Temporary Crown And Bridge Resins

The usual technique is to record an initial impression in an alginate material prior to tooth preparation

. The major impression may be recorded then or subsequently using an elastomeric impression material. The mixed temporary crown or bridge resin is applied to the prepared areas by placing it into the desired area of the alginate impression which is reseated in the patients mouth. After initial setting, the impression and the resin are removed and final hardening occurs outside the mouth. Temporary crowns can also be fabricated by placing the resins on to prepared teeth in clear plastic crown formers. In addition, the fit of prefabricated crowns can be improved by relining them with one of the temporary crown and bridge resins.

The temporary crowns and bridges are cemented into place with temporary cements, normally of a zinc oxide eugenic composition.

Requirements:

The product should ideally be non-injurious to oral tissues since it comes into direct contact with freshly cut dentine and the oral mucosa. During setting, it should not give an unduly large temperature rise whilst in contact with dentine, as this could damage the pulp. It should not undergo a large setting contraction which could make removal of the temporary crown or bridge difficult, particularly if the set material is rigid.

Sufficient working time to allow mixing, placement into the impression and seating into the mouth.

After seating in the mouth, rapid attainment of a rubbery stage which facilitates its easy removal without distortion.

Rapid hardening outside the mouth, enabling the trimmed crown or bridge to be cemented into place after a short time.

Available materials:

Five types of material are available; the auto-cure acrylic material is essentially identical with that discussed on a s a restorative resin. A dual cure methacrylate material is now available. The liquid contains both chemical and light-sensitive initiators. The material undergoes a chemical setting reaction initially to achieve a partial set, the provisional restoration can be removed from its matrix at this stage and final setting is achieved by visible light curing either in the mouth or at the chair-side. Care is required as the heat generated during this latter process is substantial. The higher methacrylate resin is similar in many ways to the product sometimes used as a reline material for dentures and described on the composite material is different from those used as filling materials, which are not suitable for temporary crown and bridgework due to their unfavorable setting characteristics.

Properties:

Setting characteristics: The composite materials have the advantage of exhibiting a distinct rubbery stage, during which the temporary crown or bridge can be removed from the patients mouth without distortion or damage.

The two types of acrylic material do pass through a rubbery stage, when removal is facilitated, but this stage is not as distinct as in the other two products. Automated mixing of provisional crown and bridge resins should enhance their properties by eliminating mixing porosity. The dual-cure composite material once again relies upon its chemical set to give the rubbery stage to facilitate removal of the provisional restoration. Final curing is achieved out of the mouth by visible-light activation.

Biocompatibility:

Certain components of some of the products are known to have an irritant effect when placed on freshly cut vital dentine. Methylmethacrylate monomer, for example, present in the acrylic material, falls into this category. When using this product it is necessary to either varnish the preparations or apply a surface layer of petroleum jelly as a protective measure. The isobutylmethacrylate monomer used in the higher methacrylate product is far less irritant than methylmethacrylate.

Mechanical properties:

The mechanical properties of the materials become most significant when minimal tooth tissue removal or shoulder less crown preparations are used. There is a danger of fracture occurring in the thin areas at the tapered margin of such a temporary crown. This is most likely in the acrylic material which is weaker and more brittle than the other products.

Appearance:

In terms of appearance the acrylic, higher methacrylate and composite materials are all available in a range of shades and a good match with tooth substance can be achieved.

by: hawary16

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