Fundamentals of Cavity Preparation PDF

Fundamentals of Cavity Preparation FUNDAMENTALS OF CAVITY PREPARATION Definitions: Fundamentals: are basic principles, rules, laws that serves as the groundwork of a system. Standardization: the process of making something conform to a standard. Cavity: is a defect i...

Fundamentals of Cavity Preparation FUNDAMENTALS OF CAVITY PREPARATION Definitions: Fundamentals: are basic principles, rules, laws that serves as the groundwork of a system. Standardization: the process of making something conform to a standard. Cavity: is a defect in the hard tooth structure resulting from dental caries. Prepared Cavity: A bio-mechanical foundation for restoration. Direct restorations: restorations that applied directly in the patient’s mouth in the same visit of cavity preparation. Indirect restorations: restorations that fabricated outside the patient’s mouth (in the lab) then cemented to the prepared cavity in another visit. A. Biologic fundamentals I- Protection of the Dentin and the Pulp Against: (4 Irritations) Mechanical Irritation: Excessive cutting of enamel (width) exposes larger area of dentin à weakening of the remaining tooth structure. Overcutting of dentin (depth); the remaining protective dentin bridge will be decreased & can mechanically injure the pulp. Unnecessary application of excessive pressure on the base of a deep cavity. 1  Fundamentals of Cavity Preparation Thermal Irritation: Keep it within the physiological tolerance of the dentin-pulp organ i.e., Thermal Tolerance Zone (TTZ) of dentin ranges between 85- 132°F (29-56°C). This is done by 1. Minimizing the frictional heat o Decreasing pressure area of cutting and time of cutting. o Tools with high cutting efficiency à Sharp & of proper size 2. The use of coolants. Air-water spray jet during working. Chemical Irritation: Using chemical agents might cause chemical irritation during cleaning of the cavity. (Like phenolic agents) The acidic dentin conditioners used with bonded restorations might cause a degree of chemical irritation to the pulp. Bacterial Irritation: Ensure the removal of bacteria causing the caries during preparation (can be aided by application of antimicrobial agents like chlorhexidine) To avoid cross infection during cavity preparation, sterilization of in- struments, burs and armamentarium is essential. The use of rubber dam à isolate the field of operative procedure. II- Control of Caries: Ø Removal of all carious Enamel & Dentin Ø Prevent Caries Occurrence: Proper Oral hygiene measures (ex. tooth brushing, flossing, mouthwash…etc.) Topical application of fluoride. Application of remineralizing agents for incipient lesions Pit & Fissure Sealing or enameloplasty for retentive grooves. 2  Fundamentals of Cavity Preparation Ø Prevent Caries Recurrence: Prevention of food accumulation in pits and fissures adjacent to the defect either by preparing these pits and fissures or by sealing them. Removal of all weak or undermined enamel that might fracture causing food accumulation. III. Protection of Investing Tissues and Adjacent Teeth: Avoid improper sub-gingival extensions. Having a smooth & caries free gingival margin. Protect adjacent tooth during proximal preparation by metal bands. Avoid injury of gingival tissues during working. B- Mechanical fundamentals Ø Preserve the structural integrity of both tooth and restoration. (Avoid fracture) Ø Retaining the restoration inside the cavity (Avoid Displacement) ØA correct cavity design with maximum conservation of tooth structure. C- Esthetic fundamentals Symmetrical cavity outline with smooth sweeping curves. Limiting area-display of restorations using conservative designs. Elimination of discolored enamel or dentin specially in anterior cavities 3  Fundamentals of Cavity Preparation Conservative approach in operative dentistry Definition: Minimally invasive approach that focus on detecting, diagnosing & removing only damaged structure, preserving the remaining healthy tooth structure and emphasizing the concepts of prevention and remineralization during restorative procedure § It refers to all procedures carried out to preserve the integrity of teeth and the masticatory system. § Prevention is the theme applied nowadays in operative dentistry rather than extension for prevention. § The concept of "Sealing for prevention"; e.g., using pit and fissure sealants, § Black's statement: "The day is surely coming when we will be engaged in practicing preventive, rather than reparative dentistry". Advantages of conservative cavity design: 1. Preservation of healthy tooth structure 2. Decreased irritation of dentin-pulp complex and investing tissues. 