Impression Material Choices Can Mean the Difference Between Success and Failure
An accurate fi t and adequate function can be achieved with any of the countless varieties of vacuum-formed appliances on the market. The key to success? The dental practice must supply certain vital information to the laboratory, in particular proper impressions and bite registrations.
As vacuum-formed appliances must have the same degree of accuracy as any other appliance or restoration, it is imperative that the correct impression is received. Laboratories prefer to receive the impression for the opposing in alginate and the impression for the working model in vinyl polysulfide, silicone, or other accurate impression materials?preferably one with a light wash for accuracy.
Many dentists prefer to send in alginate impressions because it is less expensive and takes less time than other impression materials. Using an alginate impression for the working model creates a myriad of problems for the laboratory. Foremost, it is a very technique sensitive material and can introduce inaccuracies into the final working model. Its main function is traditionally for opposing models and other procedures not requiring an accurate fit. Avoid using alginate working impressions to minimize and eliminate chairside adjustments.
Occasionally an impression is sent into the laboratory with pulls, voids, and bubbles. This creates highly inaccurate models and the laboratory is forced to ?connect the dots.? It is almost impossible to fabricate an accurate vacuum-formed appliance from impressions with these types of discrepancies. Even one pull in a crucial area could lead to extensive chairside adjustments or even a remake.
The extensions of the impressions should fl ow past all soft-tissue undercuts buccally and lingually, capturing maxillary tuberosities and mandibular retromolar pad areas. If the posterior extensions are not captured in the impression, the appliance has the distinct chance of failure due to inadequate retention in those areas.
Bite Registrations?Eliminate Distortions
Frequently a vacuum-formed case is sent to the dental laboratory without a bite registration, forcing the laboratory to hand-articulate the case. Even though the laboratory is considered an expert at setting bites, some cases present two different sets of occlusal patterns to hand-articulate the bite. This again forces the laboratory to decide which bite is correct. In the end, we use the bite that we think is best and hope that we have selected the patient?s true bite. This scenario can be frustrating to both the lab and dental practice.
The variety of materials used to record a bite relationship ranges from baseplate wax and alumina wax to vinyl polysiloxanes, addition reaction silicones, and even polysulfi des. Each of these materials may react differently in extreme weather conditions such as heat and cold. It is a time-consuming task to evaluate each material on the market. Therefore I recommend that if you have doubts about the stability of your registration material in those conditions, consult the manufacturer or read the manufacturer?s instructions.
The Key to Comfort and Function
Vacuum-formed appliances are excellent tools that can be used in a variety of case scenarios. For the laboratory to fabricate a well fi tting appliance, it is critical that the proper impressions and bite registrations are received from the clinician. By doing so, little or no adjustment should be required and the patient will receive the most comfortable and functional appliance possible.