Options for esthetic restorations

Contemporary dental practices have embraced the process of bonding to natural tooth structure to combine function and esthetics. Direct composite resin restorations have replaced amalgam and gold in many clinical situations where esthetics is of primary concern and an adequate amount of sound natural tooth structure remains.

When more extensive reconstruction is required or elective treatment requires covering larger areas of the tooth, indirect laminates of processed composite resin or ceramic materials can be used. A laminate can cover only the facial surface of a tooth or it can wrap around to inter-proximal or lingual areas where esthetics or function demands more aggressive treatment. Where conventional treatment may call for a full crown, a 360-degree laminate can accomplish the same result while requiring a thickness of as little as 0.5 mm.

While ceramic materials are inherently brittle and can be broken easily when unsupported, they become quite strong when laminated to an underlying durable material. The lamination process is why porcelain fused to metal has been so successful in dentistry. The fact that the porcelain is laminated to a metal understructure gives it the strength to hold up under heavy functional stresses in the oral cavity.

Now that we can bond ceramics and composite resins to natural tooth structure, the tooth itself becomes the supporting structure and no metal foundation is needed for many clinical situations. Because there is no need for metal and an opaque layer of porcelain to block out the color of the metal, the restoration can be thinner so less tooth reduction is required. More of the patient’s natural tooth structure can be maintained.

Restorative Materials
But as we communicate these facts about advances in materials to our patients, we must temper our enthusiasm and provide balance. The word “permanent” should be eliminated from our dental vocabulary. Even natural teeth wear away as we age. Gold is the closest thing to permanent we can use for dental reconstruction. However, today most patients prefer tooth-colored alternatives.

Bonded composite resin and ceramic materials can deliver the desired result while providing longevity that is acceptable to most patients. Still, it is very likely that the restoration a patient receives today will need replacing sometime in the future. While most dentists agree that bonded ceramics provide greater longevity than composite resin, ceramic materials are more difficult to remove.

Composite resin can be removed with carbide finishing burs and it is easy to detect when natural tooth structure is reached, so little or no additional tooth removal may result. Porcelain, however, must be ground off using diamond burs and it is very likely that more tooth structure will be removed at each new preparation. I prefer to prepare conservatively so the patient will have as much natural tooth structure as possible left for future treatment.

Direct composite resin restorations require the least tooth reduction since they are built directly on the prepared tooth. They can be paper-thin in areas and still perform well. Laboratory-fabricated processed composite resin restorations need slightly more preparation because they must have a certain thickness (as little as 0.3 mm) to be held in the technician’s hand and finished without breaking. Stacked porcelain needs only a little more thickness (0.3 to 0.5 mm) but still can be very thin and function well after being bonded to the natural tooth. Pressed ceramics require more aggressive tooth reduction (0.6 mm and up) because they are fabricated using the lost wax technique. The process simply requires more thickness to allow for fabrication of the ceramic core and overlaying of low-fusing porcelain for esthetics.

The Lamination Process
Regardless of the material chosen, the bonding process requires attention to detail and flawless clinical technique. There is no span between excellent and poor in adhesive dentistry. If the clinical result is not excellent, it is no good at all. Inadequate bonding can lead to marginal leakage, tooth sensitivity, recurrent decay, de-bonding and loss of the restoration.

Current science advocates wet bonding. Our modern bonding agents are hydrophilic, allowing for bond strengths on dentin and enamel that exceed the cohesive strength of both the natural tooth and the restorative material. The luting composites for indirect laminates of composite resin and ceramic materials are of major importance in the success of the restorations. Indirect inlays, onlays and full crowns rely on composite luting agents for their retention and marginal seal.

The bonding process actually results in a strengthening of the remaining natural tooth structure since the remaining parts are held together instead of being wedged apart, as was the case with amalgam fillings.

What About Sensitivity?
Post-operative sensitivity has been one of the major areas of concern for most dentists who perform adhesive procedures for their patients. While some clinicians have claimed no problems with sensitivity, the vast majority of dentists are concerned about this phenomenon. Cold sensitivity is the most common complaint. This can occur due to open margins and resultant micro-leakage. However, even dentists who are sure that a marginal seal has been achieved have expressed frustration that cold sensitivity can still exist.