3. Decreased fractured tooth and/or the restorations 4. Decreased displayed area of the restorations 5. Easier, quicker and standardized procedures 4  Fundamentals of Cavity Preparation Restorations Direct Restorations Indirect Restorations Metallic Non-Metallic Metallic Non-Metallic (Amalgam) (composite) (Cast Gold) (Composite) (Glass Ionomer) (Ceramic) As the most frequently used restorations are Resin Composite & Amalgam restorations, so the following cavity preparation steps are related to those two materials. N.B. Cavity preparation of the other restorative materials will be discussed later in their chapters. Knowing properties of the restorative materials has a direct impact on the cavity preparation to make it suitable to receive & retain them. Amalgam Composite - Brittle but more resilient - Brittle than Amalgam - Non bonded to tooth - Bonded to tooth structures structures - It undergoes corrosion - It is biocompatible - Bad esthetic (Silver in - Good esthetic (Tooth color) colored) 5  Fundamentals of Cavity Preparation Cavity Preparation Steps of Cavity Preparation: 1. Obtaining of the outline form 2. Obtaining of the resistance and retention forms 3. Obtaining of the required convenience form 4. Removal of the remaining carious dentin 5. Finishing of the cavity walls and margins 6. Performing the toilet of the cavity N.B. In case of deep carious lesions, where step 4 must precede step 2 1. Obtaining of the Outline Form § Cavity Outline is the shape of boundaries of the completed cavity. The external outline describes the external marginal boundaries & the internal outline describes the shape of the internal form of the preparation. § The aim is to place the margins of cavities in sound enamel at areas not carious and less stress-bearing. Essentials of the outline form: 1. All carious enamel and dentin must be included in the outline. (Amalgam & Composite) 2. All pits & fissures which are deeper than one third the enamel thickness should be included in the outline (Amalgam only, in composite we seal them using pit & fissure sealant) 3. All weak or undermined enamel either by caries or cutting must be eliminated and included in the outline. (Amalgam & Composite) 6  Fundamentals of Cavity Preparation 4. Adjacent cavities, approaching to each other, should be joined when the interfering tooth structures are: à Carious (Amalgam & Composite) à Undermined (Amalgam & Composite) à Weak (less than 1mm) (Amalgam & Composite) à Crossed by a fissure (Amalgam only, Composite à Sealing) This is specially considered with oblique and transverse ridges. 5. All severely weakened cusps and ridges that are subjected to occlusal forces must be reduced and included in the outline to be protected with appropriate restoration (cusp capping). (Amalgam & Composite) 6. The outline form must be in the form of harmonious sweeping curves to avoid stress concentration and provide better esthetic. (Amalgam & Composite) 7. The Cavo-surface angle given correct angulation according to the restorative material. (Amalgam always 90◦ (Butt-joint) - Composite is 90◦ in margins subjected to direct force + gingival margins & beveled in margins not subjected to direct force (ex. Labial) 8. Other factors including Age, Oral Hygiene, Caries Susceptibility & the Esthetic affect the outline. 7  Fundamentals of Cavity Preparation Pit & Fissure sealing: - Indicated in case of deep grooves liable to food stagnation and difficult to clean in recently erupted permanent molars. - Using pit & fissure sealing instead of preparing them to be included in the outline has preserved sound tooth structures - If used in conjunction with resin composite restorations, it is termed Preventive Resin Restoration (PRR). Enameloplasty § Reshaping of the retentive fissures to make them non-retentive and less prone to caries as a preventive method § It is performed using with the side of diamond stone, leaving a smooth surface. 8  Fundamentals of Cavity Preparation Factors Affecting the Outline of Different Cavities Occlusal Cavities: Caries Extension Occlusal Table Anatomy: differs whether these teeth are in upper or lower jaw, and from one tooth to another in the same jaw. Unusual anatomy, mal-alignment and steep cusps will change the locations and extent of the outline form. Occlusal Contacts: Cavity margins must be placed at contact-free areas of minimum stresses & should not be placed in stress bearing areas. Proximal Cavities: 1. The size and site of the contact area. 2. The width of the embrasures 3. The position and health of gingival tissues The outline form for proximal cavities demands: Extension in the gingival, buccal, and lingual directions sufficient to free, these margins of from contact, with the adjacent tooth (flaring of the margins) to make them accessible application of restoration & for oral hygiene measures (tooth brushing Posterior teeth (Class II): The junction between the occlusal cavity and the proximal surface could be in the form of Ingerham’s lines: a. Straight line à in missing contact. 