Amalgam Flashcards
What is amalgam composed of?
Powder = silver, tin, copper, zinc
Liquid = mercury
What is the purpose of zinc?
Zinc reacts preferentially to form zinc oxide (known as slag) and hydrogen - it acts as a scavenger during alloy production
What is the purpose of copper in amalgam?
It increases strength and hardness
What are the difference types of amalgam?
Traditional
High copper amalgam
What is the setting rxn for amalgam?
Powder (silver tin copper, zinc) + liquid (mercury) react to form Y (unreacted particles) + Y1 (silver mercury) + y2 (tin mercury)
Describe Y
This is the unreacted powder that increases corrosion resistance and strength of amalgam
Describe Y1
This is silver mercury that increases corrosion resistance
Describe Y2
This is tin mercury the is BAD as it decreases corrosion resistance and is weak
What is the problem in traditional amalgam and how do we overcome this?
Problem is the y2 phase as it is weak and has poor corrosion resistance - we overcome this by using highe copper amalgam which contains copper >6% which prevents/reduces formation of y2 by reacting to form copper tin
What is the benefit of not having y2 phase?
Increased strength
increased corrosion resistance
reduced creep
higher early strength
What is the problem with zinc containing amalgam?
Zinc containing amalgam reacts to form slag (ZnO) and hydrogen as it reacts with water in saliva/blood - this creates problem as the hydrogen bubbles formed within amalgam can cause:
- increased pressure leading to expansion of amalgam
- downward pressure that causes pulpal pain
- can cause restoration to protrude upwards and sit proud of tooth (creep) and eventual lead to ditched margins as fracture due to being unsupported
What are some properties of amalgam?
High strength (initially weak but sets over 24hrs)
High thermal conductivity (if close to pulp consider liner)
Long working time - 6 mins and not affectd by moisture
thermal expansion high - 3x more than tooth
radiopaque
poor aesthetics
high abrasion resistance
What is creep?
This is where low level stresses on the tooth for a significant period of time cause the material to undergo plastic deformation and then permanent deformation and as a result material protrudes from the cavity and sits proud - area of amalgam that is unsupported and higher risk of fracture and ditched margins
What is issue with amalgams thermal expansion?
Very high compared to tooth - can lead to microlekaage
What are some disadvantages of amalgam?
Poor aesthetics
high thermal expansion
contains mercury
high corrosion
