MYOFUNCTIONAL APPLIANCES

The correction of malocclusion, primarily by means of controlled movement of the developing and mature dentition into a desirable occlusal relationship.

Via tooth borne appliances such as the bionator and twin block, it controls and modify the growth of skeletal structures of the craniofacial complex.


Bionator

This appliance is introduced by Balters whereby the bulkiness of the Activator and its limitation to night-time wear lead to the development of Bionator.

The palate is free for propioceptive contact with the tongue and the buccinator wire loops hold away a potentially deforming muscle action.

It is also considered a design developed from the “Skeleton of the activator” + “Modification of Robin’s concept”.

According to Balters:

         “ The equilibrium between tongue and cheeks, especially b/w tongue and lips in height, breadth and depth in an oral space of maximum size and optimal limits, providing functional space, is essential for the natural health of the dental arches and their relation to each other ”               

Abnormal position of the tongue led to development of malocclusions

 –  Class II :  Posterior displacement

 –  Class III : low anterior displacement

 – Narrow arches and crowding : low outward pressure

 – Open bite : hyperactivity and forward posture

He was in support of:

       – Function and Form concept : Van der klaauw

       – Functional Matrix theory     :  Melvin Moss


Aims of Bionator

  • Accomplish lip closure and establish contact b/w back of the tongue and soft palate
  • Enlarge oral space
  • Incisors in edge to edge relation.
  • Lengthening of the mandible
  • Leading to an improved relationship b/w the jaws, tongue, dentition and soft tissues

Indications:

  • Well aligned dental arches
  • Functional retrusion
  • Mild to moderate skeletal discrepancy
  • No evidence of labial tipping seen

Contraindications:

  • Class II relationship caused by maxillary prognathism
  • Vertical growth pattern is present
  • Labially tipped lower incisors

Advantages:

  • Less bulky
  • Can be worn full time, except during meals
  • Constant influence on tongue & perioral muscles

Effects of Bionator:

  • Modulation of muscle activity of tongue
  • Elimination of abnormal influences of perioral musculature
  • Stimulation of myotactic muscle activity and isotonic muscle contractions
  • No vertical component except for guiding eruption of teeth
  • No viscoelastic response
  • Prevention of deleterious para-functional activity at night : relaxation of lateral pterygoid ( used for TMJ problems)

Bionator Types


Twin Block

Introduced in 1977 as a two-piece appliance resembling a Schwarz double plate and a split activator, which is further developed by William J.Clark with the replacement of occlusal inclined planes by means of acrylic inclined planes on bite blocks.

Twin-block appliance is the most popular functional appliance in the UK.

The upper & lower parts fit together using posterior bite blocks with interlocking bite planes, which posture the mandible forwards.


Mechanism of action
  • Guide mandible downward and forward
  • Favorable proprioceptive contacts of inclined planes
  • Adaptation of the muscles of mastication
  • Vertical and transverse control

Mode of action:

-> Growth modification by rapid neuromuscular response & gradual dentoalveolar response

-> Initial experiments suggests that substantial changes in skeletal structure, including condylar growth, remodelling of glenoid fossa, mandibular growth & maxillary restraint

-> Proliferation of connective tissue and blood vessels in the retrodiscal area

-> Unloading of the condyle

-> Pterygoid response: Pain while retracting the mandible. Discomfort on removal of appliance due to compression in the tension zone behind the condyle

Design:

  • Occlusal bite blocks with inclined planes
  • Midline screws for expansion
  • Retention :   Adams clasps / Delta clasps
  • Interdental clasps on lower incisors
  • Labial bows on upper incisors
  • Springs to move individual teeth
  • Provision for extraoral traction
Components of the Appliance:
  1. Midline screw
  2. Occlusal bite block
  3. U/L delta clasps
  4. Labial bow
  5. Provision of extra oral traction ie. head gear tubes when indicated

Standard twin block

Labial bow– early stages of development to:

-overcorrect incisor angulation.

-limit the scope of functional correction.

-good lip seal is achieved naturally.

Delta Clasps:-

Placed according to the area of retention

  • Mesial and distal undercuts
  • Interdental undercut.

Used on upper 1st permanent molar and lower 1st  premolar

Base plate:

Cold cure acrylic method or heat cure acrylic

Occlusal inclined planes:            


Fabricating the appliance:-

-> Once the bite is taken it is transferred on to the set of patients models.

-> This is then mounted on to a fixator.

-> Individual wire components are then fabricated

-> Wax up is done

-> This is followed by flasking, dewaxing, packing and curing.

-> Cured appliance is then finished and polished.

-> Checking on the models for proper fitting.


Advantages over other Functional appliances:

  1. Functional mechanism similar to natural dentition.
  2. Occlusal inclined planes give greater freedom of movement in anterior and lateral excursions.
  3. It is possible to modify the existing appliance rather than having to construct a new appliance, if further advancement of mandible is required.
  4. Less interference with normal function.
  5. Improved appearance and function due to absence of lip, cheek and tongue pads.
  6. Aesthetically acceptable.
  7. Can be worn 24 hrs.
  8. Independent control over upper and lower arch width.

Side-effects of Twin block appliance:

  1. Residual posterior lateral open bites at the end of functional phase because posterior teeth are prevented from erupting by the occlusal coverage of bite blocks.
  2. Seen in cases initially presenting with deep overbite.
  3. Trimming of acrylic from occlusal surface of upper block to allow lower molar to erupt is done to avoid lateral open bite.

Conclusion

Functional appliances helps in interception of developing malocclusion by changing direction and amount of growth.

Proper fabrication of functional appliances also plays important role during treatment period.

Alloy in Prosthodontics

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Photo by Pixabay on Pexels.com

WHAT IS METAL?

Metal is an element like gold or palladium which ionizes positively in solution.

  • Examples of noble metals:  gold, platinum, rhodium, ruthenium, iridium and osmium
  • Examples of base metals: titanium, nickel, copper, silver and zinc

 

WHAT IS ALLOY?

Alloy is a metallic material formed by the combination of 2 or more (major & minor) metals. In their molten state, metals dissolve to various degrees in one another, allowing them to form alloys in the solid state. In dentistry, alloys are used in inlays, long span bridges, partial denture framework etc.

 

Structure of Alloys

Alloys are crystalline in structure. This structure consists of crystals or grains abutting one another. The boundaries between the grains are referred to as grain boundaries. Size of grain determines the properties of alloy. The smaller the grains the better as more boundaries prevent dislocations in the structure.

 

General Requirements of Alloy

1) Biological

  • Non toxic & non allergic (biocompatibility)
  • Resistant to tarnish & corrosion

 

2) Functional

  • Satisfactory physical (aesthetics) and mechanical properties (yield strength)

 

3) Working

  • Easy to process and handle (ease of casting, soldering, burnish-ability)
  • Readily available, relatively inexpensive constituents

 

Desirable properties of casting alloys

  • Exhibit biocompatibility
  • Ease of melting
  • Ease of casting
  • Ease of Brazing and soldering
  • Ease of Polishing
  • Little solidification shrinkage

 

Dental applications of alloys

žCast Co/Cr alloys

  • RPD framework
  • Porcelain-metal restorations

ž

Cast Ni/Cr alloys

  • RPD framework
  • Crown and bridges
  • Porcelain-metal restorations

ž

Cast Ti and Ti alloys

  • Crowns & bridges
  • RPD framework
  • Implants

ž

Wrought Ti and Ti alloys

  • Implants
  • Crowns
  • Bridges

ž

Wrought S/S alloys

  • Endodontic instruments
  • Orthodontic wires and brackets
  • Preformed crowns

ž

Wrought Co/Cr/Ni alloys

  • Ortho wires and endo files

ž

Wrought Ni/Ti alloys

  • Ortho wires and endo files

 

žWrought beta Ti alloys( Ti/Mo)

  • Ortho wires

 


 

Classification of Dental Casting Alloys

 

Classification based on nobility by American Dental Association (1984)

A) High noble alloys:

More than 40 wt% gold + 60 wt% other noble metals (gold, iridium, osmium, platinum, rhodium)

 

B) Noble alloys:

