Triple tray crown and bridge procedures: a guide to how dental assistants can assist the dentist: Part 1.
The triple tray technique is an efficient technique for producing accurate impressions for crowns and bridges in dentistry. (1-3) A single, double-sided impression simultaneously captures an impression of the crown preparation, but also captures the maxillary arch, the mandibular arch and the occlusal relationship between the arches (Figure 1, pg. 16). (1) Two basic impression materials, polyether or vinyl-polysiloxane (VPS), both similarly accurate, (4-6) are used in the triple tray technique. (7-13)
In the basic triple tray technique, (1,14,15) the assistant takes a preoperative triple tray impression of the part of the mouth containing the tooth that will be prepared, using a medium viscosity VPS or polyether impression. Then, after the dentist has prepared the tooth, the assistant uses the pre--operative impression (that has an impression of the original, unprepared tooth) as a mold for making a provisional (or temporary) restoration for the prepared tooth. After making the provisional restoration, the assistant places a layer of low viscosity (high flow to capture fine details) VPS into both sides of the triple tray. The assistant then gives the triple tray to the dentist, who seats the tray intra--orally to reline the impression and capture the fine details of the prepared abutment and the opposing arch. (Of course, if a monophase impression material like medium viscosity polyether or Impregum is used for the pre--operative impression, then only this one phase is used again for the reline impression.)
In the United States, only a dentist, and not an assistant, is legally permitted to make or approve a final impression for a crown or bridge. The assistant then cements the provisional restoration, fills out the dental laboratory order, has the dentist review and sign the order, and packages the case for shipping or pickup. The patient is then re--appointed for the visit where the final crown or bridge is tried in and permanently cemented.
Two triple trays are often used in dentistry: the quadrant and the short anterior trays (Figure 2, pg. 16). The quadrant tray is a long quarter--arch tray that is generally used for capturing posterior (and sometimes anterior) teeth and is long enough to capture all posterior and most anterior teeth in a quadrant. The short anterior triple tray captures anterior teeth and often captures premolar teeth.
Properties of impression materials relevant to triple tray impression success
The authors suggest that an assistant only uses fast--setting (two minute set) impression materials with triple tray impressions. Slower-setting materials are not needed since the triple tray can be quickly loaded and positioned intra--orally. Use of fast--setting material helps to prevent the tray from being inadvertently removed from the mouth before the impression has set completely. Also, it is faster and less tedious to take repeated reline impressions if these are required to capture missed details.
Impression materials set faster in the mouth compared to setting on the office desktop because the mouth is a warmer environment compared to the office desktop. The intra--oral impression is set when it is does not show softness or dimpling to light finger pressure, or if the impression material that was left over after mixing, and is sitting on the cooler office desktop, has also set.
Impression materials used in crowns and bridges should be hydrophilic, meaning that they are to some extent attracted to moisture. Polyether, such as Impregum, is naturally hydrophilic and can often yield accurate impressions even if there is slight seepage of blood in a periodontal pocket, as can modern hydrophilic VPS impression materials. (16,17)
Whether to use one triple tray versus two separate arch impressions
Before taking the pre-operative impression, the assistant decides if a triple tray impression can be used for the crown and bridge procedure being performed on the patient, or if instead two separate maxillary and mandibular arch impressions should be made.
A triple tray is acceptable to use if, after the tooth or teeth are prepared into abutments, it is still obvious, from the occlusion of the unprepared teeth, how to articulate the teeth together. (12) In general, if there are two unprepared posterior teeth in occlusion after the tooth is prepared, or if there is one unprepared posterior tooth and at least one (but ideally at least two) unprepared anterior teeth still in occlusion, it will be obvious to a laboratory technician how to articulate the opposing arches represented on the triple tray model. In general, the occlusion of opposing posterior teeth is more obvious than the occlusion of opposing anterior teeth, but anterior teeth often have flat or faceted occlusal contact points that make it obvious how the teeth mesh together. After performing hundreds of crowns and bridges, the authors believe that the triple tray yields accurate and useable impressions for single unit crowns and three--unit bridge preparations, if the aforementioned occlusion reference points for unprepared teeth are present.
