Saturday, October 15, 2011

CHAPTER 4 AMALGAM


COMPONENTS:
Silver - increases strength, expansion and reactivity, decreases creep; corrosion products are AgCl and
            AgS
Tin - increases reactivity and corrosion; decreases strength and hardness; corrosion products are SnO, SnCl
            and SnS
Copper - increases strength, expansion and hardness;  decreases creep; corrosion products are CuO and
            CuS
Zinc - increases plasticity, strength and the Hg/alloy ratio; decreases creep; causes secondary expansion;
     corrosion products are ZnCl and ZnO
Mercury - wets the alloy particles; decreases strength if in excess amounts; implicated in toxic and allergic rx

Mercury and amalgam
Chemical forms of mercury: elemental, inorganic and organic
TLV: threshold limit value for elemental mercury vapor set by NIOSH and is a time-weighted average for an 8 hour day; equal to 50mg/m3
Mercury hypersensitivity
incidence in the general population and only 0.6% show clinical manifestations
usually presents as a contact dermatitis and is treated locally with antihistamines or corticosteroids most allergic reactions subside in 1-2 days

TYPES OF AMALGAM

1.      Spherical                AgSnCu + Hg  ® AgHg + CuSn +AgSnCu     (Ex. Tytin, Valiant)
2.      Lathe cut                AgSn + Hg ® AgHg + SnHg + AgSn              (Ex. Tytin FC)                               
3.      Admixed                AgSn + AgCu + Hg  ®  AgHg + Ag Sn + CuSn  ( Dispersalloy, Valiant PhD)
(No gamma 2 if copper >12%)

Phases: Gamma                        -           AgSn
                        Gamma 1         -           AgHg
                        Gamma 2         -           SnHg
                        Epsilon             -           CuSn
                        Eta                   -           CuSn
                        Beta                 -           AgSn
                        Beta 1              -           AgHg

Amalgam thickness 
cuspal coverage- 2mm over cusp
pin coverage - 1-2 mm (Burgess)

Amalgapins
     Shavel recommends using 2.5 mm amalgapins
     Roddy claims that 1 mm amalgapins are just as retentive as 3mm pins

Clinical advantages of using "pinless" retention techniques when preparing teeth for extensive restorations:
                        1.         Occlusal clearance can be minimal; less than the 4mm needed for retentive pins
                        2.         Passive in nature and does not rely on elasticity of dentin
                        3.         Can be used when replacing a fractured drill or pin without consideration for 3-5mm      spacing

Condensation
-incremental placement helps
-be methodical to ensure intimate adaptation of alloy to all features of the preparation
-maximum density to try to eliminate air voids
-lower residual mercury by incremental build-up and carve- back of excess

OPER DENT 19: 53-58, 1994. This study measured the fracture resistance of complex amalgam restorations when pins or amalgapins were either distributed around the preparation or concentrated in one area of the preparation.  Either 4 TMS stainless steel Regular pins or four amalgapins were used. The pins were either concentrated at two line angles, or distributed at the four line angles.  Distributed pins provided more resistance than amalgapins or clustered pins.

Trituration
            -overtrituration
          working time - decreases for all alloys
          compressive and tensile strengths
          lathe cut alloys -both compressive and tensile  strengths are increased
          spherical and admixed alloys-over trituration reduces compressive and tensile strengths
          increases creep- for all alloys
            -undertrituration - lowers creep

Clinically differentiating between over and under trituration
undertrituration - mass is difficult to manage crumbles and not convenient to manipulate during
     insertion; undertrituration of spherical and admixed high-copper alloys reduce compressive and
     tensile strength
overtrituration - mass often sticks to mixing capsule and is difficult to handle due to its short working
     time
Decreasing trituration time gives you a wetter mix, and increased working time; if mix is too dry, decrease
            the trituration time

Amalgamator Maintenance and Calibration (2 techniques)
run your amalgamator to warm it up before using; calibrate every 6 months to RPM 3600 +/- 200
Rupp Technique
            -select the speed specified by the manufacturer
            -set the amalgam time 6 sec less than that recommended by the manufacturer                    
            -triturate and observe the resultant mixture
            -when the 1st plastic mixture is obtained increase the time by 2 sec and this will become the setting
                        for your amalgamator
Asgar Technique
     -select the speed recommended by the manufacturer
            -select a time 5 sec. greater than that specified by the manufacturer
            -triturate and observe the resultant mixture
     -reduce the time until the mix is no longer shiny and sticky  (falls out of capsule after a light tap)