More than 25 wt% noble metals, i.e. no limit for gold content

 

C) Predominantly base metal alloy:

More than 75 wt% base metal + less than 25 wt% noble metals

 

wt% = % by weight of

 

Classification based on mechanical properties

Type I : Soft

  • Yield stress: 140 MPa
  • Hardness: low
  • % Elongation (ductility): 18% minimum
  • Restoration: Inlay

 

Type II : Medium

  • Yield stress: 140-200 MPa
  • Hardness: medium
  • % Elongation (ductility): 18% minimum
  • Restoration: Onlay

 

Type III : Hard

  • Yield stress: 200-340 MPa
  • Hardness: high
  • % Elongation (ductility): 12% minimum
  • Restoration: Crown, short bridge

 

Type IV : Extra hard

  • Yield stress: 340-500 MPa
  • Hardness: extra high
  • % Elongation (ductility): 10% minimum
  • Restoration: Crown, long bridge, post & core, partial denture

 

NOTE:

* The heating temperature increases from type I to type IV

* The strength increases from type I to type IV

* The elongation (ductility) decreases from type I to type IV

* The yield strength (force per unit area required to deform the alloy) increases from type I to type IV

 


 

Base Metal Alloy

 

Types:

1) Nickel Chromium alloy

2) Cobalt Chromium alloy

3) Titanium and titanium alloys

 

Nickel Chromium alloy

– A substitute for Type III gold alloy

 

Composition:

Major elements 90% by weight

  1. Nickel: 70-80%
  2. Chromium: 12-20%

 

Minor elements 10% by weight

  1. Molybdenum: 3-6%
  2. Silicon and Manganese
  3. Aluminium: 2-6%
  4. Beryllium: 0.5%

 

 

Cobalt Chromium alloy

  • A substitute for Type IV gold alloy
  • Almost all RPD frameworks are done by Co/Cr alloys

 

Composition:

Major elements 90% by weight

  1. Cobalt: 35-65%
  2. Chromium: 28-30%
  3. Nickel: 0-30%

 

Minor elements 10% by weight

  1. Molybdenum: 3-6%
  2. Silicon and Manganese
  3. Carbon: 0.2%

 

Titanium alloys

Composition:

  1. Titanium alloy
  2. Chromium – 5-15%
  3. Nickel 5-15%
  4. Molybdenum 3%
  5. Silicon, Manganese, iron, carbon

 

 

Role of the major elements

Cobalt: Increases strength, hardness, modulus of elasticity

 

Nickel: Increases strength, hardness, modulus of elasticity and ductility

However, nickel is allergic. Affecting more on females than males like swelling or gingival discoloration.

 

Chromium: Tarnish & corrosion resistance increases by passive layer (an oxide layer that is thin, uniform, non porous, adherent and transparent.

 

Role of the minor elements

– To increase strength, hardness & decrease ductility

Molybdenum: Grain refiner

 

Carbon: 0.2% as discontinuous precipitate in the grain boundaries. It affects the hardness, strength and ductility. Too much carbon will make alloy brittle.

 

Aluminium: Increases tensile and yield strength. It reacts with nickel forming intermetallic compound which precipitates inside the solid solution alloy -> precipitation hardening

 

– To improve cast-ability

Silicone & Manganese: increase fluidity of molten alloy, act as deoxidizer

 

Beryllium: Decreases the melting temperature (note: beryllium vapor is carcinogenic and may lead to fibrosis of the lungs)

 

 

Biological Properties

Biocompatibility – All base metal alloys are biocompatible due to the presence of passive layer except base metals containing Nickel and Beryllium.

Also, due the presence of passive layer, all base metal alloys are resistant to tarnish & corrosion.

 

 

Physical Properties

Melting temperature – most base metal alloys melt at temperatures of 1400°C to 1500°C. The addition of 1% to 2% beryllium lowers the melting temperature of Ni-Cr alloys about 100°C.

 

 

Mechanical Properties

Yield strength – the yield strength gives an indication when a permanent deformation of a device or part of a device will occur.

It is believed that dental alloys should have yield strengths of at least 415 Mpa to withstand permanent deformation.

 

Hardness – is an indication of the ease of finishing the structure and its resistance to scratching in service.

 

Tensile strength – tensile strength of cast base-metal dental alloys is greater than 800 Mpa.

 

Elongation –increasing the nickel content with a corresponding reduction in cobalt generally increases the ductility and elongation.

 

Elastic modulus – the higher the elastic modulus, the more rigid structure can be expected. Elastic modulus of base metal alloys is approximately double of type IV cast dental gold alloys.

 

Fatigue – cobalt chromium alloys possess superior fatigue resistance when compared to titanium, gold alloys. Any procedures that result in increasing the porosity or carbide content of the alloy will reduce the fatigue resistance.

 

 

– end –

 

 

 

 

 

 

References:
pocketdentistry.com/21-alloys-used-in-dentistry/

http://www.slideshare.net/UDDent/dental-casting-alloys

Fixed Prosthodontics – Interim Fixed Restoration

 

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WHAT IS INTERIM PROSTHESIS?

Interim prosthesis means a fixed or removable dental prosthesis, or maxillofacial prosthesis, designed to enhance esthetics, stabilization and/or function for a limited period of time, after which it is to be replaced by a definitive dental or maxillofacial prosthesis.

Often such prostheses are used to assist in determination of the therapeutic effectiveness of a specific treatment plan or the form and function of the planned for definitive prosthesis.

It’s synonyms are provisional prosthesis, provisional restoration.

 

A temporary restoration is not expected to last a long time. A provisional restoration is expected to last for two to six weeks; but, provisional is not only temporary but designed to be replaced by something definitive(permanent) —a definitive restoration in this case.

Interim (provisional or temporary) crown or fixed partial denture is a restoration applied to the prepared tooth temporarily to protect it and to keep the patient comfortable during fabrication of the definitive (permanent) restoration.

It influences the ultimate success of the final restoration.

 

Functions of interim restoration

  1. Protection
  2. Positional stability
  3. Mastication
  4. Esthetics

 

1) Protection

  • The interim restoration protects the pulp, the periodontium and the prepared tooth.
  • It protects the pulp from thermal and chemical irritation caused by foods, drinks.
  • It protects the peridontium from injury by food impaction due to loss of contact and gingival recession due to loss of normal buccal and lingual contours.

Interim restoration must be cleansable in order to maintain gingival health.

 

2.Positional stability

  • To maintain the tooth position and prevent mesial, distal drift or over eruption which will change the relation with the surrounding teeth.
  • To maintain the gingival tissue contour, prevent gingival hyperplasia or gingival recession.

 

3- Mastication

  • To maintain the function of the prepared teeth and enables the patient to use them in mastication satisfactorily.

 

4- Esthetics

  • To restore and maintain esthetics.

 

Mechanical Requirements

  1. The restoration should be strong enough to withstand occlusal forces without fracture.
  2. It should be retentive to avoid displacement.
  3. Easy removal for reuse without being damaged.

 

Factors to be considered in making an interim restoration

The dark red area represents the optimum, in which biologic,  mechanical, and esthetic requirements are adequately met.

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Ideal requirements of Interim restoration materials:

1. Ease of handling, adequate working time, easy moldability. and rapid setting time.

2. Biocompatibility – non toxic, non allergic, non exothermic.

3. Dimensional stability during solidification.

4. Ease of contouring and polishing.

5. Adequate strength and abrasion resistance.

6. Good appearance, color control and color stable.

7. Ease of adding to repair or correct.

8. Chemical compatibility with temporary luting cements.

 


 

Types of interim restorations

A) Prefabricated (crowns)

  • Aluminum crowns
  • Anatomical metal crown forms
  • Clear celluloid shells
  • Tooth coloured polycarbonate crown forms

 

B) Custom (crowns or fixed partial dentures)

  • Material (variety of resins)
  • Technique (direct or indirect)

 

A) Prefabricated Crowns

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These preformed crown forms are commercially available; they can not satisfy the requirement of an interim restoration, so they must be lined with autopolymerizing resin. They are available in a variety of tooth types and sizes.

