General principles of anchor preparations
Introduction
Nowadays, the imperatives of tissue economy, respect for the integrity of the dental organ and aesthetic dentistry are at the heart of our practice, this requires a perfect knowledge of the different anchors and their preparation principles in order to promote the most conservative therapeutic approach.
II. Classification of different anchoring methods
Total anchoring
Total covering crown
Partial anchoring
Is a restoration of the form and function of a portion of the crown of a tooth. All partial anchors can be designed to be cemented or bonded
General principles of preparations
According to SHILLNGBURG The contour shape of a preparation intended to receive a prosthetic element must meet five principles
- The economy of dental tissues .
Excessive removal of dental substance can lead to harmful consequences. It is then appropriate to limit the indication of peripheral restorations and to favor less mutilating partial restorations. The therapeutic choice is made with respect to the therapeutic gradient.
General principles of anchor preparations
- Maintaining periodontal health
Respect for biological space is an important factor for the success of the treatment.
The cervical edge of a crown must be at least 2 mm from the alveolar crest (overall height of the epithelial-connective attachment system or biological space
- The durability of the dento-prosthetic system.
They are essentially ensured by the geometry of the preparation which determines the orientation of the forces in relation to the tooth-prosthesis interface.
- Retention and stabilization of reconstruction .
- Retention
During preparation, the practitioner is responsible for four factors:
1) stripping of the preparation,
2) developed surface of the cement film,
3) cement surface subjected to shear,
4) surface condition of the preparation.
b) stabilization
- Height of preparation
- Width of preparation
- Insertion axis;
Is an imaginary line along which the prosthesis must be inserted or removed.
Two plans are concerned by the insertion axis:
The vestibulolingual orientation can interfere with the aesthetics of metal-ceramic or partial coverage crowns.
In the mesio-distal plane, the axis of insertion of the prosthesis must be tangent to the proximal contact surfaces
Visual inspection of the insertion axis
Is a basic way to ensure that the preparation does not have any undercut or excessive convergence,
It is monocular, 30cm from the preparation. Must allow to see the limits of the preparation. (In binocular, an undercut of 8° goes unnoticed)
is carried out using a mirror held 1.5 cm above the preparation
5) Sustainability of the reconstruction :
In order to ensure the sustainability of the reconstruction, sufficient space must be provided for the material , allowing the reconstruction to have an adequate morphology.
Occlusal reduction
- If the reduction is insufficient at the level of the furrow => risk of perforating the reconstruction
- The significant reduction causes excessive tissue mutilation and insufficient height of the axial faces
Chamfer of the external slopes of the support cusps
- The chamfer of the external slope of the support cusp is an integral part of the reduction of the occlusal face
The absence of a chamfer on the external slope of the support cusps may be the cause of insufficient material, risk of perforation at this level or the origin of the overcontour of the crown, and therefore of occlusal interference.
Axial reduction
It depends on the adequate thickness of the restorative material.
If it is insufficient: Thin and fragile thickness of the prosthetic element
if it is excessive; it is the cause of an over-contoured morphology
6) The precision of the preparation limits
- The prosthetic edges are satisfactory provided that
- Their adaptation to the limits of the preparation is as precise as possible, this to reduce the thickness of the cement joint.
- Their resistance is sufficient and supports the forces of chewing.
Their location allows their finishing and visual control by the practitioner and the maintenance of this region by the patient on the other hand.
General principles of modern preparations
More recently, several authors have abandoned traditional concepts of extended oral rehabilitation, proposing an exclusively adhesive strategy based on biomimetic principles, in order to preserve a maximum of mineralized tissues on the one hand, and to systematically maintain the vitality of the teeth on the other hand.
- The odontoprosthetic continuum
This is a principle of continuity, morphological, biological and optical between the residual dental substrate and the prosthesis.
General principles of anchor preparations
- Adequacy of residual structures and prosthetic restorations
The preparations are packaged by:
- the type of material used
- the assembly mode
- Cervical limit
- Periodontal protection
- In order to protect the marginal periodontium during clinical maneuvers, three types of periodontal tissue protection are proposed
Protective thread: Braided, incompressible and chemical-free self-thread
The Maillefer gingival retractor:
– Ultrasonic diamond inserts : They allow atraumatic intrasulcular penetration
- Adequacy of reduction thicknesses to the therapeutic project
The reduction thickness must take into consideration the type of material used.
The reduction should be done according to the final shape of the reconstruction and not according to the residual dental tissues.
- The thickness compat allows an evaluation of the dental tissues to be prepared
- The mask technique (MOCK UP) allows the preview of the final result and thus obtain the patient’s consent.
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