Casting Defects In Dentistry

Casting Defects In Dentistry

 Defects in casting.

Grouped under:

• Distortion.
• Surface roughness and irregularities.
• Porosity.
• Incomplete casting.

Read And Learn More: Fixed Partial Denture Short Essay Question And Answers

Distortion

Factors causing distortion

• Due to distortion of the wax pattern
• Uneven setting and hygroscopic expansions of the investment
• Configuration of the pattern, type of wax, and thickness influence the amount of distortion caused by expansion of investment
• Distortion during the carving, pooling and removal of wax.

Surface Roughness and Irregularities

Surface roughness is defined as finely spaced surface irregularities whose height, width and direction establish predominant surface pattern.

Surface irregularities are isolated, generally caused by improper mixing technique.

Distortion Causes

Distortion Air bubbles

• Air bubbles on margin of casting prevents proper seating
• This can be prevented by proper vacuum mixing, application of wetting agent and applying an initial layer of investment on pattern.

Distortion Water films

• Appears as ridge or veins
• This occurs due to high water–powder ratio of investment material
• Application of wetting agent to wax pattern before investment can reduce water films.

Distortion Rapid heating

• Seen as fins or spines
• When the invested material develops tensile stresses inside and compressive stresses outside, the investment cracks and forms fins.

Distortion Under heating
It is due to incomplete wax elimination.

Water–powder ratio
Increased water powder ratio causes rough casting and decreased ratio causes voids.

Prolonged heating
This causes sulphur compounds that contaminate the surface of the casting.

High temperature
Above 700°C causes breakdown of the investment and results in rough casting.

High casting pressure
Ideal casting pressure is 0.10–0.14 MPa (3–4 turns in the centrifugal casting machine.)

Composition of investment
If coarse silica is present it can cause rough casting.

Foreign bodies
Incorporated from rough crucible, sprue former, investment pieces, bits of carbon from flux.

Impact of molten alloy
Due to improper spruing.

Wrong pattern placement in the casting ring
Too many patterns placed together and patterns placed too close in the same ring should be avoided.

Porosity In Casting

Porosity In Casting  Internal

Solidification defects

Microporosities

Pin hole porosities

Subsurface porosities

Residual air in the mould.

Porosity In Casting  External

Surface roughnesses.

Solidification defects

Solidification shrinkage

Mainly occurs near sprue-cast junction.

Porosity In Casting Causes

• Incomplete feeding of molten metal
• Premature solidification of the sprue.

Suck back porosities

Near area of sprue.
This occurs when a hot metal, impinging from sprue channel on to a point on the mould wall, causes a hot spot. This hot spot causes local region to freeze last resulting in shrinkage of that area leading to a suck back porosity.

Porosity In Casting  Can be prevented by

• Flaring of the area of sprue attachment
• Lowering casting temperature by 30 degree.

Microporosities
Occurs when solidification occurs too rapidly before the micro voids can reorganise.Can prevent by lowering the mould temperature.

Pinhole porosity
Is spherical in shape whereas gas inclusion porosity is larger. During solidification absorbed gases are expelled leading to pinhole porosity.

Subsurface porosities Cause

Simultaneous nucleation of solid grains and gas bubbles as the metal freezes at the mould walls.

Can be decreased by controlling the rate of molten metal entry.

Residual air in the mould

• Causes back pressure porosity.
• It occurs as a large concave depression due to the inability of air in the mold to escape.

Residual air in the mould Causes

• Dense investments
• Low mold temperature.

Residual air in the mould Can be prevented by:

• Proper burn out
• Adequate mold temperature.
• Ideal casting pressure.

Incomplete casting Causes

• Insufficient venting
• Increased viscosity of fused metal
• Decreased casting pressure
• Incomplete wax elimination.

Question 52. Myofascial pain dysfunction syndrome.

Myofascial pain dysfunction syndrome, commonly known as a form of temporomandibular joint (TMJ) syndrome, is a vicious cycle between pain and dysfunctional use of the jaw and neck muscles.

Myofascial pain dysfunction syndrome Synonyms

• Costes syndrome
• Impostor disease
• Pain-dysfunction syndrome
• Temporomandibular joint syndrome.