 

Polycarbonate crown

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Suitable for anterior teeth as it is constructed from a color stable resin, but available in only one shade, this can be modified to a limited extent by the shade of the lining resin. They are supplied in incisor, canine and premolar tooth type.

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Aluminum / and Tin-silver Crowns

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Nickel-chromium crowns

Used mainly for deciduous teeth. They are trimmed and adapted with contouring pliers and cemented with high strength cement. They are longer-term interim restoration due to their hardness.

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Custom made interim crowns and bridges

May be constructed by Indirect or direct method using resin material.

 


 

Indirect Technique

In this technique the interim restoration is constructed outside the mouth so it has the following advantages over the direct technique:

1-There is no contact of free monomer with the prepared tooth or gingiva, which might cause tissue damage or sensitization.

2-The prepared tooth is not subjected to heat created from the exothermic reaction of resin which might cause irreversible pulp damage.

3-  The marginal fit of indirectly constructed restoration is better due to its complete polymerization undisturbed on the stone cast.

4-The indirect technique reduced the chair time.

 

Steps:

* The study cast is constructed from alginate impression before preparation.

* If the tooth or teeth to be restored has any obvious defect, it should be corrected on the study cast with red utility wax.

* Fill all the embrasures with wax or putty to eliminate undercuts.

* Construct the rubber base index for the tooth to be prepared or the index may be constructed from the patient mouth.

* Upon completion of the preparations, make alginate impression for them and pour it in fast-setting plaster.

*  Coat the cast with separating medium.

*  Mix the temporary acrylic resin in a dappen dish and put some on the protected areas of the cast, such as interproximal spaces and in grooves and boxes.

* As the resin begins to lose its surface gloss and become slightly dull, fill the index, place it over the cast.

*  Put them in pressure pot if available or warm water to accelerate polymerization (hot water causes boiling of the monomer porosity).

* The restoration is then removed from the cast, if it is not easily removed from the cast; break the cast with a heavy laboratory knife.

* The interim restoration is then finished using acrylic burs, sand paper discs with different grits.

Finally the restoration is polished with pumice, rag wheel and rubber cups to be ready for cementation.

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The connectors of an interim FPD are purposely overcontoured to improve strength. This should be limited by aesthetics and maintenance of periodontal health.

 

Indirect-direct Procedure

In this technique, the indirect component produces a “custom-made preformed ESF” external surface form”. In most cases the practitioner uses a custom ESF with an underprepared diagnostic cast as the TSF. “tissue surface form”.  The resulting mold forms a shell that is lined with additional resin after tooth preparation. This last step is the direct component of the procedure.

The indirect-direct approach offers these advantages:

  1. Chairside time is reduced.
  2. Less heat is generated in the mouth.
  3. Contact between the resin monomer and soft tissues is minimized compared to the direct procedure and there is a reduced risk of allergic reaction.

However, adjustments are frequently needed to seat the interim completely on the prepared tooth. This is the primary disadvantage of the indirect-direct procedure.

 

Step-by-step Procedure

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  • ESF “external surface form” can be prepared using silicon material or vacuum-formed polypropylene sheet.
  • Prepare the abutment teeth on accurately mounted diagnostic casts. The diagnostic preparation should be more conservative than the eventual tooth preparation and should have supragingival margins.FP_147
  • Apply resin into the ESF and complete the interim restoration.
  • Seat the newly completed interim restoration (called now custom pre-formed ESF) on cast and refine occlusion by articulator. Finish and clean then send it to dentist.

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Step-by-step Procedure – CLINICAL STEPS

  • After tooth preparation, try-in the custom pre-formed ESF.
  • To make the TSF, fill the interim with resin and seat it over prepared teeth.
  • Confirm the marginal fit and occlusion, refinish and polish, then cement the restoration.

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– end –

Fixed Prosthodontics – Components of the Dental Bridge

FP_088Each part of the bridge should be designed individually, but within the context of the overall design. The components of a bridge are retainers, pontics and connectors.

 

Retainers

Major or minor

Fixed-fixed , cantilever and spring cantilever bridges have only major retainer(s).

Fixed-movable bridges have a major retainer at one end of the pontic and a minor retainer (carrying the movable joint) at the other.

A major retainer for a conventional posterior bridge should not be less than an MOD inlay with full occlusal protection.

For incisor teeth it is usually a complete crown, although partial crowns are still sometimes used.

Minor retainers do not need full occlusal protection: a minor retainer may be a partial crown or a two – or three-surface inlay without full occlusal protection.

Minimal-preparation minor retainer(s) are also used for minimal-preparation where the occlusion is favourable.

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The Choice Between Complete Crown, Partial Crown,

Intra-coronal or Minimal-preparation Retainers

 

Criteria for choosing a suitable retainer includes:

  • Alignment of abutment teeth and retention
  • Appearance
  • Condition of abutment teeth
  • Conservation of tooth tissue
  • Occlusion
  • Cost

 


 

Pontics

The principles guiding the design of the pontic are :

  • Cleansability
  • Appearance
  • Strength

 

Cleansability

All surfaces of the pontic especially the ridge surface, should be made as cleansable as possible. This means that they must be smooth and highly polished or glazed. and should not contain any junctions between different materials.

In a metal-ceramic pontic the junction between the two materials should be well away from the ridge surface of the pontic.

It is important too that the embrasure spaces and connectors should be smooth and cleansable. They should also be as easy to clean as possible. Access to them and the patient’s dexterity should be taken into account in designing pontics.

 

Appearance

Where the full length of the pontic is visible. It must look as tooth-like as possible.

 

Strength

All pontics should be designed to withstand occlusal forces.

The longer the span, the greater the occlusal-gingival thickness of the pontic should be .

Metal-ceramic pontics are stiffer and withstand occlusal forces better if they are made fairly thick and if the porcelain is carried right round them from the occlusal to the ridge surface leaving only a line of metal visible on the lingual surface or none at all.

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Classification of pontic design

Pontic designs are classified into two general groups:

Mucosal contact & Non-mucosal contact based on the shape of the gingival side of the pontic.

 

A) Mucosal contact:

  • Ridge lap
  • Modified ridge lap
  • Ovate
  • Conical

 

B) No mucosal contact:

  • Sanitary (hygenic)
  • Modified sanitary (hygenic)

 

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PONTIC DESIGNS

 

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Sanitary (hygienic) Pontic

 

 

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Modified Sanitary (hygienic) Pontic
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Saddle Pontic
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Ridge-lap Pontic
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Tissue surface is inaccessible to cleaning devices

 

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Bridge with a ridge-lap (concave) tissue surface

 

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Conical Pontic

 

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Modified Ridge-lap Pontic

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Ovate Pontic

 

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Patient must be instructed in how to clean the gingival surface of the pontic with floss

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Sanitary or Hygenic Pontic

The design of the sanitary pontic allows easy cleaning, because its tissue surface remains clear of the residual ridge.

This hygienic design permits easier plaque control by allowing cleaning devices to be passed under the pontic.

Its disadvantages include entrapment of food particles, which may lead to tongue habits that may annoy the patient.

The hygienic pontic is the least “toothlike” design and is therefore reserved for teeth seldom displayed during function (i.e., the mandibular molars).

A modified version of the sanitary pontic has been developed . Its gingival portion is shaped like an archway between the retainers. This geometry permits increased connector size.

 

Saddle or Ridge Lap Pontic

The saddle pontic has a concave fitting surface that overlaps the residual ridge buccolingually.

Saddle or ridge lap designs should be avoided because the concave gingival surface of the pontic is not accessible to cleaning with dental floss, which will lead to plaque accumulation.

 

Modified Ridge-lap Pontic

The modified ridge lap pontic combines the best features of the hygienic and saddle pontic designs, combining esthetics with easy cleaning.

The modified ridge lap design overlaps the residual ridge on the facial (to achieve the appearance of a tooth emerging from the gingiva) but remains clear of the ridge on the lingual.

It should be as convex as possible from mesial to distal (the greater the convexity, the easier the oral hygiene).

Tissue contact should resemble a letter T whose vertical arm ends at the crest of the ridge.

The modified ridge lap design is the most common pontic form used in areas of the mouth that are visible during function.

