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: â€œA Retention lock is a prepared groove whose length is in a vertical plane and which is in dentin.â€? 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.
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-linguofacial 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)
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. Âź 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
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 axiogingivo-lingual point angles to a narrow vertex just apical to the axiopulpal line angle.
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. 4
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.
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.
III. Coves: Coves are always used to provide additional retention in preparations that utilize slots or pins. Coves are prepared with no. Âź bur.
IV. Pin Retained Restorations: A pin retained restoration may be defined as â€œAny restoration requiring the placement of one or more pins in dentin to provide adequate retention form and / or resistance form.â€? 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
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 â€“ 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 â€“ 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,
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.
- 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.
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.
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 patientâ€™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 â€œcoveâ€? 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, 15
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 Âź bur can be used to prepare pilot hole. With drill tip placed in proper position and with handpiece rotating at very low speed (300 â€“ 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
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 Âź to Â˝ 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. Â˝ 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.
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.
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 窶電entin interface.
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 â€˜amalgapinsâ€™ 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.
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. â€œAmalgapinâ€? orifices can be performed with a round end cylindrical bur (no. 1156 from SS White), with a number 33 Â˝ 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.
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.
VI. BONDED â€“ 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 â€œBaldwern techniqueâ€?. -
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. 23
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 24
(10-50Îźm), 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.
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 â€˜hybrid layerâ€™ results and a micromechanical bond is formed to the dentin surface. The bond to the enamel is formed through the use of autopolymerising resin. The amalgam bonding
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.
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.
Baratieri et al. Textbook of advanced operative dentistry, 2 nd edition.
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.
Duane R. Wacker et al. Retentive pins, their use and misuse. Dent Clin North Am 29(2): 327-340, 1985.
Gwinnett AJ et al. Adhesive restorations with amalgam; Guidelines for clinicians, Quintessence Int. 25(10); 687: 1994.
http://dentistry.ouhsc.edu/intranct-web/courses/OPDT 8451/OP Dent 100-03 / Bonded amalgam. html.
Jose Mondelli et al – Influence of proximal retention on the fracture strength of class II amalgam restoration. J Prosthet Dent. 46(4); 420424: 1986.
Robbins JW et al. Retention and resistance features of complex amalgam restorations. J Am Dent Assoc 118(4); 437-442: 1989.
Sturdevant’s art and science of operative dentistry – 4 th edi/editor – Theodore M et al.
The art and science of operative dentistry – 2 nd Edition / editor Cliffor U. Sturdevant et al.