Class I Cavity Preparation for Amalgam Restorations PDF

Class I Cavity Preparation For Amalgam Restorations Basma Ahmed Mohamed Abdelmoneim BDS, MDS (Operative Dent, Endodontics and Crowns & Bridges), PhD (Oper Dent) Senior Assistant Professor of Operative Dentistry Facult...

Class I Cavity Preparation For Amalgam Restorations Basma Ahmed Mohamed Abdelmoneim BDS, MDS (Operative Dent, Endodontics and Crowns & Bridges), PhD (Oper Dent) Senior Assistant Professor of Operative Dentistry Faculty of Oral and Dental Medicine Delta University for Science and Technology  Conservative Class I Cavity Preparation For Amalgam Restoration Conservative tooth preparation is recommended: -To protect the pulp. -To preserve the strength of the tooth. -To reduce deterioration of the amalgam restoration. -To enhance marginal integrity and restoration longevity.  Pits and fissures are frequently more retentive for food and susceptible to caries due to their defective structure. -Class I cavity preparation These are pit and fissure type cavities that involve:  -The occlusal surfaces of molars and premolars.  -The occlusal 2/3 of buccal and lingual surfaces of molars.  -The palatal pits in maxillary anterior teeth. -These lesions are characterized by: A small surface opening which may remain unnoticed until the lesion becomes of a considerable size -The carious lesion has a conical spread in both enamel and dentin. Types of Class I -Simple  include only one surface (occlusal, lingual, buccal or palatal). A -Compound  include two surfaces (occluso-buccal or occluso-lingual). B,C -Complex  include more than two surfaces (occluso-bucco-lingual). Cavity Preparation For Class I Cavity Preparation 1-Outline form Outline form for class I cavities should be very conservative. It is governed by: - The extent of caries in both enamel and dentin. - The amount of extension needed to eliminate pits and fissures to secure smooth margins. -Outline form varies in shape according to : number of cusps and orientation of pits & fissures. Operative procedure -Begin the Class I occlusal preparation by entering the deepest or most carious pit using the No. 1/2 carbide bur at high speed with air-water spray. -Extension of the cavity done with a suitable size fissure bur (No. 245 bur) -This bur has a head length of 3 mm and a tip diameter of 0.8 mm to prepare a conservative class I preparation. -The sides slightly convergent toward the shank  produces occlusal convergence of the facial and lingual preparation walls for adequate retention form for the cavity preparation. -The slightly rounded corners of the end of this bur produce slightly rounded internal line angles to make the tooth more resistant to fracture from occlusal forces. - When the bur enters the pit, the proper depth of 1.5 mm (one half the length of cutting portion of bur) should be made. - The 1.5 mm pulpal depth is measured at the central fissure. - -The necessary cavity extensions through pits, fissures and deep developmental grooves are made by keeping the bur at right angle to the surface of the tooth. -Cavity Preparation in mandibular premolar (crown tilted lingually) cutting bur should be tilted so that its long axis is parallel to the long axis of the tooth crown - -The cavity preparation should include the faulty, defective occlusal pits and fissures, all carious and undermined enamel. -Extending around the cusps to conserve tooth structure. -Running in sweeping curves with a minimum width. -Cavity width = 1/3-1/4 intercuspal distance and terminating on smooth surfaces to preserve the strength and function of the cups but it must be wide enough to allow condensation of restorative material. -The mesial and distal margins are placed in midway between the bottom of the proximal fossa and the crest of the proximal ridges. -The mesial and distal walls  should have slight divergence from the pulpal floor outward to avoid undermining the marginal ridges to preserve strength of marginal ridges. -Eliminating a weak wall of enamel by joining two outlines that come close together (less than 0.5 mm apart). -Extending the outline form to include enamel undermined by caries. -Using enameloplasty on terminal ends of shallow fissures to conserve tooth structure. -Establishing conservative depth of the pulpal wall. 2-Resistance and retention forms Resistance form achieved by: -Boxing the preparation. -Flat pulpal floor in sound tooth structure to resist forces directed in the long axis of the tooth and provide strong and stable seat for the restoration. -Minimal extension of external walls. -Strong, ideal enamel margins. -Sufficient depth (i.e., 1.5 mm) to result in adequate thickness of the restoration. Retention form achieved by: -Slight occlusal convergence of two or more opposing external walls. 3-Convenience form -Although conservation of tooth structure is very important, convenience form in some cases may require extent of the preparation to provide adequate access and visibility. 4-Removal of carious dentin -In small size cavities……..carious dentin should have been removed during making the cavity extensions. -In moderately deep and deep cavities……carious dentin is removed carefully using large spoon excavator or round bur. -Only light pressure in a direction parallel to that of the pulp is made. -This is continued until hard dentin floor is reached. -Resistance form improved with flat floor peripheral to excavated areas. 5-Pulp protection -If the cavity preparation is of ideal or shallow depth  no liner or base is indicated (varnish application is sufficient). -In deeper carious excavations (remaining dentin thickness is judged to be 0.5 to 1.0 mm)  place a thin layer of resin-modified glass-ionomer RMGI base. -In very deep cavities (less than 0.5 mm RDT)  calcium hydroxide liner is placed then the resin modified glass ionomer base applied. 6- Planning of enamel walls -The enamel walls of the cavity should be finished without any undermined enamel to meet the tooth surface at a right cavo-surface angle. -Cavo-surface margins should be well defined and well supported in order to be easier to visualize and carve and to provide marginal integrity of the restoration. -This may be done by sharp chisels and hatchets, fissure burs, or stones. -All sharp corners in enamel must be rounded. 7-Toilet of the cavity --A sharp explorer is then used to check the details of the prepared cavity and to loosen the tooth debris which are then blasted out with air. -This facilitates adaptation of amalgam to the cavity and improves the physical properties of the restoration by elimination of void or foreign material. Buccal or Palatal pits preparation -These preparations are made with No. 245 bur. -If the lesion is very small……… No. 330 or 169 bur may be used. -Depending on the extent of the caries and the angulation of the walls, retention grooves may be added with No. 1/4 or 1/2 bur. Compound Class I Cavity Preparations -In case of occluso-buccal and occluso-lingual cavities  extensions are made through the fissures and towards the respective surfaces. -The cutting is done in dentin at D.E.J using No. 56 bur  by applying pressure in a gingival direction and at the same time moving the bur mesio-distally until the occlusal ridge is undermined and removed. -If the caries is still gingival to the level of the pulpal seat  a step is indicated. -The axial wall should follow the contour of the buccal / lingual surface of the tooth. -It may be extended: *With step forming axial wall and gingival floor. *Without step in cases of extended caries or fissure at the level of the pulpal floor. *Rounding of axio-pulpal line angle to: - Prevent stress concentration. - Provide bulk to the restoration. *Axial wall direction should be parallel to the corresponding external tooth surface to: - Prevent pulp exposure. - Provide uniform thickness of the restoration. - Retention grooves (if needed) in dentin along the axio-mesial and axio-distal line angles (as secondary retention form).