Fundamentals of Cavity Preparation PDF

Chapter (5) Fundamentals of Cavity Preparation Introduction: There are some fundamental principles which must be realized in preparing cavities for the reception of restorative materials. These fundamentals must be intelligently applied and adhered to in all cases since failure of th...

Chapter (5) Fundamentals of Cavity Preparation Introduction: There are some fundamental principles which must be realized in preparing cavities for the reception of restorative materials. These fundamentals must be intelligently applied and adhered to in all cases since failure of the whole procedure might occur if any of the fundamentals is violated or neglected. These fundamentals are essentially discussed under biologic, mechanical and esthetic perspectives. A. Biologic fundamentals: These fundamentals are primarily concerned with the health and integrity of the remaining tooth tissues. I) Protection of the dentin- pulp organ (DPO) II) Prevention of caries recurrence together with working with aseptic procedures. III) Protection of the gingival and periodontal investing structures. I) Protection of the dentin- pulp organ (DPO):  The pulp which is the highly specialized and sensitive vital structure within the tooth must be protected against all sorts of irritation during cutting.  The irritation may be mechanical, thermal, chemical or bacterial. A) Mechanical protection of DPO: For the protection against mechanical irritation, the following should be avoided: a. Excessive cutting of enamel (width) since this will expose larger area of dentin. Increase in cavity width causes weakening of the remaining tooth structure and subjects it to more mechanical irritation. b. Overcutting of dentin (depth) and unnecessary deepening of the cavity. The deeper the cavity, the more the pulp will be irritated because the remaining protective dentin bridge will be decreased and the number of patent dentinal tubules will be increased. 1|Page  c. Unnecessary application of excessive pressure on the dentin forming the base of a deep cavity; only gentle pressure must be employed either during the use of hand or rotary instruments, Improper use of probes or instruments in the deep pulpal floor will transmit more mechanical pressure and might cause pulp exposure. d. Cutting across the recessional lines of the pulp; the excavation of soft dentin must be carried out parallel to the recessional lines of the pulp in scooping action and layer by layer. B) Thermal protection of DPO: There is always an unavoidable degree of frictional heat resulting during cavity preparation. The ultimate goal of the clinician is to keep it within the physiological tolerance of the dentin-pulp organ i.e. within the limits of the TTZ (Thermal Tolerance Zone) of dentin which ranges between 85-132ºF (29- 56o C). Thermal irritation during cavity preparation could be controlled by minimizing the frictional heat resulting during cavity preparation via: a. Decreasing pressure, by intermittent cutting, and cutting over wider surface area in lateral extension rather than pulpal pressure. b. Decreasing the time of cutting. c. Using tools with high cutting efficiency. The tool should be sharp, of proper size and appropriate for the desired function. d. The use of coolants. Air-water spray jet is considered to be the most appropriate since the use of air drying only could desiccate the dentin while the use of water coolant only impairs the visibility during work. The coolant must fulfill the following requirements: a. It must be copious in amount, with adequate rate. b. It should be multidirectional, to produce an all-over cooling effect on the tooth. c. It should be of the same temperature as that of the mouth i.e. 37ºC. Lower temperatures cause fogging, alters vision and may irritate the pulp. On the other hand, higher temperatures will not be that effective in cooling. 2|Page  C) Chemical protection of DPO:  Cavity cleansers (or cavity sterilizers) like phenolic agents proved to have an insignificant role in preventing or decreasing secondary caries with the possible pulp affection and thus their use was discontinued.  Only antimicrobial solutions like chlorhexidine may be used for cavity disinfection in some instances.  It was thought that acidic dentin conditioners used with bonded restorations might cause a degree of chemical irritation to the pulp. However, it is now widely accepted that the use of certified conditioners, primers and adhesives, with full respect to its manufacturer instructions, will not produce damaging effects to the pulp. This could be due to proper sealing of dentin and significant improvement of marginal adaptation of restorations, which are far more important in preventing pulpal irritation. D) Microbial protection of DPO: To avoid cross infection during cavity preparation;  Sterilization of instruments, burs and armamentarium is essential.  In case of very deep cavities with very thin remaining dentin bridge barrier, the use of rubber dam to isolate the field of operation becomes mandatory to prevent bacterial contamination from saliva or neighboring teeth if accidental pulp exposure occurs. II- Control of caries: Secondary or recurrent caries may occur at the margins of any restoration or may spread underneath it and escape notice for a long time until it may become too complicated or too late to save the tooth vitality and pulp exposure with bacterial contamination becomes evident. Control of caries recurrence requires: a. Removal of all primary carious enamel and infected dentin. b. Extension of the cavity margins should not end at any defected areas including pits and fissures and angular grooves (proper cavity outline). c. Removal of all week undermined enamel and short, loose or friable enamel rods to avoid its chipping with subsequent marginal ditching, food impingement and caries recurrence. 3|Page  d. Giving the cavo-surface angle the correct angulation that is compatible with physical characteristics of the particular restorative material used. e. All other known preventive measures including, potent oral hygiene motivation, topical application of fluorides, antibacterial and remineralizing agents should be also maintained. III. Protection of investing tissues and adjacent tooth: This is achieved by: a. Rounding and smoothening ragged cavity margins, especially those adjacent to the gingiva. b. Excision of the infectious lesions, and elimination of defects which encourage plaque retention. c. Avoid unnecessary subgingival extensions whenever possible. d. Protect adjacent tooth surface during proximal tooth preparation by insertion of steel bands or wedge guards. e. Proper guide and support for hand and rotary instruments to avoid slippage of instruments and injury to the gingiva and periodontium. B- Mechanical fundamentals: These principles are primarily concerned with (Resistance and Retention): a. Preservation of the structural integrity of both the tooth and restoration (Provision of correct resistance form against fracture). b. Retaining the restoration inside its corresponding preparation (Provision of adequate retention against dislodgement). This can be achieved through a correct mechanical cavity design and maximum conservation of tooth structure. C- Esthetic fundamentals: This includes: a. Provision of symmetrical cavity outlines with refined margins, and smooth sweeping curves. The margins must be either concealed or placed parallel to respective tooth contours and anatomic landmarks. b. Margins must be free of opaque, discolored or pitted enamel. 4|Page  c. Limiting area-display of restorations through using conservative designs with minimum lateral extensions. d. Elimination of discolored enamel or dentin. When using esthetic tooth- colored restorations, hard discolored dentin may preferably be masked with opaquers. 5|Page