Geometric consderations in hip and knee replacements Flashcards
What effect does increasing the offset of a hip replacement have on joint reaction force, torque forces and cantilever forces?
It decreases the JRF
BUT
Increases the torque and cantilever forces on the femoral fixation
What mechanisms of wear are there?
Mechanical - Abrasive, Adhesive, Fatigue
Chemical - Corrosion (Crevice, Pitting)
What are the 4 modes of wear?
Mode 1 wear occurs between the two surfaces that are articulating in the manner intended by the designer
Mode 2 wear occurs between a bearing surface and a non-bearing surface
Mode 3 wear occurs as a result of a third body particle coming between the bearing surfaces
Mode 4 wear occurs between two non-bearing surfaces
How do Morse Tapers fix in place?
By frictional forces between the two surfaces (side loading). A negative mismatch has the majority of the load near the top of the trunnion
Fretting is a wear process. What is it?
Wear occurring due to micromovement (Mode 4). Parts of passivating layer are abraded off leaving exposed non-oxidised metal. Both exposed metal and particles abraded off become oxidised. Oxide is usually harder. Localised oxidation process causes galvanic currents releasing hydrogen ions and HCl formation. Third body wear from oxidised particles - between bearing surfaces and at fretting surfaces - false brindelling
What is galling?
Adhesive wear.
Typically occurs with relatively converging sliding contact e.g. Morse taper assembly. Softer material gets gouged out and forms lumps. Some lumps stick to the harder surface. May cause jamming of surfaces
When is it possible to achieve fluid film lubrication in terms of the bearing materials?
Metal on metal or ceramic on ceramic, it is possible to achieve fluid film lubrication. With metal or ceramic on UHMW Polyethylene it will only achieve boundary or at best mixed lubrication
Wear is proportional to friction. What is the equation for friction?
Friction = friction coefficient x weight
Friction is also proportional to applied load and sliding distance
Will a larger femoral head or smaller create more wear debris?
A larger femoral head creates more wear debris. Because circumference is greater = greater sliding distance
What can help improve wear characteristics?
Oxidised zirconium - ceramic properties at the surface
Highly cross-linked Polyethylene
Vitamin E-Polyethylene
In terms of fluid film lubrication, what is the ideal Lambda value?
3 - with greater values, the internal friction in the lubricating fluid actually causes the friction to rise
What is required for fluid film lubrication?
Needs to be hard on hard bearing
Needs sufficient clearance to allow lubrication fluid to be entrained between the bearing surfaces
Needs sufficient speed pf motion between the bearing surfaces to entrain the lubrication fluid
Further enhanced by a wettable material e.g. ceramic
Stem fixation shape fill vs double taper?
Shape closed - shear forces at interface
Double taper polished stem - hoop stresses at interface
