FINISH LINE IN FPD FOR PARTICULAR TEETH

The success of a prosthetic restoration largely depends on the accuracy with which the preparation of the tooth (or teeth) is carried out, given that excessive/insufficient reduction of dental tissue can either lead to retention problems or to the thickness of the ceramic material not corresponding to the functional loads. In addition, if one does not protect the anatomically critical areas during dental preparations, the pulp or periodontal tissue could be injured. Therefore, sufficient professional knowledge and perfect control of instruments are fundamental preconditions for achieving a good final result. This applies to the anatomical preparation phase (ie, the selective reduction of dental volumes in view of the restoration’s final shape, as well as to the finishing of the abutment’s margins and surfaces).
Special care has to be taken during the positioning of the preparation margin (ie, the final phase of the preparation of a tooth immediately preceding impression taking). For greater protection of the biological structure, the literature indicates that the prosthetic margin should ideally be placed close to or even above the gingiva, thus ensuring better accessibility and improved visibility of the contours. However, the constantly growing aesthetic demands and the need to disguise the restoration between natural dental elements force the clinician to create suitable thicknesses and position the prosthetic margin from the vestibular side to the intrasulcular region (ie, near or sometimes in direct contact with the epithelial attachment).
To avoid injuries and changes to the periodontal tissue during intrasulcular positioning and to counter such problems, clinicians have been using modified chamfer preparations for years. This requires a clinician to use diamond rotary instruments for anatomical preparations whose design is more pronounced than a vertical chamfer but less than a rounded shoulder. This offers the dental laboratory technician sufficient space for the creation of stable prosthetic margins even during the firing of the ceramics and, therefore, highly precise and aesthetically pleasing prostheses. At the same time, this procedure is easier on the dental structure than conventional methods of manufacturing dentures, especially in areas where aesthetics are of great importance.

ROTARY VERSUS OSCILLATING INSTRUMENTATION
The intrasulcular positioning entails the necessity for tissue relocation, either to finish the preparation or to allow the impression material to enter the subgingival region, but above all it entails the necessity to work with instruments that come in direct contact with soft tissues. It has been established that rotary instruments that come in contact with soft tissue can cause tearing and bleeding and, even in mild cases, can lead to oozing hemorrhages that impair or considerably reduce the legibility of the impression. To prevent such issues and allow better control of the instruments during the positioning and finishing phases, it is recommended to use oscillating instruments. The advantage of oscillating instruments is that they are easier to handle during these delicate phases because they: (1) are safer on soft tissues, (2) do not cause damage and bleeding of the gum tissue, and (3) create an ideal finish of the dental surface in preparation for the subsequent cementation phase.
The author initially developed and used ultrasonic tips (diamond instruments coated at 90 µm) of the same shape and dimensions as rotary instruments. They were used at a high setting for modified chamfers to position the margin and at a low setting to finish the core. The ultrasonically driven piezoelectric power source causes the instruments to vibrate at a frequency of approximately 30,000 MHz. The different settings do not affect the frequency, but the oscillation’s amplitude. This means that, at the same frequency, high settings produce wide oscillations capable of removing dentin, therefore allowing the lowering of the preparation margin without harming the soft tissue. At low settings, the instrument vibrates with minimum oscillation amplitude, in which case it is unable to remove dentin. The instrument cannot only finish the core surface in complete safety, but can also create the ideal degree of surface roughness for a homogenous distribution and perfect function of the cement. Although these tips have sparked off a revolution in the technique of lowering the preparation margin, the disadvantage of these ultrasonic instruments is that they have a limited operational area because of the way they vibrate. They are only capable of performing linear oscillations. In other words, oscillating instruments can only move in 2 directions. Consequently, the oscillating effect in some areas around the instrument is limited or reduced. It is, therefore, essential that a certain tilted position be maintained during the preparation to avoid limiting the performance of the instrument. The dental assistant must blow air on the operative site during the preparation to remove some of the cooling water. This action must be taken because the water comes between the instrument and the dentin, shielding the dentin from the action of the instrument with an effect similar to that of hydroplaning.