Written by Gary A. Schoenrock, DDS, FAGD Sunday, 31 March 2002 19:00
After an in-depth study of viscous elastomeric impression materials and their behavior, I invented the laminar impression technique (LIT) in the mid 1970s. Since that time, I have been very gratified to observe the growing number of clinicians who are recognizing the benefits of this method and adopting it into their practices.
When using any dental technique, the temptation arises to modify it in an attempt to make it simpler and/or more effective. So it has been with the LIT. I receive many calls and faxes from doctors who are confused about these modifications and seek my advice as to whether or not they are advisable. My observation is that some of these modifications are merely optional steps I have written or talked about in the past, and as such, are completely acceptable. Other well-intentioned modifications contradict the original premise of the LIT by violating the precepts of viscous fluid dynamics upon which it was founded, and will invariably lead to results that fall short of the original technique.
This article presents a basic summary and rationale of the LIT, along with the complimentary principles of physical and biological behavior of elastomeric impression materials upon which it is based, so that it can be understood why the LIT is designed as it is and why it can do what it does. Also addressed are a few of the modifications and similar methods of which I have heard and seen in print, and I give my opinion as to why they will or will not yield acceptable results.
WHAT’S WRONG WITH “BULK COMPRESSION” METHODS?
|Figure 1. A piston (a) is forced into a cylinder (b) containing a liquid (c). Compressive hydraulic forces result as illustrated by the black arrows.||Figure 2. A similar situation develops when an impression tray (b) containing a viscous impression material (c) is pushed onto a tooth preparation (a). The resultant hydraulic forces can cause a multitude of problems.|
My education concerning impression techniques for fixed prosthodontics was very similar to what is currently being taught in the vast majority of dental schools. The basic technique is to place an elastomeric impression material in a tray (stock or custom), syringe a small amount of lighter-bodied material around the prep or preps, and then place the tray in the mouth and surround the preparation with the impression material until set. Usually, a separate impression is taken for a counter model, and a bite registration is also obtained. Another impression is usually taken to construct a temporary from acrylic resin. All these methods, whether they utilize rubber bases, polyethers, hydrocolloids, or silicones are what I call “bulk compression” methods and, as such, fail to recognize that all elastomeric impression materials behave as viscous liquids. Simple physics teaches us that liquids cannot be compressed, and when attempts are made to confine a bulk of impression material in a tray and force it into the prep area, many adverse sequelae can and do result, as will be discussed later (Figures 1 and 2).
|Figure 3. A listing of desirable characteristics of any impression technique. Check yours here!|
In addition to these problems, most bulk compression methods are burdened with excessive steps, waste of material, and uncertain results. Recognizing the problems with existing impression techniques prompted me to study the behavior of viscous fluids, especially how they behave under load flow, and to develop a multi-point objective that, hopefully, my new technique would meet (Figure 3).
WHAT’S WRONG WITH “DOUBLE-BITE” METHODS?
The LIT is a modification of the very popular “double-arch” impression method in which the working impression, counter impression, and the bite registration are all obtained as one unit utilizing unique impression trays made of metal or, more recently, disposable plastic.1 The basic double-arch technique has been well reported in the literature.2-4 Advantages to the double-arch method include the following:
(1) Increased speed and efficiency because of the capture of working impression, counter impression, and bite registration simultaneously.
(2) Registration of the involved teeth under functional load. Taking impressions with the teeth apart and bite registrations with the teeth together inevitably leads to error, as the teeth shift slightly when in occlusion. Numerous studies have confirmed the superior accuracy of double-arch methods over more traditional bite registration techniques.5-8
(3) More efficient use of impression materials, as only a segment of the arches is impressed, resulting in a significant savings in materials.
Unfortunately, double-arch methods as used by many clinicians have come under criticism from a variety of sources. Lab technicians have noticed that a great many of these impressions result in crowns and bridges that fail to seat accurately. This can be attributed to a number of factors. If the impression is taken in a single phase (that is, the patient is anesthetized, the preparation is completed, and then the impression is taken), the usual protocol is injecting a light-bodied impression material around the preparation while loading the tray with heavy- or medium-body material. While accurate results are possible, errors are frequent because of the following pitfalls:
•The time delay between placement of the light-body material on the prep and placement of the tray in the mouth results in partial polymerization of the impression materials before seat. “Working time” of elastomeric impression materials is a fallacy invented by dental manufacturers. The materials begin polymerization immediately after the base and catalyst are mixed, and the material should be placed as soon as possible after mix. The longer the delay, the greater the error.
•Accurate closure into centric occlusion by the anesthetized patient is made more difficult by the fact that proprioception is greatly diminished.
