Article
Clinical tooth preparations are one of many routine treatments carried out in general and specialist practice. It is a topic that has been extensively researched and documented within academia and the teaching environment, with literature going back many decades and its principles are well illustrated and explained in the classic bibliographical textbooks.1,2 However, a systematic review found a the lack of occlusal reduction and over tapering to be common in preparations.3 Other studies have found the average margin width to fall below the ideal reduction of 1–1.5 mm.4,5 The ideal preparation varies depending on the type of restoration and underlying tooth structure (Table 1).
| Unsatisfactory | Ideal | |
|---|---|---|
| Labial/buccal reduction | Overprepared, underprepared | 1.2–1.5-mm uniform reduction with a two-plane reduction |
| Palatal/lingual reduction | Overprepared, underprepared | 0.5–0.7-mm uniform reduction following anatomical tooth contour |
| Incisal/occlusal reduction | Overprepared, underprepared, insufficient incisal/occlusal clearance and functional cusp bevel | 1.5–2-mm uniform reduction, smooth incisal/occlusal outline form |
| Interproximal reduction | Insufficient contact/s clearance, overprepared, underprepared | 0.5–1.5-mm reduction, subject to contact area, smooth and well-defined transition between porcelain and metal interface/line-angles |
|
Margins ideally finished on sound tooth structure: |
Unclear, ill defined, rough, underprepared, overprepared, inappropriate gingival level finish line | Clear, well defined, smooth and uniform with margin reduction aligning with axial surface reduction and appropriate gingival level finish line |
| –taper | Over/under tapered | 5–7° taper is ideal, but 10–20° is acceptable |
When carrying out a crown preparation, one of the major difficulties encountered is assessing the amount of reduction. Traditionally, a putty index constructed prior to the preparation has been used, which is then sectioned to give a guide as to how much has been reduced during the preparation (Figure 1).
This has limitations insofar as it can be difficult to correctly locate the putty index after the tooth has been prepared, the silicone putty material has a degree of elasticity that can give errors in measuring the amount of reduction and it can be difficult to use on posterior teeth where only limited areas of the preparation can be assessed. This can be overcome by a the construction of a temporary or provisional crown, with the use of a putty index or alginate impression pre-preparation and then reseating after the preparation with temporary material. This temporary crown can be used diagnostically by using calipers to measure the amount of reduction in various areas, so it can give a good overall assessment of the preparation (Figure 2). It can be more time consuming and repetitive, and potentially uses more materials, increasing costs and waste, but it supports the clinician in fully evaluating the preparation. The technique can work well if the preparation is carried out conservatively, but if the tooth is overprepared, then it may provide only a limited advantage to the clinician.
