Introduction:

Preparation form of amalgam restoration has traditionally been designed to

provide adequate retention. Retention form is defined as that shape or form of

cavity that best permits the restoration to resist displacement through tipping or

lifting forces.

Historically, in class II dental amalgam restorations occlusal convergence of

facial and lingual wall and dovetail design provide sufficient retention form to

occlusal portion of cavity preparation. The occlusal convergence of buccal and

lingual proximal wall offers retention in proximal portion of preparation against

displacement occlusally.

Extensive class II dental amalgam restorations, however, demand for

additional retention measures.

These auxiliary methods of retention are:

I. Proximal retention locks.

II. Dentinal slot.

III. Coves.

IV. Pin-retained amalgam restorations.

V. Amalgapin.

VI. Bonded amalgam restorations.

This seminar focuses on such auxiliary methods of retention which are

required for extensive class II dental amalgam restorations.

I. Proximal retention locks: “A Retention lock is a prepared groove whose

length is in a vertical plane and which is in dentin.”

To enhance retention of the proximal portion, proximal locks may be

indicated to counter proximal displacement. Many operators use proximal locks

routinely to ensure that each portion of tooth preparation is independently

retentive. However, evidence suggests that retentive locks may not be needed in

conservative narrow proximal boxes.

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To prepare retention lock, no. 169L bur with air coolant (to improve vision)

and reduced speed (to improve tactile feel and control) is used. The bur is

positioned at the axio-lingual and axio-facial line angle and directed (translated) to

bisect the angle, approximately parallel to DEJ. This positions the retention lock

0.2mm inside the DEJ, thus maintaining enamel support. The bur is tilted to allow

cutting to the depth of diameter of the bur end at a point angle and permit the lock

to diminish in depth occlusally, terminating at the axio-lingual-pulpal or axio-linguo-

facial point angle. When the axio-facial and axio-lingual line angles are less than

2mm in length, reduce the tilt the bur slightly so that the proximal locks are

extended occlusally to disappear midway between DEJ and the enamel margin.

There are four characteristics or determinants of proximal locks.

1) Position.

2) Translation.

3) Depth.

4) Occlusogingival orientation.

1) Position:

It refers to the axio-facial and axio-lingual line angles of initial tooth

preparation (0.2mm axial to DEJ). Retention locks should be placed 0.2mm

inside DEJ regardless of depth of axial walls and axial line angles.

2) Translation:

Translation refers to the direction of movement of axis of bur.

3) Depth:

It refers to the extent of translation i.e. 0.5mm at gingival floor level and

diminishing occlusally.

4) Occluso-gingival orientation: It refers to the tilt of the no. 169L bur, which dictates the occlusal height of

the lock, given a constant depth.

Also, instead of 169L bur, no. ¼ bur can be used to cut proximal locks. The

rotating bur is carried into axio-linguo-gingival and axio-facio-gingival point angles

and then moved parallel to DEJ to the depth of diameter of bur. It is then drawn

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occlusally along the axiolingual and axiofacial line angles, allowing the lock

to become shallower and to terminate at the axio-linguo-pulpal or axio-facio-pulpal

point angle.

Jose Mondelli et al suggested three retention designs at axio-buccal and

axio-lingual line angles.

1) An angular area from the axio-gingivo-buccal and axio-

gingivo-lingual point angles to a narrow vertex just apical to the axiopulpal

line angle.

2) A cone shaped vertical groove which diverges towards the

occlusal surface. The greater diameter is located at the level of occlusal

dentinoenamal junction, while the minor diameter is located close to the

axio-gingivo-buccal and axio-gingivo-lingual point angles.

3) A cylindrical groove made with straight fissure bur for deciduous teeth.

Regardless of the method used in placing the locks, extreme care should

be taken to prevent the removal of dentin that immediately supports the proximal

enamel. Also, it is essential not to prepare locks entirely in the axial wall because

no effective retention is obtained and there is risk of pulpal involvement.

