Composite Bonding Frequently Asked Questions

• Why do my composite restorations occasionally dislodge?
• How do I check my equipment is working properly?
• With so many types of adhesive systems, how do I decide which to use?
• How do I maximize bond strength?
• To rubber dam or not to rubber dam?

Class V composites in particular are more prone to dislodging, as there is usually no mechanical retention and they depend entirely on the composite-tooth bond to stay in place. Hence these restorations are often used in clinical evaluations of bonding agents/adhesives.

Dislodgement could occur because of:

• Bond Breakdown. Dentine margins that are exposed to the oral environment predispose to breakdown of the composite-dentine bond over time. These restorations would generally dislodge after some time

• Insufficient Bond Strength. This could be because of:
  

• Choice of bonding agent (see below). The "convenience-pack" single bottle self-etch adhesives as a whole have poor bond strengths compared with others. In a clinical study, 28% of class V composite restorations had dislodged after x years.

• Faulty equipment. Assuming you are using the bonding agents/adhesives according to manufacturer's instructions, keeping a dry field without saliva contamination of the bonding surface, the problem may lie with faulty equipment — the light cure unit (LCU) or the triple syringe!


A surprising number of LCUs have decreased light output, either because of electricity fluctuations or aging of the bulbs or cracks in the light rod. This will reduce its effectiveness and consequently the composite's depth of cure, especially the lower portion of each increment, compromising the bond with the tooth.

Air from the triple syringe may also be contaminated with moisture and/or oil from the compressor. Use of oil-free compressors will prevent oil contamination. Air-dryers and condensers in the air-line should remove moisture from the compressed air. However, moisture may still contaminate your air after you have used the water in the air-water syringe if the 'o' rings of the syringe tip are worn. It could also happen if you use a disposable tip that is not pushed all the way in.

Polymerisation light cure unit (LCU)

Check the output of your light cure unit with a dosimeter weekly. A study found that 46% of Polymerisation light units needed either repair or replacement (Pilo et al. 1999). Most new light-cure units have built-in meters. An example is the 3M ESPE Freelight 2 that

Air-water Syringe

Spray water, then take a mouth mirror (or any mirror!) and blast air onto the mirror. If there are water droplets on the mirror, it is time to renew the worn 'o' ring(s) of the triple syringe.

If you use a disposable syringe tip, check that the tip is fully seated first. If it is, then it could be a worn 'o' ring.

There are two ways to look at this, either by solvents used or number of steps involved in the bonding procedure.

Solvent Type

All bonding agents are based on two different types of solvents — water (and alcohol) or acetone. These solvents are used as water chasers, particularly addressed to dentine; they displace the dentinal fluids in dentinal tubules and also the water that suspends the collagen exposed from acid-etching dentine (wet bonding technique), bringing with it the resins. Once this is achieved, the solvent is evaporated. The resin-impregnated collagen network created by demineralizing dentine during acid-etching, provides micro-mechanical interlocking with dentine when the resin is polymerised, forming what is known as the "hybrid layer". In enamel, they form resin tags. Together, both are responsible for the bond between tooth and composite.

Examples of both solvent-type bonding agents/adhesives is shown in the table below:

Acetone. My interpretation of the large body of bonding studies available is that higher bond strengths are achievable with acetone-based solvent bonding agents (again, that depends on the studies). However, the bond strength achieved is vary variable because:

• Significant increase in bond strength when wet bonding is performed correctly.

• Excessively moist dentine prevents the effective penetration of the acetone-based bonding agent (Tay et al. 1996). This is referred to as the "overly wet phenomenon".

• Acetone is volatile and tends to evaporate from the dispenser over time, increasing the viscosity of the bonding agent with storage, reducing its ability to penetrate demineralized dentine.

• Not adversely affected by overly moist or dry dentine (graph left) and therefore, less technique sensitive than acetone-based systems.
• The viscosity of the bonding agent remains more consistent over (storage) time as their solvents are less prone to evaporation.

Therefore, the composite-tooth bond is fairly consistent, reliable and less reliant on operator skill.

In clinical use, I personally favour the consistency provided by the less technique sensitive alcohol-based systems. If you prefer acetone-based products, I would recommend the unit dose versions for more consistent bonding.

Bonding Steps

Three Step.The other way is to consider the bonding systems by the number of steps involved.

The 3 step technique of separately acid-etching, priming dentine then applying the adhesive to bond on the whole provides slightly higher bond strengths then the other 2 step and self-etching systems. Achieving maximal bond strength to dentine requires moist bonding. The problem clinically is the difficulty in determining the difference between over-dried dentine, optimally moist dentine, and overly wet dentine, which will compromise the dentine bond, particularly with acetone-based bonding agents.

In an attempt to simplify and shorten the bonding procedure, manufacturers have tried to simplify the process by reducing the number of steps involved.....

The 2 step technique combines the primer and adhesive in a single step after acid-etching. It simplifies the bonding procedure but at a price of a slightly lower bond strength than the 3 step technique. Like the 3 step technique, achieving the maximum bond strength requires moist bonding.

