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Preparation of Inlays and Onlays

The preparation of inlays and onlays mainly follows the same basic concepts of indirect restorations. The aim of tooth preparation is to preserve more tooth tissue compared to a crown preparation, while giving an adequate amount of protection to the tooth. The preparation of opposing cavity walls should be cut in a way to avoid undercuts in order to gain optimum retention from the cavity shape for the indirect restoration. For inlay and onlay preparations that are going to be completed with all-ceramic then these cavity shapes can be slightly over-tapered as most of the retention is gained from the cement lute. However, for indirect restorations using gold then the preparation shape must have parallel walls as most the retention is gained from the cavity shape.

The process of preparation and cementation of an inlay or onlay is usually carried out over two appointments, with the preparation being carried out during the first appointment and cementation at the second. After the tooth has been prepared at the first visit a putty and wash impression should be taken of the prepared tooth to be sent to the laboratory for fabrication of the indirect restoration. Also, a provisional restoration is made using material such as ProTemp, to protect the tooth from sensitivity, loss of vitality, bacterial contamination, to prevent over-eruption of the prepared tooth and also maintains dental aesthetics for the time being.

For tooth preparation, firstly start with occlusal reduction which depending on the restorative material being used can range from 0.5mm-2mm. The best instrument to use for this is a high-speed diamond fissure bur and the reduction should follow the inclination of the cusps and grooves as this will allow the preservation of more tooth tissue. A functional occlusal bevel should be created on the occlusal loading cusp of molar teeth by holding the bur at a 45o angle to the occlusal surface. This is the buccal cusp for mandibular teeth and the palatal cusp for maxillary teeth. A high-speed tapered diamond bur has the most convenient shape to prepare the buccal, lingual and proximal reduction of the tooth.

In the majority of clinical situations an inlay preparation is being made from a tooth that already has a Class II restoration and is being prepared to protect the tooth. The restoration is firstly removed and then the cavity can be converted by ensuring any undercuts are eliminated from the preparation. There are two ways this can be done, either by blocking the undercuts out with an adhesive restorative material or by removing tooth tissue to create the divergent cavity needed.