Advantage:

A relatively conservative method for obtaining auxiliary retention in class II

dental amalgam restoration.

Disadvantage:

In case of wrong translation, there is risk of pulpal involvement if lock is

placed too far axially.

Proximal retention locks in Box-only preparations:

One of the concepts in class II dental amalgam, as suggested by Markley, is

to eliminate the occlusal portion of the preparation if no caries is present.

The Box-only preparation considered to be ideal for teeth in which there is

no evidence of any caries in occlusal portion.

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When giving retention locks in such box only preparation, retention locks

should extend from gingival floor to occlusal surface at axio-facial and axio-lingual

line angles; unlike in conventional design in which locks are extended only upto

length of axial wall, here locks are extended to occlusal surface.

Locks are also given in class II design where dovetail is used in proximal

box retention.

However, Terka, Mahler and Van Eysden have demonstrated clinically that

class II dental amalgam restoration with dovetail and retention lock serves as

satisfactory as dovetail without retention lock.

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II. Slot Retained Amalgam Restorations:

A slot is retentive groove in dentin whose length is in horizontal plane.

Slot retention may be used in conjunction with pin retention or as alternative to it.

Slots in gingival floor may be used to provide additional retention in an extensive

proximal box that has facial and lingual walls extending to or beyond proximal line

angles of tooth crown.

Slot dimension depends upon size of the proximal box. Generally slots are

prepared with the no. ¼ or ½ round burs, 0.5-1mm deep gingivally, 2-3mm in

length faciolingually and 0.2 – 0.5mm inside dentinoenamel junction.

In 1979, Outhwaits et al introduced circumferential slot, prepared with 33 ½

inverted cone bur and compared it with TMS pins. They reported that pin retained

restorations have a greater tendency to slip on their bases whereas slippage did

not occur in circumferential slot. Slot retained restorations are more sensitive to

displacement during matrix removal than pin retained restorations.

Circumferential slot has its greatest indications in teeth with short clinical

crowns and in cusps that have been reduced 2-3 mm for coverage with amalgam.

In these situations, slot provides more resistance and retention than amalgapins.

Advantages:

- Felton et al reported that medium sized self threading pins elicit an

inflammatory response if placed within 0.5 mm of pulp. Slot placed in the same

location does not. Slot is less likely to create micro fractures in dentin and to

perforate the tooth or penetrate into pulp.

Disadvantages:

Compared with pin placement, more tooth structure is removed while

preparing slots.

Pashley et al reported that shear strength of pin retention was significantly

stronger than slot retention.

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III. Coves:

Coves are always used to provide additional retention in preparations that

utilize slots or pins.

Coves are prepared with no. ¼ bur.

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IV. Pin Retained Restorations:

A pin retained restoration may be defined as “Any restoration requiring the

placement of one or more pins in dentin to provide adequate retention form and /

or resistance form.”

Since 1800s dentistry has been using various types of pins to retain filling

materials in mutilated teeth. Burgess was the first to approach pin retention from

scientific point of view and published his finding in 1917.

The first approach that was systematic was published by Markley and

Denver, Colarado in 1958.

Since that time the dental profession has through careful research and

clinical experience, developed various principles of design and usage of retentive

pins. Various problems resulting from their use have also been discovered, and

attempts made at handling them have proved to be useful.

Generally, pins are placed whenever satisfactory retention form cannot be

established with undercuts, proximal retention locks, slots or coves.

Types of pins:

There are three basic types of pins:

1) Cemented pins.

2) Friction locked pins.

3) Self threading pins.

1) Cemented pins:

In 1958 Markley described a technique for restoring teeth with amalgam

and cemented pins, using threaded or serrated stainless steel pins cemented into

pinholes prepared 0.001 to 0.002 inch (0.025 to 0.05mm) larger than diameter of

pins. The cementing medium may be either zinc phosphate or polycarboxylate

cements. The retentiveness of pins using these two materials can be

approximately equal, but, depending upon brand or size of pins used a

significantly higher retentiveness may be obtained with zinc phosphate cement.

