Tooth Preparation Theory: Application to Custom Abutment Design

The two primary principles in preparation design are retention form and resistance form, which are interrelated and often inseparable. Retention form is the ability of the preparation to impede removal of the restoration along the path of insertion (Figure 1). Retention is affected by 4 factors:

• Taper: The ideal taper of each axial wall is between 6° and 8°, for a total taper of the form between 12° and 16°. As the amount of taper increases, the retention of the form drops significantly (Figure 2). This factor is probably the most important when designing an abutment.

• Total surface area: In general, the more surface area, the better. This is affected by the tooth size and the addition of boxes or grooves in the preparation form.

• The area of cement under shearing forces: Dental cements are much stronger in shearing forces and weaker in tension. Designing a preparation form with more parallel opposing walls will restrict the path of draw for the restoration. This creates greater shear forces on the cement while reducing tensile forces.

• Surface roughness: Titanium typically cannot be chemically bonded, so cements utilizing adhesion are used to retain abutment-supported restorations. Oilo and Jorgenson showed when using dies with a 10º taper, a surface roughness of 40 µm was twice as retentive as one with a 10-µm surface roughness. Although sandblasting can create an adequately rough surface, the resulting darkening effect on the titanium surface can create unfavorable esthetic issues with all-ceramic restorations. This darkening can be avoided if a “grooved,” retentive surface is created on the axial walls in the milling process. Figure 3 and Figure 4 illustrate this “grooved” surface as machined by the author’s Versamill 5-axis mill (Axsys Dental Solutions, axsysdental.com).

Resistance form is the ability of the preparation to prevent dislodgement of the restoration by forces applied apically, obliquely, or horizontally (Figure 5).

Noting Figure 6: When a load (L) is applied to the occlusal surface of the restoration, it wants to “tip,” or be dislodged, and the restoration will want to rotate around a fulcrum point (F). The only aspect preventing this dislodgement is the shape/taper of the opposing axial wall. Note the direction of the resulting forces on points 1, 2, and 3. Whenever the shape of the opposing wall is at a 90º angle to a line drawn to the fulcrum, very little resistance (points 1 and 3) is seen. This occurs frequently with overtapered preparation forms. Note, however, that at point 2, adequate preparation form resists this tendency for the restoration to tip. Hence, it is at this point on the preparation that resistance form has been created.

The height and width of the preparation form will significantly affect resistance form. The taller and narrower the form (Figure 7), the more resistance form is present. The shorter and wider the preparation form (Figure 8), the less resistance form is present. These principles of retention form and resistance form are especially critical when working with anterior teeth. Due to the anatomy of anterior teeth, the lingual surface provides relatively little surface to create the desirable features discussed above. An adequate lingual surface (1.5 mm to 2.0 mm) must be created while accurately maintaining the proper taper relative to the facial surface (Figure 9).

The contours of the submarginal area of the abutment and the final placement of the margin are critical to healthy tissue response, esthetics, and cement cleanup. Whether the clinician is attempting to specifically shape the surrounding supporting tissues or simply transition from the round interface of an implant to the proper contours of a tooth at the level of the cervical tissue, the emergence profile of an abutment should be smoothly contoured, avoiding sharp undercuts and areas that will be difficult for the patient to maintain. Ideally, the margins should be placed within 1 mm of the gingival tissue to facilitate cement removal while maintaining specific esthetic requirements.

Also, consider the amount of clearance/reduction necessary for the final restoration. Specific numbers vary depending upon materials; however, Figure 10 provides a general guideline. In addition, providing functional cusp reduction (or second plane reduction) is vitally important. This additional 30º to 45º reduction is provided on the occlusal-buccal surfaces of both maxillary (for esthetics) and mandibular posterior teeth (for material thickness) (Figure 11). The maxillary anterior teeth should also demonstrate a second plane reduction on the facial-incisal surface to prevent the dreaded “headlight” effect of the restoration substructure.

The fabrication of high-quality, custom titanium abutments requires efficient application of the biomechanical concepts of tooth preparation and finely tuned, robust milling strategies that consistently produce complete and highly accurate results.

Geoffrey W. Sheen, CDT, DDS, MS, has a private prosthodontic practice, is the owner of Mustard Seed Dental Studio, and is a clinical assistant professor at Georgia Regents University College of Dental Medicine in Augusta, Georgia.

For more information, contact:
Axsys Dental Solutions
axsysdental.com
855-687-7941