Alloys In Dentistry Question and Answers

Material selection and failures in RBFPD.

The original Rochette’s bridge used gold alloy. Further studies proved that base-metal alloys had better bonding with resin cements than gold alloys since then nickel-chromium alloy was the choice for resin-bonded fixed partial denture (RBFPDs).

Materials Used

Gold alloy

Ni–Cr alloy

Co–Cr alloy

Metal ceramic

Fiber-reinforced composites

Zirconia.

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

Types of Retainers in RBFPD

• Fixed–Fixed
• Fixed–movable
• Single abutment single pontic cantilever
• Double abutment single pontic cantilever.

Base-metal alloys

Advantages of base-metal alloys

• Greater bond strengths with resin and tooth
• Can be made in thin section hence, bulky crown contour can be avoided (ideally should be a minimum of 0.5 mm thick).

Disadvantages of base-metal alloys

• At high temperatures base metals are difficult to cast and pre-solder
• Nickel sensitivity is possible in allergic patients.

All ceramic RBFPDs

• Were introduced in the early 1990s
• Better esthetics.

Fiber-reinforced composite (FRC)

Advantages

• Better adhesion of the composite resin luting agent to the retainer
• Superior esthetics
• Ease of repair.

Disadvantages

• Delamination and framework fracture are the most common failures
• They will also discolor over time
• Greater occlusal clearance is required (1–2 mm).

Zirconia Advantages

• Superior strength and fracture resistance and toughness
• Can be milled
• Good esthetics.

Zirconia Disadvantages

• Fracture of the veneered porcelain
• Bonding needs to be increased by selective infiltration etching.

Zirconia Bonding/cementation

• The original RBFPD frameworks were perforated for mechanical retention of the cement to the framework
• Perforations weakened the framework strength and exposed the resin to abrasion and thereby caused microleakage
• Presently adhesive bonding is commonly used in which there is enamel to resin bond, cohesive bond of the composite resin and resin to framework bond.

Commonly used Luting agent

• Panavia EX introduced in 1984 is capable of bonding cobalt chromium to enamel. It is based on bis-GMA resin and contains MDP (10-methacryloxydecyl dihydrogen phosphate)
• Panavia has a compressive strength of 200–300MPa and tensile strength of 20–40MPa.
• Basemetal bonded better than noble alloys due to oxidation of metal
• Air abrasion with 50μ alumina prior to bonding roughens the surface and provides a molecular coating of alumina which helps oxide bonding of phosphate-based adhesive systems, such as Panavia
• Moisture control is essential to optimal bonding hence, rubber dam or cotton wool isolation is required.

RBFPD Failures

• Debonding is the commonest problem and if occlusal discrepancies/parafunctional habits are present there are increased chances of debonding
• Hence recall appointments are important before caries or debonding occurs
• Creugers et al. found a survival rate of 75% for anterior and 44% for posterior bridges at 7.5-year follow-up
• Other failures observed were loss of retention and fractures of the veneering porcelain
• Hence, care need to be taken in case selection, abutment preparation, moisture control during bonding and final occlusal contacts.

 Osseointegration

Osseointegration Definition

Osseointegration is “the structural and functional connection between ordered living bone and the surface of a load carrying implant”.

Theories of osseointegration

Classification based on stimulus of regeneration.

Cell-to-cell contact
The injured cells close to the osteotomy region send chemical signals to the adjacent undifferentiated cells to form new cells.

Matrix involvement
The matrix component of injured bone releases protein signals to undifferentiated cells to form the new cell.

Piezoelectric stimulation
The piezoelectric signals elicited by the movements of the fracture ends can act as a stimulus for repair and that stimulates the progenitor cells to form the cell lines.

Mechanism/sequelae of osseointegration

The red cells, platelets and inflammatory cells of blood shift from the capillaries to the tissue surrounding the implant.

The blood cells get entrapped at the implant interface and releases cytokines and other soluble, growth and differentiation factors.

Fibrin, the reaction product of thrombin and fibrinogen released into the healing site can adhere to all surfaces and is critical in determining the integration.

The implant surfaces provide sufficient anchorage to the fibrin to withstand detachment during cell migration. The cells that reach the implant surface differentiate to synthesize bone matrix.