The irritation by use of zinc phosphate cement by acid penetrating into dentinal

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tubules is slightly higher. This irritation may be minimized by or eliminated by

placing cavity varnish into pinholes before cementing the pins. However, using

cavity varnish to pinholes can reduce the retention of pinholes almost to half.

According to Chan and Svare, cemented pins have a greater degree of

leakage than non cemented pins; those cemented with zinc phosphate cement

have a greater degree of leakage than those cemented with polycarboxylate

cement. Depth of hole for cemented pins should be 3-4 mm for maximum

retention.

Cemented pins are the least retentive of the three types of pins. They will

provide adequate retention if correctly placed in sufficient numbers.

2) Friction locked – pins:

In 1966 Goldstein described a technique for friction locked pins in which the

diameter of prepared pinhole is 0.001 inch (0.025 mm) smaller than diameter of

the pin. The pins are tapped to placed, retained by resiliency of dentin and are two

to three times more retentive than cemented pins.

Stresses are created in dentin when the pin is tapped to place and may

result in lateral cracks perpendicular to axis of pins. Also shearing of dentin occurs

apical to the leading edge of the pins. Pulpal stresses are more when lateral

surface of friction locked pin is adjacent to the pulp. Microleakage occurs to a

great degree around friction locked pins than around Thread-Mate system of self

threading pins.

The pinhole should be 2-4 mm deep. Major disadvantages with this system

are the difficulty in placement of these pins in posterior teeth, patient

apprehension during placement and lesser retention as compared to TMS pins.

3) Self – threading pins:

Going in 1966 described pin-retained amalgam using self-threading pins.

The diameter of prepared pinhole is 0.002 inch to 0.004 inch (0.038 to 0.01mm)

smaller than diameter of the pin. The pin is retained by the threads engaging

resilient dentin as it is inserted. The compression of dentinal tubules that has been

observed during insertion of threaded pins may be evidence, although speculative,

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of the elastic factor that accommodates insertion of threaded pins into hole of

smaller diameter.

Although threads of self threading pins do not engage the dentin for entire

width, the self threading pins are most retentive of three types of pins.

Pulpal stresses are more when the self threading pins are inserted

perpendicular to the pulp.

The depth of pinhole is 1.3 to 2mm depending upon diameter of pin used.

Several types of self-threading pins are available like,

- Centerlok pin (ARM laboratories, Zephyr core, Nev).

- Dolphin Retention Aid (Union broach company, Inc, NY).

- Reten pin (Dental product company, Conshohokeni, Pa).

- Stabilok Pin (Pulpdent Corp of America).

- Thread Mate System (Whale dent Inc, NY).

Thread Mate System (TMS) is the most widely used self threading pin system.

Chan and Svare have demonstrated that TMS pins exhibit less microleakage than

friction locked or cemented pins.

Advantages:

-Tooth preparation is more conservative than for alternative retentive

methods.

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- Along with retentive form in selected cases, resistance form is also

improved.

Disadvantages:

- Drilling pin holes and placing pins may create craze lines or fractures, as

well as internal stresses in dentin.

- Microleakage around all types of pins is demonstrated.

Factors affecting retention of the pin in dentin and amalgam:

1) Type of pin:

The least retentive pin in dentin is the cemented pin, followed by friction

locked pin. The self threading is the most retentive of three.

2) Surface characteristics of the pin:

Retention of the pin in amalgam is increased by increase in number and

depth of deformations on the pin. With the use of spherical or admixed amlagam

alloy instead of a conventional alloy the adaptation of amalgam to all three types

of pins is greatly improved.

3) Orientation of the pins:

Retention provided by pins is increased by placing pins in non-parallel

manner. Excessive bending of pins to improve retention in amalgam is not

desirable since bending may interfere with adequate condensation of amalgam

around the pin and thereby decreases the retention. Excessive bending may also

weaken the pins.

4) Number of pins:

Within limits, increasing the number of pins increases retention in dentin

and somewhat in amalgam. But as the number of pins increase:

- Crazing of dentin and potential for fracture increases.