Myofascial pain dysfunction syndrome Causes

• Bruxism
• Chewing gum for long duration constantly
• An habitual chewing pattern with force on one side of jaw
• Occlusal problems
• Ear infections
• Trauma
• Stress.

Myofascial pain dysfunction syndrome Symptoms

• Mainly presents as a unilateral diffuse pain in periauricular region with muscle tenderness, limitation of jaw function, clicking sound in contralateral TMJ
• Tinnitus, dizziness, hearing loss and ear pain
• The pain is sharp and searing, or dull and constant occurring during swallowing, yawning or chewing with the focus over the TMJ joint, immediately in front of the ear
• The pain can be associated with muscle spasm (muscles attachment of skull, face and jaws)
• Crepitation, clicking and cracking of joint
• Luxation and subluxation
• Difficulty in mouth opening
• Mandibular deviation on jaw movements
• Pain can be felt radiating to the side of the head, cheek, lower jaw, and the teeth.

Theories related to cause of MPD

• Psychophysiologic theory – Bruxing and other parafunctional habits leading to muscle fatigue
• Muscle theory – Constant muscle activity leads to pain
• Mechanical displacement theory – Malrelation of teeth alters occlusion thereby leading to pain.

Evaluation of type of pain by (Bell)

Intensity, cause of onset, quality, time, local or diffuse, sensations felt and description of the pain should be noted.

Function of temporomandibular joint

• When excess forces, as bruxism is loaded onto the TMJ the sliding joint uniformly distributes the load
• When the forces applied are constant and intensive the protective cartilage lining of the joint wears.

The pain can be relieved by:

• Observing a dietary regimen consisting of soft diet
• Applying dry or moist heat to the affected side three or more times for 20 minutes at a time
• An analgesic tablet needs to be taken on regular basis if pain is continuous
• Stretching exercises of the jaw and neck muscles can decrease spasm associated with those muscles.

Thorough dental evaluation is required

• Evaluate malrelation
• Faulty restoration
• High points
• Change in vertical dimension
• Change in occlusal contact points
• Abnormal wear of teeth
• Fracture and malalignment of teeth
• Any tooth that was not replaced, causing a change in occlusion.

Correction

• As splints for bruxers
• Restoring the occlusal plane by restoration
• Replacing missing tooth by prosthesis
• Correction of malaligned tooth by uprighting orthodontically
• To eliminate occlusal contact on inclined planes to enhance positional stability.

Differential diagnosis

Trigeminal neuralgia.

Aids used for diagnosis

Myotronics
With microcomputer can help to assess the resting and functional states of muscles.

Pantograph
Can detect muscle dysfunction.

CAT scan
Can locate degenerative joint diseases.

Arthrography and MRI
Can detect internal derangements.

Question 53. Implant-Supported Fixed Partial Denture.

Implant-Supported Fixed Partial Denture Definition

An implant is a substance that is placed into the jaw to support a crown or fixed or removable denture.

Indications in fixed partial denture

• For single tooth replacement where fixed denture is not possible
• In distally missing abutments implants are placed and a crown is given
• In long-span edentulous areas where fixed partial denture do not fulfil Ante’s law
• In patients who are unwilling to reduce sound tooth, yet want replacement for missing tooth.

Implant-Supported Fixed Partial Denture Advantages

• Preservation of bone
• Better retention and stability
• Better function
• Good esthetics and comfort
• Reduction of sound tooth can be avoided.

Classification of implants

Depending on tissue placement:

• Subperiosteal (epiosteal)
• Endosteal – Root form and plate form
• Transosteal – Passes the entire alveolar bone height.

Depending on materials used:

• Bio-inert – Titanium and its alloys, cobalt chromium and alloys.
• Bioactive  – Ceramics, hydroxyapatite, carbon.

Depending on edentulous spaces:

Misch and Judy classified spaces with different bone volumes (similar to Kennedy’s classification).

Implant options

• The first three classification were fixed prostheses and the other two were removable prostheses
• Classified as FP-I, FP-2, FP-3, RP-4 and RP-5
• In fixed prosthodontics, the endosteal root form implants are commonly used.

Root form implants

Types

• Cylinder type, which is tapped in
• Screw type which is threaded or non-threaded, coated or non-coated.

