• To replace missing teeth structure due to fracture or caries or existing restorative material.
• When one or more cusps need capping.
• When increased resistance and retention forms are needed. ADVANTAGES
• Conserves tooth structure(less cutting).
• Less time required.
• Resistance and retentive forms significantly increased by pins and slots.
Primary Retention Form: During initial tooth preparation, the form & shape of the preparation need to provide resistance against fracture & retention of the restorative materials in the tooth for non-bounded retentions. Often, features that enhance the retention form of a preparation also enhance the resistance form. Definition of primary retention form: It is that shape or form of the conventional preparation that resists displacement or removal of the retention from tipping or lifting forces. The retention form developed during initial tooth preparation may be adequate to retain the restorative material in the tooth. Sometimes, however, additional retention features must be incorporated in the final stage of tooth preparation. Principles of primary retention form:
• Vary depending on the restorative material used.
• Amalgam
• Convergence of the cavity walls
• Undercuts
• Amalgam retention
• Composite
• Micromechanical bond to enamel & dentin
• Walls direction
• Cast metal
• Frictional retention
• Cement inter-locking, adhesive semen.
Amalgam retention is adjunct to mechanical retention form & not a substitute. Adequate mechanical retention features still must be incorporated into the preparation. Reviewing the retention form for different classes, different restorative materials. As the number of pins increases the retention in dentin and amalgam increases but the problems created also increases. Chances of:
1.Crazing of dentin increases.
2.The amount of available dentin between the pins decreases.
3.The strength of amalgam retention decreases.
Increasing the diameter also increases. It appears that amalgam will maintain a presence into the future. There is growing evidence to suggest that it will be in the presence of a retention resin, especially for large amalgam retentions. While tooth reinforcement and conservation of tooth structure are noted advantages, bonded amalgam retentions may provide the added advantage of improved marginal seal, but laboratory significance is still being tested for clinically importance. With a decrease in micro leakage, there is sure to be less post-operative sensitivity and less secondary caries. The bonded amalgam retention relies upon a strong, durable bond between the tooth surface, the adhesive resin, and the amalgam. The tooth surface must be well prepared to assure the adaptability of the resin bond and the amalgam retention. The adhesive resin must be strong and resistant to breakdown, especially at the retention margin. The amalgam must be selected wisely and manipulated with respect to long-term service. Unfortunately, current literature does not present long-term clinical studies of bonded amalgam performance to support its superior performance over traditional amalgam retentions. Bonded amalgams may prove to be a valuable retention for the practitioner, but only if fundamental amalgam principles remain in practice. Adherence towards sound operative principles, specific to the cavity-surface of tooth preparations for amalgam retentions, has been shown to improve longevity of service and marginal integrity. It is unlikely that retention will be routinely employed in preference to non-bonded amalgam retentions, but there is growing confidence in adhesive dentistry in the placement of large, complex amalgam retentions. It is the opinion of this resident, that bonded amalgams have a future. Very small pieces of amalgam are released when a dentist places or removes retention. These pieces usually collect in the office’s wastewater, which goes into the public sewer system. Amalgam separators can be used to collect the amalgam pieces so they can be recycled rather than enter the sewer system.