- The amount of available dentin between pins decreases and potential for

further dentinal crazing increases.

- Strength of amalgam restoration decreases.

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5) Length of pin into dentine and restorative materials:

For cemented pin the retention in dentin increases linearly as the depth of

pinhole increases. For friction locked pins and self threading pins there is no

significant increase in retention when length embedded into dentin exceeds 2mm.

6) Diameter of the pin:

Within limits as diameter of pin increases, the retention in dentin and

amalgam increases. A pin technique should be used that permits optimum

retention with minimal danger to the remaining tooth structure.

Pin placement factors and techniques:

a) Determination of pin type:

As the retention provided by threaded pins is greater than friction locked and

cemented pins, threaded pins are the widely used pins. Also Thread mate system

(TMS) pins which are a type of threaded pins are the most commonly used.

b) Determination of pin size:

Four sizes of TMS pins are available:

- Regular (0.031inch) (0.78mm).

- Minim (0.024inch) (0.61mm).

- Minikin (0.019inch) (0.48mm).

- Minuta (0.015inch) (0.38mm).

Two determining factors for selecting appropriate size of pin are

- The amount of dentin available to safely receive the pin.

- Amount of retention desired.

e.g. The pins of choice in severely destructed posterior teeth are the minikin and

minim.

c) Determination of number of pins:

As a general rule, one pin per missing axial line angle should be used.

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Also fewest possible pins are used to achieve desired retention. When only

2-3 mm of occluso-gingival height of cusp has been reduced no pin is required as

enough tooth structure remains to use conventional retention features.

d) Determination of the location of pinholes:

Aids in determining the location for pinholes are knowledge of normal pulp

anatomy and external tooth contours, a current radiograph, a periodontal probe

and patient’s age.

Areas of occlusal contact on the restoration must be anticipated, since

vertical pins placed directly below an occlusal load weakens the amalgam

significantly.

Caputo and Standlec state that ideally pinholes should be located halfway

between pulp and the DEJ or external surface of root.

Standlec and others have shown that there should be at least 1mm of

sound dentin around circumference of pinhole.

The pinhole should be placed no closer than 1mm to DEJ and no closer

than 1.5mm to the external surface of tooth. Also one should provide occlusal

clearance to have 2mm of amalgam over pin. Before final decision is made

operator should carefully probe the surface gingival crevice to determine if any

abnormal contours are present on external surface of the tooth.

It may be necessary to prepare “cove” in vertical wall if position of pinhole is

close to vertical wall of tooth structure that jeopardizes condensation of amalgam.

The cove is prepared with number 245 bur to enable the preparation of

pinhole in previously described location, as well as to provide a minimum of

0.5mm dentin around circumference of the pin for adequate condensation of

amalgam. The minimal interpin distance is 3mm for minikin and 5mm for minim

pins.

When possible, the location of pinholes on distal surface of mandibular

molars should be avoided. Obtaining the proper direction of preparing pinholes in

these teeth is difficult because of abrupt flaring of roots just apical to CEJ. If

pinholes are placed parallel to the external surface of tooth crown in these areas,

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penetration into pulp is likely. Also morphological features must be considered for

mesial concavity of first maxillary premolar and furcation area of molars and teeth

that are extremely tilted.

Pinhole preparation:

The Kodex drill (a twist drill) should be used for preparing pinholes. The drill

is made of a high speed tool steel that is swaged into an aluminum shank. The

aluminum shank, which acts as a heat absorber, is color coded so that it can

easily match the appropriate pin size.

Because optimal depth of pinhole into dentin is 2mm (1.5mm for minikin

pins) a depth limiting drill should be used to prepare the pinhole. Also number ¼

bur can be used to prepare pilot hole. With drill tip placed in proper position and

with handpiece rotating at very low speed (300 – 500rpm) apply pressure to drill

and prepare pinhole in one or two movements until depth limiting portion of drill is

reached, and remove the drill from pinhole. Standard drill can also be used for this

purpose.