Mechanism of Integration

Osseointegration

(Ingvar Branemark)

Fibro-osseous attachment

(Weiss)

• In which a fibro-osseous ligament is formed between bone and implant interface.

Biointegration
Occurs when the material is coated with hydroxyapatite, which forms a physiochemical bond with bone.

Periosteal integration
The ideal attachment is osseointegration, which is an apparent direct attachment of osseous tissue to an inert alloplastic material without intervening connective tissue.

Parts of an implant

• Implant body.
• Healing screw.
• Healing caps.
• Abutments.
• Impression posts.
• Laboratory analogs.
• Waxing sleeves.
• A superstructure connected through an attachment.

Types of connections

Split external hex

• Fixed by abutment screw
• For two stage.

Internal hex/Internal octagon: (Corevent)
For single stage and two stage.

Morse taper

Surgical procedure

Procedures are done only after thorough radiographic evaluation and patient examination.

• Incision:
  For two stage implant:
  Crestal incision on buccal side and flap elevation on lingual side.
  For single stage:
  A trap door incision.
  A round drill is used to stabilize implant drill.
  Twist drills of increasing diameter are used after depth is assessed.
  A final drill, one size smaller than the implant, is used to prepare implant site. Sizes for twist drills are 3.5, 3.8 and 4.2. Depth markings are present on the twist drills.
  Counter sink is used only for two-stage implants.
  After the implant site is prepared, the implant is screwed in or tapped in with cover screw and the incision is approximated 3–5 mm from incision line and 3–5 mm apart.

Second stage surgery

• Once osseointegration is complete, a full thickness flap is raised.
• Cover screws are removed and the abutment extension is threaded into the body.
• Impression is made and the final suprastructure is attached after laboratory fabrication of crown.

Single stage systems are placed in one stage surgery with temporary crowns attached.

Implant systems

A few of the implant systems available:

• Sterioss
• Frialitt
• Branemark
• IMTEC
• Centerpulse
• Endopore
• Oraltronics.

Question 54. Full-mouth rehabilitation.

Full-mouth rehabilitation Treatment program

• Initially pain and infection are eliminated.
• Restorations like fillings are done.
• Oral hygiene prophylaxis is done.
• Reshaping, repositioning and restoring the dentition are done.

Full-mouth rehabilitation Treatment objectives

• To create occlusal contact by:
  • Recontouring
  • Reshaping
  • Repositioning
  • Surgery
  • Combinations of any of the above.
• To substitute an occlusal contact:
  An occlusal splint needs to be substituted to achieve a holding contact.
• To splint unopposed teeth:
  By splinting unopposed teeth to teeth that are opposed.

Factors to be analysed in full-mouth rehabilitation

Anterior teeth
To determine the anterior guidance in harmony with envelope of function.

Occlusal plane
During anterior guidance, if posterior tooth interference is present, the occlusal plane should be evaluated and corrected.

Posterior teeth
Posterior teeth should not cause interference with disclusive function of anterior guidance.

Pankey–Mann–Schuyler technique for full mouth rehabilitation

Principles by Schuyler

• Maximum teeth in static, coordinated, occlusal contact in centric relation.
• Anterior guidance should be in harmony with lateral excursive movements on working side.
• Disclusion by anterior guidance of all posterior teeth in protrusion.
• Disclusion of all non-working side inclines in lateral excursions.
• Group function of working side inclines in lateral excursion.

Advantages of Schuyler’s technique

• More than eight teeth are not prepared or rebuilt
• No loss in vertical dimension
• Less time
• No complicated equipments
• Laboratory procedure is controlled by dentist.

Schuyler’s technique Procedure

Order of rehabilitation:

• Lower anteriors.
• Upper anteriors.
• Lower posteriors.
• Upper posteriors.

Establishing anterior guidance:

• Coordinated centric relation stops should be present on all anterior teeth.
• Centric slide should not be present.
• An anterior guide table is customised to minimise stresses in lateral excursions.

Corrections are required for:

• Steep labioincisal angle.
• Wear of teeth.
• Unesthetic appearance as crowding and spacing.