9  Fundamentals of Cavity Preparation b. Uniform curve in most cases where a narrow proximal contact and wider embrasure exist c. Reverse curve (S shaped) in case of broad contact and narrow embrasures used in: Ø The buccal wall of lower molars because the contact is deviated buccally & distal of the lower first molar Anterior teeth (Class III cavities and Si/Sta 2.1, 2.2, 2.3): § The factors controlling the outline of proximal cavity mentioned in proximal cavities for posterior teeth are also applied, § The main deciding factor becomes the esthetic requirements. § In Class IV (Si/Sta 2.4),, the role of esthetics is maximized with the importance of the type of occlusion due to the involvement of the incisal angle. Cervical cavities: § The outline form is just limited to the defect. § Class V cervical cavities (Si/Sta 3.1, 3.2, 3.3) is controlled gingivally by the position of the gingival tissues. § On some occasions for esthetic demands, the gingival margin should be hidden sub-gingivally. 10  Fundamentals of Cavity Preparation 2. Obtaining of the Resistance and Retention Forms These are two distinct but inseparable and interrelated steps as two faces for the same coin. The Resistance Form: design features in the cavity preparation which allows both tooth and restoration to resist the functional stresses of mastication without fracture. Factors Influencing Stress Response of Tooth and Restoration: 1. Occlusal loading in terms of: § Its magnitude which differs from patient to another and from one location to another in the same patient. § Its direction, compressive, tensile or shear § Its character static, dynamic or cyclic 2. Cavity design in terms of: § Direction of walls; recommended to be parallel to direction of forces and Smooth to avoid stress concentration § Smoothness and flatness of Floor to avoid stress concentration & perpendicular to the direction of forces § Amount of bulk which distributes stress through depth (enough depth: 1.5-2.0 mm) rather than width. ↑Width à will weaken the remaining tooth. (Doubling the depth provides 4 times increase in strength, while doubling the width provides only doubles the strength but weakens the remaining structure) § The isthmus portion with appropriate width to avoid stress concentration. § Line and point angles must be rounded to avoid stress concentration § The cavo-surface angle inclination relative to the physical 11  Fundamentals of Cavity Preparation property of the restorative material as well as the direction of enamel rods. 90° in brittle restoration. § The amount of retention which provides stability for the restoration 3. Strength characteristics of the restorative material: § The tensile stresses for the brittle materials such as amalgam, and ceramics lead to fracture § Strong and ductile gold which is utilized for protection of the weakened tooth structure. The Retention form: design features in the cavity which enables it to retain the restoration securely in function & prevent its displacement. A- Chemical Retention: Chemical or true adhesion by primary valence bonding is strong and stable. It is not achievable except for the glass ionomer restorations. B- Mechanical Retention: It is the principal mechanism of retention utilized in restorative dentistry, Ø Micro-Mechanical Retention: The most conservative mechanism of retention employed for resin composite restorations. It forms microscopic resin tags created by resin impregnation in the micro-pores formed by acid etching of enamel and dentin (Acid etches Enamel & Dentin à create micro-pores à adhesive resin infiltrates/impregnates (gets inside & fills) these pores à create resin tags that locks inside enamel & dentin) 12  Fundamentals of Cavity Preparation Ø Macro-Mechanical Retention: It depends on features that are made in the cavity. It is the main retention mechanism employed for metallic restorations. a) Axial Retention: o It prevents displacement of restoration in a direction parallel to the long axis of the tooth (in occlusal or apical) Axial Retentive Features: 1) For direct restorations occlusal displacement is prevented by occlusal convergence of the cavity walls and the proximal locks (undercuts) à “The internal outline is made greater than the external outline” 2) The frictional parallelism of walls of the cavity by increasing the degree of parallelism & the length of walls. (Parallelism & Convergence) 3) Dentin Pins They can be cemented or threaded, parallel or non-parallel to each other. 4) Inter-radicular retention (posts) indicated in endodontically treated teeth 13  Fundamentals of Cavity Preparation 5) Dentin ledges which are flat seats performed in dentin with deep amalgam cavity preparations to prevent pulpward movement of restoration and to prevent fracture of the thin dentin bridge. Dentin Bridge: It is the dentin thickness between the floor of the cavity & the roof of pulp chamber b) Lateral retention: It prevents displacement of restoration in a direction Perpendicular to the long axis of the tooth (in Lateral Direction) Lateral Retentive Features: 1) Frictional opposing wall parallelism 2) Proximal axial grooves (locks): in compound or complex class II cavity preparations at the axio-buccal and axio-lingual line angles at the expense of buccal and lingual walls. 14  Fundamentals of Cavity Preparation 3) Buccal or lingual extensions in molars and extensions onto the other side in premolars or molars. 4) Dove-tail lock (common in premolar) increase lateral resistance of displacement of the restorations. 