Self-Etching Systems (One step technique). These one step systems achieve all three procedures — acid-etching, priming and bonding in one step. Besides simplifying and shortening the bonding procedure, this one step process circumvents the problems of wet bonding. However, with each simplification, the price seems to be a large drop in bond strength for the one bottle systems. However, there is less nanoleakage since the etch is only as deep as the advancing front of the self-etch bonding agent.

The acidic monomers used in these systems to enable self-etching can cause superficial chemical burns of the mucosa as a minor irritation! Therefore, if a rubber dam is not used, air-dry the tooth towards a high vacuum suction. Some exciting product development is being carried out in this field.

For the three and two step bonding systems, maximum bond strength to dentine is obtained with wet bonding. There is a significant increase in bond strength with wet bonding when using acetone-based bonding systems, whereas the difference in bond strengths between dry and wet bonding is not very different for water/alcohol based systems. Composite placement is very technique sensitive. Technique errors can almost halve your bond strength (Frankenberger et al. 2000)! The critical steps involved in bonding are:

• Etching dentine
• Moist etched dentine substrate for bonding
• Evaporation of solvents
• Adhesive application layers
• Light cure duration

Etching Dentine

As part of the total etch, both enamel and dentine are etched together. The etching time for enamel is not critical, whereas it is for dentine. If dentine is over-etched, the depth of demineralisation is

greater than the ability of the bonding agent to penetrate, leading to nano sized spaces through which leakage has been detected. This has been termed nanoleakage. With dentine, a deeper etch is not desirable.

Tip: Selectively place the acid etchant on enamel first, then apply onto dentine for 15 sec only (or other time recommended by the manufacturer). Wash.

Moist Dentine

Moist dentine in an integral part of the "wet bonding technique". Unlike enamel where acid-etching causes selective demineralisation of highly mineralised enamel prisms, acid-etching dentine demineralizes it, exposing a network of collagen fibres suspended in the water from the air-water syringe used to wash the acid off (figure below). This is the optimal state of the collagen network for bonding. The primer/adhesive needs to replace the water, so as to form a interlocking meshwork of collagen entombed in the resin of the adhesive, when polymerised. This interlocking mesh of collagen and adhesive is known as the "hybrid layer".

Excess water on the enamel and dentine surface needs to be removed as it will dilute the water based bonding agents and also prevent the acetone-based bonding agents from reaching the dentine surface.

consistent method is to use a moist cotton pellet to remove surface water without dehydrating the suspended collagen network (Pereira et al. 2001). Of course, "if you use our new self-etch bonding agents - you don't have this problem anymore!", advise some advertisements!

Solvent Evaporation

Evaporate the solvent by drying with a stream of air from the air-water syringe for 5 sec. Leaving excess solvent can reduce bond strength.

Adhesive Application Layers

After applying the requisite number of layers of adhesive and drying the surface, the tooth surface should be shiny. If it is matt, then that area is insufficiently coated with adhesive, and it must be recoated.

Bond strengths can be improved between 40 to 60% when the number of coats of adhesive is increased to four (table below) instead of the often recommended two applications (Hashimoto et al. 2004).

Light Cure Duration

Manufacturers provide a recommended light cure time as a guide. However, at the bottom of the cavity, particularly in proximal boxes, the light may be further away from the composite than optimum — the light intensity fall out has to be compensated for, otherwise, this vital first increment will not be fully polymerised to the adhesive, leading to low bond strength, leakage and premature failure.

Tip: Double the recommended light cure time for at least the first increment. If very deep, even triple it. Composite cannot be over polymerised, only underpolymerized. It's not worth the subsequent hassle from having to manage post-operative sensitivity etc. just to save 20-40 sec. Think of it as an investment in your peace of mind, not an expense! Smaller accessory light tips are available which will allow deeper access of the light and therefore, more effective Polymerisation of deeper increments of composite.

Saliva or blood will affect composite bonding (Hitmi et al. 1999). Therefore, good isolation is essential. A study has shown that using both effective cotton roll isolation and rubber dam can produce equivalent composite margins (van Dijken & Horsted 1987). When doing a small, simple composite restoration and isolation can be obtained with cotton rolls, especially in the maxilla, I do that.

I notice that one of the first things many graduates do on leaving dental school is to stop using the rubber dam, like it is a rite of passage. Once mastered, it is very easy to apply. It's there to make our work more enjoyable..... not a pain!

Where there are multiple restorations or build-up will take time, I find it simpler to put a rubber dam on rather than battle both Polymerisation shrinkage and saliva (even if there is cotton roll isolation!) all at once! Once rubber dam isolation is in place, composite restoration placement becomes a pleasure, not a chore!

There is a surprising paucity of research done on the association between rubber dam use and the quality of restorations. The only randomised clinical trial found no difference in the quality of posterior composite restorations after 10 years when effective cotton roll isolation was compared with rubber dam use (Raskin et al. 2000). Another study found it was more difficult to obtain good posterior contacts when rubber dam was used for composites in posterior teeth (Dorfer et al. 2001). This may be true if you do not pay attention to the interdental wedge placement, for any direct restorative material.