•Any interference during closure that results in distortion of the tray itself is likely to cause an inaccurate impression. The problem here is that the set impression material is not rigid enough to prevent “springback” of the tray upon removal. Inaccuracies of this nature are particularly insidious, as the impression will often look adequate, but the prosthesis will fail to seat. As noted above, plastic double-bite trays are especially vulnerable to this distortion and have been roundly criticized by lab technicians. These interferences with the tray structure are much more likely when the patient is anesthetized, as they cannot feel the impingement.
•A separate impression must be made if a custom temporary is to be fabricated.
An alternate approach is to obtain a preliminary impression using a heavy-bodied impression material in a double-arch tray before local anesthesia is administered. This eliminates the problems of accurate closure into centric occlusion, since this record is taken before local anesthesia is administered. Interferences with the tray are also more readily identified at this stage for the same reason.
This registration can also be used as a mold for a custom temporary after tooth preparation. An appropriate temporary material (ie, acrylic or bis-acryl plastic) is mixed and placed in the portion of the preliminary impression corresponding to the prepared tooth. The impression is replaced in the mouth and closure is obtained. A temporary constructed in this manner very nearly duplicates the anatomy of the unprepared tooth. Stock, out-of-the-box aluminum or polycarbonate temporaries do not faithfully duplicate the morphology, contact position, or dimensions of the unprepared tooth. Altering the anatomy of a tooth can exert a powerful orthodontic force, as illustrated by such commonly used appliances as the lip bumper for distalization of molars and certain crossbite correctors. The potential exists when using stock temporaries for the temporized tooth to shift and move from its original position between the prep and seating appointment.
A problem occurs when the preliminary impression is corrected or “washed,” typically by placing a light-bodied impression material in the original impression and reseating on the preparation. Because fluid dynamics teaches us that liquids cannot be compressed without generating hydraulic compressive forces, some or all of the following complications are likely to occur:
•The hydraulic compressive forces may expand the tray walls and distort them. If a sufficiently rigid tray is used, the preliminary impression material itself may be distorted and “spring back” upon removal, resulting in an inaccurate impression. “Venting” the tray by drilling holes has little effect, as the wash material inevitably flows into unprepared tooth areas, causing distortion.
•Impression material may be forced submucosally with significant adverse effect on the soft tissues. This complication has been reported in the literature on numerous occasions.9-11
•Salivary contaminants may be forced into the dentinal tubules that have been opened by tooth preparation. This may result in a tooth that has greater than usual postoperative sensitivity, or in extreme cases may lead to pulpal death.
|Figure 4. Pros and cons of traditional “triple-tray” impression techniques.|
A study of fluid dynamics and precision industrial molding techniques12 led to the development of the LIT, which utilizes the advantages of the double-phase double-arch impression method, but avoids the aforementioned shortcomings (Figure 4).
|Figure 5. Typical “triple-bite” type impression trays.|
(1) Double-arch disposable plastic trays. These trays are manufactured by any number of reliable sources and should be chosen on the basis of applicability to the particular situation. Posterior, anterior, and full-arch trays are readily available, and all are amenable to this method (Figure 5).
(2) A vinyl polysiloxane bite registration material (ie, Blu-Mousse [Parkell], Regisil PB [Caulk], Stat-BR [Kerr]). Cartridge form is most convenient.
(3) A low-viscosity, fast-setting vinyl polysiloxane wash material (eg, Express Light-Body/Fast Set [3M ESPE], Aquasil LV/Fast Set [DENTSPLY Caulk]). This material should be obtained in cartridge form with accompanying mixing tips and intraoral tip for direct placement.
(4) A bis-acryl temporary material for provisional fabrication (eg, ProTemp Garant [3M ESPE], Luxatemp [Zenith]). Be certain that the material you choose has a base-to-catalyst ratio of 10:1 to avoid decatalyzing the wash impression material.
(5) A 3/32-inch twist drill bit (available at a local hardware store).
(6) Retraction cord of your preference.
The patient is anesthetized using standard procedures. The impression tray is then tried in, ensuring that there are no gross interferences when the patient closes in centric occlusion.
|Figure 6. A preliminary impression is taken for crown preparation on No. 30.||Figure 7. The preliminary impression will be useful both as a temporary matrix, a bite registration, a counter impression, and a custom injection chamber.|
The tray is then loaded with the preliminary impression material and placed on the upper arch first. The patient is instructed to close rapidly into centric, and the opposite side of the arch is checked to make sure accurate closure has been obtained. The impression is allowed to set and is then withdrawn. An impression obtained in this manner will function not only as a final impression tray but also as a very accurate matrix for the custom temporary (Figures 6 and 7).
