Material Selection, Tooth Reduction, and Margin Placement for Anterior Indirect Esthetic Restorations

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Material Selection, Tooth Reduction, and Margin Placement for Anterior Indirect Esthetic Restorations
Oral Health Journal
November 2007
By: Frank Spear, DDS

Choosing a method of restoration and a material comes down to evaluating several different parameters. These include the condition of the existing tooth, colour of the existing tooth, position of the tooth in the mouth, (anterior vs. posterior) esthetic desires of the patient and the skills and preferences of the laboratory technician doing the restoration.1,2 One of the more frequent questions I am asked as an educator is, what material should I use on a particular patient? The truth is, there is no one right answer. It is very dependant upon the variables described above, and even then, several different materials may be acceptable.

Having said that, it is important to inform you of this author's basic philosophy of treatment planning, that all teeth should be restored with the most conservative restoration that satisfies the patient's esthetic and functional requirements. That means that even though a full crown or aggressive reverse-3/4 crown prep might be esthetically acceptable, if a conservative veneer would be equally successful I believe it should be the first choice.

MATERIAL OPTIONS

For the purpose of treatment planning and choosing a material, I prefer to categorize materials into two large groups. Those that are homogenous in composition, meaning uniform in structure throughout, or heterogenous (non-homogenous), meaning they differ structurally from inside to outside (Fig. 1). Examples of homogenous materials would be powder and liquid ceramics fabricated on refractory dies or platinum foil, or pressable ceramics.

Examples of non-homogenous materials would be anything that uses a high strength core with a different material fired on it's surface. Examples would be metal ceramics and a variety of non-metallic restorations using alumina, zirconia, or lithium disilicate as high strength core materials. As a general rule, the homogenous vs. non-homogenous groups have distinctly different properties in several areas. With regards to tooth reduction, the homogenous materials can be used with less reduction than the non-homogenous materials. Optically, the homogenous materials are usually able to be fabricated with greater translucence than the non-homogenous ones. This leads to the fact that homogenous restorations must be bonded to improve their predictability vs. the non-homogenous ones that can be cemented to place or bonded without significant differences in performance.3-6

Because of the differences described above, most non-homogenous materials are used as traditional full crowns or bridge restorations, while homogenous restorations can be used as full crowns, but are particularly useful for more conservative bonded restorations.

Ultimately the material chosen is chosen based upon the needs of the tooth being restored. For the purposes of simplicity, I like to think of restorations as fitting into four major categories on anterior teeth. Those that replace primarily enamel, those that replace enamel and dentin but aren't full crowns, traditional full crowns that have normal coloured dentin, and full crowns with discoloured dentin or metal posts which must be covered.

Identifying which of these four situations exists will determine what material to choose, how much tooth needs to be reduced, and where the margin will be placed.

ENAMEL REPLACEMENT
The most conservative of all anterior indirect restorations is the one that essentially replaces enamel with minimal, if any, preparation into dentin. These restorations are useful when the overall tooth colour is pleasing and the goal of the restoration is to place a new, more pleasing external surface on the tooth. This type of restoration can also be used to alter tooth shape easily when the need to change the colour of the tooth is minimal.7,8 Tooth reduction for enamel replacement is minimal but is dependant upon the skills of the technician and the material chosen. The enamel thickness of a natural tooth varies from .4mm on the facial in the cervical 1/3, to .8 -1mm on the facial in the incisal 1/3. Therefore, true enamel replacement restorations are typically .3 - .5mm thick and require minimal preparation (Figs. 2-4).9

There has been a tremendous amount of marketing recently concerning no-prep veneers which fall under the category of enamel replacement, having the same limitation in changing tooth colour dependant upon the thickness of the porcelain. While there may be some teeth where true no-prep veneers are possible, physics is physics and adding material to the external surface of a tooth increases it's bulk. If that bulk is acceptable because the tooth was small enough to begin with, then no prep is possible. In general, even for enamel replacement restorations, some tooth preparation is desirable to allow for ideal facial contours.10,11

For the thickness of porcelain used for enamel replacement, homogenous materials are the only options, either powder and liquid ceramics or ultra thin pressed restorations. In general, the powder and liquid based restorations have the advantage of being able to be fabricated with less thickness, can have variations in colour built into the restoration without the necessity of any external stain, and therefore can be finished with a bur if necessary without removing any external colour.

The disadvantages of powder and liquid ceramics are that they are typically harder to fabricate than a pressable restoration, may not fit as well, and in a laboratory setting aren't as strong. Pressables have the advantages of easier fabrication, usually a better fit, and higher strength in a laboratory setting. Their disadvantages relate to colour management. For these reasons, external colour usually has to be applied to the ultra-thin pressed restoration to keep it from being monochromatic. The risk is that if any finishing or recontouring is done, this external colour can be removed. Suffice it to say that different clinicians and technicians have been able to do enamel replacement using both powder and liquid ceramics and pressable ceramics with excellent results, the choice being largely dependant upon the individual clinicians and technicians.

Margin placement is one of the big advantages of enamel replacement restorations. The same ultra-thin, highly translucent ceramics that makes changing colour difficult with these restorations, allows them to have supragingival margins placed that are invisible.12,13 It also allows margins to be placed interproximally and incisally without the need to carry the margin through the contact or over the incisal edge and down the lingual surface.

In fact, in a clinical study where enamel replacement veneers were used and a comparison done between covering the incisal edge or preparing the facial, half of the incisal edge and leaving the lingual half enamel, there was no differences in the success seen at three year recalls.14 This is not to say that the incisal edge can't be covered with enamel replacement restorations, but if the existing incisal edge position is pleasing, it is not necessary to reduce it's length to do these types of restorations (Figs. 5-9).

One might question why you would choose this type of restoration over one that does a more aggressive tooth preparation and uses thicker ceramics, but is easier to fabricate. There are several reasons you might choose the more conservative approach. We know that as we remove the facial enamel from an anterior tooth, it becomes more flexible under loading, illustrating that the enamel provided the primary rigidity to the tooth.15-17

By leaving as much enamel as possible, we therefore maintain a more rigid foundation to bond our ceramics to. This ultimately results in less stress being applied to the bond under function since the stresses of flexion always end up accumulating at the junction between materials of differing rigidities. In addition, enamel is a highly predictable and durable substance to bond to, so our bonds are very technique insensitive. Also since minimal dentin is exposed, the risk of sensitivity is almost non existent. Finally, just as we treat occlusal caries in a posterior tooth with an occlusal restoration, not an MOD or a crown, it only makes sense, when possible, to perform the most conservative anterior restorations whenever possible.

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