Taking separate arch impressions
If the occlusion reference points are not present or are not obvious, such as if preparing a bridge where there would be no unprepared posterior tooth distal to the bridge abutments, or if a single triple tray impression will not capture all abutment teeth, then two separate impressions of the arches should be made. Also, if a patient has a gag reflex when a triple tray is used, taking separate mandibular stock tray impressions may be more tolerable. If the patient cannot tolerate mandibular stock tray impressions, quarter arch stock tray impressions with a bite registration may be used to reduce the total amount of gag--stimulating impression material used. Occasionally a patient's dentition is malposed, with teeth in many different directions, so that the patient may not be able to dose into a triple tray when the assistant is making a provisional restoration. Here, separate arch impressions prevent the malposed teeth from the two arches from interlocking or interfering with one another. (18)
Typically the separate arch impressions are made using a medium sized plastic mandibular stock impression tray (Figure 2, pg. 16), but a larger or smaller sized tray may be used as needed. A maxillary stock tray, which needs more impression material, is not necessary because only an impression of the teeth and the edentulous areas between the teeth are required, and not an impression of the palate.
For the counter-model impression of the arch that does not contain prepared teeth, the assistant fills half to two--thirds of the impression tray with medium viscosity VPS, and then fills the rest of the tray with low viscosity VPS. The assistant then suctions out pools of saliva in the mouth and dries the teeth with gauze, and seats the tray.
For the arch that contains the tooth that will be prepared, the assistant takes an initial, medium--viscosity VPS impression of that arch. After the tooth is prepared, the assistant will fill this impression tray with low--viscosity VPS and the dentist will seat this relined triple tray intra--orally to take a final impression to capture the abutment details.
Alginate versus VPS or polyether for counter model impressions
The authors advise against making the counter--model impression using alginate impression material. Alginate can distort unless a stone model is poured up within about 10 minutes of making the alginate impression. (19-21) Making this stone model takes up the assistant's time and trimming the model on a model trimmer risks injury. The assistant may use an incorrect stone to water ratio when mixing the liquid stone or may incorporate air bubbles into the mixture when vibrating it into the impression, resulting in an inaccurate counter--model.
Also, some assistants may make the alginate counter--model impression but not pour it up in the office immediately. Instead, the assistant may wrap the impression in a moist paper towel, place it in a plastic headrest cover to prevent moisture evaporation, and send it to the dental laboratory to have the dental laboratory pour up the counter--model. However, the dental laboratory technician may not notice for days that an alginate impression had been sent to them for pour up and by then the impression will have distorted.
A counter--model impression made with poly--vinyl siloxane or polyether is more accurate than alginate. VPS is stable for weeks in open air without distorting, while polyether is stable for about a week in air before it starts to distort. (4,22) Unlike with alginate impressions, VPS or polyether impressions should not be wrapped in a moist paper towel before sending them to the dental laboratory, but instead should be dried and stored dry. (21) Assistants do not need to immediately pour up the impressions; an experienced dental laboratory can do so instead.
Manual versus automatic mixing of impression materials
Impression materials can be mixed manually, via hand spatulation, or automatically, by using a hand--powered impression gun or an automated mixing machine (Figure 3, left). (12, 23-24)
To mix impression material by hand, the base and catalyst components of the impression material are placed on a mixing pad. Typically, equal lengths of each material are dispensed so that the total amount of impression material that will be mixed will fill the triple tray. A wide hand spatula is used to mix the two components by smearing the components together and then using the edge of the spatula to plow the impression material into a pile, followed by smearing the material again into the pad surface. This process of mixing is repeated until the mixture is fully mixed and of uniform color and there is no marbling or individual streaks of the colors of the individual components in the mixture. Then, the assistant uses the spatula to place the mixture in the triple tray.
A manual impression gun mixes impression material more accurately than hand spatulation. With the impression gun, a disposable plastic cartridge has two tubes in it that hold the base and catalyst. This cartridge is locked into the impression gun which, when the gun handle is squeezed, pushes two rods into the disposable plastic cartridge. These rods push the impression material out of the end of the cartridge and into a disposable plastic spiral tube mixing tip that is attached to the cartridge. The two ingredients emerge perfectly mixed from the tip of the spiral mixer.
Another precise way of mixing impression material is to use an automated mixing machine. (23-24) A jumbo sized cartridge containing the base and catalyst tubes is loaded into the machine. This cartridge provides enough impression material for multiple triple tray fillings. A disposable plastic spiral mixing tip is attached to the machine for each mix. The automated mixing machine also provides a perfectly proportioned and homogeneous mix of impression material, and does not require the assistant to handle different impression guns. (24)
Spiral mixing tips
Each type of cartridge of VPS may have its own type of spiral mixing tip that should be attached to the VPS cartridge. Different tips are color-coded for different types of impression materials. Using the wrong tip may result in the material being not completely mixed when it emerges from the tip of the spiral mixer. The assistant should know which types of mixing tips are used in the office, which tip goes with which material, and monitor when the office is running low on stock of a required mixing tip.