After tooth preparation, tissue retraction can be carried out using preferred methods. If I wish to place my margin subgingivally, I place a thin retraction cord after gross reduction of the tooth, and then place my margin to the level of the retracted gingiva (the tissue will be retracted apically about 0.5 mm). After complete preparation, I place another length of retraction cord loosely in the sulcus, which retracts the gingiva slightly laterally. This ensures a gingival trough that is slightly wider coronally than apically, which prevents tearing the impression material. Of course, tissue management can be at your discretion. The LIT can be used regardless of the method employed.
The preparation is then lubricated (I prefer Cortisporin Otic suspension, which contains a broad-spectrum antibiotic as well as a corticosteroid to disinfect and reduce inflammation). The temporary material is then mixed and placed in the preliminary impression in the involved areas. The impression is reseated into the patient’s mouth, and they are again instructed to close into the tray. When the temporary material is nearly set, the impression is removed (temporary should remain on the tooth). The temporary is then removed (the second retraction cord will usually come out with it; if not, remove it now). Inspection of the occlusal aspect of the temporary will indicate the need for further tooth reduction if the temporary appears thin (ie, if you can see light through it). When the preparation is complete, the preliminary impression is then modified for the final impression injection (Figure 8).
|Figure 8. Custom temporary crown fabricated using preliminary impression after tooth preparation.||Figure 9. A 3/32-inch drill bit is used to drill two holes through buccal aspect of tray into prep area.|
|Figure 10. Drilled tray is reseated into mouth.||Figure 11. Cartridge “gun” with intraoral tip is placed into one hole and injection begins.|
|Figure 12. Injection continues until material flows clean from second hole.||Figure 13. Final impression.|
With a large carbide bur, a slight amount (0.4 to 0.6 mm) of impression material should be removed at the margins of the involved tooth to allow for thickness of wash. Then, using the 3/32-inch twist drill bit and a straight handpiece, two holes are drilled through the buccal side of the tray at the mesial-proximal and distal-proximal of the involved tooth (Figure 9). The final step is to swab the preliminary impression clean with rubbing alcohol on a cotton applicator stick. This removes the greasy layer left behind by the bis-acryl temporary material and ensures a good bonding of the wash material to the tray. The No. 1 cord may be removed at this time if necessary, but if all margins can be visualized, it more commonly is left in place during the final impression. The tray is reseated and the patient is instructed to bite into maximum intercuspation (Figure 10). Using the cartridge dispenser gun with the small intraoral tip or an impression syringe, the light-body wash material is injected in one hole until it comes out clean from the second hole. (The material will have a flushing effect on fluids, therefore continue the injection until the excess comes from the second hole clear of any contaminate.) Material is allowed to set and then removed (Figures 11 through 13).
|Figure 14. Small plastic parts obtained by precision injection molding in an identical procedure to the LIT.|
The unique injection phase makes the LIT the only fundamentally new impression method in decades. Compressively generated hydraulic forces cannot be generated, as the wash material is injected into an open system. Actually, injection of plastics and elastomerics for the fabrication of precision parts is not new. Readers who assembled plastic model kits in their youth will undoubtedly remember the small parts with exquisite detail which were attached to the “trees” formed by injecting the molten plastic into molds (Figure 14). Crowns and bridges are also made in the same fashion, attaching a sprue to a wax pattern and injecting molten metal into the mold. Why should impressions be any different? Impressions taken using the LIT will result in fixed prostheses of excellent fit, requiring at most only minimal occlusal adjustment. This is a direct result of the avoidance of hydraulic compressive forces, and the fact that all impressions (working and opposing) are taken with the teeth together, eliminating the major cause of high crowns.
Time and material savings using the LIT are also significant, which is of increasing importance in the economically conscious practice. Temporaries will be of previously unmatched precision. The combination of construction of the temporary with the teeth together and the accuracy of the bis-acryl material will yield temporaries that do not have to be “ground down” or “reamed out” to seat. These temporaries will become uncemented much less often than their more conventional counterparts and will maintain spatial relationships of their underlying preparations flawlessly. The bis-acryl materials are also chemically compatible with light- and self-cured composite resins, which makes modification of shade and shape for functional and aesthetic verification simple.