When the location for starting pinhole is neither flat nor perpendicular to

desired pinhole direction, either correct the located area or use this drill, whose

blades are 4-5mm in length to prepare pinhole that has effective depth of 2mm. To

measure depth of pinhole omni-depth gauge can be used.

Pin insertion:

a) Cemented pin technique:

Hold the pin with a lock-in or magnetized tweezer or a hemostat. Try it in

the pin channel for proper fitting and protrusion in the restoration. Be sure to mark

each pin channel end as well as cavity end of every pin.

Zinc phosphate cement or polycarboxylate cement is mixed (luting

consistency) and then introduced swiftly into pin channel using explorer tip or

lentulo spiral at very low speed. Pin is firmly held into the pin channel to ensure

complete seating. After cement has completely set excess is removed with an

excavator. A lateral facet is placed on the side of pin using a carborandum disk to

create an escape way for the cement during cementation and to reduce friction

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during seating into the channel. As claimed by Courtade, this procedure will

increase retention of cemented pins within the pin channel.

b) Friction grip pin techniques:

Pin is held by hemostat or a tweezer and seated at the pin channel orifice.

Then with a specially made seater with a concave head is firmly applied on the pin

head, being sure that its axis is parallel to that of the pin. With a hammer apply

light strokes to the seater until the established mark on pin comes to the cavity

floor. Finally remove all holding devices and check the cavity floor, walls and

surrounding tooth surface for any crack or gross fracture.

c) Threaded pin techniques:

Two instruments for insertion of threaded pins are available.

- Conventional latch type contra angle handpiece.

- TMS hand wrenches.

When using the latch type of handpiece, insert a link series or link plus pin

into the handpiece and place the pin in the pinhole. Activate the handpiece until

the plastic sleeve shears from the pin. Then remove the sleeve and discard it.

A standard design pin is placed in the appropriate wrench and slowly

threaded into pinhole until a definite resistance is felt when pin reaches bottom of

hole. The pin should be then rotate ¼ to ½ turn counter clockwise to reduce

dentinal stress created by end of pin pressing the dentine. Carefully remove hand

wrench from the pin. Hand wrench should not be used without rubber dam or

throat shield.

To cut excess length of the pin, use a sharp no. ½ or 169L bur at high

speed oriented perpendicular to the pin.

Bending of pins:

Pins are not to be bent, to make them parallel or to increase their retentiveness.

However occasionally bending of pins may be necessary to allow for condensation

of amalgam occluso-gingivally. When pins require bending the TMS bending tool

must be used.

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The bending tool should be placed on the pin where the pin should be bent and

with firmly controlled pressure, the bending tool should be rotated until the desired

amount of bend is achieved. Abrupt or sharp bend increases the chance of

breaking the pin.

Complication during pin placement:

1) Drill penetrates into the pulp:

Sometimes during pin placement or during drilling the pinhole, pulp

exposure occurs. It occurs mostly due to wrong orientation during placing the drill

or due to incorrect radiographic measurement.

In such cases, bleeding should be controlled from exposed site with

sterilized paper points and calcium hydroxide liner should be placed. New hole is

drilled at least 2mm away from exposed site.

2) The drill penetrates into the periodontium:

If exist point is above alveolar crest, the pin inserted and trimmed flush with

root surface or pin is removed and external aspect of pinhole slightly enlarged and

restored with amalgam.

If the perforation is apical to the gingival attachment then two treatment

options are available.

- Reflect the tissue surgically, remove the necessary bone, enlarge

pinhole slightly and restore with amalgam, OR

- Perform a crown lengthening procedure and place margin of cast

restoration gingival to perforation.

3) Pin fails to bind and shear but keeps rotating within its channel:

The best course if this occurs is to cement the pin or use larger pin.

4) The pin shears off well short of its intended depth:

Unscrew the pin with finger wrench or small hemostat, clean the channel

with finger held drill and try again.

5) The dentin fractures away peripheral to the pin:

Remove the loose fragments and extend the preparation to include defect.

