Building up a marginal wall restores the contact area while creating a marginal seal and proximal emergence profile. The centripetal buildup technique (CBT)1 can be used to convert a Class 2 cavity to a Class 1 with the aim of simplifying the occlusal restoration procedure. This chapter describes the instruments and techniques to restore the interproximal wall. Once this has been done correctly, the occlusal surface can be modeled effectively and predictably.
Once the substrate has been hybridized, a Class 1 cavity will only require appropriate occlusal modeling for the specific clinical case.
For a Class 2 cavity, it is advisable to restore the missing proximal wall.1 This requires anatomical knowledge of the affected area: Proximal contact area varies in terms of width, position in the arch, and the individual patient. With premolars, for example, the contact area can occupy a more coronal position, while in molars it is positioned close to the middle third.2
- Physiologic preservation of tooth tissue
- Protection of underlying periodontal structures
- Transfer of masticatory force from tooth to tooth
Contact area location, cervico-occlusal extension, and anatomical profile can be assessed both radiographically and clinically (Fig 6-1). The proximal surface curves in both the buccopalatal/lingual and cervico-occlusal directions. The tightness of the interproximal contact area and anatomical variability of proximal curvatures can be reproduced by means of direct reconstruction using preformed sectional matrices with multiple convexities.
A matrix acts as a support for restoration material during proximal wall construction. It carries anatomical information spanning an area from the proximal cervical margin to the top of the marginal ridge.5 Given its non-anatomical nature, band matrices have proven less effective in establishing correct contact areas than sectional matrices with multiple convexities used with external (eg, Tofflemire, Waterpik) and automatic stabilization systems (Fig 6-2).6–14
Band and sectional matrices are made out of steel or acetate. Clinical and in vitro studies reveal no difference in marginal seal quality in Class 2 restorations, whether performed with acetate or metal matrices.12,13 The emergence profile created using band matrices is flat and oblique, the contact area is carried toward the occlusal surface, and the marginal ridge crest is displaced toward the proximal wall of the adjacent tooth (see Fig 6-2a), creating a greater fracture risk in the marginal ridge.5–15
The contact area established between the premolars tends toward the occlusal surface; the position of the contact area between premolars and molars and between molars and molars tends toward the cervical area (see Fig 6-1a). For both premolars and molars, the marginal ridge curves toward the center of the occlusal surface and allows masticatory forces to be discharged axially because it is close to the tooth’s center of gravity. Sectional matrices are shaped with multiple convexities so that they resemble the proximal emergence profiles of proximal teeth. They are the only matrices able to simulate the cervico-occlusal and buccolingual curvature typical of premolars and molars in a relatively natural manner (see Fig 6-2b). Interproximal tooth convexities display considerable intra-individual and interindividual anatomical variability, and the best sectional matrix must be chosen for each individual clinical case based on a rational approach. Depending on the manufacturer, sectional matrices can come in different heights (3.5 to 7.5 mm), thicknesses (0.025 to 0.04 mm), and curvatures (Figs 6-3 and 6-4). Some steel matrices have a coating that prevents the composite from sticking to the matrix surface (Fig 6-5). However, ordinary steel matrices are more than sufficient for the task (Fig 6-6).
VIDEO: CHOOSING A MATRIX
Choosing a sectional matrix
Metal matrices are the best choice as far as material is concerned. Acetate matrices do not allow the marginal fit to be checked as accurately given the lack of optical contrast.
The choice of sectional matrix is determined by matching the matrix’s point of maximum convexity with the tooth’s maximum circumference, which establishes the contact area. This measurement can be taken using a periodontal probe that records the distance between the gingival sulcus with inverted dam and the tooth’s maximum interproximal circumference, which is used to establish the contact area (Figs 6-7a and 6-7b). The measurement is transferred to the chosen matrix, and the practitioner checks that it matches the point of maximum curvature (Fig 6-7c). If two adjacent interproximal cavities are present (Figs 6-8), two adjacent sectional matrices are positioned. The future contact area is evaluated and considered in the context of neighboring contact areas.
Wedges are made out of wood or plastic. The latter can be regular or anatomical. The cross section of a wedge is always triangular. Wedges essentially perform four functions:
- Stabilize the matrix.
- Adapt the matrix to the step to avoid overcontours or undercontours.