Dental laboratories will appreciate always having adequate clearance to fabricate their prostheses, as the LIT sequence allows for temporary fabrication before the final impression. This allows the clinician to inspect the occlusal aspect of the temporary and affect further tooth reduction if it appears too thin. The LIT can be used in a variety of situations, including multiple-unit bridges, opposing units, adjoining units, and larger cases that are best treated with full-arch impressions. Full-arch double-bite trays are available, and I have efficiently and successfully performed full-arch reconstruction using the LIT. The nuances of these various advanced techniques are beyond the scope of this article and are best learned from seminars or videotapes.
The reader should be cautioned that the LIT is not an excuse for poor tissue management. Although the LIT has the advantage of a “flushing” effect of blood and crevicular fluid from around the preparation, excessive bleeding and grossly subgingival preparations should be managed with appropriate hemostatics and/or crown-lengthening procedures to expose adequate tooth structure and preserve biological width between attachment and crestal bone. My rule of thumb is “if you can’t see it, you can’t impress it.” Unfortunately, the predilection for compressive methods to register long “fins” of material past the prep has deluded operators into thinking that this constitutes accuracy. The price one pays for an extremely accurate impression technique that does not generate compression is the necessity for all margins to be clearly visible.
In summary, the LIT is an efficient, precise, and predictable advancement built on the foundation of the double-arch impression systems. Its versatility makes it adaptable to the vast majority of situations in the dental practice that require the fabrication of precision-fixed prosthodontic restorations.
MODIFICATIONS: BETTER OR WORSE?
Since the introduction of this technique to the profession in 1977, a number of clinicians and dental manufacturers alike have attempted to modify the LIT. Here I review some of the obvious modifications and similar techniques, and comment on their acceptability. Please understand that this is not an attempt to “put down” other techniques, as I am of the firm belief that if you are using a particular method and you are obtaining optimal results, you should not change things (if it ain’t broke, don’t fix it!). However, because of the many calls and faxes I receive daily from doctors wanting my comments on a modification that they have learned about from a lecture, article, or commercial promotion, I would like to “clear the air” concerning my feeling about a few of them.
THE “ONE-HOLE” MODIFICATION
One of the current modifications of the LIT is identical to the original technique except for the final injection phase. Instead of drilling two holes in the tray, this technique utilizes only one. The premise is apparently to gain more compression, thereby hopefully increasing the chance that deep margins will be caught by the wash material.
As part of my lectures on the LIT, I cover the method of using only one hole. The advantage is as stated previously; deep subgingival marginal areas can be picked up more readily. Unfortunately, when using only one hole, tremendous hydraulic compressive forces are generated, which increase the chance that impression material will be forced submucosally, causing a delayed foreign-body reaction. In addition, the “end point” of injection cannot be visualized, which may lead to inadvertent over-injection, forcing impression material into unprepared tooth areas. This will “raise” the entire tray and lead to a “high” crown. There is also the possibility that the compressive forces will distort the preliminary impression material itself. When the impression is removed, the distortion will “spring back.” The resultant crown will more often than not fit very tightly or fail to seat at all.
I caution doctors who wish to occasionally utilize one-hole injection for deep margins to have retraction cord in place while injecting to block material from submucosal entry, and have their lab place multiple coats of die spacer to obviate a “tight” crown. I feel a better solution is to expose all margins using cord, electrosurgery, rotary finishing burs, or any other method to make sure they can be visualized. While this method has its place as an occasional alternative, I cannot recommend its routine use.
THE ALTERNATIVE MATERIALS MODIFICATION
A number of clinicians have recommended different materials for the LIT. One modification uses a very watery wash silicone that sets very stiffly (Mach II, Parkell) for the injection phase. The rationale here is to add additional stiffness to the final impression. Others have recommended entirely different materials for the LIT, ranging from polyethers to hydrocolloids and alginate.
There are many acceptable materials for both the preliminary and wash phase of the LIT. Most of these materials are classified as polyvinylsiloxanes (addition reaction silicones). Happily, this class has very nearly cornered the market with practicing dentists, enjoying a huge market share. Because all of the addition reaction silicones are chemically compatible, you can “mix and match” them to your liking. You must be sure that your preliminary material is a very high durometer set to avoid distortion. The first and currently most popular material that fits this description is Blu-Mousse from Parkell. If considering another material, compare it with Blu-Mousse to see if the set durometer is comparable. If so, everything will be just fine. The wash material I currently use is Express light-body/fast set (3M ESPE). Because you are not playing around with slapping material in a tray and injecting a mound of wash around a prep, you want the fastest set possible. I have had no trouble injecting around an entire arch with this material. I also like its “flat” light blue color when set, which gives a nice contrast to the royal blue preliminary and is easily read. It may sound picky, but some wash materials are glossy yellow, blue, or green when set. These washes are very difficult to read, and I avoid them (maybe its just the “bifocal” factor of my veteran eyes).
