Mechanical Principles tooth preparation

Mechanical Principles tooth preparation. Dr. Kamal Ebeid Lecturer of Fixed Prosthodontics.

The design of the prepared tooth should possess some mechanical features so as to resist dislodgment of the restoration or even its fracture during service..

Mechanical princeples are divided into. Providing Retention form. Providing Resistance form. Preventing Deformation of the restoration (structural Durability)..

1- Retention form. It is the quality of a preparation that prevents the removal of the restoration along its path of insertion or long axis. is dependent basically on two opposing vertical surfaces in the same preparation.

Factors affecting retention form. Magnitude of dislodging forces. Geometry of the prepared tooth Taper Length (surface area). Preparation features (groove , pinholes) Roughness of fitting surface of restoration Materials being cemented. Type of luting cement..

Magnitude of force. page11image46665168. A picture containing food Description automatically generated.

Geometry of prepared tooth A- Taper. page12image46886064.

Slight convergence or taper is necessary when preparing the tooth Too large taper lead to non retentive preparation. Recommended convergence between opposing walls is 6 degrees..

'5 20 25 30 35 DEGREE OF TAPER 40 45 so.

Geometry of prepared tooth B- Length. page17image46889392.

The basic unit of retention = two opposing walls with minimal convergence If the opposing walls are not available due to: - Excessive destruction (decay or attrition) - Partial coverage restorations - Greater than desirable inclination Added means of retention are used.

3- Roughness of the fitting surface:. Roughening = increasing the retention Done by air abrasion of the fitting surface with 50 μm of alumina Airborne particle abrasion increases in-vitro retention by 64% for metal fitting and polycrystalline ceramics. Done by etching using Hydrofluoric acid and silane coupling agent for glass ceramics. Recently, laser technology has been used for polycrystalline..

4- Materials of Fabrication:. 1- Glass Ceramics – Highest retention due to their high bond strentgh structure through etching with HF acid and use of silane coupling agent. 2- Zirconia (polycrystalline) – High retention and bonding but less than glass ceramics as they are unable to be etched; however the use of an MDP containing cement with air-abrasion of fitting surface increases bonding and retention. 3- Metal – The least retention as it mainly depends on frictional retention and luting cement..

5- Luting agent:. Adhesive cements are the MOST retentive Adhesive resin cement > Glass ionomer cement > Zinc phosphate and polycarboxylate > Zinc oxide and eugenol.

2- Resistance form. It is the quality of a preparation that prevents dislodgement of the restoration by forces directed in an oblique or horizontal direction.

Tipping path. It is the path along which the restoration could be displaced under the displacing occlusal forces. Tooth structure lying outside the tipping path will resist displacing forces.

Factors affecting resistance form:. Dislodging forces Luting Agent Geometry of tooth preparation.

1- Dislodging forces:. Mastication and parafunctional activity cause substantial horizontal or oblique forces Lateral forces displace the restoration by: - causing rotation around the gingival margin.

2- Geometry of tooth preparation:. Type of preparation: - Resistance is greater in full coverage than in partial coverage Addition of boxes and/or grooves increases resistance – Greatest if walls are perpendicular to the direction of force.

Type of axial grooves. V-shaped b) U-shaped c) Box-shaped The U-shaped groove or flared Box provide more resistance than do the V-shaped ones as the first two types have definite walls that are perpendicular to the direction of applied force..

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3- Structural durability. The restoration must have bulk of material so as to provide sufficient strength capable of withstanding masticatory forces without permanent deformation..

Structural Durability:. A restoration must contain a bulk of material that is adequate to withstand the forces of occlusion Bulk should be confined to the space created by the tooth preparation to provide adequate bulk: - Occlusal reduction - Functional cusp bevel - Axial reduction.

Occlusal Reduction:. Full metal restoration: - 1.5 mm – functional cusp 1mm – non functional cusp Metal-ceramic crowns: - 1.5 to 2mm – functional cusp - 1 to 1.5mm – non functional cusp All ceramic crowns: - 2 mm over all.

Functional Cusp Bevel:. Wide bevel on: - lingual inclines of the maxillary lingual cusps - buccal inclines of mandibular buccal cusps Adequate bulk of metal in areas of heavy occlusal contact.

Lack of functional cusp:. thin area in casting. Over-contouring.

Axial Reduction:. Thin walls of casting are subject to distortion Over-contouring has disastrous effect on the periodontium.

Thank you kamal_ebeid@dent.asu.edu.eg.