Also, some composite materials that are used to make temporary restorations may come in a cartridge that attaches to a different gun compared to a VPS gun and may use a different spiral mixing tip.
Taking the pre--operative triple tray impression
The assistant positions the tray by aligning the curvature of the mesh surface of the tray with the curvature of the patient's arch so that essentially the patient occludes into the center of the mesh. For posterior triple trays, the posterior plastic loop of the tray should be placed distal to the most distal posterior teeth of the arch so that the distal loop of the tray goes behind the teeth. Ideally, there should be no interference of the teeth, the hamular notch or the retromolar pad into the distalloop of the tray. (25-28)
Prior to placing the triple tray, the assistant observes the patient's natural occlusion without the triple tray in place to see how the patient's teeth mesh together in maximum inter-cuspation. When the triple tray is in the mouth, the assistant makes sure that the patient is occluding into the same maximum inter-cuspation as when the tray was not in the mouth. (29) If the patient is shifting the bite when occluding into the tray, the assistant can instruct the patient to "bite on the back teeth," a command which often causes the patient to bite into the proper maximum inter-cuspation position.
After making an anterior pre-operative triple tray impression, the assistant rinses the tray in water to remove saliva, blood and other protein material. The assistant then identities the midline of teeth eight and nine in the maxillary impression. The assistant then drills an orientation notch into the plastic rim of the tray that originates at that midline and extends about 3-5mm in length (Figure 4, pg. 17). This orientation notch indicates which side of the tray is the maxillary side and which point on the tray corresponds to the anterior midline. Later, after the tooth is prepared, when the assistant places low viscosity impression material in both sides of the anterior tray to reline the pre-operative impression, the low-viscosity impression material may cover up all of the anterior teeth. Without the orientation notch, the assistant or the dentist may forget which side is the maxillary side and how to center the tray in the mouth for making the reline impression. Placing the reline impression upside down, or not aligned with the midline, can ruin the impression and require a re-make of the impression, using a new triple tray.
Introduction to provisional restorations
After the pre-operative impression has been made and the dentist has prepared the abutment tooth, the assistant makes a provisional restoration to cover and protect the abutment tooth. The temporary restoration helps to maintain the inter-proximal contacts and occlusion on the abutment tooth. Without a temporary restoration, the neighboring teeth may shift into the abutment tooth in the two weeks it takes for a laboratory to make the final crown.
Also, without a firm occlusal contact, the opposing tooth that occludes into the abutment tooth may extrude into the abutment tooth and the abutment may also extrude into the opposing tooth. This shifting may result in the final crown requiring extensive adjustment to fit it onto the abutment tooth during the crown try-in visit, or even a remake of the crown. (15, 30-33) There are various techniques for making provisional restorations described in the literature. (15, 30-33)
Modifying the preoperative impression prior to making the provisional restorations
Sometimes, the pre-operative impression requires modification before the assistant can make the provisional restoration. The provisional restoration must have contact with teeth next to the abutment tooth. If, in the pre-operative impression, there is a wall of impression material between the abutment tooth and the neighboring tooth, the assistant should use a hemostat to remove the excess impression material to allow the liquid temporary mixture to flow into contact with the neighboring tooth.
In the pre-operative impression prior to making the provisional restoration, the assistant should also try to ensure that the patient can occlude completely into it. While making the pre-operative impression, impression material can sometimes flow into undercuts in the mouth in the gingiva or in tuberosity areas, or under pontics of existing bridges in the patient's mouth. This material may prevent the patient from fully occluding into the pre-operative impression (Figure 5, pg. 20). The assistant should cut away such obstructions using scissors or (carefully!) using a scalpel (Figure 6, pg. 20). In general, undercut areas within tooth structure, excess impression material that flowed under bridge pontics, or long protrusions of impression material in the posterior lingual area should be cut away until the patient can close completely into the pre-operative impression.
