retaining large amalgam restorations Flashcards
how is amalgam retained
convergenet cavity walls (mechanical locking)
what does loss of retention lead to
poor margins
Plaque accumulation
features that improve retention
grooves
channels, slots
bonding
pins
slots
inverted cone bur
generates a dove tail for locking
amalgampins
0.8mm diameter
2mm deep
slight angle
risk of amalgapin
damage the pulp
shelves
need to be 2mm deep
cut around the tooth
when can undercut fail
when the restoration moves bucally
success of a restoration retentive
place features where they will resist loads
destribute forces around the tooth
consider direction of forces
adhesive amalgam process
develops adhesions through molecular entanglement
micromechanical locking between resin and amalgam
clinical trial results for bonded amalgam
lower sensitivity of the tooth
retention rates exceed that of pins
less iatrogenic damage
better restoration survival
problems with bonding amagalm
loss of bond layer
oxygen inhibition, need to prevent oxygen getting in
weakening of amalgam
adhesion to matrix band
pins
restoration can lock onto the screw ontop
must have 0.5mm sound dentine to surround
one pin per missing wall/cusp
avoid placement of pins at
bifurcation
curved roots
thin dentine
pulp
how to place pin
maximise all other retention find flat anatomically safe location - mark a centre with ½ round bur - select drill angle - drill hole - clean debris - place pin - adjust pin if required