Before considering embarking on indirect restorations, patients should be assessed to ensure that their periodontal condition has been stabilised and their caries risk is low.
The restorative assessment of the individual tooth involves:
• sensitivity/vitality tests
• long cone periapical radiograph
• examination of the quality of any existing restorations
• assessing whether the remaining tooth structure after preparation will have sufficient strength
• assessing the need for crown lengthening prior to treatment
• occlusal considerations.
The occlusal assessment should involve consideration of the tooth position relative to the opposing as well as the adjacent teeth, as this will influence preparation design. If there are occlusal interferences, these may place such a crown under high functional stresses and will require removal at a prior visit.The surfaces of the crown will need to be duplicated so that either the group function or canine guidance occlusion is maintained. The tooth may be a key unit in the arch, i.e. partial denture abutment, and the shape of the surface should be modified to allow the subsequent placement of the denture. In such situations, mounted study casts are a useful aid in planning the preparation design as well as carrying out the occlusal assessment. Any tooth preparation for a crown should follow the appropriate biomechanical principles below , and when planning replacement of a failed indirect restoration, it is important to identify the cause(s) of failure so that this may be corrected at the time of preparation.
Biomechanical principles of tooth preparation
• Preservation of tooth structure and pulp vitality
• Obtaining adequate retention and resistance form
• Obtaining adequate structural durability of the restoration
• Obtaining adequate marginal integrity
• Preservation of periodontal health
• Appropriate aesthetics
Common causes of failure include:
• poor preparation design/shape resulting in lack of retention and/or resistance form
• insufficient reduction or lack of support/thickness for ceramic or composite
• undercut preparations
• failure to identify and/or correct occlusal problems
• poorly fitting restorations resulting from poor impression procedures or faulty laboratory technique
• inappropriate prescription/planning; no preventive regime
• incorrect shade.