- Simplify sectional matrix insertion.
- Assist the separator ring in temporary movement of the teeth between which it is positioned (Fig 6-9).
After reconstruction, separator ring removal will reposition the teeth to establish interproximal contact with appropriate tightness. The wedge generally must be inserted where the embrasures are wider to leave enough space for the square end of the wedge, which is usually in a palatal to buccal direction (Fig 6-10). Regardless of insertion direction, the important thing is to ensure the wedge and matrix are a perfect fit to correctly restore the interproximal profile (Fig 6-11). The wedge tip and square end should be equally exposed on both sides (buccal and palatal/lingual) of the interproximal area to allow correct interaction with the separator ring.
When a wooden wedge is positioned, it first can be soaked in water-soluble liquid soap to help prevent rubber dam displacement (Fig 6-12).
The curved part of the tip is positioned on top of the dam located above the interproximal papilla, which is slightly compressed (Fig 6-13a). The wedge rotates downward toward the papilla (Fig 6-13b). While it is rotating, the wedge is gradually pushed in a buccal direction (Fig 6-13c). As it advances into the interproximal area, the wedge stabilizes between the teeth, exerting a slightly distalizing and extrusive thrust (like a lever) (Fig 6-13d). During wedge insertion, the clinician must maintain pressure and sense the resistance of the tooth walls. If the wedge slides too easily between the teeth and the perceived resistance is light or if only the tip engages, the wedge must be considered unsuitable. Figure 6-14 demonstrates some tips and tricks for using wedges.
The use of the wedge is also important during Class 2 cavity preparation. Interproximal cavity access leads to a risk of iatrogenic damage to the adjacent tooth and can also damage the rubber dam, which will have to be replaced. The most vulnerable point of the dam is the part covering the papilla.
- Temporary orthodontic distraction, slightly separating the contact area
- Protection from rubber dam rupture due to contact with rotary instruments
The case shown in Fig 6-15 illustrates the part played by the wedge in protecting the dam in the interproximal area. The progressive wedge technique is also used to slowly increase the interproximal space. This can be useful, for example, when inserting a matrix into tight spaces. The first wedge (see Fig 6-15e) is positioned to begin temporary orthodontic distraction. After approximately 8 to 10 seconds the first wedge is removed, and the sectional matrix is positioned and stabilized using a larger-sized wedge (see Fig 6-15f).
Wooden wedges are the most commonly used wedges. They were the first available on the market and are still probably the most reliable. They are used to fit the sectional matrix to the cervical shoulder and operate as separation instruments. One very useful aspect of wooden wedges is that they soak up water, expand, and swell, which improves the marginal fit of the matrix. By exerting thrust between the teeth, they bring about minimal, gradual displacement without damaging periodontal tissues. Inserting a wedge between two teeth produces a lever effect. The teeth are subject to a thrust that extrudes them slightly from the gingiva. The separator ring exercises significant temporary orthodontic separation. The combined action of both instruments produces a very tight contact area.
The cross section is triangular at the tip and along the body and square or rectangular at the end. The square cross section of the rear part allows the wedge to achieve a stable grip when tweezers are used for insertion and removal. The flat part of the tweezer grip exercises thrust against the flat rear part of the wedge.
Commercially available wooden wedges are color coded, which varies from manufacturer to manufacturer and grades them according to size (height and width). Some wedges are the same height but different widths (rectangular cross section), while others are the same width but different heights (rectangular cross section). Some are the same height and width (square cross section). Smaller wooden wedges are very useful for pediatric patients and in all cases where the interproximal space is very tight.
A standard wedge can be modified by creating an intermediate bulge using Howe or universal pliers (Fig 6-16). The tips of the Howe pliers are rounded with a flat working part, which is ideal for flattening the wedge tip and body. This modification will ensure smooth insertion and a more decisive thrust, which will make for a better sectional matrix seal. This procedure is appropriate for specific anatomical configurations of the cervical shoulder that occur on the mesial surfaces of maxillary premolars. Modifying the tip alone (see Fig 6-16b) creates access without interference, maintaining a more consistent thrust on the cervical shoulder.