Customised anterior guide table:

• Centric relation, at the correct vertical dimension, is made and facebow transfer is done.
• Cast is mounted and articulated after full mouth preparation.
• A plastic guide table is placed and incisal pin is raised 1 mm.
• After applying monomer on table, the manipulated acrylic resin is placed on table.
• The pin is lowered and upper cast is brought in edge-to-edge relationship.
• The guide pin records the movement of the path followed by cast from centric to edge-to-edge relationship.
• Lateral excursive movements are also carried out before the acrylic sets.

Evaluation of Upper Anteriors

• Tooth position in the arch.
• Lip support.
• Lip closure.
• Phonetics.
• Neutral zone.
• Smile line.

Evaluation of occlusal plane:

• Correction can be done by selective grinding.
• By restoration to alter occlusal plane in malaligned tooth.
• By occlusal plane analysis (Broadicks).

Evaluation of Lower Posterior Teeth

Restored with cusp tip to fossa contact.

Evaluation of occlusal plane:

• Each lower buccal cusp tip position and contouring of lower fossa.
• Distance between the lower buccal and lower lingual cusp should not be greater than half the total buccolingual width of tooth at its widest part.

Evaluation of Upper Posterior Teeth

The last to be restored.

Evaluate type of upper occlusal incline:

• Group function.
• Partial group function.
• Canine guided (total disclusion).

Question 55. Contraindications for implant placement

Implants are contraindicated in patients with abnormal bone metabolism, poor oral hygiene and previous irradiation of the implant site.

They can be classified as follows:

• Medical/Systemic
• Dental/Local.

Medical Contraindications Implant Placement

Medical contraindications given by the National Institutes of Health Consensus Conference, is as follows:

• Acute illness
• Terminal illness
• Pregnancy
• Uncontrolled metabolic disease (e.g.: diabetes)
• Tumoricidal irradiation of implant site
• Low-dose irradiated patients
• Patients with psychiatric problems
• Haematological systemic disorders.

Dental contraindications Implant Placement

Some of the relative contraindication in the oral cavity includes:

• Disease affecting the mucosa of the mouth
• Periodontal conditions
• Severe bruxism
• Heavy masticatory force application
• Occlusal disharmony
• Infections in the neighbouring teeth, such as pockets, granuloma
• Underlying cysts or pathology in the implant site
• Impacted teeth at the implant site
• Root dilacerations of adjacent teeth to implant site
• Anatomic limitations in a specific region.

Question 56. Laboratory procedures in fixed partial denture.

Steps in lab procedures

Working cast and dies

Pindex System

Preparing the master cast

• The final impression is poured in stone
• After the stone sets, the retrieved cast is trimmed to a horseshoe-shaped form
• The buccolingual width of the base of the horseshoe is 13–18 mm, and thickness 15 mm between the gingival margins of the preparations and the inferior border.

Pin Placement

• After the under surface of the cast is trimmed perfectly flat, drill two holes for each removable section as far apart to provide space for the pins and sleeves
• The pins are cemented into holes using low viscosity cyanoacrylate cement
• The short index pins are cemented in the lingual holes
• Long pins cemented into the buccal holes
• The white sleeves are placed over the long pins and the gray sleeves over the short pins.

The Secondary Base

• A strip of carding wax is placed over the extensions of the long pins and the gray sleeves
• The master cast is boxed or placed in a rubber base former and a secondary base is poured with cast stone.

Removable Dies

• After the secondary base sets, remove the master cast and trim the secondary base on the model trimmer
• Dies are sectioned with a die saw and trimmed carefully to expose the finish line
• Clean the stone grindings from the dies, pins, base and pin holes
• Reseat the dies
• The master cast is mounted.

Fabrication of wax patterns

• Wax pattern is the duplicate replica of the finished cast restoration, which will be placed on the prepared tooth
• Wax pattern is invested and cast to form the final restoration.

Indirect technique
Pattern is waxed on die from an accurate impression of prepared tooth.

Cusp Marginal Ridge

• Functional cusp contact with marginal ridges and occlusal fossae
• Opposing occlusion is a single tooth opposing two teeth.

Advantages

This is the most natural type of occlusion and is found in 95% of all adults. It can be used for single restorations placing very little lateral stress on the tooth.

Disadvantages

Food impaction and the displacement of teeth may arise if the functional cusps wedge into a lingual embrasure opposing them.

Uses

Most cast restorations done in daily practice.