If the assistant will be making a provisional bridge, he or she should (carefully!) cut away the edentulous ridge between the abutment teeth, using a scalpel (Figures 7 and 8, pg. 21). The assistant should remove a trapazoid-shaped section of the impression material so that the occlusal aspect, of the part of the pre-operative impression that is between the abutment teeth, is level with the occlusal aspect of the abutment teeth. The buccal-lingual width of the cut-out portion should be approximately the same as the bucco-lingual width of the abutment teeth. The buccal and lingual walls of the cut out portion should flare slightly in the preoperative impression, so that it is not undercut, since undercutting this area would make the provisional restoration lock into the preoperative impression.
Lubricating the pre-operative impression or the abutment
Some assistants like to lubricate the preoperative impression with petroleum jelly or mineral oil to prevent the provisional restoration mixture from sticking to the impression. However, the authors suggest not to use lubrication in the pre-operative impression since temporary material is easy to remove in general, even from a dry pre-operative impression. The lubrication can form a film on the preoperative impression material that may make it difficult for the reline impression material to bond to the pre-operative impression when the final reline impression is taken.
Also, the authors suggest that the patient's own saliva is all the lubrication that is needed on the abutment. Lubrication is not necessary to prevent the provisional material from locking onto the abutment because the assistant will prevent such lock-on by moving the material on and off the abutment repeatedly while the material is setting.
Mixing the temporary restoration material: composite versus acrylic resin
Some temporary restoration material, such as a self-cure composite material, is mixed automatically. The material comes in a tube that has the base and catalyst ingredients that is attached to a dispensing gun and is dispensed through a spiral mixing tip. (34) The assistant points the tip into the hole in the preoperative impression that corresponds to the abutment tooth and fills the hole.
Another type of temporary restoration material, acrylic resin, consists of powder and monomer ingredients that are hand-mixed using a cement spatula. The assistant dispenses 1-2 cubic cm of the powder into a dappen dish. The assistant then eye-drops the monomer liquid into the powder, enough to create a thick, barely viscous--but not visibly powdery--acrylic/monomer mixture, which is then spatulated into the abutment tooth hole in the pre-operative impression.
Some assistants mix acrylic resin material directly in the hole in the pre-operative impression of the unprepared tooth instead of using a separate mixing container. The assistant fills the hole with acrylic powder and then eye-drops the acrylic monomer into the powder while mixing it with a thin rod. The authors advise against mixing the acrylic directly in the impression hole because the uneven surfaces of impressions of teeth in the impression make it difficult to homogeneously mix the acrylic. Unmixed acrylic powder may remain in the hole, which may result in a stiffer mix of acrylic that the patient cannot fully occlude into and which will not flow enough to create a smooth, detailed, porosity-free temporary restoration.
A flexible silicone dappen dish is perhaps the best container to use for mixing the acrylic/monomer mixture since the hardened mixture can easily be removed from the container by flexing it and the container is re-usable and sterilizable. A silicone container also has a stable, non-slipping base (versus plastic disposable dappen dishes that can slip while the assistant is mixing impression material inside them).
If the assistant mixes the acrylic resin material in a glass dappen dish, the assistant should immediately dean out any excess, unused mixture from the dish using gauze while the mixture is still soft. The assistant then immediately disposes of the gauze and also of the gloves that the assistant was using while wiping the dish, to prevent monomer from penetrating the gloves and contacting the assistant's fingers.
[Editor's note: In the second part of this article, appearing in the next issue of The Dental Assistant, Dr. Mamoun and dental assistant Javaid will discuss provisional restoration fabrication in more detail, making final triple tray impressions, filling out laboratory slips and packaging cases, cementing provisional restorations, and the special topic of malting impressions for round-house crown and bridge cases.]
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John S. Mamoun, DMD, is a 2003 graduate of the University of Medicine and Dentistry of New Jersey. He completed a one-year Advanced Education in General Dentistry residency at the Eastman Dental Center, in Rochester, NY, and later earned his Fellowship award from the Academy of General Dentistry. He is currently in private practice in Manalapan, NJ.
Mariam Javaid, BDS, graduated from the Altamash Institute of Dental Medicine (AIDM) in 2009, in Pakistan. She completed a residency at the AIDM in 2010, and an externship at the Karolinska Institute in Stockholm, Sweden, then came to the United States, where she is a dental assistant in Philadelphia, PA.
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|Author:||Mamoun, John S.; Javaid, Mariam|
|Publication:||The Dental Assistant|
|Date:||May 1, 2013|
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