Common problems with anterior crown preparations

Crown preparations are simple in concept in that they require a uniform reduction of tooth structure to allow enough room for indirect restorative material and the creation of a margin to finish the restoration. However, multiple aspects of this procedure can be performed poorly resulting in substandard results. Anterior teeth crown preps are unique due to the importance of aesthetics in this zone and their different form to posterior teeth. on one hand, anterior teeth tend to be longer than posterior teeth occlusogingivally which bodes well for their being sufficient wall height to provide retention form. On the other hand, their smaller buccolingual dimension means that it is more important to avoid overreduction in this dimension as the pulp and fracture resistance of the tooth can be compromised.

Technical issues that can occur with anterior crown preps include:

-Most commonly, there can be insufficient reduction of the tooth on the incisofacial third. This occurs when a margin is prepared on the facial aspect but a second plane of reduction isn't performed nearer to the incisal edge. It will result in a preparation with insufficient room to place porcelain with ideal tooth contours. The technician will be forced to overbulk the porcelain to ensure there is enough material thickness to avoid material fracture. This will also result in a non ideal facial contour of the tooth. If they don't and the crown is made to ideal tooth form then there will be a thin spot on the porcelain that will show through yellow dentine tooth structure. This also happens to be in the most aesthetically important position in the mouth as it is visible on low and high smile.
-On the other hand, failing to separate the facial reduction into at least two planes of reduction and making a straight facial wall of reduction from the margin will result in overpreparation of the labial surface. This may compromise the pulp status and increase the fracture risk of the tooth. It will not result in an aesthetic issue if enough tooth is reduced on the incisofacial area however it will result in overtapering of the tooth. If the crown height is sufficient then this may not be an issue but if the crown is short, the increased taper can result in poor crown retention.
-A similar issue can occur on the palatal surface where the reduction doesn't follow the palatal tooth contours and instead a flat reduction is performed. This will result in overreduction and overtapering of the tooth. Due to the angled nature of the tooth incisal to the cingulum, this area doesn't provide a high degree of resistance to the preparation as it must be tapered to provide enough restorative room. Therefore, the angulation of preparation at the neck of the tooth on the palatal is very important in providing parallelism against the labial surface and resistance form to the tooth.
-Due to the importance of parallelism in the preparation, it is easy to introduce undercuts in the preparation. The removal of these undercuts will result in excessive tooth structure to be removed but must be done as if it is accepted will cause temporary crowns to lock in and will cause excessively thick cement spaces (to allow seating of a crown) which increase the risk of dislodgement.

A) Insufficient reduction of the incisofacial due to lack of secondary plane reduction B) Excessive reduction of the facial surface C) Excessive reduction of the palatal surface D) Undercuts in the tooth preparation E) Ideal tooth preparation with even reduction of the tooth surface, contoured facial surface and parallel palatal surface