Composite Bonding Frequently Asked Questions

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:

1. 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:


1. 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.
2. 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.
2. Bond Breakdown. Dentine margins that are exposed to the oral environment predispose to breakdown of the composite-dentine bond over time. In-vitro studies seem to indicate that these composite-dentine interfaces, such as the gingival margins of non-carious cervical lesions (NCCL) will break down within months. Based on these studies we should expect our NCCL restorations in our patients to fail within the year. However, we all know from our clinical experience and numerous clinical studies that these NCCL restorations do indeed last for many years. So obviously some other factors not captured in in vitro studies occur in the oral environment. However, we cannot expect dentine-composite bonds to last indefinitely, and they will fail at some time - and dislodge.

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 cure units (LCU) in the private practices studied needed either repair or replacement (Pilo et al. 1999).

Most new light-cure units have built-in meters (fig. left). Remember to use them! If your LCU's output is below the recommended minimum light output of 300 mW/cm², and a minimum heat output of <50 mW/cm², try a new bulb. If this does not remedy the situation, ask the manufacturer to assess your unit. If it is too old......buy yourself a new toy! The new LED lights are cool.... (whichever way you interpret it!). As LED lights use very little current, LED LCUs are all cordless which make them very convenient to use! For LCUs without built-in light meters, buy a separate light meter.

To check the integrity of the LCU's light rod, place one end of the rod over a printed page or pattern and check the other end of the rod - you should see the letters clearly (figure left). If there is a break in the lettering there is a defect in your rod. You'll need to buy a new one - the cost will only make a small hole in your pocket, but your patients will be grateful for restorations that are predictably cured and better longevity!

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 (fig. left), 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.

Tip: You can only get dry, oiless air if the compressed air is delivered from an oiless compressor with an inline moisture filter, preferably with an air dryer! It is the price of achieving consistent, predicatable results and clinical excellence!

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 to infiltrate occupy the empty spaces around the exposed collagen network. 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". Together, both are responsible for the bond between tooth and composite - but the resin-impregnated collagen network is the major contributor to the adhesion. The resin tags play only a minor role in dentine bonding, unlike in enamel where the resin tags are the sole conributor to adhesion.

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

No. of Steps	Acetone-based	Alcohol/Water-based
3: etch, prime, bond	Bisco All-Bond 2	3M-ESPE Scotchbond Multipurpose/Plus, Ivoclar Syntac, Kerr Optibond/FL
2: etch, prime & bond;	Bisco One-Step/Plus, Dentsply Prime & Bond NT	3M-ESPE Single Bond 2, Ivoclar Excite, Hereus Kulzer Gluma Comfort Bond, Kerr SoloPlus
1: two bottle self-etch & prime & bond		Ivoclar AdheSE, Kuraray Clearfil SE Bond
1: one bottle self-etch & prime & bond	Dentsply Xeno IV, Hereus Kulzer iBond	3M-ESPE Universal Adhesive, Clearfil S3 Bond, Dentsply Xeno V+, Ivoclar Adhese Universal, Kerr SoloPlus Self Etch

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 extremely volatile and tends to evaporate from the dispenser over time, increasing the viscosity of the bonding agent with storage. The increased viscosity reduces its ability to penetrate demineralised dentine. Thus high bond strengths are usually achieved when a new bottle of adhesive is opened, then it starts deteriorating after that. How much reduction in bond strength occurs depends on how long the bottle is stored and how quicky it is recapped after each use.

Alcohol/Water

In general, water/alcohol-based bonding agents have a slightly lower bond strength to dentine (but there are always exceptions!). However, they are:

• 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

The other way is to consider the bonding systems by the number of steps involved:

Three Step

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. The alcohol/water-based systems are less sensitive to how dry or moist dentine is before application of the primer.

As there is only a certain depth to which the primer and adhesivee can penetrate the demineralised dentine, it is very important not to over-etch dentine, otherwie there is a small space between the hybrid layer (the maximum extent of primer/adhesive infiltartion) and the mineralised front of dentine where the dentin is demineralised but is not infiltrated with the adhesive, leaving bare collagen exposed. This gives rise to the phenomenon known as "nanoleakage".

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

Two Step

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, though some brands are less sensitive than others to the moisture level of dentine before the primer-adhesive application. Again, like the three-step technique, it is important not to over-etch dentine to prevent nano-leakage.

