Lecture 6 - Implants for Joint Replacement Flashcards
Why and when are joints replaced?
- end-stage, last resort treatment
- OA, RA, Gout, Injury
Metals
- metals used are alloyed –> improves properties
Stainless steel:
- iron + chromium + nickel + carbon
- fatigue and corrosion resistance
Cobalt-chromium alloys:
- cobalt + chromium + others
- Wear and corrosion resistant
- for both surfaces of one bearing
Titanium alloys:
- titanium + aluminium + vanadium
- lighter
- good corrosion resistance
Ceramics
- hard and brittle, low wear
Alumina:
- high E and hardness
- wear and corrosion resistant
Zirconia:
- alloying constituent with alumina
Bioactive glass:
- surface reactive
- bond to bone and enhance tissue formation
- not for load bearing
Polymers
- can incorporate other substances
PMMA:
- solid material for femoral heads
- bone cement
UHMWPE:
- lower friction than metal on metal
Modern THA: three types of ball-sockets
Metal head on plastic (UHMWPE)
- most widely used
Ceramic head on plastic acetabulum / ceramic on ceramic
- younger, active patients
- squeek
Metal on metal
- rare
- metallosis - carcinogenic effect of metal ions
Hip resurfacing
- metal cap on femoral head
- less bone removed + revision easier
- more common in younger patients
Anatomic approach to TKA & issues
- just articular surfaces replaced
- soft tissue constraints
Issues:
- complex geometries difficult to manufacture
- surgery difficult
- ACL resection necessary to correct deformity
Functional approach to TKA
- take away condyles and ACL/PCL
- mechanics of knee simplified
Mobile bearings
In a mobile bearing the polyethylene component can move relative to the components that
are fixed to the bones
- unrestricted rotational movements Large congruent surfaces which provides - lower contact stresses and thus minimal wear - lower constraint forces which should result in decreases loosening - less bone resorption
Modern TKA
- cobalt-chrome alloy femoral component
- cobalt-chrome / Ti alloy tibial tray affixed to proximal tibia
- UHMWPE tibial bearing component fixed into tibial tray
- UHMWPE patella component (metal backing)
Partial vs total
Successful implant design parameters
- biocompatible
- load-bearing
- long-lasting
- reliable
- reviseable
- restore function
- relieve pain
TKA complications
- aseptic loosening (fibrous encapsulation, mechanical integrity lost
- wear particles –> osteolysis
UHMWPE Wear - what happens? And solution?
Problems:
- particulate debris –> pseudosynovial membrane at interface between implant and bone –> macrophages –> cytokines –> bone resorption –> aseptic loosening
Solution:
- additives / irradiation –> affect biocompatibility
Three types of wear with UHMWPE
Adhesive (adhere to metal –> polymer film)
Abrasive (particles rub on metal surface)
Fatigue (creep –> folds + cracks form –> particles break off)
Alumina wear resistance
- 10x lower than PE against metal
- stripe
- squeaky hips
- surgical technique crucial to success
