The Five Dimensions
Before starting to prepare a tooth, it is imperative that you have a clear vision of the final tooth preparation and a rational for tooth reduction. The rational for preparing teeth in a particular manner is driven by the desired restorative outcome, determined by the changes required to achieve that outcome, and based on factors that are unique to each patient.
These factors are:
- Tooth position and arrangement within the patient's facial makeup
- The tooth’s appearance, including size, shape, contour, surface topography, shade and brightness
- Functional considerations
This first article in a four part series will begin to define five dimensions that influence preparation design and should be evaluated before beginning tooth preparation. Where the tooth problems are located, what esthetic changes are planned and how functional issues will be addressed with the new restorations all influence the diagnosis and treatment plan. The dimensions address clinical and technical factors that affect the clinician’s ability to achieve predictable patient outcomes. If the dimensions are not comprehensively assessed, the treatment outcome will be compromised.
The five dimensions that need to be evaluated to determine the current conditions and complete a comprehensive diagnosis are:
- Tooth length (incisal edge position)
- Esthetics (as it relates to hue, chroma, value and translucency)
- Labial aspect of tooth morphology and position
- Interdental issues
- Palatal/lingual morphology
#1 Tooth Length
The definitive incisal edge position of the tooth or restoration is primarily determined by its esthetic position, but can be influenced by functional considerations when addressing mandibular anterior teeth. Changes in tooth length can be accomplished through an additive process (restoration) or a subtractive process (enamel-plasty or restoration depending on the extent the tooth is shortened).
Evaluation of the esthetic aspect relates to assessment of the current appearance of the tooth (hue, chroma, value and translucency) and the changes required to achieve the desired outcome.
First, determine if the problem is in an isolated area or if it involves the overall appearance of the tooth. Isolated areas such as decalcifications or brown spots may be best resolved with microabrasion or a conservative composite restoration. An example of an overall or more extensive esthetic problem would be a change in tooth shade from A2 to A1 (requiring one shade change), or a change from A4 to A1 (requiring five shade changes).
The three factors that will determine the depth of tooth reduction on the facial aspect of the tooth are:
- The degree of change in color or saturation of color desired
- The degree of change in value/brightness desired
- The degree of relative translucency vs. opacity required to achieve the desired outcome
The greater the changes in color, saturation of color and value, and the higher the opacity required to mask the underlying tooth structure, the greater the depth of tooth reduction/preparation required.
#3 Labial Aspect of Tooth Morphology and Position
The labial aspect of tooth morphology pertains to the tooth size, shape, contour and surface topography. The tooth shape can be classified as triangular, ovoid or square. The tooth contour is concave, flat or convex. The surface topography can be smooth, undulating or rough.
All of these attributes can be altered by reductive or additive restorative techniques. Improvements in tooth position or rotation will also affect the extent and depth of tooth preparation. If a tooth is significantly rotated and the desired outcome is to achieve proper alignment, if the tooth is positioned within the arch too far labially or palatally, or if it is proclined or retroclined, more tooth structure will need to be reduced in additional areas of the tooth’s surface to allow the problems to be corrected.
#4 Interdental Issues
Six interdental issues can influence preparation design. They are:
- Existing restoration
- Fracture (Click here for more on traumatic fractures.)
- Morphology change
- Soft tissue influence
All of these problems require a solution that moves the preparation finish line palatal/lingual to the interproximal contact. The more significant the problem, the farther it is positioned palatally/lingually.
Extending the preparation through the interproximal to position the finish line on enamel is essential if there is decay, previously restored areas, or fractures. If there is discoloration interdentally that requires masking with a restorative material, the finish line is moved palatally/lingually. If there are significant morphology changes in tooth form because of its shape, rotation or position, or if there is a need to close open interdental spaces such as diastemas or gingival embrasures, the preparation finish line must be positioned more palatally/lingually. The finish line movement is farther palatally/lingually relative to the interdental contact and depends on how large the space is, or how significant the morphology change.
The goal is to create a smooth transition from the palatal/lingual aspect of the tooth to restoration with convex shapes, avoiding concavities at the finish line.
#5 Palatal/Lingual Morphology
Loss of tooth structure on the palatal aspect of maxillary anterior teeth can be caused by attrition (wear with the opposing teeth) and erosion from acid. In addition, the palatal aspect of the maxillary anterior teeth may be changed in order to alter the functional angle of disclusion (anterior guidance). Positional changes of the incisal edge to solve specific problems such as significant rotational issues or the tooth being in a proclined or retroclined position will also influence changes in the palatal/lingual morphology. Other reasons to include the palatal/lingual aspect in the preparation design are decay, existing restorations, if there is a fractured tooth structure, or if the opposing tooth contacts the preparation finish line. Generally, you want to avoid tooth contacts on the margin of the restoration.
In order to finalize the design of an outcome-based preparation, it is imperative that you complete a thorough evaluation of the five dimensions. While generally there are multiple problems that influence preparation design, Part II of this series will focus on cases where the patient presents with a problem that involves only one dimension.
Bob Winter, D.D.S., Spear Faculty and Contributing Author