Several modifications have been proposed for in class II cavity design for amalgam restorations . These modifications are changes from the classic preparation design . They are indicated for those situations which are not ideal. Modifications are based on following factors:

1. Extent of caries
2. Location of proximal caries and relationship of adjoining teeth
3. Esthetic considerations
4. Involvement of several surfaces on the same tooth
5. Prosthetic considerations

MODIFICATIONS :

1. SIMPLE BOX PREPARATIONS: It consists of only a proximal box preparation without an occlusal step and minimum facial and lingual extensions . It is indicated for a small proximal carious lesions in the absence of occlusal caries , moderate proximal caries, when there is narrow inter-proximal contact. The facial and lingual walls converge occlusally to provide retention form. For added retention axiopulpal grooves are placed gingivally.

2. SLOT PREPARATION : It is similar to class V amalgam cavity preparation except that it is done on the proximal surface of the tooth. It is indicated in proximal root surface caries with gingival recession. The design is similar to that of the slot preparation except it is approached from the facial aspect without disturbing the contact area.

3. CONSERVATIVE DESIGNS : It is recommended in all posterior teeth where caries incidence is very low and all occlusal pits and fissures are not involved. The conservative design preserves all the remaining sound tooth structures such as transverse ridge or oblique ridge and thereby protects the cuspal strength .

4. MODIFICATIONS TO PRESERVE ESTHETICS : To preserve the esthetics in the critical area during a mesio-occlusal cavity preparation , the facial wall of the mesial box should be prepared straight , i.e parallel to the long axis of the tooth rather than gingivally divergent . Another modification is to avoid breaking the facial contact whenever caries is limited only to the mesio-lingual embrassure.

5. MODIFICATIONS OF ROTATED TEETH : Depending on the degree of rotation , the proximal box is displaced facially or lingually.

6. ADJOINING RESTORATIONS : The intersecting margins of the two restorations -should be at right angles . Care should be taken so as to prepare the outline of the new cavity without weakening the amalgam margin of the existing restoration .

7. MODIFICATIONS FOR ABUTMENT TOOTH: The facial and lingual walls of the proximal box must be extended more so that the entire rest seat can be prepared in the amalgam without encroaching on the occlusal margins.

The pulpal floor is deepend an extra 0.5 mm apical to the region of the rest seat so as to provide an adequate thickness of amalgam.

8. CUSP CAPPING : It is indicated where there are extensive caries underneath the cusps and facio-lingual extensions of the occlusal preparation is more than 2/3rd the distance between the facial and lingual cusp tips. For cusp capping 1.5 to 2 mm of cusp reduction is necessary . The reduced cusps must meet the adjacent unreduced cusp at 90 degree cavosurface angle to provide both adequate edge strength of amalgam.

References: STURDEVANTS 7TH EDITION

LASER: Light Amplification by Stimulated Emission of Radiation .Laser dentistry is easier, more precise ,less traumatic for patients . Lasers provide a state-of-the-art approach for modern dentistry.

DISCOVERY : It was discovered by Shallow and Towns on 1958. First working laser was built by Maiman of Hughes research laboratory in 1960.

COMPONENTS OF LASER :

1. OPTICAL CAVITY : This is an internally polished tube occupying the center of the device .
2. ACTIVE MEDIUM : This consists of chemicals that fill the core of the optical cavity . When stimulates , the active medium emits laser light . The active medium may be gas , crystals, solid state semiconductors.
3. EXCITATION SOURCE : This surrounds the optical cavity and provides energy for exciting the active medium . This may be a flash light , arc light or an electromagnetic light .
4. OPTICAL RESONATOR: This consists of two parallel mirrors placed at each end of the optical cavity .It amplifies the laser beam.
5. COOLING UNIT : To dissipate the heat air or water -resistance coaxial coolants are provided in the unit.
6. DELIVERY SYSTEM : The laser light is delivered by a quartz , fiber-optic , a flexible hollow wave guide or a hand piece.
7. CONTROL PANEL : This provides control over the power output.