Differentiating osteogenic cells are also derived from bone remodeling sites and perivascular connective tissue cells around the implant site.

Migration of the connective tissue cells occurs through fibrin that forms during clot resolution and connective tissue cells replace the fibrin to act as a source of osteogenic cells.

The osteogenic cells secrete the bone matrix leading to bone growth.

Theories of Osteogenesis

Distance osteogenesis
Formation of new bone arising from the bone adjacent to implant site. The bone formation occurs from bone towards the implant. The implant is surrounded by new bone rather bone forming on implant surface.

Contact osteogenesis
Formation of new bone starting from the implant surface. The implant surface gets colonized by osteogenic cells and the new bone formation arises to join the adjoining bone interface.

Stages of osseointegration are

Stage -1 – Woven bone formation
Occurs within the first 4–6 weeks after surgery. It starts growing from the adjacent bone towards the implant and fills the space between the implant and adjacent bone.

Stage -2 – Adaptation phase
Starts from second month, the microscopic structure of newly formed bone changes towards the well-known lamellar bone (most highly structured type of bone tissue).

Stage -3 – Remodeling
This is the last stage of osseointegration which improves bone quality by replacing the necrotic and primitive woven bone with lamellar bone. It starts around the third month.

Factors affecting Osseointegration

Implant surface and design
Increased roughness and threads increases the bone contact area to 30% comparing to that of smooth, non-threaded implants (20%).

Biocompatibility of the material
The oxide layer that is present on the surface of titanium (Ti) makes it more biocompatible (cpTi and Ti-6Al-4V Titanium 6 Aluminum 4 Vanadium).

Host variables
Osseointegration of the implant is dependent on the quality and density of the bone.

Surgical procedure
Minimal trauma to the bone during placement of implant is required for good osseointegration.

Infection control
A strict sterilization protocol to be followed during the implant placement procedure.

Implant site
The osteotomy site should be free from any barriers, or increased space between bone implant interfaces.

Primary stability
Good stability will prevent movement of the implant during healing period.

Occlusion

The delayed loading protocol is reported to be more ideal for osseointegration. Criteria for successful osseointegration given by Albrektsson et al. 1986.5

• The individual must have an immobile implant
• No evidence of peri-implant radiolucency in radiograph
• The vertical bone loss must be less than 0.2 mm
• A success rate of 80% after 10 years posttreatment.

 Alloys used in Fixed Partial Denture

Precious metal alloys

• Gold-based alloys.
• Palladium-based alloys.

Base-metal alloys

• Nickel–chromium–beryllium (Ni–Cr–Be).
• Cobalt–Chromium alloys (Co–Cr).

Precious metal alloys

Gold-based alloys

Gold alloys

Used since 1907.

Composition

Main composition is gold. It has platinum content of 8%, palladium – 4.6%, silver – 1.3% and tin, indium and iron to promote oxide bonding, harden the alloy and refine the grain structure.

Advantages

• Excellent bonding to porcelain
• Good castability
• Resistance to tarnish and corrosion
• Freedom from toxicity
• No discoloration.

Disadvantages

• Expensive
• Low elastic modulus and poor sag resistance.

Gold–palladium–silver alloys

Composition

Gold 42–50%, palladium 25–32%, silver 6–16% and indium and tin contribute 8%.

Advantages

• Substituting palladium for gold raises the melting range and lowers the coefficient of expansion
• Less susceptibility to dimensional changes with good corrosion resistance and clinical working.

Disadvantage
Greening effect due to the silver content.

Gold–palladium alloys

Composition
Gold 42–55%, palladium 36–40% with minor quantities of tin, indium and gallium.

Advantages

• Greening effect not present
• Lower coefficient of thermal expansion of gold and hence, more compatible with porcelain
• Strength is improved
• Corrosion resistance is good with better sag resistance.

Disadvantage
Thermal expansion incompatible with higher expansion porcelain.

Palladium-based alloys

Used since 1970s.

Palladium in alloys:

• Increases strength
• Reduces the cost
• Produces white color.

Palladium–silver alloys

Composition

Palladium content of 50–60%, silver 30–40% and traces of tin and indium.