Plastic wedges (Fig 6-17a) can be standard or anatomical. Standard wedges are overly rigid and do not adapt to the tooth anatomy. Sometimes they are equipped with a carry handle that simplifies the positioning procedure. Like wooden wedges, they are color coded and graded according to size. An anatomical wedge cannot absorb water and expand, but its anatomical shape makes up for this shortcoming because it is well suited to the gingiva (it does not compress the papilla) and the specific anatomical shape of the cervical shoulder.
FenderWedges (Directa; Fig 6-17b) are plastic wedges that are equipped with a matrix plate and are only recommended for use when preparing a Class 2 box-form preparation (or box). While the wedge protects the dam, the matrix plate protects the adjacent tooth from iatrogenic damage. Alternative methods for preventing damage to neighboring teeth are available (Fig 6-18).
One recently introduced type of plastic wedge (Fig 6-19) is equipped with silicone wings that expand into the available space. Their ability to adapt to the cavity margin makes them a practical choice in cavities with a concave cervical shoulder like the one shown in Fig 6-20.
Choice of wedge in complex anatomical situations
Where the cervical shoulder is concave, wedges with silicone wings are useful but not essential. Wooden wedges can be inserted by rotating their axes 90 degrees. The upper part of the wedge is convex in the transition from the middle third to the anterior third (tip). They are well suited to concave anatomical configurations such as the one shown in Fig 6-20. This clinical situation, characterized by a low cervical shoulder with a concave profile, is not a suitable candidate for conventional wedge placement. In this case, an acetate sectional matrix has been used to allow a clinical view of the wooden wedge placement.
The following strategies can be followed in this situation:
- Placement of a plastic wedge with silicone wings.
- Placement of a wooden wedge inserted parallel to the occlusal plane (rotated 90 degrees).
- Placement of a modified wedge.
- Placement of compatible material (polytetrafluoroethylene [PTFE]) between matrix and wedge.
- If a wooden wedge cannot be positioned and/or the matrix cannot be correctly fitted, a completely different strategy must be considered, such as resective bone surgery and indirect restoration procedures.
Correct use of sectional matrices and separator rings helps achieve a tight proximal contact area and a well-defined anatomical contour to minimize the use of rotary instruments during finishing.7
The separator ring essentially performs two functions:
- Temporary orthodontic tooth separation to resolve the problem of the gap left by the sectional matrix following removal
- Allowing a sectional matrix to be fitted to the buccal and linguopalatal walls
Separator rings tend to lose their elastic memory with use. The arms must be compressed and moved together periodically to reactivate the elastic memory. When the ring presses and adapts the end of the matrix, the mesial part bends closer to the adjacent tooth, with which a contact area will be established.
VIDEO: RING REACTIVATION
Standard separator rings
Standard rings (Fig 6-21) are the most versatile and can be adapted to any clinical situation. Short arms are ideal for single Class 2 restorations. Long arms are useful for long teeth and multiple Class 2 cavities when they are fitted to overlap with short-armed rings (see Fig 6-21d).
First-generation rings offered limited separating power and were subsequently improved to overcome this drawback. They can be positioned in three different ways, according to cavity design. This allows the sectional matrix to be fitted more effectively to the residual buccal and lingual walls.
Positioning a standard separator ring
Ends interposed between the matrix and wedge
This is the ideal positioning (Fig 6-22). Wedge deformation as a result of the bending induced by the separator ring arms improves cervical adaptation of the matrix, while the position of the ends on the outer sides of the box walls improves buccal and palatolingual matrix adaptation.
Ends of both arms above the wedge
This method is ideal for cavities with a small proximal box (Fig 6-23). The ring closes, imposes a separation force, and seals the sectional matrix tightly. This type of positioning must be avoided at all costs for wide box preparations because the ring arms would close on the matrix, bending it toward the cavity. If reconstructed, the emergence profile of the proximal wall would be irregular, without contact with the neighboring tooth.
Placement of one or both ends behind the wedge
This solution (Fig 6-24) is preferable when the box is very wide at the buccal and/or palatal level and the separator ring end could collapse into the cavity if interposed between the matrix and the wedge. This placement type is also recommended with a very apical cervical shoulder because the deformation induced by the ends on the wedge often allows a better fit.
Resin and silicone separator rings
Separator ring systems feature rigid resin or silicone ends and are equipped with two V-shaped indentations to house the wedge tip and end (Figs 6-25