Fixation Flashcards
What is the area between the bone and the fixator known as?
bone-implant interface
What is the main operational difference between screws and nuts and bolts?
Screws require access to one side of the bone only whereas nuts and bolts need two
What is interference fit?
fixation technique relying on a tight contact between the implant and the bone providing friction to prevent movement at the interface
What happens if the interference fit is too tight?
the bone spits
What type of joint replacement is this technique used for?
cementless implants
Is bone cement adhesive ?
No
What material is bone cement more like?
grout - a filling material
what advantage does bone cement give to implants?
it can fill gaps so a perfect geometrical match is not required
Why is it difficult to apply adhesives to bone?
Bone is wet and difficult to access for cleaning
What are two methods of biological fixation?
beaded coating the same material as the metallic implant with pores
HAp coating - plasma spray coating
Why are prosthesis stems tapered?
press outwards on bone to prevent subsistence
What is biocompatibility?
acceptance by the body tissues and systems
What two factors are important in biocompatibility?
extent of which body fluids and tissues affect a material (corrosion)
Extent of which the material adversely affects body tissues (tendency to cause abnormal tissue changes)
What is corrosion?
the progressive unwanted removal of a material by an electrochemical process.
How does corrosion occur?
it occurs when two electrodes are immersed in a liquid that conducts electricity. a current flows from one material to another through the electrolyte, allowing a chemical reaction to take place.
What makes up the electrolyte in the body?
body fluids which contain salts of variable kinds (they are very corrosive)
What does corrosion cause in implants?
pits and craters - loss of material
What makes corrosion worse?
electrodes are different materials
How does corrosion occur in a single metal component?
non-homogenous regions within the material like altered distribution of alloy components
How can corrosion occur between two electrodes of the same material?
slight differences in the manufacturing of the two materials
Some metals become more resistant to corrosion by mixing together certain metals together to form an….?
alloy
Which metal is very resistant to corrosion without alloy components?
titanium
Why is titanium inherently resistant to corrosion?
it has a passivation layer of titanium oxide when exposed to a corrosive environment
What is fretting corrosion?
abrasion of materials that removes the protective metal oxide coating and corrosion occurs
What is crevice corrosion?
occurs in crevices between implants where body fluid becomes trapped - it is starved of oxygen and creates a high concentration of acid leading to corrosion
What are two methods for improving corrosion resistance?
nitric acid immersion
titanium nitride coating
What toxic metals are released from corrosion?
vanadium
aluminium
Name the most important tissue reactions to implanted materials
growth of thin fibrous layer between implant and body tissue
local infection - before, during or after surgery
body sensitisation to metals (chromium, cobalt, nickel)
inflammation in regions of corrosion
tissue necrosis
immunological reaction to wear particles
tumours
What causes tissue necrosis after implant insertion?
bone cement generating intense heat as it cures
What can happen as a result of immune response to wear particles?
cell mediated bone resorption
What metals can the body become sensitised to as a result of implants?
chromium
nickel
cobalt
Why are ceramic components undesirable?
they fail in a brittle manner with no warning
What one material has provided a satisfactory alternative to traditional implant materials?
carbon fibre reinforced plastic
