FINISH LINE IN FPD FOR PARTICULAR TEETH

[sushantpatel_doc]

PFM FPD-UPPER INCISORS
1) Incisal edge reduction: 2mm
2) Lingual Reduction (Fossa/Cingulum Area): 1.25-1.5 mm
3) Preparation centered on long axis of tooth, cusp tips centered
4) Retention and Resistance: 6-10 degrees taper with 2-3 mm lingual wall height
5) Facial-Proximal axial reduction: 1.2-1.3 mm on both facial planes
6) Lingual wall—Axial depth half the depth of the chamfer bur
7) Shoulder finish line 90 degrees 0.5 mm supragingival
8) Chamfer finish line 0-0.5 mm supragingival
9) Shoulder-Chamfer blend: mesial just lingual to contact, distal at the contact

PFM-PREMOLARS
1) Buccal cusp reduction: 2mm (cusp tip to midway down triangular ridge)
2) Lingual occlusal surface reduction: 1.5 mm reduction over entire area (including ½ the buccal cusp, central groove)
3) Preparation centered on long axis of tooth, cusp tips centered
4) Retention and Resistance: 6-10 degrees taper
5) Facial-Proximal axial reduction: 1.2-1.3 mm on gingival plane
6) Lingual wall reduction – Axial depth half the depth of the chamfer bur
7) Shoulder finish line 120 degrees 0.5 mm supragingival
8) Chamfer finish line 0.5 mm supragingival
9) Functional bevel placement 2.5 mm occlusal-apical height
10) Non-functional (Aesthetic) bevel placement 2.5 to 3 mm occlusal-apical height
11) Shoulder-Chamfer blend: mesial just lingual to the contact, distal at the contact

PFM-LOWER INCISORS
1) Incisal edge reduction: 2 mm
2) Preparation centered on long axis of tooth, incisal edge centered
3) Retention and Resistance: 6-10 degrees taper with 2-3 mm lingual wall height
4) Facial-Proximal axial reduction: 1.2-1.3 mm on both facial planes
5) Lingual reduction (Fossa/Cingulum area): 1.0-1.25 mm reduction over entire area
6) Lingual wall—Axial depth half the depth of the chamfer bur
7) Shoulder finish line 90 degrees 0.5 mm supragingival
8) Chamfer finish line 0.5 mm supragingival
9) Shoulder-Chamfer blend: blend occurs just lingual to contact points

¾ Crown
1) Occlusal Reduction: 1.5 mm (L) to 1.5 to 0.5 mm (B)
2) Preparation centered on long axis of tooth
3) Retention and Resistance: 6-10 degrees taper with adequate wall height
4) Retention-Resistance Boxes (Axial Depth) Proper location and depth (0.8-1.2mm)
5) Functional Bevel Placement correct angulation and reduction
6) Non-functional Bevel Placement (Buccal cusps “shoe”): correct angulation and reduction (0.2-0.3 mm)
7) Buccal Angles of Exit (from box forms): 135 degrees with 0.75 mm clearance
8) Isthmus depth and width: 2.0-2.5 (D) and 1.5 mm (W)

        

[Drsumitra]

radiographic image showing decay under crowns

        

[Drsumitra]

Porcelain crown restorations can fail for a variety of reasons. Product failure however is seldom the cause … it is rare that the integrity of high grade porcelain or the fabrication process (in the dental lab) becomes suspect. Dental science and engineering have all but eliminated those types of failures.

How, when and in what fashion a crown fails is actually a function of two key elements. One consideration usually focuses on the preparations made for placing the crown. Under-preparation or over-preparation of the tooth structure can be suspect. Bonding issues related to cementation characterstics and possible contamination of the bonding surface can cause crowns to go bad.

A second consideration focuses on where the restoration was used in the patient’s treatment. Different locations involves different preparation and different bonding procedures for anterior teeth as compared to posterior teeth. In many instances… even the cementation requirements can differ.

Loose Fitting Crowns
Bite and occlusal issues are common causes… as well as generalized trauma to the affected tooth. Bite problems, if they are immediate, may suggest inaccurate treatment. If the bite problems occur over an extended period of time… it may be due to issues involving other tooth structures and the overall occlusal relationship.

Inadequate existing tooth structure or poor preparation of the existing tooth structure may be the culprit. In some instances, with Dr. Briglia’s patients, a build up and post may be implemented to create the needed structure for a crown to adhere to.

If certain physical properties aren’t monitored closely, even a brand new build up can fail, due to the inadequate relationships of physical forces (biting) and retention qualities of surface areas.