 

Conical Pontic

Often called egg-shaped, bullet-shaped, or heart-shaped.

The conical pontic is easy for the patient to keep clean. It should be made as convex as possible, with only one point of contact at the center of the residual ridge.

This design is recommended for the replacement of mandibular posterior teeth where esthetics is a lesser concern.

 

Ovate Pontic

The ovate pontic is the most esthetically appealing pontic design. Its convex tissue surface resides in a soft tissue depression or hollow in the residual ridge.

Socket-preservation techniques should be performed at the time of extraction to create the tissue recess from which the ovate pontic form will emerge. When an adequate

volume of ridge tissue is established, a socket depression is sculpted into the ridge with surgical diamonds or electrosurgery.

 

Advantages include its pleasing appearance and its strength. its emergence from the ridge appears identical to that of a natural tooth. In addition, its recessed form is not susceptible to food impaction.

Disadvantages: Meticulous oral hygiene is necessary to prevent tissue inflammation resulting from the large area of tissue contact. Other disadvantages include the need for surgical tissue management and the associated cost.

 


 

The occlusal surface

The occlusal surface of the pontic should resemble the occlusal surface of the tooth it replaces. Otherwise it will not serve the same occlusal functions.

 

The approximal surfaces

It is important that the embrasure space between the connector and the gingival tissue be as open as possible to ensure that there is good access for cleaning. particularly if the pontic is a ridge-lap or saddle pontic.

 

The buccal and lingual surfaces

The buccal and lingual surfaces of a pontic will be designed as a result of deciding the ridge surface.

 


 

Connectors

Fixed connectors

There are three types of fixed connector:

  • Cast
  • Soldered
  • Porcelain

 

A) Cast connectors

They are made by wax patterns of the retainers and pontics connected by wax being produced so that the bridge is cast in a single piece.

This has the advantage that a second soldering operation is not required. Cast connectors are stronger than soldered connectors.

 

B) Soldered connectors

They are used if the pontics and retainers have to be made separately .

This is necessary when they are made of different materials. for example a complete gold crown retainer with a metal-ceramic pontic.

 

C) Porcelain connectors

They are used only in conjunction with all-porcelain bridges.

 

Movable connectors

Movable connectors are always designed so that the pontic cannot be depressed by occlusal forces.

This means that the groove or depression in the minor retainer (the female part of the attachment) always have a good base against which the male part of the attachment can seat.

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– end –

Fixed Prosthodontics – Dental Bridges

woman wearing black eyeglasses
Photo by Daniel Xavier on Pexels.com

WHAT ARE BRIDGES?

The fixed partial denture is a prosthetic appliance, permanently attached to remaining teeth, which replaces one or more missing teeth. This type of restoration has long been called a “Bridge”.

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Some other important terms…

The abutment is any tooth, root or implant which gives attachment and support to the fixed partial denture.

The retainers, are extra-coronal restorations that are cemented to the prepared abutment teeth.

A pontic, is the artificial tooth replacing the missing tooth in the fixed prosthesis. Pontics are attached to the retainers.

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The connectors are the portions of the bridge uniting the individual parts of the bridge (pontic and retainer). They may be rigid (solder joints or cast connectors) or non-rigid (precision attachments or stress breakers).

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A span is the space between natural teeth  that is to be filled by the bridge.

A unit, when applied to bridgework, means either a retainer or a pontic. A bridge with two retainers and one pontic would therefore be a three-unit bridge.

 


 

Conventional and minimal preparation bridges

Conventional bridges involve removing tooth tissue, or a previous restoration, and replacing it with a retainer.

The alternative, minimal-preparation bridge involves attaching pontics via a metal plate to the unprepared (or minimally prepared) lingual surfaces of adjacent teeth. The attachment is made by a composite resin material, retained by the acid-etch technique to the enamel. They are called also known as resin-bonded bridges.

 

Ante’s Law

The combined pericemental area (root surface area) of all abutment teeth supporting a fixed dental prosthesis should be equal to or greater in pericemental area than the tooth or teeth to be replaced.

Ante IH. The fundamental principles, design and construction of crown and bridge  prosthesis. Dent Item Int 1928;50:215-32.

 


 

The Four Basic Bridge Designs

  • Fixed-fixed bridge.
  • Fixed-movable bridge.
  • Cantilever bridge.
  • Spring cantilever bridge.

 

A) Fixed-fixed Bridge

A fixed-fixed bridge has a rigid connector at both ends of the pontic. The abutment teeth are therefore rigidly splinted together, and for a conventional bridge must be prepared parallel to each other so that the bridge, which is a minimum of three units, can be cemented in one piece.

The retainers should have approximately the same retention as each other to reduce the risk that forces applied to the bridge will dislodge one retainer from its abutment, leaving the bridge suspended from the other abutment.

To minimize this risk, it is also important for the entire occluding surface of all the abutment teeth for a conventional bridge to be covered by the retainers.

The opposing teeth cannot then contact the surface of an abutment tooth, depress it in its socket and break the cement lute.

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Advantages

  1. Robust design with maximum retention and strength.
  1. The construction is relatively straight-forward in the laboratory because there are no movable joints to make.
  2. Can be used for long spans.

 

Disadvantages

  1. Requires preparations to be parallel.
  2. Has to be cemented in one piece, so cementation is difficult.

 

 

B) Fixed-movable Bridge

 

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A fixed-movable bridge has a rigid connector, usually at the distal end of the pontic, and a movable connector that allows some vertical movement of the mesial abutment tooth .

 

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Advantages

  1. Preparations do not need to be parallel to each other.
  2. More conservative of tooth tissue because preparations for minor retainers are less destructive than preparations for major retainers.
  3. Allows minor movements of teeth.
  4. Parts can be cemented separately, so cementation is easy.

 

Disadvantages

  1. Length of span limited.
  2. More complicated to construct in the laboratory than fixed-fixed.
  3. Difficult to make temporary bridges.

 

 

C) Cantilever Bridge

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A cantilever bridge is a fixed partial denture that attaches to adjacent teeth on one side of the bridge only.

 

Advantages

  1. The most conservative design when only one abutment tooth is needed.
  2. Construction in the laboratory is relatively straight-forward.
  3. Most suitable in replacing anterior teeth, where there is little risk of the abutment tooth tilting.

 

Disadvantages

1.The length of span is limited to one pontic.

  1. The construction of the bridge must be rigid to avoid distortion.
  2. Occlusal forces on the pontic of small posterior bridges encourage tilting of the abutment tooth.

 

 

D) Simple Cantilever Bridge

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A cantilever bridge provides support for the pontic at one end only. The pontic may be attached to a single retainer or to two or more retainer(s) splinted together. but has no connection at the other end of the pontic.

 

 

E) Spring Cantilever Bridge

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Spring cantilever bridges are restricted to the replacement of upper incisor teeth. Only one pontic can be supported by a spring cantilever bridge.

This is attached to the end of a long metal arm running high into the palate and then sweeping down to a rigid connector on the palatal side of a single retainer or a pair of splinted retainers.

The arm is made long and fairly thin so that it is springy. but not so thin that it will deform permanently with normal occlusal forces.

Forces applied to the pontic are absorbed by the springiness of the arm and by displacement of the soft tissues of the palate.

The abutments are usually the two premolar teeth splinted together, or a single premolar or molar tooth.

 

Combination designs

The four basic designs can be combined in a variety of ways. In particular, the fixed- fixed and cantilever designs are often combined. Similarly, it is possible to combine fixed-fixed and fixed-movable designs.

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Hybrid design

This term refers to a bridge with a combination of conventional and minimal-preparation retainers.

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Minimal preparation bridges or Resin‑bonded bridges consist of a metal framework with a pontic tooth with wing-like extensions coming from the proximal sides.

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These metal wings are prepared to have a porous surface so that they can receive a bonding agent, and then the wings are bonded to the back sides of the teeth on either side of the missing tooth.

 

Examples of minimal-preparation bridge:

Macro-mechanically retentive bridges (Rochette). have large undercut perforations through the cast-metal plate. through which the composite resin cement flows.