Very stiff washes such as the Mach II mentioned before will function perfectly, except perhaps if undercuts are present. Care taken to block out prep uncercuts using a light-cured resin-modified glass ionomer (ie, Vitremer [3M ESPE]) will pay dividends here. As far as other classes of impression materials, the only one I have found to be acceptable is the polyethers, which will maintain a sufficient stiffness to yield a great impression. I would recommend Ramitec (3M ESPE) for the preliminary impression, which is very stiff, and Permadyne Garant (3M ESPE), which comes in cartridge form for easy injection. Please forget using rubber bases, hydrocolloids, alginates, or anything else. Ask any manufacturer who promotes these materials for this method if they will pay for the remakes!
There is much interest in a method I have seen that obtains a preliminary impression after tooth preparation using a “double-bite” tray and a material similar to Blu-Mousse. After this impression is made, the patient is instructed to open the mouth while the impression is held against the opposing arch. While maintaining this opening, a fine wash material is injected into the tray in the prep area, and the patient closes into it. The rationale is to hydraulically force wash material into the sulcus and “correct” the impression. No venting or relief is recommended.
Aside from the time and material disadvantage of having to obtain a separate impression to construct a temporary, my feeling is that this method definitely courts disaster by ignoring fluid physics. Because the initial impression is of the prepared tooth, attempts to further “wash” it by compressing a mass of material around it (confined in an extremely tight space) will generate tremendous hydraulic force. Because liquids cannot be compressed, and two things cannot occupy the same place at the same time, either the tray walls or the preliminary impression material itself will become distorted and will “spring back” when the impression is withdrawn. Although visually these impressions will be impressive (pun intended), with long fins of material evident beyond the finish line, the feedback I am receiving from many doctors and labs who have experience with this method is a very high incidence of tight crowns that fail to seat. This makes sense to me. Other doctors who use this technique because of its claims to avoid all retraction methods, and who have their labs use multiple coatings of die spacer, recognize this limitation and are having good results. Go figure!
THE FINAL ANALYSIS
No impression technique can be considered universally applicable. Indeed, I once had a lab technician tell me that if I came up with an impression technique that was as simple as throwing a bucket of manure on the wall, he had some accounts that would miss the wall! This being said, my feeling is that the LIT uniquely respects and takes advantage of the laws of fluid dynamics and represents the state of the art in fixed prosthodontic impression methodology. Whatever method you choose, it is my hope that this article helps you to understand and better choose your particular favorite.
1. Schoenrock G. The laminar impression technique. J Prosthet Dent. 1989;62:392-395.
2. CRA Newsletter. Double-arch impression update. Clin Res Assoc. 1986;12.
3. Costello W. An efficient impression-articulation technique. Dent Survey. 1997;53:44-45.
4. Wilson EG, Werrin SR. Double arch impressions for simplified restorative dentistry. J Prosthet Dent. 1983;49:198-202.
5. Schwartz R, Davis R. Accuracy of second pour casts using double-arch impressions. Am J Dent. 1992;5:192-194.
6. Schwartz R, Davis R. Marginal adaptation of castings made with dual arch and custom trays. Am J Dent. 1992;5:253-254.
7. Parker M, Cameron S, Hughbanks J, et al. Comparison of occlusal contacts in maximum intercuspation for two impression techniques. J Prosthet Dent. 1997;78:255-259.
8. Douglass G. The cast restoration: why is it high? J Prosthet Dent. 1975;34:491-495.
9. O’Leary T, Standish S, Bloomer R, et al. Severe periodontal destruction following impression procedures. J Periodontol. 1973;44:43-48.
10. Gullett C, Caulder SL. Residual fragment of rubber-base material. Oper Dent. 1978;3:131-132.
11. Clark SM. Rubber-base foreign body. J Prosthet Dent. 1974;31:439-440.
12. Hughes WF. An Introduction to Viscous Flow. New York, NY: McGraw-Hill Book Co; 1979.
Dr. Schoenrock has maintained a private practice in Holland, Ohio for over 25 years. His practice offers advanced crown and bridge treatment, implantology, orthodontics, endodontics, and oral surgery with intravenous sedation. Dr. Schoenrock has a special interest in sharing “real world” dental techniques with the profession, which he accomplishes by writing for many dental journals and presenting his “Common Sense” seminars to regional and state dental meetings and internationally. Dr. Schoenrock is the founder of the Midwest Dental Evaluation Group, which publishes INTERFACE, a newsletter that reviews new and practical materials and techniques for the profession. He can be reached for seminar information, questions, or sample issues of INTERF
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