The bonding process and bonding materials can often lead to crowns becoming loose in a very short time. Issues of over-preparation or under-preparation can be the cause. Matching the ideal cement product with the type of bonding surfaces can be critical. Contamination of the bonding surfaces is a frequent cause, arising from air borne bacteria, saliva and even a patient’s breath can compromise a bonding procedure.

Older crowns often become loose due to decay occuring under the crown. Decay can become manifested either through leakage between the bonding surfaces that allow bacteria ot accumulate. In other instances the preparation process did not remove 100% of the existing decay.

This decay-causing-looseness event occurs frequently in certain types of bridgework applications. Anchor teeth restorations, due to the biting forces operating on the bridge, may cause bonding surfaces to flex and eventually leak.

Discolorations
Many of the older crowns produced during earlier generations of dentistry science become discolored because underlying metal characteristics. Errors in creating and assuring secure margins of the crown can also cause the “dark line effect” on a crown. The use of all porcelain crowns or porcelain fused to gold crowns, as used in our office, overcomes this age old problem.

Sensitivity – Pain
Tooth sensitivity (reactions to hot and cold temperatures) that occurs immediately after placement of a new crown can be caused by indequate bonding procedures. Internal structures of the original tooth structure are indequately sealed due either inaccurate preparation or contamination of bonding compounds.

Pain associated with biting pressure may indicate a crown treatment that may have too much height or poor match of occlusal surfaces of an adjacent tooth. In some severe instances, stories of new crowns being ground down to the point of exposing underlying metal structures are not unheard of. Precision use of Articulators enables dentists to overcome this issue. Unfortunately, articulators are not used as often as they should be.

Doctors who readily accept and/or seek out the development of complex multifaceted treatment plans tend to have the technical know-how of overcoming what should be common error issues in dentistry. Because of the vast experience acquired in successfully treating chronic dental problems, routine and common place errors in judgement and/or application of treatment rarely occur.

        

[drsushant]

Objective
The goal of this study was to test the hypothesis that fracture toughness of the veneers in clinically failed zirconia-based fixed partial dentures (FPDs) is not significantly different from that of the in vitro group and to determine the potential reasons for their failures.

Methods
Fracture toughness values of the veneer layers in clinically failed zirconia core/glass veneer FPDs (n=4) and laboratory prepared glass veneer bar specimens (n=6) were determined using fractal analysis. A modified slit island technique was employed to measure the fractal dimensional increment (D*) of the two studied groups. The fracture toughness (KC) values were estimated using equation , where E is the elastic modulus and ao is a characteristic length parameter. Fracture toughness (KC) values of the specimens calculated using fractal analysis and fractography were statistically compared using a paired t-test.

Results
The average fracture toughness of the veneer in clinically FPDs (0.5±0.05MPam1/2) is not significantly different (p>0.05) from that of the bar specimens (0.6±0.1MPam1/2). The reasons for the early failures in FPDs could be occlusal overloading, stress corrosion, fatigue or improper structure design.

Significance
Fractal analysis is shown to be an alternative analytic tool for clinically failed ceramic restorations, especially for those with fracture origins chipped off during mastication and hence could not be analyzed using other techniques, such as fractography.

        

[drmithila]

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.
 

        

[drmithila]

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 level1

        

[drmithila]

DIGITAL IMPRESSIONS

See if it Fits Your Practice Needs and if so, Purchase as Soon as You Can Afford it
Are these devices making better impressions than the popular polyvinylsiloxanes or polyethers? In our own studies (and those of others), the answer is YES! However, for somewhere between $20,000 and $30,000, are they appropriate for your practice at this time? Almost all of the users of the CEREC (Sirona Dental Systems), E4D (D4D Technologies), iTero (Cadent), or Lava Chairside Oral Scanner (C.O.S) (3M ESPE), are very satisfied with the digital impression concept and its effectiveness. Further advancements are coming constantly. We predict that some form of this concept will gradually dominate the impression marketplace. How long that will require is unknown, but we predict between 5 and 10 years.

        

[drsushant]

 Impression Materials

Typical elastomer impression materials are being challenged for accuracy by digital impressions (to be discussed later), but conventional impressions are still by far the most popular procedure. Vinyl polysiloxane (VPS) has the majority of the US market, while polyether is still used by many practitioners. Two of the most popular and well-proven premium brands of VPS material are Aquasil Ultra Monophase (DENTSPLY Caulk) and Imprint 3 VPS (3M ESPE). Impregum (3M ESPE) dominates the polyether market. Either type of material (used properly) provides extremely accurate impressions. For those practitioners who have significant financial constraints, lower cost, proven brands are available from example companies such as Parkell, Pentron Clinical, and others.

