Factors That Influence Anterior Preparation Design - Part I

Since treatment planning is the key to determining the proper preparation design, you must first establish what problems exist.  This means comprehensively evaluating the patient through clinical examination, and interviewing the patient to develop a clear understanding of their treatment goal.  Are there tooth problems? Esthetic concerns? Functional issues? A combination of any or all of the above? The clinician must then clearly communicate to the dental technician all of the changes that need to be made along with the goal of treatment.  In addition, they need to inform the technician about the preparation design and the desired restorative material to be used, so when the diagnostic wax-up is completed, the necessary morphology changes are incorporated.  Precision in the diagnostic wax-up is necessary because preparation reduction guides are made from the diagnostic wax-up, and are then used to confirm there is adequate space for the restorative materials to meet the esthetics, strength and functional goals of the case. 

Ten factors influence and are critical to consider, when designing tooth preparation.  Part one of this article will discuss the five factors that have the greatest influence in this process. 

The five factors that have the greatest influence on the preparation design are:

• Esthetics
• Tooth morphology
• Tooth arrangement
• The structural integrity of the tooth
• Function

Esthetic changes in hue, chroma and value have the greatest effect on the amount of facial reduction needed during tooth preparation  This is highly dependent on the patient’s desired change in color (hue) and intensity (chroma), and how bright (value) they want their new restorations to appear.  

The most important consideration when prepping teeth is to make sure there is uniformity in the depth of tooth reduction for all teeth being prepared, if they are the same color.  The greater the changes desired by the patient, the more tooth reduction required to create the space necessary for the appropriate restorative material to mask the current underlying tooth’s appearance and meet the esthetic goal. 

• An extensive veneer preparation reduces the facial aspect of the tooth 1.2 mm in the incisal third, tapering to 0.8 mm gingivally. This reduction is suggested when making 2 – 3 shade changes
• An extensive crown preparation reduces the facial aspect 1.7 mm reduction in the incisal third, tapering to a 1.2 mm gingival shoulder design.  This is required for the most severely discolored teeth when
  an opaque core is required to totally mask the tooth preparation.

Desired changes in tooth morphology will determine:

• How much tooth structure is removed from the incisal edge 
  • If there is an increase in length planned, there may be little or no reduction necessary. 

• Changes in tooth proportion (width) will require the preparation to extend interproximally, moving the veneer interdental finish line palatally. 
  • This allows the technician to make the restoration(s) wider or narrower as required. 

• Changes in contour influence the depth of the facial reduction.  They can be:
  • Additive (less preparation needed to accommodate for the require material thickness to accomplish the morphology changes, or
  • Subtractive (more reduced off the tooth to create space
    for the restorative material) and to accommodate the morphology changes.

If the natural teeth are proclined, retroclined or rotated, and the objective is to change the tooth arrangement to straighten and align the restorations, the preparation design must be appropriate to reach the desired goal.  When changing tooth position or rotation, you will remove significantly more tooth structure on certain aspects of the tooth relative to other areas.

For example, if the maxillary anterior teeth are retroclined and the desired goal is to move the incisal edge facially to bring the facial aspect perpendicular to the occlusal plane, minimal preparation is required on the facial incisal third because it will be an additive restorative process.  More reduction will be required on the palatal aspect in order to make the desired incisal edge position change without making the incisal edge excessively thick.

Previously restored teeth, decay, fractures, attrition or corrosion, can affect the structural integrity of a tooth, and will affect the amount of tooth reduction required and the location of the preparation finish lines.  Placing the finish line on enamel and avoiding wear facets, will create the best long-term seal of the margin.

When working to establish or maintain function, if there are any anterior guidance changes planned which would include redesigning the palatal morphology of the maxillary anterior teeth, the preparation margin may have to be positioned more gingivally.

• Normal tooth reduction is dictated by the required thickness of the restorative material based on manufacturers’ recommendations for strength.
• If the desired functional outcome is to decrease the angle of disclusion creating more freedom in the anterior guidance, tooth reduction will be required on the palatal aspect.  Considerably more reduction may be needed because restorative material is required to cover the prepared tooth to establish the desired guidance.  There needs to be adequate room to meet the manufacturers guidelines as to the amount of restorative material needed to maintain strength, in addition to what is needed to make the desired changes in the angle of disclusion.  

When considering the factors that most critically influence anterior tooth preparation design, usually there is a combination of tooth problems, esthetic concerns, and functional issues.  Part two of this article will discuss five additional factors that influence the anterior preparation design process.

Bob Winter, D.D.S., Spear Faculty and Contributing Author