Complex Amalgam Restorations
Cuspal Coverage, Retention & Resistance Features
M. R. Bouschlicher, DDS, MS
Cuspal coverage

Cuspal coverage
            Centric holding cusps
                        (stamp cusps) = 2.5 mm reduction
            Maxillary premolar facial cusp
                        < 2.5 mm for esthetics

Anatomic cusp reduction
            Depth cuts recommended
            Length of head of bur must be known
                        Measure with perio probe or Boley gauge
                                    #330 @ 2.0 mm
                                    #169 @ 5.5 mm

Anatomic cusp reduction
        Adequate strength for amalgam (bulk fracture resistance)
        Preserves tooth structure

Cusp reduction
        Make a drawing prior to cusp reduction
        Note cusp height

Make a drawing prior to cusp reduction
        Note cusp tip location mesiodistally

Cusp reduction
        Measurement allows cusps to be built & carved back to original morphology prior to rubber dam removal
        Less potential for fracture while checking occlusion

Self-Threading Pins

        Self-threading more retentive than cemented of friction-locked pins (Dilts et al., J.A.D.A. 1981)
        Optimum depth in dentin 2.0-3.0 mm (Dilts et al., J.A.D.A. 1981)
        Pin length in amalgam @ 2.0 mm provides optimum retention (Moffa et al., J.A.D.A. 1969)
        Larger diameter pins are more retentive (Dilts et al., J.A.D.A. 1981)

        Pins provide retention & resistance form
        Pins do not strengthen either the tooth or the restoration (actually weaken both)
        Therefore, limit use to minimum # necessary

        Mechanical interlocking of amalgam with undercuts in the pin
        Pin retention depends on resiliency and firmness of dentin
        Place only in healthy dentin
        Demineralized dentin will not hold pins (use other methods)
        Post-operative sensitivity if placed close to the pulp

How many pins?

        1 pin / missing cusp
        1 pin / missing line angle
        1 pin / missing proximal surface
                    Caution: 3 surface preparations (e.g. MOL, DOL, MOB & DOB have lost line angle + cusp-- place 1 pin

Pin placement
        1 mm inside the DEJ
        1 mm inside external surface of root if apical to CEJ
        < 2.0 mm inside the DEJ
        2.0 mm into dentin
        2.0 mm into amalgam

Pin placement

        Inter-pin distance dependent of pin diameter
                    (3 mm for Minikin 0.019" dia. pins)
                    (5 mm for Minim 0.021" dia. pins)
        Pilot holes enter dentin @ @ 90º Ð
        Access to condense amalgam around full circumference of pin

Cove placement
        Pin located optimal distance from DEJ w dentinal wall adjacent to pin
        Allows access to condense amalgam around full circumference of pin

Pin placement
        Each pin must be opposed by another retention feature
        Retention groove
                    Segmental circumferential slot
                    Segmental peripheral shelf
                    Another pin

Correct number of pins to place depends upon:
        Size of pins
        Larger dia. ® more retention
        Larger dia. ® greater inter-pin distance
        Amount of remaining tooth structure
        Other resistance features used
        Functional requirement of final restoration

Pilot hole preparation:
        R.D. in place
                    Protect from aspiration
                    Prevent saliva contamination in event of pulp exposure
        Starting point with 1/4 round bur
                    Prevents drill from "walking"

Pilot hole preparation:
        Prepare parallel to nearest external tooth surface
        Align twist drill with external tooth surface
        Clockwise rotation
        Drill hole in one continuous motion
        Keep drill revolving until withdrawn from dentin
                    Avoid pumping or "stop & go"
        Maintain constant angulation
        Effect of twist drill side deflection (illustration on Web page)

Cavity varnish
        Application prior to insertion of pin
        Thinly coats the pilot hole
        Does not coat the pin

Dentin bonding agent
        Applied after pin placement
        Adhesive will occlude pilot holes if applied prior to pin placement
        Pilot holes may need to be redrilled
        Amalgam can be condensed immediately after placement of bonding materials

Pin placement w Autoclutch (illustration on Web page)

Pin placement with hand wrench (illustration on Web page)

Bending pins
        Use fork shaped pin bender or small hemostat
        Keep it within bulk of planned restoration
        Keep it within crown base-up
                    Visualize planned crown preparation in 3-D
                    Avoid cutting retention pins during preparation

Shortening pins:
        Approach perpendicular to pin if possible
        Stabilize with cotton pliers or hemostat

Flat gingival seat & pulpal floor

Pins available in clinics:
        Minikin pin (0.019" dia.) red
        Minikin drill (0.017" dia.) red
        Minim pin (0.023" dia.) gray
        Minim drill (0.021" dia.) gray

Potential risks:
        Voids around pin due to inadequate condensation
        Pin at or just below surface of amalgam
        Dentinal crazing & enamel cracks
        Pulp exposure
        Root perforation

Pin at amalgam surface
        Shorten with round diamond
        Potential for microleakage
        Weakens amalgam
        Amalgam too thin over pin

Stress concentration at end of pin ® dentinal crazing
        Crazing or root fracture with subsequent microleakage


Pulp exposure
        Cover exposure with Ca(OH)2
        Vitrebond applied over Ca(OH)2

Root perforation
        Condense amalgam into perforation
        Remove amalgam from outside of root & burnish thoroughly

Where to place pins? (illustrations on Web page)
        Best locations: line angle areas
        Worst locations:
                    Above bifurcation
                    Above trifurcation
                    Where root concavity is expected
                    Where root concavity is identified (not expected) Look for it!

                    Retention placement: locations for maxillary premolar (illustration on Web page)
                    Retention placement: locations for maxillary molar (illustration on Web page)

                    Retention placement: locations for mandibular premolar (illustration on Web page)
                    Retention placement: locations for mandibular molar (illustration on Web page)

                    High risk locations summary diagram (illustration on Web page)

Horizontal Pins use in resisting horizontal displacement (illustration on Web page)
Vertical Pins use in resisting horizontal displacement illustration on Web page)


Non-Pin Resistance & Retention Features

Non-Pin Resistance & Retention Features:
        Parallelism in all walls of preparation
        Proximal box form
        Retention grooves in proximal line angles
        Box form in buccal & lingual groove areas of molars
        Boxes in areas other than in interproximal surfaces

Circumferential slot
        Place with 33-1/2 inverted cone
        Resistance @ 4 pins (TMS Minim) Outhwaite et al. (1979)
        More sensitive to displacement during matrix removal than pin retained

        Placement locations similar to conventional pins
        Resistance to displacement = conventional pins (Davis et al. 1983)
                    Depth 1.5-2.0 mm
                    Diameter 0.8 mm
                    Place with #330 or #56 (0.8mm dia.)


Peripheral Shelves
            Segmental use more common than circumferential
            Use only in area of preparation where needed
            2.0 mm wide
            1.0 mm height

Bonded Amalgam Restorations
"Until clinical research data are available to support the efficacy of bonding as the sole provider of resistance and retention, resin bonding agents should be used only in conjunction with proven mechanical forms of resistance and retention." (Swartz, Summitt & Robbins, Fundamentals of Operative Dentistry1996)


"Complex Amalgam Restorations; Cuspal Coverage, Retention & Resistance Features" coming soon to the Operative Dentistry II Lecture Home Page.

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