The most likely explanation is that a hybrid layer has not been achieved in the dentin bonding process or there are open dentin tubules which have not been sealed. By acid etching the enamel for 15 seconds with 37% phosphoric acid, part of the enamel matrix is dissolved, leaving thousands of tiny micro-pores in the enamel surface.

A liquid resin is flowed into these pores and allowed to harden, forming thousands of fingers of resin into the enamel. The bond is tremendous and is stronger than the tooth. The same acid removes the smear layer from the dentin in just five seconds, leaving the dentin tubules open. A hydrophilic resin flows into these open tubules and into the inter-tubular dentin, forming a “hybrid” layer consisting of resin and dentin which is mechanically and chemically locked together at a strength which is higher than the tooth itself. If this hybrid layer is incompletely formed or absent, the dentin tubules remain open and a negative pressure results when temperature or osmotic changes occur in the oral cavity. This results in the sensation of pain.

Given enough time, cold sensitivity usually goes away. I believe many endodontic procedures have been prematurely performed by well-meaning dentists who did not understand this process. However, with proper technique, post-operative sensitivity can be nearly eliminated today.

A dentin desensitizer and wetting agent used after acid etching and before applying the bonding agent can be very beneficial in helping form this important hybrid layer. Several formulas are currently on the market. One of the popular brands contains gluteraldehyde as its anti-microbial agent. This chemical can tan gingival tissues, so it must be used carefully in order to confine it to the tooth structures. I prefer another well-known brand (Hurriseal, Beutlich Pharmaceuticals) that uses benzalchonium chloride as the anti-microbial agent. It is kind to all tissues and can be used anywhere in the oral cavity.

Bonding Agents
Fourth-generation bonding agents utilized a primer followed by an adhesive. Fifth-generation bonding agents combine these into one solution. Both can work effectively when used properly. Film thickness is the major concern for one-step bonding agents since they must be light cured before a luting agent is placed between them and the restoration.

Many current one-step bonding agents exhibit a very low film thickness and can be used successfully for indirect procedures. When in doubt, a fourth- generation bonding agent can be used since it is cured only after the restoration is placed.

Luting Agents
Luting composites can be either microfills or micro-hybrids. Handling properties usually determine the choice by the clinician. Some are less viscous and flow readily. Others exhibit a higher viscosity. I like the higher viscosity resins and find clean-up procedures less difficult with their use.

Luting composites can be light cured or dual cured. I find dual-curing resins more efficient to use as long as a distinct gel state occurs during the curing process. At this stage (approximately three minutes after the start of the mix of the catalyst and base for most materials), the excess can be easily peeled away with a gentle touch of a scaler or explorer. Nearly total clean up can be accomplished before light curing.

For those who are concerned about stresses that may be caused by high-speed curing, the dual-cure technique will eliminate this problem. Some clinicians have expressed concern that dual-cure resins can darken with time, as did self-cure resins of the past. I must say that I have never seen this phenomenon clinically and I have been using dual-cure luting agents routinely for years.

Finishing procedures
It is hypothesized by some that sensitivity is created by aggressive finishing procedures. Care should be taken to minimize use of rotary instruments at the marginal areas where dentin and cementum are present. Careful technique must be used with light-cured luting resins. You must be sure to remove as much excess resin as possible before light curing. This will minimize the need for finishing. By using a dual-cure resin, most of the excess can be removed before light-curing, so post-cure finishing is automatically minimized. Of course, excellent margins that are clinically undetectable and flow into the natural tooth anatomy allow for maximum dental health and reduce the likelihood of any post-operative problems. Esthetic supra-gingival margins are usually possible and preferable when metal-free restorations are used.

Finally,
The choice of direct or indirect composite resin, stacked porcelain or pressed ceramic materials depends on many factors. When considering the restorative material, conservation of tooth structure will often lead to the correct choice. Sometimes other considerations, such as the presence of old restorations or broken down tooth structure, may dictate or allow the use of materials which require more aggressive preparation.

A desensitizer and wetting agent can help eliminate post- operative sensitivity and ensure a complete hybrid layer. The use of a fourth- or fifth-generation dentin and enamel and bonding agent will assure bond strengths that exceed the cohesive strength of the tooth structure or the restoration. Modern luting composites can be light cured or dual cured. The dual-cure technique can result in easier clean up and less need for marginal finishing which may also result in less post-operative sensitivity.