#9 What I Learned Today

Introduction to Operative Dentistry

Today's simulation lab assignment focused on the preparation of Class I amalgam restorations on premolars and molars. Like any preparation, three specific forms need to be met in order to perform a competent preparation. The retention form ensures that the restorative material remains intact in the preparation once placed. This can be done through the use of parallel walls for deep preparations (roughly in excess of 4 mm), convergent walls (in instances less than 4 mm in depth), and dovetails (rounded walls at mesial and/or distal portions in order to resist lateral movment of restorative material). The resistance form makes sure that the placed restoration will be able to withstand occlusal forces and not fracture. This includes providing adequate thickness for restorative material (amalgam requires at least 2 mm, composite requires less), keeping the preparation small in order to maintain strong cusps and ridges, and making a flat pulpal floor that is perpendicular to occlusal forces. Finally, the convenience form needs to be met in order for the dentist to easily access (and see) the preparation and place the restorative material.

We performed Class I preparations on teeth #12 (permanent maxillary left first premolar) and #14 (permanent maxillary left first molar). We used a #330 bur, as we were only looking to penetrate the tooth to a depth of 2 mm (the same length as the working part of the bur). A #245 can be used for deeper preparations. In each case, the tooth was penetrated to the depth of 2 mm at the mesial/distal pit of the central groove. The bur was brought straight back up, and the resulting hole was measured with a periodontal probe to ensure proper depth. The bur was reinserted, and the groove was traced towards the opposite mesial/distal.

When making a Class I preparation, it is important to make all cavosurface line angles rounded. The walls of the buccal and lingual are made to be convergent in order to increase retention of the amalgam material. Distal and mesial walls are made to be divergent to 6 degrees. Pulpal floor is made to be flat from the buccal to the lingual, and slightly rounded from the mesial to the distal (in order to match the curvature of the DEJ). Finally, no unsupported enamel (enamel without dentin underneath) should be left, due to it being easily fractured. This means that any preparations nearing the mesial/distal marginal ridges should leave up to at least 1.6mm of enamel in order to be supported. Otherwise, a Class II preparation might be necessary.