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Medium-mechanical retentive systems. all involve retentive features cast as part of the metal framework.

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Micro -mechanical retention (Ex, Maryland Bridge) is produced by casting the metal retainer and then etching the fit surface by one of two methods: electrolytic etching in acid in  the laboratory or chemical etching with a hydrofluoric acid gel either in the laboratory or at the chairside.

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Reasons for Bridge Failure

  • Loss of retention
  • Mechanical failure
  • Abutment failure
  • Design failure

 

A) Loss of Retention

  • One of the more common ways in which they fail is by one of the retainers becoming loose from the abutment tooth.
  • Caries develops rapidly across the whole of the dentin surface of the preparation when there is a gap between the bridge retainer and unprepared tooth surface.
  • It happens if the preparation is not adequately retentive. However, it may be possible to provide additional retention.
  • Alternatively it may be necessary to include additional abutment teeth in a bridge to increase the overall retention or to change the design in some other way.

 

B) Mechanical failure of crowns or bridge components

Typical mechanical failures are:

  • Porcelain fracture
  • Failure of solder joints
  • Distortion
  • Occlusal wear and perforation
  • Lost facings

 

Porcelain fracture

Pieces of porcelain fracturing off metal-ceramic crowns, or the loss of the entire facing due to failure of the metal-ceramic bond.

 

Failure of Solder Joint

Occasionally a solder joint that appears to be sound fails under occlusal loading. This may be due to:

  • A flaw or inclusion in the solder itself.
  • The solder joint not being sufficiently large for the conditions in which it is placed.

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Distortion

In metal-ceramic bridges distortion of the framework can occur during function or as a result of trauma.

 

Occlusal wear and perforation

Perforation can be the result of normal wear.

 

Lost facings

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It can be re-placed and are reasonably satisfactory and less costly alternative to replacing the whole restoration.

 

C) Changes in the Abutment Tooth

  • Periodontal disease
  • Problems with the  pulp
  • Caries

 

D) Design Failures – inadequate bridge design

FP_090A) Under-prescribed bridges

These include designs that are unstable or have too few abutment teeth.

Another ‘under-design’ fault is to be too conservative in selecting retainers.

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B) Over-prescribed bridges

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Where it includes more abutment teeth than are necessary.

 

 

 

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Example of a poor pontic design

 

 

– end –

Fixed Prosthodontics – Pontic Design

 

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Pontic Fabrication

Pontics can be made with the metal-ceramic technique, which provides the best solution to the biologic, mechanical, and esthetic challenges encountered in pontic design. Their fabrication, however, differs slightly from the fabrication of individual crowns.

A well-designed metal-ceramic pontic provides easy plaque removal, strength, wear resistance, and esthetics.

 

Anatomic Contour Waxing

For strength and esthetics, an accurately controlled thickness of porcelain is needed in the finished restoration.

To ensure this, a wax pattern is made nearly to the final anatomic contour and then cut it back.

This also permits an assessment of connector design adequacy.

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Armamentarium

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  • Bunsen burner
  • Inlay wax
  • Sticky wax
  • Waxing instruments
  • Cotton cleaning cloth
  • Die-wax separating liquid
  • Zinc stearate or powdered wax
  • Double-ended brushes
  • Cotton balls
  • Fine-mesh nylon hose

 

Step-by-step Procedure

  1. Wax the internal, proximal, and axial surfaces of the retainers.
  2. Soften the inlay wax, mold it to the approximate desired pontic shape, and adapt it to the ridge.

(Prefabricated pontic shapes are also available as a starting point.)

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Prefabricated pontic shapes
  1. Lute the pontic to the retainers and, for additional stability, connect its cervical aspect directly to the master cast with sticky wax. Then wax the pontic to proper axial and occlusal (or incisal) contour.
  2. Complete the retainers and contour the proximal and tissue surfaces of the pontic for the desired tissue contact. The pontic is now ready for evaluation before cut-back.

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Evaluation
The form of the wax pattern is evaluated and any deficiencies are corrected.

Particular attention is given to the connectors, which should have the correct shape and size.

The connectors provide firm attachment for the pontic so it does not separate from the retainers during the subsequent cut-back procedure.

 

CUT-BACK PROCEDURE

  1. Use a sharp explorer to outline the area that will be veneered with porcelain. The porcelain-metal junction must be placed sufficiently lingual to ensure good esthetics.
  2. Complete the cut-back as far as access will allow with the units connected and on the mastercast.
  1. Section one wax connector with a thin ribbon saw (sewing thread is a suitable alternative) and remove the isolated retainer from the master cast.
  2. Refine the pontic cut-back where access is improved by removal of the first retainer.
  3. Reseat the first retainer, reattach it to the pontic, section the other connector, and repeat the process.
  4. Sprue the units and do any final reshaping as needed.
  5. Invest and cast

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The metal coping on the gingival aspect of the pontic follows the same contours that the porcelain will, rather than being just a straight bar of a metal between the retainer copings.
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Evaluation of the metal substructure

The porcelain must cover the labial surface, the incisal portion of the lingual surface, and the entire area adjacent to or contacting the ridge.

About 1 mm of porcelain thickness is needed on the gingival surface.

Porcelain tissue contact allows for better esthetics and removes the potentially rough porcelain –metal junction from contact with the tissue where it could cause irritation.

The tissue contact of the porcelain should be a modified ridge-lap.

There must be no saddle contact.

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Porcelain Application

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Apply a porcelain separating liquid to the stone ridge so that the additional gingival porcelain can be lifted directly from the cast.

Mark the desired tissue contact and contour the gingival surface to provide as convex a surface as possible.

The porcelain on the tissue surface of the pontic should be as smooth as possible. Pits and defects will make plaque control difficult and promote calculus formation.

The metal framework must be highly polished, with special care directed to the gingival embrasures (where access for plaque removal is more difficult).

 

Connector Design

The size, shape, and position of connectors all influence the success of the prosthesis.

Connectors must be sufficiently large to prevent distortion or fracture during function but not too large; otherwise, they will interfere with effective plaque control and contribute to periodontal breakdown over time.

Adequate access (i.e., embrasure space) must be available for oral hygiene aids cervical to the connector.

If a connector is too large inciso-cervically, hygiene is impeded, and over time periodontal failure will occur.

For ease of plaque control, the connectors should occupy the normal anatomic interproximal contact areas because encroaching on the buccal, gingival, or lingual embrasure restricts access.

In addition to being highly polished, the tissue surface of connectors is curved facio-lingually to facilitate cleansing. Mesio-distally, it is shaped to create a smooth transition from one FPD component to the next.

In a facio-lingual cross section, most connectors have a somewhat elliptical shape.

Do not close embrasure spaces. Tissue surface is Curved.

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However, to improve appearance without significantly affecting plaque control, anterior connectors are normally placed toward the lingual.

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Most recommend vertical height for the connectors is 3 to 4 mm.

 

Rigid Connectors

Rigid connectors must be shaped and incorporated into the wax pattern after the individual retainers and pontics have been completed to final contour.

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– end –

Fixed Prosthodontics – Tooth Preparation for Crown Restorations

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The crown restorations can be used to rebuild a single tooth or as a retainer for a fixed prosthesis.

 

Principles of tooth preparation

A good preparation ensures that subsequent techniques (e.g., interim restoration, impression making, pouring of dies and casts, waxing) can be accomplished.

The design and preparation of a tooth for a cast metal or porcelain restoration are governed by five principles:

A) Preservation of tooth structure

B) Retention and resistance form

C) Structural durability of the restoration

D) Marginal integrity

E) Preservation of the periodontium

 

Successful tooth preparation and subsequent restoration depend on simultaneous consideration of all these factors. Improvement in one area often adversely affects and may lead to failure in another area.

Communication between the clinician and dental laboratory regarding any deviation from “ideal” criteria is essential and can prevent misunderstanding, frustration, and ultimate failure.

 

A) Preservation of tooth structure

Excessive removal of tooth structure can have many ill effects. If a tooth is over-tapered or shortened too much, there will be an unnecessary sacrifice of retention and resistance.

Preservation of tooth structure

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Tooth structure removed following minimal recommended dimensions.