Pontic placement

After the wax pattern on abutment tooth is complete, a pontic pattern is made and connected to the retainer wax patterns.

Spruing

A sprue former is a wax, plastic or metal pattern used to form the channel allowing the molten metal to flow into the mould to make a casting.

Sprue placement

• Should be attached at 45° angulation to the thickest portion of the pattern. This prevents porosities and air entrapments
• The area of sprue attachment is flared for smooth flow of metal.

Sprue attachment

• One end of the sprue is attached to the wax pattern with a reservoir.
• The other end is attached to the crucible formers cone shaped structure.
• The wax pattern should have 6 mm space between the tip of pattern and casting ring.The space allows for the air to escape out of the pores in investment.
• The wax pattern should have uniform space within the casting ring (it should be placed in the center of the casting ring).

Investing

• After the wax pattern is attached to the crucible former, a surface tension reducing agent is applied on the wax pattern.
• The vacuum mixed invested material is brushed on to the pattern before filling the entire ring.
• The filled ring is placed away for one hour.

Burn out

• The furnace temperature is brought to 200°C and held at the same temperature for 30 minutes
• The final burn out temperature is raised to 650°C and held for 45 minutes
• The pattern is placed for burn out procedure.

Casting

• Casting machine is given three or four clockwise turns depending on the alloy
• The cradle and counterbalance weights are made to approximate with casting ring used
• Alloys can be melted with gas air torch or electricity
• Only after the alloy is melted the mould is removed from burn out furnace and the lever is released to complete the casting.

Recovery of casting

• After the red glow is disappeared, the casting ring is plunged in running water
• A knife is used to remove the investment material and the rest of investment material is tapped out with blunt instrument.

Finishing of metal

• The recovered casting is trimmed and finished
• Some times a metal try in is done for ceramic restoration.

Other procedures

Soldering

If it is a long-span FPD or a complex design FPD, soldering of certain parts might be required.

Veneer application (acrylic or porcelain)

• In acrylic veneer application, wax is built up to the correct contour, dewaxed and packed in the matched tooth colored heat-cure acrylic resin and cured
• Porcelain application can be achieved by platinum foil technique or direct lift technique
• The condensed porcelain mass is placed in a preheated furnace [approximately 650°C (1200°F) for 5 minutes]
• Initially the opaquer is placed, and then the body porcelain is built and finally the incisal porcelain
• When the final contour is satisfactory, characterization is completed and the porcelain is glazed.

Question 57. Tensofrictional resistance.

Definition

Frictional resistance is the resistance to penetration developed by the friction sleeve, equal to the vertical force applied to the sleeve divided by its surface area.
The resistance is the sum of friction and adhesion.

Application of tensofrictional resistance

In cement retained crown with vital tooth only.

Principle of tensofrictional resistance

Retention is provided by two parallel walls and the friction that is generated between the tooth and crown surface.
Friction becomes relevant when the tooth and crown surfaces are in close proximity.

Function

During the initial phases of the evolution of fixed prosthodontic treatment, retention without the use of a luting agent was routine, hence space for the luting was not provided with die spacers.

As there was no space provided for the luting cement during the process of cementation, hydraulic pressure built up in the crown causing vital dentin to undergo an elastic change creating a resultant tension.

It was believed that the elastic dentin tried to regain its shape by releasing the forces, thereby creating additional retention for the crown.

A few relevant dentin values

• Elastic modulus of dentin – 18,600 MN/m2
• Yield strength of dentin – 165 MN/m2.

Relevance of tensofrictional resistance

Presently this theory has no relevance because of superior quality luting agents and the space provided for luting by die spacer.

Suggested Readings

• Anusavice KJ, Shen C, Rawls HR. Phillips’ Science of Dental Materials, ed 12. Elsevier.
• Freedman. Contemporary Esthetic Dentistry.
• Misch CE. Dental Implant Prosthetics.
• Rosensteil SF, Land MF, Fujimoto J. Contemporary Fixed Prosthodontics, ed 4. Elsevier; 2006.
• Shalu Bathla. Periodontics Revisited.
• Shillingburg HT, Hobo S, Whitsett LD, et al. Fundamentals of fixed prosthodontics, ed 3. Quintessence; 1997.
• Touati B. Esthetic dentistry and Ceramic restorations.