Principles of Tooth Preparation in Fixed Prosthodontics

In fixed prosthodontics, the principles of tooth preparation can be studied under following headings:

Mechanical principles.
Biological principles.

Important features that needs consideration during tooth preparation include:

Taper.
Freedom of displacement.
Finish lines.

Mechanical Principles of Tooth Preparation

Retention

Prevents removal of the restoration along the path of insertion or long axis of the tooth preparation.
The essential elements of retention are the two opposing vertical surfaces of the preparation : External surfaces in extra-coronal restoration and internal surfaces in intra-coronal restoration
Factors affecting retention:
- Magnitude and direction of functional forces.
- Geometry of tooth preparation : Taper, freedom of displacement, occluso-gingival length and substitution of internal features.
- Path of insertion.
- Roughness of the fitting surface of the retainer, casting and crown.
- The materials being cemented.
- Type of luting agent (cement used).

Resistance

Prevents movement of the restoration under occlusal forces and dislodgement of the restoration by forces directed in an apical or oblique direction.
Factors affecting resistance :
- Magnitude and type of functional forces.
- Geometry of tooth preparation : Taper, freedom of displacement, occluso-gingival length and substitution of internal features.
- Type of luting agent.

Structural durability

The restoration must be of adequate thickness to withstand the occlusal forces, while preventing any periodontal problems due to any over-extension.

Occlusal reduction (Occlusal clearance) : To provide adequate bulk of material and strength to the restoration.
- Gold alloys - 1.5mm.
- Metal-ceramic crowns - 2.0mm.
- All-ceramic crowns - 2.0mm.
Functional cusp bevel : The functional cusps (i.e, palatal inclines of maxillary palatal cusps and buccal inclines of mandibular buccal cusps) are areas of heavy occlusal contact and hence, require wide bevel, to provide space for adequate thickness of material that can sustain more force.
- Lack of functional cusp bevel may result in over-contouring of the restoration and poor occlusion, or a thin restoration at the junction between the occlusal and axial surface (in an attempt to achieve proper occlusion).
- If the axial surface is cut (without functional cusp bevel) to obtain space for adequate thickness of the restoration, there will be over-inclination of the axial surface. This will destroy excessive tooth structure and lessen retention.
Axial reduction : If a restoration is made with inadequate axial reduction, the resulting restoration will be thin and may be distorted. Also, an attempt to compensate for this, may result in over-contouring the axial surfaces.

Biological Principles of Tooth Preparation

Prevention of Damage

To the adjacent teeth, periodontium and the dental pulp.

Conservation of Tooth Structure

The remaining tooth structure must be preserved as much as possible, keeping the mechanical principles in mind.
By virtue of orientation groove or depth cuts.

Marginal Integrity

Essential for the restoration to survive in the biologic environment of the oral cavity.
Can be achieved by close adaptation of the restoration margin and the cavosurface finish line of the preparation.
Factors affecting the marginal integrity includes :
- Placement.
- Geometry of the finish line.
- Adaptation.

Taper

Taper refers to the total convergence or divergence of the axial walls. Two opposing surfaces, each with 3-degree angle of inclination, would result in a 6-degree taper.

The axial walls of prepared tooth surface requires taper to :
- Allow adequate visualisation.
- Prevent undercuts.
- Compensate for any inaccuracy in fabrication process.
- Permit proper placement of restoration during cementation.
Also, the degree of taper is inversely related to retention and it should be kept minimal to achieve maximum retention.
Theoretically, the more nearly parallel the opposing walls of a preparation, the greater should be the retention and hence, parallel walls will have the most retention. However, parallel walls are impossible to create.
Optimum taper to minimise stress in the cement interface between the prepared tooth surface and restoration : 2.5 - 6.5 degrees.
Mach² have estimated that a minimum taper of 12 degrees is necessary just to ensure the absence of undercuts.
A taper of 16 degrees has been proposed as being achievable clinically with adequate retention.
Optimum taper (Angle of convergence) for different tooth :
- Maxillary and Mandibular anterior teeth : 10 degree (All four walls).
- Maxillary premolars : 14 degree (All four walls).
- Mandibular premolars : 16 degree (Mesial/ Distal walls) and 12 degree (Buccal/ Lingual walls).
- Maxillary molars : 17 degree (Mesial/ Distal walls) and 21 degree (Buccal/ Lingual walls).
- Mandibular molars : 24 degree (Mesial/ Distal walls) and 20 degree (Buccal/ Lingual walls).

Freedom of Displacement

It refers to the number of paths along which a restoration can move.

Maximum retention is achieved when there is only one path for removal of restoration form the prepared tooth eg. A full veneer preparation with long, parallel axial walls and grooves.
The resistance of a restoration can also be increased by limiting the freedom of displacement from torquing or twisting forces in a horizontal plane. eg. Grooves prepared perpendicular to the axial wall provides good resistance to displacement.

Finish Lines

It can be defined as the peripheral extent/margin that forms the junction between the prepared tooth structure and the unprepared tooth.

Types:

Chamfer finish line : Veneer metal restoration.
Classic Shoulder finish line : All-ceramic crown.
Shoulder with a bevel : Proximal box of inlays and onlays and occlusal shoulder of onlays and mandibular three-quarter crowns.

Points to Note

Forces acting of teeth : Tipping (non-axial force), Twisting and forces along the path of insertion.
Angle of inclination : Relationship of one wall of a preparation to the long axis of that preparation.
Angle of convergence : In extra-coronal preparations, the external walls should converge occlusally and the relationship between the opposing walls is referred as angle of convergence.
Angle of divergence : In intra-coronal preparations, the opposing internal walls should diverge occlusally and the relationship between the opposing walls is referred as angle of divergence.
Axial wall height or Occlusogingival length : The length must be great enough to interfere with the arc of rotation of the prosthesis. In short-walled preparation, the resistance can be improved by placing grooves in the axial walls. (The grooves reduce the rotational radius and wall of the grooves near the occlusal surface interferes with displacement).
Arc of rotation : Formed about a point on the cavosurface margin of the prepared tooth surface on the opposite side of the restoration.
Internal Features that may provide space for restoration and improve the rigidity an durability of the restoration include the offset, occlusal shoulder, isthmus, proximal groove and the box. The isthmus connects the boxes, and the offset ties the grooves together to enhance the reinforcing “truss effect.”
Path of Insertion : Imaginary line along which the restoration will be placed onto or removed from the preparation.
If height and taper of a prepared tooth surface are same, which tooth will have better resistance to dislodgement : Premolar or Molar ?
- Premolar (Arc of rotation : Bigger in molars, hence, more tooth height is required in molars to prevent dislodgement).

References

Fundamentals of Fixed Prosthodontics, 4th Edition, Herbert T. Shillingburg, Quintessence Publishing Co.
Mack PJ. A theoretical and clinical investigation into the taper achieved on crown and inlay preparations. J Oral Rehabil 1980;7:255-265.

*This article is an excerpt from the above mentioned sources and Medical Sutras does not make any ownership or affiliation claims.