Acrylic Resin Crowns
Prefabricated forms are ready for use as temporary crowns, and made of polycarbonates or cellulose acetate – for anterior teeth. They should be always modified by a process of relining (rebasing) with materials of the same chemical composition, and further contouring and shaping.
Temporary forms made of aluminium, silver-tin alloy or nickel-chrome alloy, can be used for posterior teeth. Ni-Cr ESF are intended (designed) for use in cases of severely damaged primary posterior teeth (molars) in children. They are not relined or rebased, but need careful adjustment and fixing, cementation with a permanent cement.
The basic procedures for fabrication of temporary crowns are: indirect, direct and combined – indirect/direct method. Indirect method means that the crown or bridge is fabricated on a master cast.
The first step is taking an impression from the prepared abutment teeth and soft tissues, pouring a stone model, master cast, wax modeling of the restoration. Modeling can be performed not in wax, but with special acrylic resin material and then a heat-curing process is accomplished.
The indirect method has a number of advantages and is widely used in practice. All adverse effects of the liquid component of the acrylic resin material (monomer, or methyl-methacrylate) can be avoided – allergy, sensitivity, overheating the pulp due to the exothermic reaction during polymerization, greater accuracy and precise fit of the crown margin to the finish line and gingival margin – the direct procedure is always associated with shrinkage and deformities of the resin material.
The rigid stone model prevents the material from such volumetric changes and deformities.
Due to all these reasons the indirect or combined procedures are the preferred approach in temporary resin crown fabrication.