 Life expectancy of Amalgams
Class I amalgam lasts 19.5 years
Half life complex amalgam 11.5 years
Mean age of replacement of amalgams of all classes was 11 years
Reasons for replacement
secondary caries
residual caries not removed
inadequate condensation
inadequate retention
insufficient bulk for strength, isthmus fracture, etc.
failure to remove unsupported enamel 

Criteria for selection of an amalgam
Composition
Cu > 12%
eliminates gamma 2 phase
­ strength over low Cu alloys; < susceptible to marginal corrosion, deterioration and creep
less susceptible to manipulative and operator variables
restorations of superior longevity
The ADA classifies "high copper" amalgams as those having a copper content of 9% or higher.  However for the complete elimination of Gamma-2 phase, there must sufficient copper to combine with the tin liberated during amalgamation.  Copper content must be at least 12% to eliminate all of the Gamma-2. 
Ref.:  Dental Materials Syllabus, CAPT J.C. Meiers.
Zinc
more plasticity
low oxidation
in high copper alloys, decreased marginal breakdown
particle size: want variety to increase packing fraction

Galvanism
Galvanic corrosion is an electrochemical reaction between dissimilar metals that have the potential to
     cause unpleasant and even painful biologic effects intraorally.  The highest current density of
     electromotive force between materials commonly used intraorally occurs between silver amalgam
     restorations and type III gold.  The first method of treatment is to place a rubber separator between
     the dissimilar restorations.  Other barriers used to separate the teeth include silver nitrate on the
     surface of the new restoration, coating the entire surface of the restoration with unfilled composite
     resin and curing, and replacing the contact area with resin.  Galvanism can occur between a
     temporary aluminum crown and a gold crown.
Spherical vs. admixed - clinical trials
     Spherical
-contains spheres of various sizes, each of which contains silver, tin and copper.
-need 10% less Hg to amalgamate due to less surface area.
-higher strength 15 min after condensation than admixed.
-less condensation force but voids easier to occur
-greater capsule-capsule consistency
-smaller particles-smoother surface
-excellent corrosion resistance
Spherical alloys amalgamate very readily (due to increased surface area).  Therefore, amalgamation can be accomplished with smaller amounts of mercury than is generally required for many lathe-cut alloys. . . The condensation pressures are less. . . Forcing the band against the opposing tooth to develop a positive contact becomes more of a problem with spherical alloys.  . . Spherical alloys have greater compressive strength at 15 minutes and 1 hour.  Ref.:  Baum L, Textbook of Operative Dentistry.  W.B. Saunders, 1995.
            Admixed
-more resistance to condensation
-contain both spherical and irregularly shaped particles
-easier adaptation to cavity walls and better contacts
-medium to high strength
-very good corrosion resistance
-handling characteristics
-setting time: personal choice, range 3-20 mins.
-condensability: spherical easier than lathe-cut or admixed, however, contacts harder to achieve
          with spherical
-carvability
-tensile strength of spherical improved over pure lathe cut
-early compressive strengths (15min.-1hr.) are higher for  spherical than those for lathe-cut alloys-
good for cores or large buildups
            Delivery system/cost
-pre-encapsulated- for mercury hygiene
-variety of spill sizes- saves material
-presence of diaphragm in mixed alloy
-budgetary constraints
Osborne's clinical assessment (June 1990)- after 3 years amalgam restorations were evaluated for surface
            luster and marginal fracture
-least marginal failure- Dispersalloy, Indiloy, Cluster, and Unison
-highest surface luster- Unison and Tytin
-greatest factor in long term success of restoration is the size

Conservative amalgam preparation
Degree cavo-surface angle
B-L dimension of occlusal preparation no more than 1/4 intercuspal width
Depth 0.5mm into dentin
Reverse "s" curve
Independent surface retention
Proximal outline just below proximal contact
No longer advocate GV Black’s “extension for prevention” into all groves
Slot preps for class 2 preparations; if occlusal extensions less than 1.2 mm wide, augment retention of
approximal box with retention locks on facial/lingual walls

Butt cavosurface margin
Gives bulk of amalgam at margin and resists fx.

