Making fixed prostheses that are not too high
Gordon J. Christensen, DDS, MSD, PhD
It has been my observation that one of the most frustrating situations in practicing fixed prosthodontics is receiving a fixed prosthesis (also called a "fixed partial denture" [FPD]) from the laboratory and trying it in the patient’s mouth, only to find that the occlusion on the bridge is too high. The dentist then has to spend several minutes correcting the occlusion by removing structure from the FPD or the opposing teeth.
Every dentist has experienced this situation many times, and the reason for the occlusal inaccuracy usually remains obscure. The dentist often blames the technician, but soon finds that the stone teeth of the cast on each side of the FPD are in contact and, thus, that there has to be some other explanation for the occlusal discrepancy. The technician sometimes blames the dentist for a faulty interocclusal record (IOR), an inadequate working impression or an inadequate opposing cast.
Most dentists are using the double-arch impression technique for one or two units, and, when used properly, this technique usually produces crowns that are not too high. However, most dentists use a full-arch stock or custom tray to make an impression when a FPD of three or more units is being fabricated. In spite of detailed recommendations from manufacturers and some clinicians, many FPDs are too high when returned from laboratories.
In spite of detailed recommendations from manufacturers and some clinicians, many fixed partial dentures are too high when returned from laboratories.
There are numerous factors that contribute to FPDs’ being too high when returned from the dental laboratory. In this article, I discuss several of the most significant reasons for this dilemma and make suggestions to reduce the problem.
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ALGINATE IMPRESSIONS AND STONE OPPOSING CASTS
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Often, the least experienced staff person in the office is assigned the task of making alginate impressions and pouring them to fabricate an opposing cast for use by the laboratory technician. Alginate impressions can contribute significantly to an FPD’s being too high. The following factors should be observed and incorporated into the education of staff members making and pouring alginate impressions.
- – Mechanical mixing of alginate makes a more consistent mix of alginate every time and allows predictability concerning the amount of time that will expire until the alginate sets. However, hand mixing of alginate still is the most commonly encountered method of mixing alginate. Depending on the staff member mixing the alginate, and the physical condition of that person at the time of mixing, each mix is different. Mechanical mixing of alginate in devices such as the Alginator II (Dux Dental, Oxnard, Calif.) or the Combination Unit (Whip Mix, Louisville, Ky.) ensures that the alginate mix is the same each time. If room-temperature water and weighed alginate powder are used in a mechanical mixer, each mix is the same viscosity, and the time from mixing to setting is predictable.
- – The occlusal surfaces of the teeth should be blown off with an air syringe to remove debris and saliva. They should not be overdried. The mixed alginate should be rubbed onto the occlusal surfaces with a gloved finger to fill the occlusal groves, allowing accurate reproduction of the occlusal tooth anatomy.
- – The time before pouring is critical. After being removed from the mouth, alginate impressions should be washed with a water spray, disinfected by means of the practitioner’s choice of disinfection procedures and dried until the shine just disappears. Wrapping the impression in a wet paper towel, or placing it in a sealable sandwich bag with a few drops of water, allows a comfortable period of a few minutes before the impression should be poured.
- – A liquid surfactant should be sprayed on the surface of the alginate to reduce the surface tension of the set alginate impression before stone is poured into the impression. The surfactant assists in preventing air bubbles in the surface of the cast.
- – Use of dental stone instead of plaster for the opposing cast is necessary to provide strength and high wear resistance for the opposing cast. Either slow- or fast- (five-minute) setting stone may be used. Many laboratories use plaster, which has higher setting expansion than stone and, thus, greater inaccuracy.
Poorly made alginate impressions make distorted stone representations of the mouth’s actual anatomy.
In my opinion, inadequate alginate impressions and the pouring of inadequate opposing casts are two of the major reasons for FPDs’ being too high when returned from the laboratory. Poorly made alginate impressions make distorted stone representations of the mouth’s actual anatomy. Inadequately poured impressions have both internal and external bubbles; irregular, soft surfaces produced by mucus or blood being left on the casts to incorporate with the stone; and overall soft composition because of the use of plaster instead of stone.
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INTEROCCLUSAL RECORDS
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The most commonly observed IOR are mandibular closures into centric occlusion, making full-arch interdigitations in vinyl polysiloxane (VPS), wax, polyether or zinc oxide–eugenol paste (so-called "mush bites"). The most popular IOR technique is placement of fast- and rigid-setting VPS between the maxillary and mandibular arches and making the IOR in the desired occlusal relationship. Most of the IORs are made over the entire arch, instead of just in the area of the prepared teeth. The IORs come to the laboratory untrimmed, with the IOR material touching soft tissue, lodged between open contacts and underneath tooth undercuts. Such IORs force the laboratory technician to spend significant time trimming the record, often cutting off the portion of the record that extends to the side of the arch where there are no tooth preparations.
I suggest the following technique to produce an IOR that provides optimum transfer of the correct full-arch maxillary/mandibular relationship to the technician’s articulator.
- – Place the IOR material on the prepared teeth and extend it only a tooth or two beyond the prepared teeth. If the prepared teeth are in the posterior portion of the mouth, I suggest extending the IOR to include the canine.
- – Close the mandible into the desired position (centric occlusion, centric relation or centric relation occlusion). Make sure that the patient has closed into the position that you are reproducing to be transferred to an articulator. Usually, dentists prefer to have the FPDs constructed in an adjusted centric occlusion, the most interdigitated position.
- – Remove the IOR after setting and trim it with a sharp scalpel to eliminate all IOR material that touches soft tissue, extends into undercuts or extends more than a tooth or two beyond the prepared teeth.
- – Try the trimmed IOR in the patient’s mouth, close the patient’s mandible into the desired position and check the IOR carefully to ensure that the patient has closed into the proper position.
- – The technician should be advised to sit the working cast into the IOR and to place the opposing cast into the IOR with a rubber band wrapped around the upper and lower casts, making sure that the two casts fit tightly together.
I contend that inadequate IORs are one of the greatest contributors to FPDs’ being too high on seating.
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SUMMARY
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It has been observed that a high percentage of multiple crowns or FPDs arrive from laboratories too "high." This article suggests methods to create accurate alginate impressions, opposing casts and IORs. If dentists use these techniques, it is anticipated that fewer FPDs will be too high as dentists attempt to seat them in the mouth.
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FOOTNOTES
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Dr. Christensen is co-founder and senior consultant, Clinical Research Associates, 3707 N. Canyon Road, Suite 3D, Provo, Utah 84604. Address reprint requests to Dr. Christensen.
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ALGINATE IMPRESSIONS AND STONE...
INTEROCCLUSAL RECORDS