One Step

These one step systems achieve all three procedures — acid-etching, priming and bonding in one step. Some come with the solutions in one bottle and others come in two bottles that require mixing just before use. 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. However, these self-ecthing systems all do not etch uncut enamel reliably. Therfore when bonding to enamel, it is imperitive that the enamel is etched with phosphoric acid the traditional way, before using the self-etch primer-adhesive combination, which brings us back to a two-step technique. This fact is something the manufacturer's hide in the small print in the directions for use, as it defeats the convenience of a one step technique!

Typical microtensile bond strengths (Munck et al. 2005) for the different categories by steps are shown in the graph (left) and the table (below).

No. of Steps	Enamel/MPa	Dentine/MPa
3	38	53
2	40	37.5
1-two bottle	30	40.5
1-one bottle	16	24.5

For normal use, the three-step, non-acetone systems provide the most reliable bond being the least technique sensitive to the wetness of dentine, and remain the "gold standard". The two-step non-acetone systems provide a good compromise between efficiency and slight drop in bond strength. The two bottle self-etch systems hold promise, and the dentine bond seems less prone to breakdown over time compared with other systems. I would use the one bottle self-etch systems only for non-critical procedures where speed is the essence - like in primary teeth in not very co-operative children! Under normal circumstances if I use the all-in-one bottle system, I would still etch the enamel margins with phosphoric acid first, since these all-in-one bottles do not etch uncut dentine well, which makes it a two step technique anyway!

Convenience always has a price. Are you willing to settle for less than optimum bond strength? I prefer not to have complaints of post-operative sensitivity or dislodged fillings, and sleep peacefully at night.........zzzzzzzzz

Maximum bond strength to dentine is obtained with wet dentine bonding compared to dry dentine bonding for both the three and two step bonding systems 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)!

• 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.

Excessive drying of etched dentine, however, will not only remove the surface layer of water, but also the water suspending the collagen network, causing it to collapse (rollover image left). This is similar to sea weeds attached to the tidal beach floor - it floats graciously in the water, but when the tide is out, is found slumped on the sand. In this collapsed state, the penetration of resin to form the hybrid layer is less effective, resulting in a lower dentine - composite bond.

It is always difficult to describe how wet dentine should be for optimal wet bonding! If the tooth surface is over dried, the frosted surface of etched enamel becomes visible. The collagen network collapses , preventing the bonding agent from penetrating it fully, compromising the bond. If water is visible on the etched surface, it is overly wet. What's in between is just right!

The best analogy will be the sea shore at high tide. Just as the wave just recedes, the sand there will be over wet. Beyond this the moist sand is just right. Higher up the beach where the sun has dried the sand and it is lighter in colour - it is over dried!

A 1 sec air blast from 1cm away (Kanca 1996) will dry the tooth surface enough to leave sufficient moisture. Probably the most 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 manufacturer recommended two applications (Hashimoto et al. 2004).

No. of Coats	Kerr Optibond Solo Plus		3M-ESPE Single Bond
	Bond/MPa	SD	Bond/MPa	SD
1	13.6	6.6	38.5	22.1
2	49.1	15.3	55.5	12.1
3	58.1	16.7	54.1	23.3
4	69.5	20.5	87.3	15.1
6	60.0	12.2	75.1	19.0
8	69.4	12.2	64.8	10.5

Whether this is applicable for other systems has not been determined. This is certainly a simple way to boost the tooth-composite bond (and reduce leakage at the same time)! Manufacturers do not highlight this fact as it means more work for lazy dentists who like convenience, and they may gravitate towards products that demand less layers - but they should, they will sell double the amount of adhesives! They do to me!

I was sold on this idea of increasing the number of applications of the adhesive when I tried it out when restoring non-cervical carious lesions (NCCLs) with dentine hypersensitivity (without using local anaesthtic). My patients would invariably whince each time I air-dired each adhesive layer. Only when I applied either the third or forth layer, would the whincing stop and they say whew, no more sensitivity! Try this yourself, you will be sold on the idea too!

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.

Light Tip Distance from Composite

The intensity of light that reaches the composite is determined by the formula I ∝ 1/D² where I=intensity of light and D=distance of light cure tip from the composite. From this formula you will appreciate that doubling the distance between the light cure tip and the composite will reduce the light intensity by ¼ or 25%! Therefore, try to rest the light cure tip on the tooth (it is less tiring that way too!) where possible or into the cavity of proximal boxes (depending on the size of your proximal box in relation to your light cure tip). Tip: Where the distance is compromised, always compensate by increasing your light cure time by 75% for each doubling of the distance!

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.