Advantages

• Good tarnish and corrosion resistance
• Good bond strength
• Lower sag tendency.

Disadvantage

Porcelain discoloration.

Palladium–copper alloys
Used since 1982.

Composition
Palladium—70–80% and 15% copper.

Disadvantage
Copper discolors porcelain.

Palladium–Cobalt alloys
Available as silver-free, nickel-free, gold- and beryllium-free commercial alloys.

Composition
78–88% palladium and 4–10% cobalt.

Disadvantage
Difficult to melt and has high thermal expansion.

Palladium–Tin Alloys

• The silver is substituted with tin and gallium
• Recent clinical use, hence clinical data is not available.

Non-precious alloys

• Base-metal Alloys
• Due to the high cost of gold and palladium, base-metal alloys were developed.

Nickel–chromium alloys

Composition

Nickel 70–80%, chromium 13–20% with or without small quantities of beryllium (0.5–2%).

Advantages

• Good strength hence, can be used in thin sections permitting excellent crown contour
• Physical properties as hardness, yield strength and modulus of elasticity is superior to all noble metals with regard to sag resistance.

Disadvantages

• High casting shrinkage of 2.4%
• Questionable biocompatibility
• Requires modified casting techniques.

Cobalt–chromium alloys

These base metals may contain 55–68% cobalt and up to 25–27% chromium but no beryllium.

Cobalt–chromium–nickel alloys

Advantages

• Base-metal alloys are cheaper than noble alloys
• They have good strength
• Can be used along with metal ceramics.

Disadvantages

• Fusion temperature is high
• Marginal fit is poor compared to gold alloys
• Cannot be burnished
• Nickel-containing alloy can cause allergy to some.

Question 39. Non-rigid connectors/stress breakers in fixed partial denture.

• Intracoronal retainers are non-rigid connectors that confines within proximal axial contours or within crown of tooth
• It consists of a matrix component part housed within the coronal portion of abutment tooth.

Types of intracoronal systems

• Key and keyway system
• Locking mechanism
• Frictional mechanism
• Spring retained types.

Principle of non-rigid connector

• This connector employs a stress breaking principle, which consists of a keyway (female part) and a key (male part)
• This connector when placed between pontic and retainer permits some amount of movement.

Stress Breakers Indications

• Resilient intracoronal used for removable bounded prosthesis
• Non-resilient intracoronal used for fixed movable design.

Stress Breakers Advantages

• Easy alignment
• Allows individual movement of tooth
• Force distribution can be controlled
• Torsional forces are not transmitted to the teeth
• Contour of tooth is not altered as in extracoronal types
• Can overcome problem of non-parallel abutments.

Stress Breakers Disadvantages

• Used only for posterior fixed prosthesis
• Cannot be used when opposing teeth are missing because the key tends to displace
• Cannot be used in long-span bridges
• The size of intracoronal retainers can encroach upon pulp chamber
• Requires extensive tooth preparation.

Different types of intracoronal attachments

• McCollum attachment
• Chayes attachment
• Stern precision attachment.
• Crismani attachment.
• Schatzmann attachment
• ASE attachment.

Stress Breakers Method

• A wax pattern for retainer is fabricated on working cast.
• If a plastic pattern is used for the keyway, a deep box form is carved into the distal surface of die wax pattern to create space for placement of plastic keyway.
• Path of insertion should be accurate while preparing distal box.
• The working cast with wax pattern is surveyed to locate the ideal path of insertion by locking the extension of key into the surveying arm of surveyor.
• The surveying table is adjusted till the assembly seats well into distal box.
• This key and keyway assembly is luted in place, invested, burned out and cast.

Question 40. 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.

Methods of fabricating wax pattern

Direct technique
Pattern waxed in mouth on prepared tooth.

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

Types of wax used

Type 1
Used for intraoral patterns.

Type 2
Used for extraoral patterns.

Steps in wax pattern fabrication

• Coping fabrication.
• Forming axial contours.
• Occlusal morphology.

Coping fabrication
A die lubricant is coated on the die before fabrication of a coping.

