Tooth preparation: current concepts for material selection
In this chapter:
■ Minimally invasive preparation techniques (veneer preparations in anterior and posterior regions)
■ Defect-oriented preparation techniques for posterior teeth: onlays, overlay-veneers, and partial crowns
■ Conventional crown and fixed dental prosthesis (FDP) preparation technique: the universal tooth preparation
■ Resin-bonded fixed dental prosthesis (RBFDP) preparation
It is the ultimate goal of the present restorative concept, to preserve the tooth substance as much as possible and to be as minimally invasive as possible for all types of restorations. After the development of the diagnostic plan, each patient situation is evaluated carefully by assessing each individual tooth with the aim of providing the least invasive, defect-oriented type of restoration. At the treatment planning, various types of partial coverage and restorations for the defective tooth are considered before full crowns or conventional FDPs are planned.
Hence, with aid of the following checklist, the patient-related defect situation is evaluated, including the etiology of the respective problem, in order to determine the indicated type of restoration and the most appropriate restorative material. In addition to these factors, the main desire of the patient is to be involved in the selection of restoration type and material (also see Part I, Chapter 1). Lastly, the age of the patient can play a role.
Case analysis checklist:
1. Analysis of the etiology: What is the main clinical problem and why did it occur?
2. Pretreatment diagnostics: What is the treatment goal? Communication of wax-up with patient and technician
3. Analysis of dental situation: How can this goal be transferred to the clinical situation with the least invasiveness?
4. Analysis of the tooth substance conditions with respect to general prerequisite for less invasive types of restorations: Can adhesive cementation be performed with good predictability?
5. Definition of the type of restoration and the defect-oriented preparation: What is the tooth preparation design according to defect and the treatment goal?
In order to accomplish the different preparation designs in an efficient way, preorganized preparation diamond sets can be helpful. For the abutment tooth preparations within the present concept, a specifically organized preparation set with rough and fine-coarse diamond instruments was developed, which allows for the different preparation designs with 11 preparation instruments (Universal Prep Set, Intensiv, Montagnola, Switzerland) (Fig 1-6-1).
With the more recently introduced “Guided prep set,” the step-by-step sequences of the diamond instrument application are indicated for the different preparations (Fig 1-6-2). The preparation set also contains a mandrel for the use of polishing disks (Sof-Lex disks, 3M ESPE, Seefeld, Germany), needed for the finalization of the preparations. The finalization includes the elimination of any sharp edges, and the smoothening of transition lines and of incisal/occlusal boarders – a crucial step at tooth preparations for all-ceramic restorations.
1.6.2 Minimally invasive preparation techniques
The classic indications for anterior veneers include:
■ morphological problems, anomalies of shape
■ inappropriate tooth length
■ discolorations resistant to bleaching
■ fractures of coronal parts
■ erosive or abrasive loss of enamel
■ black interdental triangles1
The veneer preparation design is defined by the initial problem and the treatment goal, ie, the diagnostic wax-up. A silicone key of the wax-up is used as reference method during the preparation. In case of virtual diagnostics, a resin model of the virtual wax-up can be fabricated by means of 3D printing and can be used for the manufacturing of a conventional silicone key.
Different types of veneers are applied today.
Non-prep partial veneers, etch pieces
The least invasive types of veneers, the non-prep partial veneers (also known as etch pieces) are indicated for the modification of the width or shape of sound anterior and posterior teeth without discoloration2.
As no or almost no preparation is performed, the etch pieces tend to exhibit slightly overcontoured margins which can be difficult to manage in esthetically crucial, visible areas. The type of preparation-veneer margin exhibits significant influence on the outcome.
In general, horizontal margins are more difficult to mask than vertical margins (Fig 1-6-3).
Direct composite restorations may be esthetically more advisable in clinical situations with horizontal margin lines, than ceramic etch pieces. Yet, the clinician needs to master esthetic composite restorations in clinical situations with high translucency and/or internal staining.
If planning an etch piece, a vertical margin preparation can be placed into the area of the mesial or distal vertical line angles (Fig 1-6-4). Minimal preparation of vertical grooves can be performed in order to:
■ mask the margin of the etch piece
■ guide the positioning of the etch piece during the adhesive cementation.
The additional veneers are more extended than etch pieces, but also belong to the low-or non-prep veneer types3. They are indicated at ideal quality and color of the enamel for the esthetic improvement of the shape of the anterior teeth. These more extended types of veneers imply minimal tooth preparation to establish a path of insertion and to deliver space for the ceramics.
Again, a thorough analysis of the clinical situation and pretreatment diagnostics with conventional or virtual wax-up is mandatory; furthermore, an analysis of the study casts and the definition of the preparation design is recommended. Preparation guides, fabricated out of resin, help transferring the treatment goal into the clinical situation, while maintaining the least invasiveness during the preparation.
In case of horizontal fracture lines, a gradually decreasing preparation of the buccal enamel from the incisal to the marginal regions needs to be performed, in order to deliver sufficient space for the dental technician to mask the horizontal tooth–ceramic transition line with the veneer.
In order to be as minimally invasive as possible, a thorough analysis of the diagnostics is performed comparing the planned outline with the actual tooth shape. This analysis includes the determination of the future restoration–tooth transition zone. If a preparation has to be performed, the area to be slightly reduced is marked with a waterproof pencil on the tooth surface (Fig 1-6-5).
For the additional veneers, the preparation is performed without a clearly marked margin line unlike with the conventional veneer preparation, more resembling a feather-edge preparation design. This “margin-less” preparation delivers entire liberty to the technician to design the additional veneer according to the esthetic goal.
The schematic step-by-step preparation procedure for an additional veneer is shown in Fig 1-6-6. The clinical step-by-step procedure is presented in Part II, Chapter 1.
With slightly to moderately discolored abutment teeth, the thickness of the ceramic veneer has to be increased to allow for a masking of the discoloration inducing the need for a conventional veneer preparation4 (Fig 1-6-7). Furthermore, for extended modifications of the shape, size, or position of abutment teeth a conventional veneer preparation may be indicated.