Rangos de movimiento

The basis of current inlay cavity preparations has come from techniques used in the cast method restoration. These were derived from original work by Black on cavity preparation with some modification for insertion of rigid materials. The cast inlay simply replaced missing tooth structure, does not reinforce that which remains and could tend to wedge cusps apart (Ingraham 1950; Werrin et al. 1980; Shillingburg et al. 1997). Therefore the proximo-occlusal inlay is only indicated for premolars with minimal caries or where a small restoration done previously was present that needed a mesio-occlusal or disto-occlusal restoration. MOD inlays, where the bucco-lingual distance is kept narrow, are acceptable for molars (Shillingburg et al. 1997). If the premolar or molar is damaged badly enough to warrant even a conservative MOD cast restoration, consideration should be given to occlusal protection, such as an onlay, because inlays act as wedges between the facial and lingual/palatal cusps of teeth. The cast gold inlay offers a superior material and margins that will not deteriorate with time (Shillingburg et al. 1997). However, the

use of cast metal inlays, one time considered to be the mark of quality dental care has declined in recent years. A number of dental authorities in North America concluding that the true cast gold inlay should have only limited use in conservative dentistry (Nuckles 1980; Clark and Smith 1984). This ^combined with concern over the use of amalgam, has brought about the development of an aesthetic replacement of the inlay design for posterior teeth (Martin 1991).

The general principles of cast gold inlavs which are recommended in most texts on conservative dentistrv. are:

(i) The outline form for inlays is generally more extended because of the need to eliminate undercuts (Gilmore et al. 1992), and includes all the defects and related fissures and grooves. Although proximal margins may be opened in the form of a flare to prevent undercuts, which may be visible on premolars, careful extensions on mesio-buccal flares may keep the display minimal (Shillingburg et al. 1997). The cervical margin of the restoration is located out of contact with the adjacent tooth and, where possible, supragingival, above the cement/enamel junction. Margins are preferably placed to permit finishing. Further they should be sited in cleansible areas to help prevent plaque formation and caries on the edge of the restoration (Gilmore et al. 1982).

(ii) The resistance form of the inlay is produced by smooth full-length walls with definite internal line angles. Resistance prevents dislodgement of the restoration by forces directed in an apical or oblique direction and prevents any movement of the restoration under occlusal forces (Shillingburg et al. 1997). Cavities are normally placed just into dentine except when caries is present. Here, further removal of tooth tissue is required. The mechanical properties of gold allow for less bulk of material and therefore possibly a reduction in the tooth destruction. Depth, combined with width, decreases fracture strength of teeth in studies by Blaser et al. (1983). Therefore, deepening an isthmus to increase resistance or inlay strength is not good practice (Shillingburg et al. 1997).

(iii) The retention form is developed from dovetails, the taper of the walls and pin retention holes. Retention prevents removal o f the restoration along the path of insertion or long axis o f the tooth preparation (Shillingburg et al. 1997). The essential element of retention is two opposing vertical surfaces in the same preparation. Grooves as well as cusp capping and surface reduction are sometimes used for

retention of large castings. The amount of wall taper and surface coverage is an important factor in the retention of the casting (Gilmore et al. 1982; Shillingburg et al. 1997).

Most cements act by creating mechanical interlocks between the inner surface of the restoration and the axial wall o f the preparation. Therefore, the greater the surface area o f a preparation, the greater its retention (Kaufman et al. 1961). Preparations on large teeth are more retentive than preparations on small teeth. Retention is improved by geometrically limiting the number of paths along which a restoration can be removed from the tooth preparation. Maximum retention is achieved when there is only one path (Shillingburg et al. 1997).

(iv) The cavo surface margin of the preparation is normally bevelled where the tooth approaches a right angle. This creates a finishing margin on the edge of the tooth structure, which is helpful in waxing the pattern and finishing the casting to maintain the seal of the restoration (Pitt Ford 1992). However, bevels are suggested to be minimal because compressive stress increases as the inclination of the bevel increases (Farah et al. 1977). The bevel is likely to produce some stress, but it is a necessary risk to produce a fmishable casting (Shillingburg et al. 1997). The good margin is enhanced by the bevel and reduces the amount of the cement liner that will be exposed to oral fluids. Some studies have challenged this concept by demonstrating that over expansion can create a closer fitting margin with a butt type of joint. However, most castings are undersized and butt joints obviate burnishing, making such margins unacceptable (Gilmore et al. 1982). More recent development in casting alloys preclude the burnishing techniques (McCabe 1990).

5.2 Importance of taper