  • Anatomically prepared occlusal surface results in adequate clearance without excessive tooth reduction.
  • A flat occlusal preparation will result in either insufficient clearance (1) or an excessive amount of reduction (2).

Anatomic occlusal reduction is conservative of tooth structure and gives rigidity to the restoration.

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B) Retention and Resistance

The feature of a tooth preparation that resists dislodgement of a crown in a vertical direction or along the path of placement is known as retention.

The features of a tooth preparation that enhance the stability of a restoration and resist dislodgement along an axis other than the path of placement is known as resistance.

Retention and resistance form

  • Taper of about 6º between opposing walls (No undercuts or over reduction).
  • Surface area of the occlusal surface.
  • Length of the axial walls.

 

Factors influencing the retention of a cemented restoration:

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Factors influencing the resistance of a cemented restoration:

Higher resistance                                                                                                                              Lower resistance

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Taper

Theoretically, maximum retention is obtained if a tooth preparation has parallel walls. However, it is impossible to prepare a tooth this way; slight undercuts are created that prevent the restoration from seating.

“An undercut is defined as a divergence between opposing axial walls in a cervical-occlusal direction”.

A slight convergence, or taper, is necessary in the completed preparation. Too large will no longer be retentive.

Degree of convergence (taper) is recommended to be 6-degree.

 

C) Structural Durability

The casting must be rigid enough not to flex and break.

Sufficient tooth structure must be removed to create space for an adequate bulk of restorative material to accomplish this.

 

Structural durability

  • Adequate occlusal reduction to allow bulk of metal.
  • Functional cusp bevel to allow for adequate thickness of metal.
  • Sufficient axial reduction.
  • Rounded angles.

 

Occlusal reduction

Enough tooth reduction must be removed from the occlusal surface of the preparation, so that metal will be thick enough to prevent wearing or distorting.

A flat occlusal surface is undesirable, because metal in the area of the grooves will be too thin, with a risk of perforation.

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Functional cusp bevel

A wide bevel should be placed on the functional cusps of posterior teeth to provide structural durability on this critical area.

Failure to place functional cusp bevel can result in thin, weak areas in the restoration.

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Proper articulation of opposing casts is the responsibility of the dentist. This is particularly critical as the complexity of treatment increases.

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D) Marginal Integrity

Finishing line: is the junction between a cemented restoration and the tooth.

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The more accurately the restoration is adapted to the tooth, the lesser is the chance of failure.

Margins should be easily discernible and accessible on the casts submitted to the technician. The saying “If you can’t see it, you can’t wax it” describes the situation well.

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Marginal integrity

  • Finishing line is clear, smooth and continuous.

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Feather-edge and chisel finishing lines: are more conservative to tooth structure, but they are not recommended because they do not provide sufficient bulk and the location of the margin is difficult to locate.

Chamfer finishing line: has distinct margin, adequate bulk. It is used in full metal crowns, lingual margin (if unveneered) of ceramo-metal crowns.

Shoulder finishing line: provides bulk of restorative material. It is used in facial margin (veneered) of ceramo-metal crowns, and all-ceramic crowns.

 

The features of a tooth preparation a and the function served by each

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Problems with Teeth Preparations

 

Problem 1: Under-reduced occlusal surface -> crown will be too thin

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WHAT TO DO? : Further occlusal reduction

 

Problem 2: undercut within one surface

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WHAT TO DO? : Further reduction of wall

 

Problem 3: Opposing walls diverge (Undercut)

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WHAT TO DO? : Taper walls more

 

Problem 4:  Finish line too light; walls are under-reduced

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WHAT TO DO? :increase reduction

 

Problem 5:  Finish line not continuous -> Inadequate reduction where proximal and buccal/lingual surfaces meet

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WHAT TO DO? : Finish line is placed

 


 

Other Troubleshootings on Tooth Preparation

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The recommended convergence angle is 6 degrees. This is a very slight taper. The angle between the hands of a clock showing 12:01 is 5 ½ degrees.

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Example of how a clinician checks the tooth with a mirror.

 


 

Complete Cast Crowns

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The full-metal complete cast crown should always be offered to patients requiring restoration for badly damaged posterior teeth, although esthetic factors may limit its application.

The complete cast crown can be used to rebuild a single tooth or as a retainer for a fixed prosthesis.

It has the best longevity of all fixed restorations.

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The features of a preparation for a complete cast crown and the function served by each

 

Criteria for tooth reduction

The occlusal reduction must allow adequate room for the restorative material from which the cast crown is to be fabricated.

Minimum recommended clearance is 1 mm on nonfunctional cusps and 1.5 mm on functional cusps.

The Functional Cusps: The Lingual Upper and The Buccal Lower

Non-Functional Cusps: The Buccal Upper and The Lingual Lower (BULL)

 

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Recommended dimensions for a complete cast crown.

 

The occlusal reduction should follow normal anatomic contours to remain as conservative of tooth structure as possible. Anatomic occlusal reduction is conservative of tooth structure and gives rigidity to the restoration.

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Proper placement of the functional cusp bevel achieves optimum restoration contour with maximum durability and conservation of tooth structure.

The functional cusp bevel is prepared by slanting the bur at a flatter angle than the cuspal angulation.
On most teeth, the functional cusp bevel is placed at about 45 degrees to the long axis.

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Axial reduction should be parallel to the long axis of the tooth but allow for the recommended 6-degree taper or convergence, which is the angle measured between opposing axial surfaces.

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The amount of axial reduction recommended is about 1 mm while following the contours of the tooth (occlusal 2/3), and about 0 .5 mm (gingival 1/3) to produce a chamfer finish line.

The margin should have a chamfer configuration and is ideally located supragingivally.

The chamfer should be smooth and distinct and allow for approximately 0.5 mm of metal thickness at the margin.

Retention form of an excessively tapered preparation can be increased by adding grooves, because these will limit the paths of withdrawal.

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Metal-ceramic Crowns (PFM)

The metal-ceramic restoration, also called a porcelain-fused-to-metal (PFM) restoration, such a restoration combines the strength and accurate fit of a cast metal crown with the cosmetic effect of a ceramic crown.

With a metal substructure, metal-ceramic restorations have greater strength than restorations made of the ceramic alone.

To be successful, a metal-ceramic crown preparation requires more tooth reduction wherever the metal substructure is to be veneered with dental porcelain.

The metal should be 0.3 to 0.5 mm thick if it is a noble metal alloy, while a metal coping made of the more rigid base metal alloys can be thinner to 0.2 mm.

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The metal coping in a metal-ceramic restoration is covered with two or three layers of porcelain:
1. Opaque porcelain conceals the metal underneath, initiates the development of the shade, and plays an important role in the development of the bond between the ceramic and the metal.
2. Body porcelain, or dentin, makes up the bulk of the restoration, providing most of the color or shade.
3. Incisal porcelain, or enamel, imparts translucency to the restoration.

 

Without the space for a sufficient thickness of ceramic material, two things can happen:

(1) The restoration will poorly contours, adversely affecting both the cosmetic effect of the crown and the health of the surrounding gingiva, and

(2) The shade and translucency of the restoration will not match adjacent natural teeth.

 

Recommended minimum dimensions for a metal-ceramic restoration:

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Anterior metal-ceramic crowns preparation

FP_061The labial reduction is carried out in two planes: the gingival portion to parallel the long axis of the tooth, the incisal portion to follow the normal facial contour.

Reduction in one plane parallel with the cervical plane may result in insufficient space of porcelain in the incisal half and an over-contoured restoration.

One-plane reduction may come dangerously close to the pulp.

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Reduce the lingual concavity of the lingual surface with wheel-shaped or football-shaped diamond to provide adequate clearance for the restorative material.

Typically, 1 mm is required if the centric contacts in the completed restoration are to be located on metal. When contact is on porcelain, additional reduction will be necessary.

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Axial Reduction of the Proximal Surfaces

Reduce the proximal surfaces with the diamond held parallel to the intended path of withdrawal of the restoration.

These walls should converge slightly from cervical to incisal/occlusal.

A taper of approximately 6 degrees is recommended.

A long-needle diamond is used to remove the contact area.

