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Written by Won-suk Oh, DDS, MS, and Ju-mi Park, DDS, MS, PhD Wednesday, 17 October 2012 19:22
A master cast for the fabrication of a removable partial denture (RPD) should be intact without demonstrating abrasion, chipping, or fracture of the stone teeth. The cast, however, does not present a higher tensile strength and often breaks in the cervical aspects of the stone teeth when being separated from an impression. This problem is more likely to occur when associated with isolated teeth, excessive tissue undercuts, advanced gingival recession, and relatively rigid silicone impression materials.1,2
Several methods have been introduced to prevent the breakage of the stone teeth when separating the cast from an irreversible hydrocolloid impression material.3-5 When involving a custom tray of acrylic resin, the tray is often sectioned and cut off from the impression material to avoid the risk of tooth fracture. When confronted with a tooth fracture, the broken teeth are related against the cast by matching the fracture surfaces. However, orientation of the broken teeth to the cast is not possible when the fracture surfaces do not demonstrate self-aligning features.
This article describes an accurate method of repair in which the broken teeth are aligned to the cast by means of the original silicone impression; and the broken teeth are contained in the impression to keep them in their original relation to the cast. This cast repair method is simple and reliable in reorienting the broken teeth to the cast using a relatively rigid and dimensionally stable addition reaction silicone impression material.
Assess the fracture pattern of the broken stone teeth from a master cast for a mandibular RPD (Figure 1). Note the absence of self-aligning features of the fractured surfaces of the broken teeth against the cast. To keep their original relation to the cast, do not separate the broken teeth from the vinyl polysiloxane (VPS) impression material (Aquasil Monophase [DENTSPLY Caulk]). Section the VPS impression in 2 pieces along the midline of the arch to isolate the area of the impression containing the broken teeth. Cut the visible light-cured resin custom tray (Triad [DENTSPLY International]) in multiple pieces using a disk (Cut-Off Disk [Buffalo Dental]), and then gently separate them from the VPS material. Next, clean the fracture surfaces of the teeth and the cast with a compressed air stream and discard secondary small chips of the stone. Reposition the VPS impression against the cast to orient the broken teeth, as a reference to the edentulous area of the ridge, and evaluate its fit on the cast along the borders of the impression (Figure 2). Set aside the VPS impression from the cast after confirming passive and complete seating of the impression without demonstrating a gap formation along its borders. Apply a thin layer of Krazy Glue (Elmer’s Products) on the fracture surfaces of the broken teeth and reseat the VPS impression against the cast as evaluated. Take care to avoid the use of excessive amount of Krazy Glue and ensure that the stone cast and teeth are completely dry. Do not disturb the setting of the Krazy Glue. Gently separate the repaired cast from the VPS impression and confirm the orientation accuracy of the broken teeth to the cast by checking for a continuation of the surface at the repaired site (Figure 3). Then, apply additional Krazy Glue at the fracture sites of the cast to reinforce the reattachment of the broken teeth.
Note the accurate orientation of the broken teeth to the cast (Figure 4) as indicated by precise intraoral fit of the metal framework for a mandibular RPD (Figures 5 and 6).
|Figure 1. A master cast fractured when being separated from a vinyl polysiloxane (VPS) impression. Note the lack of self-aligning features in the fracture surfaces.||Figure 2. The repositioned and self-standing VPS impression orienting the broken teeth against the cast. Note the absence of a gap along the border of the impression against the cast.|
|Figure 3. The repaired master cast, demonstrating accurate alignment of the broken teeth on the cast.||Figure 4. Removable partial denture (RPD) metal framework fits well on the repaired cast.|
|Figure 5. Intraoral fit of the RPD metal framework indicating the accuracy of the cast repair.||Figure 6. Excellent intraoral fit of the completed RPD.|
The cast fracture requires another impression to be made when its integrity is determined as unrestorable involving multiple patterns of fracture. The orientation of the broken pieces is complicated and not predictable. To avoid this problem, additional steps are taken prior to pouring the impression and when separating the cast.2-5 However, the tooth breakage is prone to occur when combined with a relatively rigid addition reaction silicone impression material contained inside a custom tray.
The cast can be repaired when the fracture surface presents a self-aligning feature. The broken pieces are approximated and reattached using a bonding agent. However, the orientation of the individual pieces of broken teeth against the cast is not accurate when the fracture surfaces do not demonstrate self-aligning features.
It is important to choose an elastomeric VPS impression material that is highly rigid and dimensionally stable to maintain its relation to the cast.6,7 The seating of the impression against the cast should not be manipulated and one must avoid binding against the cast, for which the custom tray is cut off from the impression to affirm tactile sensation of a snap fit. The orientation should be passive and self-standing, without demonstrating a gap formation along the border of the impression against the cast.
The cast should be free of water, presenting clean fracture surfaces without retaining secondary chips of stone before attempting to reseat the impression against the cast. Care should be taken not to disturb the setting of the bonding agent before separating the impression from the cast. Another impression should be made when a misfit is found at the periphery of the bonded site or when encountering a failure of the bonding due to accumulation of the intermediate layer of Krazy Glue between the fracture surfaces of the teeth and the cast. Any subsequent attempt to reattach the broken part is likely to result in a misfit of the RPD metal framework. This method is also contraindicated when combined with dimensionally unstable or deformed impressions.
A rigid VPS silicone impression material can aid in the orientation of the broken stone teeth to the cast when the fracture surfaces do not present self-aligning features. This method of RPD cast repair is accurate and predictable when carried out properly, and solves the problem arising from the relatively weaker tensile strength of dental stone when combined with a highly rigid impression material.
- Earnshaw RG. Gypsum products. In: O’Brien WJ, ed. Dental Materials and Their Selection. 4th ed. Hanover Park, IL: Quintessence Publishing; 2008:38-61.
- Sato Y, Takaki N, Tsuga K, et al. Effect of abutment tooth reinforcement techniques on the fracture resistance of removable partial denture master casts. J Prosthodont. 2001;10:22-25.
- von Krammer R. Avoidance of cast breakage during removal from the impression. Quintessence Int. 1990;21:217-220.
- Moon MG, Holmes RG. Modifications of the impression to prevent cast breakage. J Prosthet Dent. 1997;77:323-325.
- Galindo D, Hagan ME. Procedure to prevent cast breakage during separation from elastomeric impressions. J Prosthet Dent. 1999;81:37-38.
- Ceyhan JA, Johnson GH, Lepe X. The effect of tray selection, viscosity of impression material, and sequence of pour on the accuracy of dies made from dual-arch impressions. J Prosthet Dent. 2003;90:143-149.
- Caputi S, Varvara G. Dimensional accuracy of resultant casts made by a monophase, one-step and two-step, and a novel two-step putty/light-body impression technique: an in vitro study. J Prosthet Dent. 2008;99:274-281.
Disclosure: Dr. Oh reports no disclosures.
Disclosure: Dr. Park reports no disclosures.
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