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).
Fig 1-6-1 Universal Prep Set, Intensiv, Montagnola, Switzerland (reproduced with permission from Intensiv).
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.
Figs 1-6-2a to 1-6-2c Guided Universal Prep Set, Intensiv (reproduced with permission from Intensiv).
1.6.2 Minimally invasive preparation techniques
Anterior veneers
The classic indications for anterior veneers include:
■ morphological problems, anomalies of shape
■ diastema
■ 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).
Fig 1-6-3a Highlighted horizonal fracture and preparation lines after incisal fractures due to a trauma on teeth 11 and 21.
Fig 1-6-3b Rounded and smoothened incisal borders.
Fig 1-6-3c Feldspathic ceramic incisal etch pieces.
Fig 1-6-3d Patient’s smile several days after the adhesive cementation of the etch pieces. Note the slightly visible margin lines of the etch pieces.
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:
Fig 1-6-4a Initial situation – diastema after orthodontic treatment, patient desired a restorative solution.
Fig 1-6-4b Conventional impression taking with ultra-fine cords, no preparation.
Fig 1-6-4c Two feldspathic etch pieces 12, 13 with vertical margin lines following the anatomical structures of the abutment teeth for masking.
Fig 1-6-4d Note how thin the ceramic etch pieces are.
Figs 1-6-4e and 1-6-4f Final situation after adhesive cementation with a light-curing flowable composite filling material.
■ mask the margin of the etch piece
■ guide the positioning of the etch piece during the adhesive cementation.
Additional veneers
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).
Fig 1-6-5a Initial situation of a patient seeking esthetic improvement of the smile, ie, anterior maxillary region. Note the old metal-ceramic crowns on 11 and 21 with discolored gingival margin.
Fig 1-6-5b Extraoral anterior situation with relaxed lips and exposed anterior dentition.
Fig 1-6-5c Diagnostic mock-up made in the laboratory.
Fig 1-6-5d Try-in of mock-up, definition of treatment goal.
Fig 1-6-5e Regions in need for minimally invasive preparation according to mock-up marked on study cast.
Fig 1-6-5f to 1-6-5i Preparation guides indicating the needed preparation and markings on the teeth indicating regions needing minimally invasive preparation.
Fig 1-6-5j Intraoral view of situation after insertion of the minimally invasive additional ceramic veneers and two new all-ceramic crowns 11 and 21.
Fig 1-6-5k Final smile of the patient with improved anterior esthetics.
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.
Figs 1-6-6a to 1-6-6cc Schematic step-by-step preparation procedure for an additional veneer.
Figs 1-6-6a to 1-6-6cc Schematic step-by-step preparation procedure for an additional veneer.
Conventional veneers
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.
Fig 1-6-7a Anterior region of a male patient aged 35, who had experienced a trauma and the loss of vitality of the incisor 21. In addition, his entire anterior teeth were worn due to bruxism. He wished for esthetic improvement.
Fig 1-6-7b The first step in the present minimally invasive treatment concept was the internal bleaching of the discolored abutment tooth 21, accompanied by a pretreatment with a Michigan splint to reduce the parafunctions.
Fig 1-6-7c Then, the treatment goal was defined with a diagnostic wax-up. The treatment plan was veneers on teeth 12–22 and build up of the canine guidance by means of additional incisal veneers.
Fig 1-6-7d By means of a silicone key of the approved diagnostic wax-up, the need for abutment tooth preparation was evaluated. Note the arrows indicating where a slight change of the interproximal contact zones was planned. In these regions, the need for opening of the interdental regions was necessary.
Fig 1-6-7e In the incisal regions of the teeth prepared for veneers, flat palatally angled preparation finishing lines were delivered.
Fig 1-6-7f Conventional veneer preparation oriented at the diagnostic silicone key.
Figs 1-6-7g and 1-6-7h Virtual files (.STL) of the situation before and after the preparation for analysis of the amount of tooth structure to be removed.
Fig 1-6-7i Superimposition of the .STL files of the situation before and after the preparation to analyze the amount of removed tooth substance (mm). Note the predominantly minimal removal of tooth substance only increased in the interproximal areas where needed for the planned changes.
Fig 1-6-7j Feldspathic conventional (12–22) and additional (13, 23) veneers, manually made on refractive dies.
Fig 1-6-7k Anterior situation after the adhesive cementation of the minimally invasive anterior restorations.
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