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6) Dentinal cracks / crazing:

Micro cracks are usually not noticed during the procedure and sometimes

do not form until weeks after pin placement. If cracks are suspected, the pin

should be removed and smaller sized pin cemented.

Failures of pin retained restorations:

The failure of a pin retained restoration might occur at the following different

locations:

a- Fracture of restorative materials;

b- Separation of pin from restorative material;

c- Fracture of pin;

d- Separation of pin from dentin;

e- Fracture of dentin;

However fracture is most likely to occur at the pin –dentin interface.

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V. AMALGAM PINS (AMALGAPIN):

In spite of great acceptance and proven clinical efficacy of techniques using

prefabricated pins on dentin to retain large amalgam restorations, such techniques

have been the target of criticism because of potential disadvantages and clinical

complications.

Shavell and Seng et al in 1980 introduced the amalgapin technique for

complex amalgam restorations.

Advantages and Disadvantages of Amalgapin over technique employing

prefabricated pins:

Advantages:

- The correct drilling of amalgapin orifices is easier and faster, representing

considerably less chair side time.

- Strong, vigorous, inner dentinal pressures caused by self threading pins

could result on crazing, fissures or fractures or fracture of the dentinal

element whereas with amalgapins such problems do not exist.

- Contrary to amalgapins, the prefabricated pins exert a harmful effect over

the restorative material, cutting down on restoration resistance.

- The ‘amalgapins’ do not imply in any additional cost, constituting a

simpler and less expensive treatment form as compared to prefabricated

pins.

- Amalgam pins can be used in situations where loss of dental structure is

less than 4 mm.

Disadvantages:

- Amalgam pins require an orifice diameter usually greater than that of

prefabricated pins, and for this reason should not be used in situations

where dentin thickness is too limited.

- Restorative technique becomes more critical where dislodgement or

premature removal of matrix might determine treatment failure.

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Clinical procedure:

- Planning, field isolation and cavity preparation care is practically same

as that given to self threading pins.

- Cavity preparation concluded, similarly to technique using prefabricated

pins, the site and number of orifices to be performed are determined

(number of amalgapins). Ideal places to drill the pinholes as well as criteria

to determine such places are same as with the prefabricated pins. As to

number of orifice, one each per absent cusp would suffice, exception made

at cases where whole clinical crown needs to be reconstructed. In this

instance, it would be recommended to perform more than one orifice at

gingival wall corresponding to each proximal box.

“Amalgapin” orifices can be performed with a round end cylindrical bur (no.

1156 from SS White), with a number 33 ½ or 34 inverted truncated cone bur or

with a number 330, kept parallel to external surface of tooth and preferably half

way between enamel-dentin junction and the pulp. Selection of one of above bur

will depend on the available amount of dentin and desired degree of retention. In

case available amount of the dentin is criterium, a resin stop must be prepared on

the active part of the bur, to limit pinhole depth to 1 or 2mm.

Orifices must be drilled in one only pass as the repeated insertion and

withdrawal of the bur might enlarge them too much, resulting a greater risk of

perforation at pulpal or periodontal level. Orifices 1mm deep supplies as much

retention as those with 2or3mm. To make possible an additional amalgam volume

and consequently, greater resistance to the amalgapin, a cavosurface chamfer

must be performed at each orifice using a smooth round bur at low speed. This

bur shall have greater diameter than that of the orifice.

The orifices concluded the cavity must be washed with a calcium hydroxide

solution and dried with gently blown air. After that, a 2% sodium fluoride solution is

applied for 2min. to all cavity walls and orifices. Cavities having a mean depth and

also the deep ones must receive calcium hydroxide cement on the bottom walls.

Then selected matrix is now positioned and stabilized with wooden wedge

and amalgam is condensed carefully into orifices and with all cut cavities.

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Also, sturdvent introduced preparation of dentinal chamber for modified

amalgapin technique. In this technique several dentinal chambers are prepared

with no. 245 bur and using appropriate size round bur chambers are beveled to

provide additional bulk of amalgam. Amalgam is carefully condensed into the

chamber and restoration is completed.

