Finishing the Direct Posterior Composite Restoration: A Visual Essay

The direct posterior composite resin restoration is one of the most performed procedures in general dentistry. And in recent years, significant attention has been paid to optimizing outcomes in terms of the Class 2 box and managing polymerization shrinkage stress.

However, for all the discussion around this topic, one aspect of the procedure often does not get the attention it deserves: the mechanical finishing protocol. Mechanical finishing is defined as the procedures associated with contouring the restoration, eliminating excess resin at the margins, and performing the final polishing. It is a process that may be divided into three components:

Removal of oxygen-inhibited layer
Gross finishing
Final polishing

In this article, we will walk through what is involved in these components and outline a clinically efficient approach to finishing direct posterior composite restorations.

Removing the Oxygen-Inhibited Layer

The oxygen-inhibited layer (OIL) is a complicating factor after any procedure involving composite resin—both with direct and indirect restorations. (Indirect restorations are affected because they are conventionally bonded with composite resin cements.)

The OIL is an unpolymerized, sticky, resin-rich layer on the surface of all composite resins, which are polymerized in the presence of air¹. The OIL is formed when oxygen in the air interferes with the polymerization reaction. Put briefly, oxygen reacts more with the free-radical initiators in the composite resin than the monomers. The oxygen acts as a scavenger, binding the highly reactive free radicals and converting them into more stable hydroperoxides. The result? The monomers within the composite resin remain unpolymerized².

The OIL varies between 4 and 40 microns in section³. The thickness depends on many factors, such as the type of monomer, initiator-activator systems, particle morphology, the concentration of free radicals, and oxygen consumption rate.⁴

The Problem:

The presence of the oxygen-inhibited layer results in the following:

Reduced speed and quality of finishing procedures, since the unpolymerized resin clogs the cutting surfaces of the rotary finishing instruments.
Discoloration and wear in the mid-term. This particularly affects areas such as the interproximal, where it is challenging to remove the OIL with finishing instruments.

The Solutions:

The three leading solutions suggested in the scientific literature are:

The matrix band acting as a physical barrier to the oxygen during photo-polymerization (light-curing).
Photo-polymerization in an argon-rich atmosphere. Although enhanced degrees of conversion (DC) have been demonstrated in the lab, this approach is currently of limited practical application; however, it merits future research.
Placement of a layer of glycerine on the surface of the composite resin after initial photo-polymerization and further polymerization for 20-40 seconds (depending on the output of the polymerization light) at a distance of 1mm. This is the approach I recommend.

The 5 Phases of Gross Finishing

The function of the gross finishing phase is to remove any flash or overhangs from the margin to ensure the restoration finishes at a 90-degree butt joint, to create an anatomical “peripheral rim”⁵to facilitate interproximal oral hygiene measures and correct occlusal scheme, and to adjust and optimize the occlusal scheme.

The gross finishing procedure proceeds in the following 5-phase manner:

PHASE 1

An aluminum oxide course grit, medium-sized (1/2-inch) finishing disc (e.g., FlexiDisc®, Cosmedent) is worked over the mesial and distal marginal ridges and cuspal inclines at an angle of around 45 degrees to the long axis of the tooth (see Figure 1).

The disc is run at 5,000-15,000 RPM with or without a water spray. I prefer lower speeds without a water spray because it enhances visibility and precision. The shaping is conducted in bursts to avoid overheating the enamel resin interface. The assistant will copiously wash with an air-water spray from a 3-in-1 syringe between each burst to improve visibility and cool the restoration.

The restoration should be finished in a direction from composite resin to tooth structure to reduce the creation of marginal gaps and defects⁶.

The aim of this phase is two-fold:

Creating a rounded mesial and distal marginal ridge contour, which is more esthetically pleasing, allows improved interdental cleaning without snagging of floss, and enables the creation of a more ideal occlusal scheme (since space is created for the opposing cusp tip).
Removal of marginal excess resin to form a 90-degree butt joint between resin and tooth structure. A butt joint is preferred since it is mechanically superior. If excess flash remains and the patient occludes on the flash, it will fracture in short order, leaving a marginal defect.

