Optimizing the Prosthetic Margin With Sonic Instrumentation

INTRODUCTION
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.

INTRODUCTION OF THE SONIC HANDPIECE
To overcome these limitations, a sonic handpiece (KOMET SF1LM [KOMET USA]) was developed and introduced. It works with sonic vibrations (less frequent oscillations, approximately 6,000 Hz), but with an equal efficiency thanks to its adapted amplitude of vibrations (< 200 µm, depending on setting). Contrary to ultrasonic instruments, sonic instruments are not driven by an electrical power source. They are air-driven, allowing a sonic handpiece to be used like an ordinary handpiece. It simply attaches to the air tube of the dental turbine. The KOMET SF1LM has a MULTIflex connection. (MULTIflex is a registered trademark of KaVo Dental) The oscillation generated by this type of power source expands in all directions. Therefore, it is not linear; however, it traces an elliptical shape. This has led to the development of an instrument where, when vibrating, every part of its surface is active: the range sonic tips are available for different indications. This enables the sonic tip to be used in every possible angle of inclination. Unlike ultrasonic tips, sonic instruments do not need to be held at a strenuous fixed tilt angle during use. This makes the sonic tips easier to control, even by less experienced users. The vibrations' lower frequency does not limit the action of the instrument (ie, excellent cutting through dentin at high settings and optimal finishing at low settings), but presents an advantage of creating a roughness of a few microns while smoothing the dentin's surface. This promotes the retentive properties of the cement, which in these conditions flows perfectly, limiting the formation of undesirable and dangerous voids.
Two sonic tips with different diameters in the shape of a modified chamfer are now available: SF979.000.014 (0.14 mm) and SF979.000.016 (0.16 mm) with 0.64 µm for the positioning of the finishing line and the finer grit version SF8979.000.014 and 016 with 0.25 µm for perfect finishing of the finishing line. The KOMET SF1LM offers 3 settings (different amplitudes). For preparation and finishing of crown margins the KOMET SF1LM should be set to level 1.

CASE REPORT
Technique for Using the Sonic Handpiece
A patient presented requiring aesthetic restorations with the need to prepare the teeth for full coverage crowns on teeth Nos. 6 through 11 (Figures 1 and 2). To avoid injuries and changes to the periodontal tissue during intrasulcular positioning, the author initially used a rotary diamond instrument designed to cut anatomical preparations with modified chamfers margins (Figure 3). To further refine the preparations, a KOMET SF1LM sonic handpiece with a medium grain oscillating tip (SF979.000.016) was used on the margins (Figure 4), followed by 2 fine grain tips (SF8979.000.014/016) for finishing buccal, palatal and interproximal areas (Figures 5 to 8). As stated previously, oscillating instruments are easier to handle during these delicate phases because they are gentler on soft tissues, do not damage the gum tissue, and create an ideal finish of the dental surface in preparation for the subsequent cementing phase.
The author finished the preparations by manually using a rounded chisel with a similar modified chamfer design (Figures 9 and 10). Inspection of the completed preparations revealed that no damage was done to any soft tissue (Figures 11 and 12). An impression was taken of the cores using polyether material (Permadyne H [orange] and Permadyne L [blue] [3M ESPE]), showing a clear description of the sulcus without tears or pulls (Figure 13). The accurate impression allowed the dental technicians to create flawless restorations that were then delivered with minimal adjustments (Figure 14).

Figure 1. Preoperative frontal view.	Figure 2. Preoperative sagittal view.
Figure 3. A rotary instrument and tip with a modified chamfer design.	Figure 4. Sonic handpiece (KOMET SF1LM [KOMET USA]) and medium grain oscillating tip (size 016 = diameter of 1.6 mm) for positioning the preparation margin.
Figure 5. A fine grain sonic tip (016) was used in the buccal and palatal area.	Figure 6. A fine grain sonic tip (014) was used in the interproximal area.
Figure 7. A medium grain (64 µm) sonic tip (016) for positioning the preparation margin.	Figure 8. A fine grain (25 µm) oscillating tip was used for finishing the buccal area.

Figure 11. Buccal view of the finished preparations. (Note that there was no injury to the soft tissue.)	Figure 12. Occlusal view of the finished preparations. (Note that there was no injury to the soft tissue.)
Figure 13. The final restoration.	Figure 14. The patient smiling, showing the completed final restoration.

CLOSING COMMENTS
The success of prosthetic treatments can be largely influenced by the performance of the instruments and the use of suitable materials. This allows for consistent quality, while enhancing the practitioner’s skills. In this context, sonic instruments represent an advancement in ultrasonic technology that has revolutionized the way in which we position and finish the prosthetic margin, completely eliminating the disadvantages related to linear vibration. Today, there is a variety of versatile precision instruments that allow preservation of the dental and gingival tissue during surgical interventions. In the author’s opinion, these instruments are easy to control even by those who have only just started using this method.

Dr. Massironi earned his degree in medicine and surgery from the University of Pavia (Italy) and specialized in dentistry there as well. He is a Diplomate of International Congress of Oral Implantologists, and an active member of the European Academy of Esthetic Dentistry and the Academy of Microscope Enhanced Dentistry. He has also held numerous courses and congresses internationally. He has a well-established private practice in Milan, Italy, where he specializes in implantology and prosthesis. He has published several articles and 2 books entitled Precision in Prosthetic Restoration and Precision in Dental Aesthetics (both with Quintessence) which were translated in numerous languages. He can be reached via e-mail at info@gandini-massironi.it.

Disclosure: Dr. Massironi reports no conflicts of interest.