BASIC PRINCIPLES OF LASERS

1. LIGHT : Light is a form of electromagnetic energy that exits as a particle , traverse like a wave , at a constant velocity . Laser light is monochromatic , coherent and collimated.
2. AMPLIFICATION : This process occurs inside the optical cavity , where the photons released by the stimulated active medium are amplified by the optical resonator to produce the laser beam .
3. STIMULATED EMISSION: Occurs in active medium due to excitation source . The photons energize more atoms which further causes emission of additional atoms .
4. RADIATION : The light waves produced by the laser are a specific form of radiation of electromagnetic wave . All dental lasers emit visible light wavelength or an invisible infrared light wavelength in the portion of non ionizing spectrum called thermal radiation.
5. LASER EMISSION MODES: They are two basic modes of laser emission , continuous wave emission and free running pulse emission.

LASER TISSUE INTERACTION :

1. Reflection
2. Transmission
3. Scattering
4. Absorption

TYPES OF LASERS :

1.BASED ON WAVELENGTH :

• Argon lasers
• Diode lasers
• Nd:YAG lasers
• Er,Cr: YSGG
• Er :YAG
• CO2 lasers

2. BASED ON TARGET TISSUE

• Soft tissue laser
• Hard tissue laser

LASER APPLICATION IN CONSERVATIVE DENTISTRY :

1. CARIES DETECTION : This is achieved by using quantitative , laser induced fluorescence . Natural teeth show green florescence whereas carious shows red fluorescence and thus enables detection.
2. CARIES PREVENTION : Argon lasers alter the characteristics of enamel to make it more resistant to caries without endangering the pulp. Lasers make enamel more acid resistant and increase its fluoride uptake when topical fluorides are applied.
3. CAVITY PREPARATION USING LASERS : Hard tissue lasers are used for cavity preparation . The laser light vaporizes the water present in the enamel and thee dentin and causes micro explosion of the hydroxyapatite. This create micro craters on the tooth surface.
4. DISINFECTION OF PREPARED CAVITY : Lasers can be used for the photo-activated disinfection of prepared cavities to destroy any residual microorganisms. This employs a solution of tolonium . chloride along with red light lasers . This combination releases nascent oxygen that can destroy the bacterial cell walls .
5. LASER ETCHING OF ENAMEL AND DENTIN : Lasers produce a chalky white , rough surface of enamel that increases the retention of composite resin restorations .
6. LASER CURING OF COMPOSITE RESINS : Argon lasers emit blue light which is suitable for curing composite restorations.
7. GINGIVAL TISSUE MANAGEMENT : CO2 lasers , Nd:YAG , argon lasers are used for soft tissue surgery. Diode lasers are used for the retraction of tissues during the restorative procedures .or for re contouring the gingiva during smile design procedures.

REFERENCES: STURDEVANTS 7TH EDITION

Written by – Dr. Urusa I Inamdar

Hot tooth:

A tooth that is difficult to anesthetize is known as a ‘hot tooth ‘. This is most commonly encountered in a mandibular first molar tooth wherein after the anesthetic block , the patient may describe profound numbness of the ipsilateral lip and tongue but still may experience acute pain during the access opening procedure.

Most common sites of occurrences:

• primary and permanent teeth
• sites of recent or defective restorations
• Sites of recent trauma
• mandibular molars
• patients with anxiety about dental treatment or patient who have been in pain for several days usually require a more sophisticated approach.

Signs and symptoms

Hypothesis

Mechanism

There is a special class of sodium channels on C – fibers, known as tetrodotoxin-resistant (TTXr) sodium channels. The expression shifts from TTX- sensitive to TTXr during neuroinflammatory reactions and the TTXr sodium channels play a role in sensitizing C-fibers and creating inflammatory hyperalgesia. One of the clinically significant characteristics of these sodium channels is that they are relatively resistant to lidocaine. These channels are five times more resistant to anesthetic than TTX- sensitive channels. Hot tooth may be explained by the fact that the TTXr sodium channels have not been adequately blocked by the anesthetic.

Management

Bupivacaine has been found to be more potent than lidocaine in blocking TTXr channels and may be the anesthetic of choice when treating ‘hot tooth.’ Supplemental intraligamentary or intraosseous injections are most helpful to ensure profound local anaesthesia.

References:

• Grossman’s Endodontic Practice (13th edition)
• Article – International journal of Biomedical research ( Hot tooth – A challenge to endodontists)