Elastomer impression materials may have reached their optimum potential. Most elastomer materials used with excellent techniques can result in impressions that are not only accurate, but are also easily used by dental laboratory technicians. In contrast, digital intraoral scanning technology is just beginning to realize its potential. Continued improvement in digital scanning equipment and manufacturer investment in this technology will probably produce impressions that surpass the desirability of conventional elastomer materials. However, because of the high cost of digital devices for impressions, many years will probably be required before the majority of dentists to change to digital impressions.

Some practitioners do not realize the relatively high cost of elastomer impressions, which can cost $30 to $40 for a full-arch impression in a stock tray. The cumulative impression material and accessory cost of conventional impression techniques compared to the initial and ongoing cost of digital impression devices will be important as dentists determine their predicted return on investment for digital devices for impressions.

        

[drsushant]

 PERIO IMPLICATIONS OF RESTORATIVE MATERIALS AND MARGIN PLACEMENT

Dental crown restorations are considered a major contributing factor in the etiology of periodontal disease. Löe²⁴ in his classic 1968 paper, reviewed the reactions of the periodontal tissues to restorative procedures. His paper reviewed the reactions of the periodontal tissues and the effect of certain restorative materials on the periodontal tissues. 

While optimal periodontal health is expected to the improve success of dental care, it cannot overcome the effect of a crown restoration that extends into the subgingival area and may be responsible for causing damage to the periodontal tissue. This can occur by increasing the possibility for mechanical retention of bacteria and/or by a direct irritation effect from using a restorative material that retains more dental plaque.

Amsterdam²⁵ wrote a classic treatise on periodontal prosthesis more than 30 years ago, establishing the standard of care for a crown. He described the optimal margin/finish line which should be placed in a healthy sulcus at a minimal depth, just shy of the junctional epithelium. In guidelines still followed today, he suggested that to prevent plaque buildup, it is necessary to create optimal crown contours with proper coronal form, embrasure form, and subgingival fit at the margin.

If periodontal health is to be considered the future of success for dental care, it is imperative to use restorative materials which can help maintain a healthy periodontium. Captek (Argen) provides one example of a cosmetic (crown) restorative material that is compatible with excellent periodontal health, helping to satisfy the goal of optimal tissue health. This ceramometal crown incorporates the use of a unique metal composite gold cosmetic coping. The metal can be extended to the edge, developed into a collar, or cut back from the margin for a ceramic butt margin depending on the preparation, margin placement, and/or underlying prep tooth color. When the Captek gold was in the approximation of the margin, Goodson et al²⁶ were able to document a reduction of up to 91% in the number of bacteria surrounding this tooth versus normal tooth surfaces in the same mouth. In addition, they documented 96% less bacterial adhesion compared to conventional ceramic-fused-to-noble-metal crown restorations. These crowns have been described by the author as the "periodontal crown,"²⁷ since healthy tissue can be achieved from placement of the 22K gold margin against the subgingival tissue on any periodontally-affected tooth (Figure 4). 

For success to be achieved in dental care, it is essential that materials be fabricated in a manner that makes every attempt to develop periodontally healthy crowns, so as to prevent and/or reduce tissue inflammation. With the introduction of zirconia metal-free crowns, produced from pucks made from high quality zirconia powder (produced by TOSOH), the author employs a coping design modification to maximize soft-tissue health and protection against inflammation. One benefit of zirconia crowns is that they can be adapted to accommodate multiple finishlines at the margin, much like a traditional PFM. The "Periodontal Collar" for zirconia crowns was designed by the author and fabricated by Shaun Keating, owner of Keating Dental Arts Lab. (This dental laboratory was selected by Dr. Gottehrer based on its ability to reengineer new technology successfully.)

The collar is designed to allow removal of bacterial plaque on a regular basis to maintain optimal periodontal health. The subgingival collar is polished using a special material and, because of the special polishing process a very smooth surface is established. 

Preliminary observations by Dr. Gottehrer and Keating Dental Arts Lab, with insertion of 50 Periodontal Collar zirconia crowns, have produced healthy gingival responses. The zirconia can be made in a CAD/CAM environment and offers benefits in the way of high hardness factors and fracture resistance. Becoming more familiar with the healthiest coping designs can create optimal periodontal health resulting in long-term success. 

Restoring a tooth with compromised periodontal health with an appropriate full-crown, using modern materials that are shown to reduce the risk of additional plaque retention or a negative inflammatory response, should now be considered the treatment of choice for maintaining prolonged dental health.

        

[Author]

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