Method

• The lubricated die is dipped in melted wax, placed in a metal cup, to form a uniform thimble of wax on the die.
• The wax is added on to the die with quick strokes of a hot No. 7 wax spatula.
• The margins are re-flowed and perfect adaptation is ensured.
• The coping should be easily removed and placed back on die.
• The coping is transferred to the articulated working cast for further build up.

Axial contours

The proximal contact of wax pattern should be slightly oversized mesiodistally so that there is allowance for the casting to be finished and polished without causing an open contact in the finished restoration.

Proximal contacts

• The proximal contacts for posterior teeth are located in the occlusal third of the crowns
• Proximal contacts between maxillary first and second molars are located in the middle third
• The contact must be more than just a point occlusocervically
• The axial surface of the crown cervical to the proximal contact should be flat or slightly concave to prevent encroachment of the interdental papilla.

Facial and lingual contours
The height of contour on the facial surface of all posterior teeth and lingual surface of maxillary premolars and molars occurs in the cervical third.

Facial contour
Facial contour on maxillary and mandibular posterior tooth extends 0.5 mm beyond the contour of the cementoenamel junction.

Lingual contour
0.5 mm beyond the contour of the cementoenamel junction for maxillary teeth and mandibular first premolars, 0.75 mm on mandibular second premolars and 1.0 mm on mandibular molars.

Occlusal morphology

Types

• Cusp fossa.
• Cusp marginal ridge.

Cusp Fossa

• The functional cusp contact is only with occlusal fossae
• The occlusion with opposing tooth is tooth to tooth
• Each centric cusp should make three-point contact with the occlusal fossa of the opposing tooth
• The contact points are on the mesial, distal and the inner facing incline of the cusp.

Advantages
Parallel to long axis of tooth and force direction is through center of teeth.

Disadvantages
Not found in natural teeth, used only in restoration of tooth (full mouth reconstruction).

Cusp-fossa waxing up procedure:

Developed by P K Thomas.

Placement of functional cusp for mandibular teeth

• The mandibular buccal cusps will contact the fossae of the opposing maxillary teeth
• Cones of mandibular buccal cusps are placed with a PKT No 1 located approximately one-third the distance from the buccal to lingual surface and positioned mesiodistally.

Cones for the maxillary lingual cusps

• Positioned buccolingually
• The mesiolingual cusp cones should be located as far distally as possible
• The distolingual cones should have no contacts with opposing teeth.

Placement of nonfunctional cusps

• The maxillary buccal cusp cones and the mandibular lingual cusp cones should be formed slightly shorter than the functional cusp cones
• The mandibular lingual cusps are placed as far to the lingual and as far from each other and the lingual cusps are shorter than the buccal cusps
• The highest points on the occlusal surfaces are the tips of the cusp cones.

Placement of marginal ridges

• Should never be higher than the cusps
• The buccolingual dimension of each occlusal table formed by the ridges should be approximately 55% of the overall buccolingual dimension of the respective tooth.

Thomas notch

• When evaluating the waxed upper and lower teeth in both working and nonworking lateral excursions, interferences should be removed
• During the working movement, the buccal cusp of each maxillary premolar passes distal to the buccal cusp of its counterpart in the mandibular arch
• A small depression is placed in the distal incline of the buccal cusp of the mandibular premolar to allow the buccal cusp of the maxillary tooth to pass through without interference. This depression is called Thomas notch
• The maxillary and mandibular lingual and buccal ridges are waxed using PKT No 1
• The final contours are smoothened by PKT No 4
• The rest of the procedure remains same as for cusp marginal ridge wax up.

After final finishing
The nonworking movement of the mesiolingual cusp of a maxillary molar should pass through the area distal to the distobuccal cusp of the mandibular molar.

Stuart’s groove
This groove begins in the central fossa and is directed mesiolingually to provide an escape way for the mandibular distobuccal cusp in a nonworking movement. To prevent, such an interference, a groove is placed on the mesiolingual cusp of the maxillary molar.

Margin finishing
After the pattern is completed remove the pattern from the working cast and place it back on the lubricated die.

• Re-melt the entire marginal periphery with a hot PKT No 1 making sure that the wax is melted through to the die
• Add wax with a hot beavertail burnisher
• Carve the excess wax almost to the margin with a PKT No 4
• Finish “carving” the margin with a warm beavertail burnisher
• The margin should be closely adapted to die margin and smoothed with no irregularities.

