptable restoration, without having the doctor take a new impression and a proper bite registration.
Photo 5 - This close-up of the impression of the bicuspid abutment shows excellent marginal detail and the presence of material beyond the margin as well, making it simple to read these margins. The excellent marginal fit of this restoration can almost be guaranteed.
Photo 6 - The lack of subgingival definition, as shown on this impression, is usually indicative of inadequate retraction methods. Whether a dentist utilizes a one or two cord technique, any type of retraction is always preferable to no retraction.
Photo 7 - Monophase impression materials often lack the flow characteristics to capture subgingival detail. Most dentists are much more successful using a heavy body/light body simultaneous impression technique.
Photo 8 - An impression of a fixed bridge of this size should never be taken in a double-arch tray. Much more successful is the use of a custom tray with a heavy body/light body simultaneous technique, or a perforated stock metal tray with a simultaneous putty/wash technique.
Photo 9 - In addition to a lack of sharp marginal detail on both abutments, the use of a double arch tray for a three-unit bridge is questionable. Double arch trays are ideal for a single crown or two adjacent crowns. For cases of more than two units, custom trays or metal stock trays are always recommended.

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Dr. Michael DiTolla, DDS, FAGD, obtained his dental degree from the University of the Pacific School of Dentistry in 1988. He is a faculty member at PAC~live and is currently Director of Clinical Research & Education at Glidewell Laboratories in Newport Beach, CA.

Dr. DiTolla can be reached by calling 800-854-7256 or by email at: mditolla@glidewell-lab.com. Visit Glidewell Labs online at www.glidewell-lab.com.