5) Slots: A slot is a retentive groove in dentin, whose length is in a horizontal plane is approximately 0.6mm, depth 0.6mm and 0.5 to 1mm inside dentin-enamel junction. 6) Axial coves Prepared (with No ¼ bur) in dentin in the vertical plane to provide additional retention. Slots and Axial Coves 7) Dentin Pins 8) Intra-Radicular Posts 9) Reverse Bevel performed in compound class II cavity for cast gold restorations in dentin of the gingival floor 15  Fundamentals of Cavity Preparation Factors Controlling the Selection of Retention Features: 1. Size of the cavity and remaining amount of tooth structure: the greater the size of the cavity, the greater the need for retentive means. 2. Number of missing walls require more retention to serve for both axial and lateral directions, e.g., Complex class IV 3. Site of the cavity and occlusal stresses: e.g., class II cavity need more retention features than cervical class V cavity. 4. Type of restoration: acid etching for resin composite, undercuts & friction for amalgam, while glass ionomer undergoes chemical retention. 5. Pulp vitality 6. Esthetic Requirements Resistance and retention forms act together to prevent fracture of the tooth and the restoration and any tendency for the restoration to get displaced out of the cavity during mastication. Ø The Mortise Box or Modified Box Form it provides resistance for both tooth and restoration against fracture under masticatory forces. The use of this box-like form is advocated because: 1. Pulpal and gingival wall is placed at right angle to the direction of functional stresses à neutralization of these stresses. 16  Fundamentals of Cavity Preparation 2. It prevents the wedging action of the restoration inside the tooth (no splitting of the buccal and lingual cusps) 3. It provides retention by friction due to relative parallelism of the axial walls. Ø Retention may partly be provided automatically during making the Resistance form and vice versa: § A box form preparation, which is a form of resistance (walls parallel to direction of enamel rods & occlusal force), will at the same time provide retention through friction & undercut from the occlusal convergence of walls, providing an occlusal lock. § The inverted truncated cone form prevents wedging action of restoration (resistance form) and meanwhile provides undercuts for retention. § The bulk provided for distribution of stress and prevention of fracture of the brittle restoration also augments retention by more frictional retention and increasing the undercut areas Proximal axial locks aids retention & prevent development of excessive tension at the isthmus which might cause isthmus fracture (resistance) 17  Fundamentals of Cavity Preparation 3. Obtaining the Convenience Form Definition: features in the prepared cavity, making it more conveniently seen, approached and/or restored. § It includes a slightly more extension of a cavity outline to enable: better preparation and manipulation, Ø Accentuation of line or point angler, Ø sweeping curves for easier restoration of anatomy, Ø elimination of undermined enamel § Another example is the occlusal access to proximal lesions in Class II and the palatal access to class III § Simple class II or III could be if the adjacent tooth is missing or the embrasure is widened due to gingival recession & in old age patients. Lately, the “CONSERVATIVE APPROACH” encouraged development of more precise tools & restorative materials to access & restore defects with Maximum Conservation to tooth structure and Minimum Cutting, for example: 1) Micro-Sized Burs: Help prepare smaller sized, more conservative cavities. 2) Magnification “Dental Loupes and Microscopes”: Improve vision for minute defects to allow for precise preparation for small defects without additional overcutting for vision. 3) Visibility “Fiber-Optic handpieces & Loupes with Auxiliary Light”: Increase visibility & illumination. 4) Restorative Material: New restorative materials that can treat defective lesions without the need of excessive cutting of tooth structure “Ex. Pit & Fissure Sealants. 18  Fundamentals of Cavity Preparation 4. Removal of the Remaining Caries In the average cases, most of carious dentin is removed during the previous procedures. Following the first 3 steps in cavity preparation, the Cavity floor can be composed of: A. Hard sound viable dentin: If this cavity seat is 0.5 mm beyond the dentin-enamel junction, this most accepted cavity depth B. Hard but discolored dentin: Could be left in posterior teeth while in anterior teeth it must be carefully removed for esthetic reasons or masked by an opaquer. C. Carious dentin: May be discolored or not but it is Soft, if this layer constitutes the floor of deep and moderately deep cavities, it has to be carefully removed. Ø The Carious Process in Dentin & The Types of Carious Dentin: Cariogenic bacteria stagnating in retentive areas, ferment simple sugars to produce acid, this acid causes “Demineralization” of dentin (removal of minerals “inorganic part” à followed by destruction of organic part & subsequent bacterial invasion à produce more acid that demineralizes the deeper part à and the cycle goes on. Therefore, in Dentin we will find: 1) Area where acid from the bacteria caused demineralization of dentin but the organic part still didn’t break & bacteria still didn’t proceed to invade à “AFFECTED DENTIN” à This dentin is demineralized, Firm in consistency & can be remineralized & saved. 