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The finishing line must be smooth and continuous with other surfaces.

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Rounding of any sharp angles on the incisal edges and all around the prepared tooth.

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All-ceramic Crowns

All-ceramic crowns are some of the most esthetically pleasing restorations.

Because there is no metal to block light transmission, they can resemble natural tooth structure better in terms of color and translucency than any other restorative option.

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The preparation sequence for a ceramic crown is similar to that for a metal-ceramic crown; the principal difference is the need for a 1-mm-wide finishing line  circumferentially.

To prevent stress concentrations in the ceramic, all internal line angles should be rounded. The shoulder should be as smooth as possible to facilitate the technical aspects of fabrication.

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Incisal (Occlusal) Reduction

The completed reduction of the incisal edge should provide 1.5 to 2 mm of clearance.

This will permit fabrication of a cosmetically pleasing restoration with adequate strength.

  • If the restoration is used for posterior teeth (rare), 1.5 to 2 mm of clearance is needed on all cusps.

 


 

Partial Veneers Crowns

An extra-coronal or intra-coronal metal restoration that covers only part of the clinical crown with preservation of one or more tooth surface.

Can be used as a single tooth restoration or as a retainer for FPD on both anterior and posterior teeth.

 

Types of partial veneers

For Posterior teeth :

  • Three quarter crown.
  • Seven eighths crown.
  • MOD onlay.

 

For Anterior teeth :

  • Anterior three quarter crown.
  • Pin ledge crown.

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Proximal Grooves

  • Function: Retention, Resistance, and structural durability.
  • Direction: Both grooves should be parallel to each other and parallel to path of withdrawal.
  • Length: should extend to the full length of the proximal surface.

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Occlusal Offset

  • It is formed on the lingual incline of the buccal cusp to join the two proximal grooves (0.5 mm deep).

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Post-and-core Restorations

Definition:

A post and core is a dental restoration for an endodontically treated tooth used to sufficiently build-up tooth structure for future restoration with a crown when there is no enough tooth structure to properly retain the crown.

Post and cores are therefore referred to as foundation restorations.

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Cast post-and-core:

A one-piece foundation restoration for an endodontically treated tooth that comprises a post within the root canal and a core replacing missing coronal structure to form the tooth preparation.

 

One-piece post crowns

Where the post, core and final crown are constructed as one piece and are firmly attached to each other.

The crown could be all-metal or a metal with aesthetic facing.

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Two-piece Restorations

Where the post and the core are constructed and cemented as one piece, then the crown is constructed and cemented as the second piece.

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Superior results can now be obtained with a two step technique consisting of a post-and core foundation and a separate crown (instead of one-piece post-crowns).

With the two-step approach of fabricating a separate crown over a cast post-and-core, achieving a satisfactory marginal fit is easier because the expansion rate of the two castings can be controlled individually.

The two-step approach further permits replacement of the crown, if necessary, without the need for post removal.

Be sure there is a positive stop for the post/core so that the casting does not act as a wedge (which may split the tooth).

This may be a flat area (90 degrees relative to the path of draw) or a slight contrabevel around the perimeter of the preparation.

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—Rotation of the post must be prevented by preparing a flat surface parallel to the post.

— If insufficient tooth structure for this feature remains, an antirotation groove should be placed in the canal.

Rotational resistance can be obtained by preparing a small groove in the root canal. This must be in the path of placement of the post-and-core.

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– end –

Fixed Prosthodontics – The Introduction

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WHAT IS FIXED PROSTHODONTICS?

Fixed prosthodontics is the branch of specialized area of dentistry, involved in the replacement of missing teeth with a cast prosthesis permanently cemented in place.

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The fixed prosthesis can be in the form of single cast crown or multiple unit of cast crowns joined together, commonly referred to as fixed partial denture or bridge.

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Objectives of fixed prosthesis

  • To restore masticatory function
  • To improve appearance
  • To Improve speech
  • To promote good oral hygiene
  • To stabilize arch form and occlusion

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Indications for fixed partial denture

  • One or two adjacent teeth are missing in the same arch (short span edentulous area)
  • When the supportive tissues are healthy.
  • Suitable abutment teeth are present.
  • The patient is in good health and desires to have the prosthesis placed.
  • The patient has the skills and motivation to maintain good oral hygiene
  • Patients preference
  • Good oral hygiene

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Contraindications for fixed partial denture

  • Lack of supporting tissue and alveolar bone
  • Presence of periodontal disease
  • Excessive mobility of abutment teeth
  • Patients with poor oral hygiene
  • Patients who cannot afford treatment

Types of fixed Prostheses

1. Inlays

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2. Onlays

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3. Cast crowns:

a) Full coverage crown

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b) Partial coverage crown

A restoration that restores all but one coronal surface of the crown is usually not restoring or covering the facial surface.

Types:

i) ¾ crowns

ii) Seven eight crown

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Partial Coverage Crowns

4. Fixed partial denture or Bridges:

It is a type of cast restoration which replaces or restores one or more teeth that are cemented on the fixed or cemented onto the adjacent prepared tooth/ teeth.

FPD’s are usually referred in units i.e. 3-unit fpd which replaces one missing tooth and so on.

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5. Veneers:

It is a layer of material placed over a tooth, either to improve the aesthetics of a tooth or to protect the tooth’s surface from damage.

a. Direct veneers are prepared directly on the patient’s tooth/teeth in the clinic. E.g.: direct composite veneers

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b. Indirect veneers: are prepared in the laboratory and is cemented into the tooth. E.g.: porcelain laminate veneers

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Crowns and Bridges

1. Classification of Crowns

Based on number of covered surfaces:

A) Full coverage (5 surfaces)

B) Partial coverage (Less than 5 surfaces)

A) Full Coverage Crowns

According to: A. Material B. Retention

B) Partial Coverage Crowns

According to: Retention

2. Classification of bridges

  1. Fixed – Fixed
  2. Fixed – Supported
  3. Fixed – Free (Cantilever)
  4. Spring Cantilever
  5. Removable
  6. Adhesive

1. Fixed-Fixed Bridge

It is a bridge in which the pontic is joined at both ends to the retainers by rigid connectors.

FP_013

2. Fixed-Supported Bridge

It is a bridge in which the pontic is joined at one end to the retainer by a rigid connector, and the other end by a non rigid connector.

3. Fixed-Free Bridge (Cantilever)

It is a bridge in which the pontic is joined to the retainer at one end only, and the other end is free or unsupported.

FP_014

4. Spring Cantilever Bridge

It is a bridge in which the pontic takes its support from a remote abutment by a resilient curved arm (palatal spring).

FP_015

5. Removable Bridge

It is a bridge in which the connector is made up of two parts : one fixed to the retainer, and the other soldered to the pontic. The bridge can be removed by the patient for cleaning purposes.

FP_016

6. Adhesive Bridge

It is a conservative bridge with minimum tooth preparation and retained on teeth by using a adhesive resin cement. Eg : Resin bonded bridges

FP_017


Common Terminologies in Fixed Prosthodontics

  1. Tooth preparation: Is a clinical process where the natural tooth/teeth is prepared to receive a crown.FP_020
  2. Master cast and Die: Is the positive reproduction of the prepared tooth and consists of a suitable hard substance of sufficient accuracy (usually an improved stone, resin, or metal).FP_021
  3. Die pin or die system: Is a method of preparing the die on the master cast.FP_022
  4. Die spacer: A paint-on material that dries to a known thickness, used to provide space between a crown and the underlying tooth preparation.FP_021
  5. Wax pattern: A pattern of wax that, when invested and burned out or otherwise eliminated, will produce a mold in which a casting may be made.FP_024
  6. Dental Casting: Is the process by which a wax pattern of a restoration is converted to a replicate in dental alloy.
  7. Metal try-inFP_025
  8. Porcelain build-up or applicationFP_026

Materials used in fixed prosthesis

Dental casting alloys: are combination of two metal produced in specific proportion for fabrication of dental casting.

I. Classification by function:

a. Type I (soft): inlays

b. Type II (medium): inlays, onlays, crowns

c. Type III (hard): inlays, onlays, short span bridges

d. Type IV (extra hard): partial denture frameworks

Type III and type IV are generally termed as ‘crown and bridge alloys’.