Even if the flash is outside the zone of occlusal function, it will still tend to fracture. Why? Stress is transferred to the restoration during occlusal function, and this stress is dissipated to the bone via the dentin of the tooth and the periodontal ligament. In large bulks of resin, however, the stress is released efficiently, whereas small areas of flash create areas of stress concentration (stress risers).

This is a well-known concept in engineering and orthopedics. When the equivalent stress (stress within the resin) exceeds the yield stress (the stress at which the resin fails), the resin will fracture, leaving a marginal defect. This is why marginal ditching is seen at mid-term reviews in areas that are not subject to occlusal stress. Anecdotally, I have noted that this is most common in premolar teeth. This may be because there is less tooth structure in a premolar to dissipate stress (compared with a molar), and therefore the composite resin absorbs proportionately more stress.

The process is then repeated with a medium grit disc to create a more uniform margin. The disc is used on the backhand and “flexed.” This approach, rather than the forehand, will give a more rounded, flowing, organic profile rather than the more man-made-looking flat spots that are created on the forehand.

A clinical example is seen here:

Figure 2: The patient presented with a failing Class 1 silver amalgam restoration and Class 2 caries. The tooth was isolated with a rubber dam.

Figure 3: After tooth preparation, bonding, and resin placement. (Note some resin flash is visible at the mesial box axial walls.)

Figure 4: After gross finishing, the peripheral rim is smooth and anatomically correct.

PHASE 2

The gingival margin of the Class 2 box (if appropriate) is then finished with a diamond metal strip. Even with excellent matrixing and wedging, a small amount of flash/bonding agent remains. If left in situ, it can result in oral hygiene issues, periodontal problems, or secondary caries.

It is a simple matter to smooth this area with a coarse and medium grit metal strip. Be sure the strip goes apical to the contact point before actively removing excess resin so that the contact point is not inadvertently removed.

PHASE 3

The axial (vertical) margins of the Class 2 box are then planed using either a Number 12 (curved) scalpel blade or a heavy curette type scaler. This should be used in a scaling motion with a firm finger rest. Take care to plane parallel to the margin rather than at 90 degrees to avoid creating negative margins.

These phases are all conducted with the rubber dam in situ for efficiency and to avoid iatrogenic damage to the patient's tissues. The dam is then removed.

PHASE 4

Occlusal adjustment in MIP and all excursions are conducted with the operator's burr of choice and articulating paper. I tend to employ a small round diamond burr in a 1.5 speed increasing handpiece with a water spray at around 5,000-10,000 RPM.

PHASE 5

The margins and cuspal inclines are now smoothed with a coarse diamond-impregnated silicone point (e.g., Venus Supra, Kulzer). The aim is to eliminate flash and remove scratches created by the diamond burr during occlusal adjustment. The point is run at 8,000-10,000 RPM with a water spray to improve visibility, reduce thermal damage to the margin and increase the lifespan of the point. Light pressure should be employed (See Figure 5).

Figure 5: Diamond-impregnated silicone points.

By following this sequence, the gross finishing stage should be completed in two to three minutes per unit.

Removal of flash and creation of a butt joint margin is illustrated in the following clinical sequence.

Figure 6: Class 1 restorations were placed in the first molar and second premolar teeth. Flash at the margins is visible (highlighted with red arrows).

Figure 7: The margins are finished with coarse silicone polishing points.

Figure 6: Class 1 restorations were placed in the first molar and second premolar teeth. Flash at the margins is visible (highlighted with red arrows).

Figure 7: The margins are finished with coarse silicone polishing points.

Figure 8: Margins after gross finishing.

Final Polishing of Direct Posterior Composite Restoration

The final polish stage aims to create a smooth “enamel-like” surface luster that is comfortable for the patient's soft tissues, is aesthetically pleasing, and resists staining and discoloration. This is accomplished in four phases.