Mechanical Principles tooth preparation

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

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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..

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

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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.

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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..

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

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Geometry of prepared tooth A- Taper. page12image46886064.

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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..

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

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Geometry of prepared tooth B- Length. page17image46889392.

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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.

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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..

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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..

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

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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.

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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.

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

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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.

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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.

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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..

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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.

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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.

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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.

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

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

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Thank you