Restoration of endodontically treated teeth/Build-ups, posts & pins
Factors to consider when deciding to restore a tooth with a core build-up
Extent of the carious involvement
Restoration/remaining tooth structure
Success of endodontic treatment
Periodontal health status
Occlusion/function requirements


Restoring endo teeth with amalgam
To resist fracture restorative techniques should conserve as much dentin as possible and also provide
cuspal protection
Reagan concluded that a properly completed amalgam restoration with cuspal coverage has the same
resistance to fracture as an unrestored tooth
Composite and glass ionomer lack the physical and mechanical properties necessary to function as
     cusp coverage restorations and load bearing restorations in posterior teeth; however, excellent
     results have been obtained in using composite to restore root canal treated teeth with minimal
     access preparation and most coronal tooth structure remaining
Cuspal coverage restorations of endodontically treated posterior teeth can be either a core material followed by
     a casting; or an amalgam core combination amalgam crown                                        
Adequate retention can be achieved from the pulp chamber alone if its height is 4mm or greater
Condensing amalgam into canals, slots, amalgapins, and boxes improves retention                                         
Self threaded pins are an option to increase retention but have inherent risks of crazing and cracking of
the dentin or perforation into the PDL 
Premolar teeth require post retention more often due to the smaller pulp chamber and dentin available
for other internal retentive features
When a post is indicated, the following guidelines should be used for the best results:
     Posts should be at least as long as the crown, longer if possible, and still leave 3-5 mm of gutta percha for
                 an apical seal 
Increased post length increases retention; they should be the smallest diameter that will adapt to
          the minimally prepared canal; tapered posts increase the risk of fracture of the tooth; there
          should be at least a 1mm ferrule affect no matter what type of post and core is utilized
     Antirotational devices should be incorporated into the core if the post is cylindrical
     Core material should have good compressive and tensile strength and be dimensionally stable 
     Glass ionomer cores have the advantage of direct placement, fluoride release, dimensional
                 stability and bonding to dentin but have a low tensile strength
     Composite core advantages are direct placement and lack of corrosion, but has the inherent
                 disadvantage of low tensile strength and dimensional instability due to water sorption 
     Amalgam and cast alloys have the advantage of better strength and dimensional stability than
                 other core materials and are usually the appropriate materials
     The tooth with less than four complete coronal dentin walls may require a cemented post and
                 additional internal retentive features for adequate retention of the final restoration; clinical
                 assessment of remaining tooth structure will determine the combination of retentive features
                 used by the clinician

Pins
Types
cemented
friction-lock
self-threading
Advantages
more conservative and less time involved than castings
enhances retention form (adds walls) and is an economical alternative to castings
Disadvantages
can cause dentin crazing
microleakage can occur at pin channel
pins weaken amalgam alloy


Number of pins
one per cusp or marginal ridge may be excessive
3-5 mm between pins
0.05 to 1.0 mm inside dentinoenamel junction
sizes of pins in inches
    
Self threading (TMS)                            drill size                        pin diameter                                 
            regular                                      0.027                                       0.031
            minum                                      0.021                           0.024  
     minikin                                      0.017                           0.019
     minutia                                      0.0135                         0.015

When pins are placed nearer the occlusal surface, as in cuspal coverage areas, the pins should project only
minimally into the restorative material; long pins near an area of occlusal loading will significantly
weaken the amalgam; additionally, the purpose of the pin in cuspal coverage areas is to bind the cusp
to the restoration and to resist lateral displacement with occlusal function; should have 2 mm of       amalgam thickness above pin

Retention vs. resistance form
-retention form resists displacement through tipping or lifting forces
-resistance form enables tooth to withstand stress on  restoration and remaining tooth during function
-the effectiveness of pins, slots, box-forms (ledges), or  amalgapins can be maximized when used in
combination and properly distributed
Cuspal coverage
cover cusps not because of drying but because of weakness created due to access opening, existing           restoration, etc.
1/4 intercuspal distance = to intact tooth
1/3 intercuspal distance 1/3-2/3 as strong


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