Impressive Impressions by Gary Schoenrock, DDS This article is a based on a piece that first appeared in Interface Newsletter. It is with the kind permission of the publisher that it appears here. If you want to start a food fight in a mixed group of dentists and technicians, just voice an opinion about crown-quality using double-arch impressions. Some (usually CDTs) will swear that the double-arch tray is the spawn of Satan ... the number-one cause of remakes ... high crowns ... extensive occlusal adjustments. The primary reason so many hardworking lab owners can’t make a decent living. Others (usually dentists) will swear just as passionately, that the double-arch tray has virtually eliminated remakes in their practice. In fact, since they started using a triple-tray technique, remakes have become just a hazy memory. They almost never experience high crowns or extensive occlusal grinding. So who’s right? Oddly, they both are. Disposable double-arch trays are highly flexible and prone to distortion. So they’re easy to mishandle both in the operatory and in the lab. And when they’re mishandled, double-arch trays yield narrow castings with alarming regularity. Back in 1979, I developed the Laminar Impression Technique. This approach, involves injecting the wash material into holes drilled in a very stiff primary impression. And it yields exquisite precision. In fact, when both dentist and lab know what they’re doing, it produces crowns that slide home without any adjustment at all at least 95% of the time. (That’s not just the boasting of a proud parent. The Laminar Technique has been adopted by thousands of dentists, and they report similar success.) Now here’s my point in mentioning it: the Laminar impression technique REQUIRES a disposable double-arch tray. When you use it with the appropriate materials, it eliminates virtually all sources of operatory error. However, if the dental laboratory mishandles these impressions, all of your efforts will be wasted, as the results will be no better or, in most cases, worse than the traditional methods. Advanced impressions demand advanced lab methods and here are our recommendations for all “Triple-Bite” type impressions. If ignored, none of these directives by themselves will result in a huge degree of error, but remember that crown and bridge is a game of tenths of millimeters. Mess things up a few times and bingo - your beautiful crown is doomed to becoming a porcelain coffee table as it is ground down to a flat nub after your patient utters the feared words, “Feels real high, Doc!”. In my case, of course, I’m sending the lab Laminar impressions - But these suggestions should improve the processing of any double-arch impression.
This is probably the single most abused stage in the laboratory process (and also the one most laboratories deny ignoring). The beauty of the triple bite impression is the precision with which it captures the inter-occlusal relationship. The common lab practice of pouring the die side and removing it to locate pins, then attempting to replace it in the impression to articulate, is total madness, as it completely annihilates any chance of an accurate occlusion on the final crown. (If you doubt this, try it yourself! Once a model is removed from an impression, it can never be reseated accurately!) Fortunately, this can be overcome by using one of two methods. 1) The laboratory can place die pins in the wet stone using locator pins pushed through the silicone. After the stone sets, the second pour will articulate the models and they can then be separated. This method will yield an articulated set of casts, and the die can be sawed out and trimmed.
We have yet to see a plastic hinge articulator which does not have an unacceptable amount of “slop” in it. Laboratories love them because they are cheap and they do not have to worry about investing in a huge inventory of metal articulators which will slowly but surely vanish into their dental clients’ offices over a period of time. Unfortunately, these little Fisher Price toys cannot lend the needed precision to the occlusal adjustment of the crown. You must insist on metal. There is a simple solution if your lab balks at this suggestion. Merely purchase your own supply of metal hinge articulators (a dozen or so should get you started) and send them to your lab on a rotating basis. This also serves notice to your lab as to your commitment to this method. #5 Treat dies carefully When your next crown comes back from the lab, take a minute and examine the working dies. Chances are you’ll notice that chips and abrasions of the stone die are very common during the multiple steps involved in crown construction. To create the strongest die possible, I recommend that final die trimming be done with a sharp Bard-Parker blade to avoid chipping the margin. After placement of die spacer on the working die (more about that in a minute), the stone can be sealed with a thin coat of good old super-glue. Studies have shown that the cyanoacrylate layer does not change the dimensions of the die appreciably and will yield a die that is much more durable. #6 Tin-Foil the opposing model for cement compensation Many times a crown which has perfect occlusion will be high after cementation. This is due to the fact that no crown seats completely because of the cement film thickness. A layer of tinfoil, burnished to the opposing model in the area of occlusion with the crown only, will compensate for this nicely. Tinfoil which is placed over the entire surface of the opposing cast will merely interfere with the occlusion of the casts and guarantee a high casting. #7 Discuss the amount of die-spacer!