19  Fundamentals of Cavity Preparation 2) Area where acid from the bacteria caused demineralization of dentin, followed by destruction of organic matrix & bacterial invasion à “INFECTED DENTIN” à This dentin is demineralized & infected with bacteria, Soft in consistency & must be removed. Ø Strategies for Caries Removal: Complete Caries Removal: Conventional strategy set by “G.V. Black” to remove all carious dentin (Soft “infected” & Firm “affected”) and have only hard sound dentin floor. Partial Caries Removal: Conservative approach strategy where only soft “infected” dentin is removed, but firm “affected” dentin that can be remineralized is left to avoid pulp injury (often covered with indirect pulp capping material that allow pulp to lay down tertiary dentin & sealed with a tightly adapted & sealed final restoration). In case of Composite, cavity can be within Enamel if the minimal depth for composite restoration bulk is achieved (1.5 mm) – But in case of Amalgam, CAVITY MUST EXTEND BEYOND DEJ TO: a. Ensure no more lateral spread of caries at DEJ b. Avoid lateral cutting and placing the restoration at the most sensitive area of dentin (DEJ). c. Provide the necessary bulk required for the restoration (l.5- 2mm). d. Provide retention in dentin if required 20  Fundamentals of Cavity Preparation Ø Techniques for Caries Removal: 1. Manual Excavation by use of Excavators (Soft Carious Dentin) 2. Mechanical Excavation by use of Burs (Large Low-Speed Round Bur for Harder Carious Dentin Parts) 3. Air abrasion, tiny aluminum oxide particles (20-50 µm) under a stream of compressed air. 4. Laser debridement using thermal energy with water coolant (WaterLase) 5. Chemo-Mechanical Debridement (Carisolv: combination of amino acids and weak solution of sodium hypochlorite makes the caries easier to be removed by blunt instrument. 6. Enzymes that dissolve carious parts such as Pronase enzyme. Can the Sequence of Steps of Cavity Preparation be Violated? Yes: If after the first 3 steps, there was no remaining caries then we are “SKIPPING” the fourth step. Also, in case of EXTENSIVE CARIOUS LESIONS, “Removal of Remaining Carious Dentin” becomes STEP NUMBER TWO instead of being number four, then obtaining the Resistance & Retention becomes number three & obtaining the required Convenience form becomes number four. 21  Fundamentals of Cavity Preparation 5. Finishing of cavity walls and margins Removal of undermined enamel. Adjust cavo-surface angle inclination. Give the cavity wall correct inclination. Make the walls smooth for better adaptation. Make the outline in sweeping curves Roundation of line and point angles. Finishing is performed either by Ø Hand cutting instruments as enamel hatchet Ø Rotary instruments as fissure bur and finishing abrasive stones. Factors Affecting the Cavo-Surface Angle: 1. Type of restorative material. 2. Stresses acting on the cavity margin. 3. Location of cavity margin and the direction of the enamel rods. 4. Condition of enamel. 5. Pulp vitality, root canal treated teeth require reinforcement. 6. Esthetic demands, labial undermined enamel is left for esthetics. Importance of cavo-surface bevels with Resin Composite Restorations: Ø Exposes ends of rods rather than sides, Ø It refreshes the enamel surface Ø Improves esthetics by color degradation. Ø For convenient etching 22  Fundamentals of Cavity Preparation 6. Toilet of the cavity § It includes all procedures to prepare the cavity to receive the restorative material according to its type. § No medicaments should be used § The cavity should be perfectly clean and dry § The cavity should be lined and/or sub-based and based to provide pulp protection. Objectives: Elimination of bacteria, debris and saliva. Removal of remnants of dentin chips and temporary restoration. To improve adaptation and prevent the recurrence of decay. Prepare and condition the tissues to receive restorative material. 23  Fundamentals of Cavity Preparation Technique: Scrap the cavity walls and floors with a hand instrument, flush the cavity with water and dry it gently with air blowing. Contraindications: 1. Cavity sterilizers as phenol à irritate the pulp especially in deep cavities. 2. Alcoholic cavity cleansers may cause desiccation of dentin 3. Excessive dryness with air blast since. Special Considerations: (Smear Layer) § The microscopic smear layer was formerly thought that it has to be removed since it is a contaminant and contains bacteria. § Its importance in pulpal protection à the smear plugs obliterating the dentinal tubules § The smear layer has to be modified for bonding resin restorations to dentin. 24