II. Crown and bridge alloys:

This category of alloys include both noble and base metal alloys that are used in fabrication of full metal crowns and partial veneer crowns.

A) Noble metal alloys

  1. Gold based alloys: type III and type IV gold alloys
  2. Non-gold based alloys: silver-palladium alloys

B) Base metal alloys

  1. Nickle based alloys
  2. Cobalt based alloys

III. Metal-ceramic alloys:

A) Noble metal alloys for porcelain bonding

  1. Gold-platinum-palladium alloy
  2. Gold-palladium-silver alloy
  3. Gold-palladium alloy
  4. Palladium-silver alloy
  5. High-palladium alloy

B) Base metal alloys for porcelain bonding

  1. Nickle-chromium alloy
  2. Cobalt-chromium alloy

C) Removable partial denture alloys

  1. Nickle-chromium alloy
  2. Cobalt-chromium alloy
  3. Cobalt-chromium-nickle alloy

Dental Investment material

They are the mold materials used in the casting of dental gold alloys with liquidus temperatures no more than 1080 ºc.

FP_027

Classification:

A. Gypsum bonded investment

B. Phosphate bonded investment

C. Silica bonded investment

FP_028

  • A tooth serving as an attachment for a fixed partial denture is called an abutment.
  • The artificial tooth suspended from the abutment teeth is a pontic.
  • The pontic is connected to the fixed partial denture retainers, which are extracoronal restorations that are cemented to the prepared abutment teeth.
  • The connectors between the pontic and the retainer may be rigid (ie, solder joints or cast connectors) or non-rigid (ie, precision attachments or stress breakers).

Steps in the Fabrication of a Fixed Partial Denture

FP_029

– end –

Orthodontics – Retention

ORT_203b

RETENTION  –  Period after active treatment when passive fixed / removable appliances were worn to stabilize the occlusion that had been created

—> appliance —>

RETAINER  –  Passive orthodontic appliances that help in maintaining and stabilizing the position of a single tooth or group of teeth to permit reorganizing of the supporting structures

 

WHY RETENTION IS NECESSARY?

  1. The gingival and periodontal tissues are affected by orthodontic tooth movement and require time for reorganization when the appliances are removed.
  2. The teeth be in an inherently unstable position after the treatment, so that soft tissue pressure constantly produce a relapse tendency.
  3. Changes produced by growth may alter the orthodontic treatment results.

 

Causes of Relapse

  • Failure to remove cause of malocclusion
  • Lack of normal cuspal interdigitation
  • Incorrect axial inclination
  • Tooth size disharmony and improper contacts
  • Arch expansion
  • Failure to manage rotations

 


Retention Planning

Time of usage Cases
NO
RETENTION
REQUIRED
a. Cross bite
b. Dentition treated with serial extraction
LIMITED
RETENTION
6 months =
whole days
and
nights
a. Class I non-extraction cases with spacing and protrusion of maxillary incisor
b. Class I and II extraction cases
c. Early correction of rotated teeth to their normal position before root completion
d. Cases involving ectopic eruption or the presence supernumerary teeth
e. Corrected deep bites
PERMANENT
RETENTION
a. After arch expansion especially in mandibular arch
b. Cases of considerable generalized spacing
c. Severe rotation or severe labiolingual malposition
d. Spacing between maxillary central incisor with an otherwise normal occlusion

 


Classification of Retainers

1. Fixed retainers -> cemented or bonded to the teeth

ORT_204
0.0195 twisted wire

ORT_205

 

 

2. Removable retainers -> can be removed and reinserted by the patient

 

Removable Retainers

Hawley’s retainer

ORT_181

  • With short labial bow
  • With long labial bow
  • With contoured labial bow
  • Continuous labial bow soldered to clasps

ORT_206

 

Begg’s retainer (wrap-around retainer)

ORT_207

 

Kesling’s tooth positioner

 

Essix (vacuum-formed retainers)

ORT_208

 

– end –

 

References:
Gill, D.S. 2008. Orthodontics at a Glance. Blackwell Munksgaard.
Proffit, W.R; Field, H.W; Sarver, D.M. 2007. Contemporary of Orthodontics. 4th edition. St. Louis: Mosby Elvevier.
Singh, G. 2008. Textbook of Orthodontics. 2nd edition. New Delhi: Unipress.

 

 

 

Orthodontics – Anchorage

selective focus photography of anchor pendant
Photo by ABD NIMIT on Pexels.com

WHAT IS ANCHORAGE?

Anchorage is the resistance to the unwanted forces generated in reaction to the active component of the appliance.

 

Sources of Anchorage:

A) Intraoral sources

B) Extraoral sources

 


 

Intraoral Sources

  1. The alveolar bone
  2. The teeth
  3. The basal bone
  4. The cortical bone
  5. The musculature

 

1. The alveolar bone – Less dense alveolar bone offers less anchorage

ORT_188

 

2. The teeth –

  • Root form

ORT_189

The distribution of the periodontal fibers on the root surface aid in anchorage

 

ORT_190

Tripod roots aids in increasing anchorage

 

  • Size of the Root

ORT_191The larger or longer the roots the more is their anchorage potential

 

  • Numbers of the Roots

The greater the surface area the greater the periodontal support and hence, greater the anchorage potential.

Multirooted root provide greater anchorage

 

  • Position of the tooth

The position of the teeth in the individual arches helps in increasing the anchorage potential.

 

  • Axial Inclination of the Tooth

ORT_192

When the tooth is inclined in the opposite direction to that of the force applied, it provides the greater anchorage.

 

  • Root Formation

Teeth with incomplete root formation are easier to move and are able to provide lesser anchorage.

 

  • Intercuspation

ORT_193

Good intercuspation leads to greater anchorage potential.

 

 

3. Basal bone

ORT_194
Nance Palatal Button

Hard palate lingual surface -> Can be used to augment the anchorage

 


 

Extraoral sources

ORT_195
Headgear

The anchorage unit situated outside the oral cavity.

 


 

Classification of Anchorage

 

1) According to the manner of force application

  • Simple anchorage
  • Stationary anchorage
  • Reciprocal anchorage

 

2) According to the number of anchorage units

  • Single or primary anchorage
  • Compound anchorage
  • Reinforced anchorage

 

3) According to the jaws involved

  • Intramaxillary
  • Intermaxillary

 


 

Simple Anchorage

ORT_196

Active movement of few teeth versus several anchor teeth.

 

Stationary Anchorage

ORT_197

Bodily movement of one group of teeth against tipping of another.

 

Reciprocal Anchorage

ORT_198

When two teeth or two sets of teeth move to an equal extent in an opposite direction.

 

Single or Primary Anchorage

ORT_199

The tooth to be moved is pitted against a tooth with a greater alveolar support area.

 

Compound anchorage

ORT_200

Provides for the use of more teeth with greater anchorage potential to move a tooth or group of teeth with lesser support.

 

Reinforced anchorage

ORT_201

The anchorage units are reinforced by use of more than one type of resistance units.

 

Intramaxillary

ORT_202

The elements providing the anchorage as well as those to be moved are situated within same jaw.

 

Intermaxillary

ORT_203

When the anchorage units situated in one jaw used to provide the force required to move teeth in the opposing jaw.

 


 

Anchorage Planning

  1. The number of the teeth to be moved
  2. The type of the teeth to be moved
  3. Type of tooth movement
  4. Periodontal condition
  5. Duration of tooth movement

 

Classifying Anchorage Requirements

a. Maximum anchorage

b. Moderate anchorage

c. Minimum anchorage

 

– end –

 

References:
Gill, D.S. 2008. Orthodontics at a Glance. Blackwell Munksgaard.
Mitchell, L. 2007. An Introduction to Orthodontic. 3rd edition. New York: Oxford University Press.
Proffit, W.R; Field, H.W; Sarver, D.M. 2007. Contemporary of Orthodontics. 4th edition. St. Louis: Mosby Elvevier.
Singh, G. 2008. Textbook of Orthodontics. 2nd edition. New Delhi: Unipress.