Tooth Preparation for Full Coverage Restorations
Christopher C.K. Ho
The preparation of teeth for a full coverage restoration involves reduction of teeth to provide adequate mechanical and aesthetic properties in the restoration. This should be carried out atraumatically, with the conservation of tooth structures as an integral objective to minimise any pulpal or periodontal consequences from the procedures. Traditional preparation design for full coverage restorations has been based on retention and resistance form; however, the advent of adhesive dentistry has resulted in a paradigm shift towards less invasive preparations. There is increasing literature reporting on the ability to bond aesthetic materials adhesively with minimal or partial coverage preparations, although there is still concern in relation to a long-standing adhesive interface when dentine is the adhesive substrate.
Goodacre et al.1 outline several scientific guidelines to ensure mechanical, biological and aesthetic success for tooth preparation in full coverage restorations.
This is the angle of convergence between opposing axial walls. The more parallel the opposing walls of the preparation, the greater the retention. Jorgensen described the inverse relationship between taper and retention and found a 6° taper to be ideal.2 It is more common to see preparation TOC angles ranging from 12–27°. Goodacre et al. have proposed that the TOC ideally should range between 10° and 20°.1
This dimension is an important factor for both retention and resisitance, with longer preparations possessing greater surface area and more retention. It is proposed that 3 mm is the minimal occluso-cervical (OC) dimension for premolars and anterior teeth, while 4 mm is suggested for molars when teeth are prepared within the TOC range of 10–20°.
Masticatory function develops forces that are normally facio-lingual (FL) in direction. A critical factor is that adequate resistance is developed when the OC/FL dimension is 0.4 mm or higher for all teeth.
Teeth that possess a round tooth morphology after preparation lack resistance and should be modified by the creation of grooves or boxes in axial surfaces.
Subgingival margins are frequently required for retention and resistance form, to extend beyond caries, fractures or erosion/abrasion, to allow adequate ferrule or aesthetics. Orkin et al. have demonstrated the relationship of the position of crown margins and tissue health3 and, despite research favouring supragingival margins, the majority of crown margins are placed subgingivally, which results in more recession, a higher plaque index, bleeding on probing and gingivitis. Biologic width is the distance established by the junctional epithelium and connective tissue attachment to the root surface of a tooth, or in simple terms it is the height between the deepest point of the gingival sulcus and the alveolar bone crest. Based on the 1961 paper by Gargiulio et al.,4 the mean biologic width was determined to be 2.04 mm, of which 1.07 mm is occupied by the connective tissue attachment and another approximately 0.97 mm is occupied by the junctional epithelium. Placing crown margins that violate biologic width may lead to chronic inflammation of the gingiva, discomfort and loss of alveolar bone.
Different margin designs include chamfer, knife-edge, shoulder and bevelled shoulder designs. Knife-edge preparations should be avoided, as they provide insufficient bulk at the margins and often over-contoured restorations result.
- All-metal – chamfer margins are recommended as they are distinct and well demarcated, providing sufficient bulk of material. This is recommended to be 0.5 mm in thickness.
- Metal-ceramic – different designs have historically been used, including chamfer, shoulder and bevelled shoulder. The selection of finish lines is based on personal preference, aesthetics and ease of fabrication (Figure 6.1.1