Attempts to improve the seat of fixed prostheses have ranged from simple and practical to idiotic (ex. drilling a hole in all new crowns to provide a vent for the hydraulic pressure of the cement and then restoring this divot later with a separately fabricated plug).
Many clinicians, however, leave the choice of number of layers completely up to the lab, which does not take full advantage of the concept and may even hinder success in certain cases. Always specify the number of layers of die spacer desired on your prescription slip. The exact number will depend on your individual preferences (i.e. tight vs. passive fit), but a good starting point can be obtained by beginning with two coats on the typical prep with average taper. Bridges usually require an additional coat and multiple abutment bridges or splints (especially those with long parallel preps) can require as many as four or five coats! The spacer should always stop shy of the margin, and if the prep has very short walls and/or requires an axial groove or other retention features, you may not want to use die spacer at all to take advantage of the increase in frictional retention that this will yield. Experiment with various numbers of layers of spacer to determine what works best for your situation - experience will be your best guide!! #8 Don’t alter casts Many labs will lightly scrape the proximal surfaces of the stone teeth which will be in contact with the casting, thinking that tight contacts are better than no contact. Besides, the doctor can always adjust the proximals of the crown with a rubber wheel at chairside. This kind of thinking is exactly why some crowns take forever to seat. You should make it clear to the lab that altered casts are definitely out-of-season for your cases! #9 Fine-tune final prosthesis on a solid model As stated above, the poor stone die stands little chance of getting through the lab undamaged after the crown has been finished. As a result, a crown which appears to fit the die perfectly often fails to seat on the tooth prep. This frequently precipitates a round of accusations back and forth between the doctor and the lab tech, each blaming the other for faulty technique. To eliminate this once and for all, the final step the lab should take before delivering the case is to check its fit on a solid pour model. Almost invariably minor adjustments will be required to the inner surface of the crown and/or the proximal surfaces for complete seat. #10 Return all cases mounted on the articulator It is most useful to have the complete case related on the articulator in order to check the occlusion and cuspal position as it relates to the opposing teeth. Unless you specify this for your crowns and bridges, you are liable to receive your models already “knocked off” and sometimes in pieces. It is no hardship for your laboratory to leave things alone, especially if you supply them with your own articulators, as mentioned earlier. #11 Communicate. Communicate. Communicate! All of the hints above are not worth a hill of Minikin pins if your desires are not clearly communicated to the lab on a regular basis. Written sheets should be drawn up and sent to the lab with your specific instructions for handling the case. Feel free to copy or paraphrase the above so there will be no question as to your directives. Labs, demand clear and sharp impressions and precise instructions for each and every case. Be aware that your problem accounts are the same folks who bad-mouth you every time a crown fails to seat, so get rid of the doctors who insist on shoddy work - You can’t afford them! If you enjoyed this article, Dr. Schoenrock edits INTERFACE, a bi-monthly newsletter which reviews new materials and techniques. Inquiries for seminars, subscriptions, and sample issues can be made at (419) 866-1238. Dr. Schoenrock can be reached by e-mail at MDEGINTERFACE@sbcglobal.com.
About the author - Gary A. Schoenrock, DDS, operates a private practice in Holland, OH. Creator of the Laminar impression technique, Dr Schoenrock is Director of the Midwest Dental Evaluation Group (MDEG), and a highly respected lecturer and writer who has previously authored articles for Parkell Today.
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