Final finishing

• A small cotton pellet dipped in die lubricant and held with cotton pliers is run carefully through the grooves
• Axial walls are finished with cotton rolls dipped in die lubricant
• A dry end of the roll is buffed across the wet wax to obtain a smooth surface
• All traces of lubricant should be removed.

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.

Cusp-marginal ridge arrangements for molars

Cusp-marginal ridge arrangement for maxillary molars (E V Payne):

• Cones for buccal cusp
  • Wax pattern cones are placed on the buccal cusps, after determining the position of buccal cusp, by protrusive and a working lateral excursion, as on articulator as far buccally as possible, with a PKT No. 1 instrument.
  • The cone placed should be evaluated with the opposing cusp.
• Placement of buccal ridge of buccal cusp
  Wax is added to the buccal aspect of buccal cones giving the buccal surface its contour.
• Adding triangular ridges
  • Triangular ridges extend from the central groove of the tooth to the cusp tip (wider at base than cusp tip)
  • Added with PKT no 1 instrument
  • They should be convex to allow point occlusal contacts, which are evaluated on articulator and modified.
• Adding mesial and distal cusp ridges
  • Mesial and distal ridges are formed as inclines away from the cusp tip with PKT No 1
  • After placement they are evaluated in lateral and protrusive excursions
  • The inclines of mesial and distal ridges should coincide with inclines of the mesial and distal cusp ridges of the opposing teeth without contact.
• Adding cones for lingual cusp (functional cusps)
  • The cones are located mesiodistally in line with the opposing fossa or marginal ridge
  • The mesiolingual cones are centered between the two buccal cusps
  • Contacts should occur on the sides of the cone but not on the tip.

• Adding mesial and distal ridges to lingual cusp
  • The added cusp ridges should decrease in height from cusp tip to marginal ridges
  • The pattern is dusted with zinc stearate and occlusion is checked to complete the lingual axial contour
  • They are smoothed with a PKT No 4.
• Addition of triangular ridges to lingual cusp
  The triangular ridges should be convex to form point contacts with opposing cusps.
• Forming marginal ridges
  • Marginal ridges are formed by uniting mesial and distal ridges of the buccal cusps with the mesial and distal ridges of the lingual cusps
  • Their height is determined by the height of the cusp tips of the opposing teeth.
• Final finishing
  • The junctions between triangular ridges and adjacent cusp or marginal ridges are joined with PKT No 5
  • The ridges and the grooves are smoothed with a PKT No 3.

Cusp-marginal ridge for mandibular teeth

• Placement of buccal cusp (functional cusp)
  • Placed at the junction of the buccal one third and the lingual two thirds of the mandibular tooth mesiodistally
  • They will occlude with the opposing fossa or marginal ridge, which is evaluated by dusting with zinc stearate and closing the articulator.

 

• Placement of buccal ridges on buccal cusps:
  • Wax is applied from the tip of the cone to its base with a PKT No 1 producing the outline of the buccal surface
  • Check the buccal ridges on articulator in centric and lateral excursions.
• Adding mesial and distal ridges to the buccal cusps
  The inclines of these ridges added should be evaluated by moving the articulator through excursions.
• Placement of triangular ridges
  • They are added to the buccal cusps with a PKT No 1
  • The base of these ridges should form the central groove of the occlusal surface
  • The convex ridges should have point contacts with opposing teeth.
• Placement of lingual cusp
  • Placed as far to the lingual and as far apart mesiodistally to prevent working interferences
  • The lingual cusps should be shorter than the buccal cusps.
• Placement of lingual ridges
  Lingual ridges added to the lingual cusps form the outline of the lingual contour.
• Adding triangular ridges to the lingual cusp
  • Added with a No. 1 PKT and placed to converge slightly towards the central fossa
  • The contacts formed by opposing cusps should have a tripod form.
• Forming marginal ridges
  Are formed by joining the buccal and lingual cusp ridges.

• Final finishing
  • All grooves and fossae are smoothed with the PKT No 3
  • Ridges are rounded and finished with the PKT No 5.