The five additional factors to consider are:
- Whether the restoration is bonded or cemented
- The restorative material used
- The predictability of the restoration fabrication
- The predictability of the clinical insertion process
- Whether there are single or multiple restorations
In general, when designing preparations for restorations that are bonded (to enamel or dentin) versus cemented, cemented restorations require preparation that is more extensive. This is based on the manufacturers’ recommendations for minimal thickness of all-ceramic restorations that can be cemented. If the thickness of the restorative material is less than the manufacturers recommendations, it should always be bonded to the tooth (whether to enamel or dentin), in order to achieve maximum strength.
Cemented restorations rely on the classic principles of resistance and retention preparation form. Retention form counteracts pulling-off forces. Resistance form counteracts shearing/tipping forces. The preparation design can generally be more conservative for bonded restorations, because of the micro-mechanic retention facilitated by the process of etching both the tooth and ceramic restoration.
The following are basic guidelines for materials for anterior restorations:
Materials that must be cemented:
- Metal ceramic crowns
- Zirconia crowns (monolithic and bi-layered)
- Alumina crowns
Materials that must be bonded:
- Feldspathic crowns and veneers
- Lucite reinforced crowns and veneers
- Lithium disilicate veneers
Materials that can be cemented or bonded (depending on the circumstances):
- Lithium disilicate crowns
The restorative material used for the restoration will influence the preparation design. When all-ceramic translucent materials are used to fabricate the restoration, it is possible to use a more conservative preparation. If the tooth preparation is normal color/value, the resulting esthetic outcome will be determined by the combination of the appearance of the tooth preparation, resin cement, and ceramic characteristics. If opacity is required in the restorative material to mask the underlying tooth, a more extensive preparation is required. The opacity minimizes or eliminates the appearance of the tooth and resin cement. There needs to be additional space to allow for veneer ceramic to re-establish the translucency needed to simulate natural tooth structure. In areas of a restoration that undergo functional loading, the degree of tooth reduction required is dependent on the thickness of the material recommended by the manufacturer in order to obtain maximum strength. For example, if lithium disilicate is bonded to enamel, it can be thinner than if it is cemented to dentin. If it is bonded to enamel, it needs to be 1.0 mm. If it is cemented to dentin, then it needs to be a minimum of 1.5 mm.
The predictability of the restoration fabrication process can influence the preparation design and the case outcome. The clinician must know the capabilities of their laboratory. When completing a case prescription form, it is important that the materials and techniques ordered are those that the laboratory uses. They should be the ones in which they have the most experience in creating predictable esthetic and functional outcomes. The materials they use limit what each laboratory can produce, as do the skill set of their technicians, and the price point they have established based on their clientele. There must be clear collaboration on all cases, especially those that are difficult or complex.
The predictability of the clinical insertion process is dependent on how the case was designed and the tooth/teeth prepared, considering all the influencing factors. Ideally, tooth preparation was completed so that the esthetic and functional goals of treatment are realized including changes in shade (hue, chroma, and value), tooth arrangement, tooth morphology, and function, and adequate space was created for the chosen material. This allows for a predictable path of insertion, easy cleanup of cement, and margin locations that meet the required morphology changes. Ideally, if the preparation design is correct, the individual path of insertion of a particular tooth or teeth should allow the clinician to make a decision on the order in which the restorations are inserted. If the preparations do not have a common path of insertion and the tooth preparations are diverging, this will then dictate the order in which the clinician will be required to insert the restorations. For example, if the first restoration seated has a canted preparation and path of insertion, and the adjacent restoration to be inserted has a straight preparation and path of insertion, the angle of the resulting interproximal surface will prevent the second restoration from being completely seated. To solve the problem, the dental technician may be forced to angle the interproximal contacts to allow the restorations to be inserted, negatively affecting the esthetics of the case.
If a single tooth will be restored, that particular tooth dictates the determining factors in the preparation design. If the treatment involves multiple teeth, the preparation design is altered to increase the predictability of restoration fabrication. For example, the maxillary four anterior teeth are to be restored for esthetic and structural reasons. One central and lateral incisor were treated endondontically and both are significantly discolored. The conservative restorative approach would be to crown these two teeth, and veneer the contralateral central and lateral incisor. Can the technician manage the masking of the discolored teeth, and with what materials? If the technician’s material preference for the two crowns is metal ceramic, then how predictable will it be to match two metal ceramic crowns and two all ceramic veneers, especially if they are thin? If the preference of the technician for the two crowns is HO (high opacity) e.max as a core for the crowns, and MO (medium opacity) e.max for the veneers (which both require a layering technique), then how extensive must the clinician prepare the teeth that will be veneered? In this case, both the crown and veneer preparations should be extensive (see Part 1). If the dentist and technician are truly engaged in a collaborative relationship, these discussions should be occurring pre-treatment, not after the technician receives the case. The ultimate decision in the above example may be to crown all four teeth, in order to fabricate the restorations with the same materials and techniques needed to achieve the most predictable outcome.
As you can see, numerous factors influence preparation design. Care must be taken to consider all aspects of the case before tooth preparation begins, no matter the number of teeth that will be prepared. Failure to do so can lead to compromises in esthetics, function, and the overall goals of the case.
Bob Winter, D.D.S., Spear Faculty and Contributing Author