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VI. BONDED – AMALGAM RESTORATION: Bonded amalgam restorations are indicated for large restorations that

require additional retentive features or strengthening of remaining unprepared

tooth structure.

Even if amalgam restoration is to be bonded, retention form must be

provided by auxiliary retentive features such as locks, slots, coves, pins and

amalgapins. Amalgam bonding is an adjuvant to mechanical retention form not a

substitute. Adhesion of amalgam is not necessary in clinical circumstances when

satisfactory retention form already exists.

Advantages:

- A more conservative cavity preparation may be possible.

- Additional retention may be gained through bonding procedure.

- Teeth can be strengthened as a result of bonding process.

- The teeth will be sealed. Microleakage and post operative sensitivity

will be reduced or eliminated.

Disadvantages:

- It is technique sensitive procedure. Proper isolation while the procedure

being performed is a must.

- The technique is more tedious and time consuming.

- Amount of retention achieved is not as significant as obtained with

other auxiliary methods of retention, so it can only be used as an adjuvant

procedure.

Historical Review:

- In 1920s aiding the retention of amalgam with phosphate cement was

advocated and was known as “Baldwern technique”.

- In 1955, Buonocore introduced the concept of adhesive dentistry, a

method to increase the adhesion of resin material to enamel.

- In 1977, Fusayama et al advocated the conditioning of enamel and

dentin with phosphoric acid and coating both with chemical adhesive resin

system.

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- The first reports of experiments involving the use of adhesives under

amalgam restorations were published in 1986 by Varga et al. They

assessed the bond between amalgam and human enamel as well as their

effect on the marginal seal.

- In 1987, Shimizu et al studied use of an adhesive liner to reduce

microleakage with or without glass ionomer base and fluoride treatment.

- In 1988, Staninec and Holt measured tensile strength of amalgam to

tooth structure as well as the microleakage at amalgam tissue interface.

They reported that amalgam can adhere to acid treated enamel and dentin

through a thin coat of Panavia resin.

- In 1991, Nakabayashi et al showed the formation of resin reinforced

dentinal zone located between cured resin and the dentin. The zone, also

called as hybrid layer seems to be responsible for inhibiting the marginal

microleakage and also in the high resin to dentin adhesion strength.

- In 1992 Eakle et al showed the effect of bonded amalgam restoration in

relation to resistance of teeth to fracture. They reported that a tooth

restored with bonded amalgam requires a significantly greater load to

fracture than does a tooth restored with amalgam and no adhesive.

Amalgam bonding system:

Amalgam bonding systems may be used to seal underlying tooth structure

and bond amalgam to enamel and dentin. They require dual characteristics to

achieve optimal wetting. Amalgam is strongly hydrophobic, whereas enamel and

dentin are hydrophilic. Therefore the bonding system must be modified with

wetting agent (comonomer) that has the capacity to wet both hydrophobic and

hydrophilic surfaces.

Typical bonding agent systems may be used, but special 4-methyloxy ethyl

trimellitic anhydride (4-META)-based systems are used frequently. This monomer

molecule contains both hydrophobic and hydrophilic end. Macro shear bond

strengths for joining amalgam to dentin are relatively low (2-6 MPa). The bond that

develops between dentin and amalgam is essentially a micromechanical bond and

no chemical bonding occurs. To accomplish micromechanical bonding at the

amalgam-bonding surface interface, system is applied in much thicker layers

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(10-50μm), so that amalgam being condensed against resin adhesive layer will

force fluid components of amalgam to squeeze into unset bonding adhesive layer

and produce micromechanical laminations of two materials, several laboratory and

clinical studies have shown the dentin adhesive system such as All-Bond 2 (Bisco,

Inc, Schaumburg, Illinois), Amalgambond plus (parallel Fermingdale, NY) Panavia

(Kuraray, Osaka, Japan) and Scotchbond Multi-purpose plus (3M, ESPE, St. Paul,

Minhesota) can be used to bond amalgam restorations. This bonding mechanism

actually may depend on type of amalgam used; for example, spherical amalgam

alloy typically have higher bond strength than dispersed phase or admixed

amalgam alloy.

