- Tray rim distortion can occur even when an impression appears to be acceptable. If, during the impression procedure, the tray rim is placed under compression by any of the myriad oral structures, the rim can be torqued out of its passive shape. When the impression is removed, the reflex memory of the tray rim can override the resistance of most impression materials. This “rebound” phenomenon can create an immediate dimensional distortion in the accuracy of the impression recording. Some of these tray distortions can be predicted prior to the impression procedure by trial-fitting the empty tray intraorally in the occluded position and observing any encroachments or interferences occurring between the oral structures and the tray rims. Other distortions can be spotted after the impression procedure by checking the impression carefully for areas or spots of the tray rim showing through the impression indicating that the rim came into contact with structures and cause spreading or torquing of rim which would indicate the strong possibility of distortion from tray “rebound”. However, at times NO issues may be visible…but can present at time of seating.
- Another common distortion factor occurs during the “bite” phase when the patient unknowingly occludes onto the tray rim and traps a portion of the tray. This affects not only the potential for impression distortion but also introduces distortion of the bite registration and the opposing arch.
- Aberrant centric occlusion or other exaggerated bite discrepancies that exceed the boundaries of the tray frame (e.g. crossbite, open bite, extreme overbite, and over jet) will not create the desired records of a triple tray impression and it should be considered inappropriate.
- Unilateral or bilateral “free-end” edentulous ridges beyond the first premolars do not have adequate posterior occlusal stops to prevent overclosure or unilateral mandibular torquing/tipping during the bite registration phase. The likely result will usually be a “compressed” occlusal plane with restorations that will be shy of occlusal contact. This compressed occlusal surface error has its greatest significance if the restorations are to be monolithic (e. max or full contour zirconia) and adding to the occlusal surfaces of these materials is not desirable.
- If the restorations are in the anterior region and the patient has no bilateral posterior stops, the patient may have no perception of his centric relation, centric occlusion, vertical dimension, overbite, or overjet. In such cases it is beneficial to utilize a bilateral acrylic base plate with bite rims to record vertical and centric relation as you would for a full denture patient. This is best performed prior to administration of anesthesia and preparations of teeth.
The Gardali research & development team has found that a full arch rigid plastic/metal tray or the Quad Tray with a mono-phase, medium body impression material consistently produces the most accurate, predictable results.