The bonding system used for amalgam bonding should be essentially self

cure system. Some studies also suggest that use of dual cure bonding systems

may be beneficial for bonding amalgam to dentin.

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Method of use and theory of amalgam bonding:

After removal of carious lesion, proper isolation of the affected tooth is

carried out using a rubber dam.

The tooth is etched using 33-37% phosphoric acid. The acid is washed

away by a stream of water. The preparation should then be briefly dried, resulting

in moist and glistening dentin surface. As an alternative to drying, the preparation

may be blot dried with a damp cotton pellet. If preparation is over dried, it may be

rewetted with water or with HEMA and gluteraldehyde based desensitizer an

applicator tip. The primer should then be applied using technique described by the

manufacturer. After primer application preparation should be dried, but not rinsed.

After drying the primed surface should be glossy in appearance. If it is not, primer

should be reapplied until surface is glossy. Separate applicator tips should be

used for primer and adhesive components. Before mixing base and catalyst from

the adhesive, the amalgam should be triturated and ready to be inserted into the

preparation. Base and catalyst should be mixed, following manufacturers

instructions. After resin is placed amalgam should be condensed into the cavity

and carved.

Thus, in this method of restoration the acid is used to decalcify the dentin

surface, followed by the use of hydrophilic primer which penetrate the remaining

layer of collagen network. With subsequent application of the adhesive the

formation of a ‘hybrid layer’ results and a micromechanical bond is formed to the

dentin surface. The bond to the enamel is formed through the use of auto-

polymerising resin. The amalgam bonding agent employs a 4 META

(Metheryloxyethyl trimetallic anhydride) system. The HEMA acts as a vehicle

which carries the tri-n-butyl borane catalyst and 4 META base into the dentin

where oxygen and water supportably serves as co-catalyst for polymerization.

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Conclusion:

Although not absolute, there are indications for each of the retention forms

described.

Amalgapins and slots have their greatest indications in teeth with short

clinical crowns and in preparation where the cusps that have been reduced 2-3

mm for coverage with amalgam.

When technical requirements for placement of vertical pins can be met,

they provide excellent retention form.

Proximal retention locks, coves and amalgam bonding can be used

whenever indicated.

The literature suggests that distribution of retention features to all areas of

preparation is necessary for maximum effectiveness.

Pins, amalgapins, locks, coves, slots, amalgam bonding may be used

independently in many clinical situations.

However, effectiveness of these retention features can be maximized when

used in combination and proper distribution, which leads to successful class II

dental amalgam restorations.

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References:

1) Baratieri et al. Textbook of advanced operative dentistry, 2nd edition.

2) Crockett WD et al. The influence of proximal retention groove on

retention and resistance feature of class II preparation for amalgam. J

Am Dent Assoc 91(5): 1053-1975.

3) Duane R. Wacker et al. Retentive pins, their use and misuse. Dent Clin

North Am 29(2): 327-340, 1985.

4) Gwinnett AJ et al. Adhesive restorations with amalgam; Guidelines for

clinicians, Quintessence Int. 25(10); 687: 1994.

5) http://dentistry.ouhsc.edu/intranct-web/courses/OPDT 8451/OP Dent 1-

00-03 / Bonded amalgam. html.

6) Jose Mondelli et al – Influence of proximal retention on the fracture

strength of class II amalgam restoration. J Prosthet Dent. 46(4); 420-

424: 1986.

7) Robbins JW et al. Retention and resistance features of complex

amalgam restorations. J Am Dent Assoc 118(4); 437-442: 1989.

8) Sturdevant’s art and science of operative dentistry – 4th edi/editor –

Theodore M et al.

9) The art and science of operative dentistry – 2nd Edition / editor Cliffor U.

Sturdevant et al.

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