Polyethylene

Question 0

How is UHMWPE made?

Answer 0

• By CONDENSATION POLYMERISATION and SINTERING ( fusing) of the polyethylene fine granular powder into a prefabricated cast in a LOW PRESURE (4-6 bar) and LOW TEMP (66-88oC) with addition of CALCIUM STERATE to prevent yellowing.

Question 1

What is UHWMPE?

Answer 1

A polyethylene polymer with a least 3 million units

Question 2

Why is the process of formation of UHMWPE important ?

Answer 2

• If the particules are not SINTERED together well they may have FUSION DEFECTS.
• Early failure of UHMWPE in literature due to high wear, because of fusion defects from manufacturing process.

Question 3

What is the structure of UHMWPE?

Answer 3

• Semi crystalline polymer
• but tends to form a Amorphous structure with random arrangements and entangling of structure

Question 4

What is the importance of the UHMWPE structure?

Answer 4

• Gives UHMWPE unparalleled STRENGTH and RESISTANCE TO WEAR cf HDP as used previously

Question 5

Can you draw the monomer ethylene that forms UHMWPE?

Answer 5

H H

I I

H H

C======C

Question 6

What are the problems with sterilisation of UHWMPE?

Answer 6

• Can’t be sterilised by AUTOCLAVE
  
  • • as softens at 70^oC and melts at 135^oC so needs to be IRRADIATED GAMMA RADIATION-
  • advantage= promotes X LINKING OF PE-> INCREASED tensile strength and WEAR resistance

Question 7

what are the problems of irradiating UHMWPE in O2 atmosphere

Answer 7

• Causes OXIDATION of the PE by BREAKING DOWN THE C-C bonds -> formation of more FREE RADICALS -> decrease in MOLECULAR WEIGHT-> detoriation in mechanical properties

Question 8

How is oxidation prevented in sterilisation of UHMWPE?

Answer 8

• Irradiation in ARGON/ NITROGEN atmosphere
• then VACCUM PACKED BUT post irradiation oxidation of UHWMPE does occur

Question 9

What are the disadvantages of gamma irradiation ?

Answer 9

• Reduces RESISTANCE to FATIGUE and FRACTURE reduces elastic modulus , tensile strength, ductility and yield stress

Question 11

What is UHMWPE use?

Answer 11

• In weight bearing components of TKA

Question 12

What is the process of ANNEALING? Why is it used?

Answer 12

• Heating to below melting point
• Aim to reduce free radicals and not disrupt crystalline structure

Question 13

What is UHWPE wear debris associated with?

Answer 13

• HISTIOCYTIC OSTEOLYTIC response

Question 14

What size UHMWPE particles are the most reactive?

Answer 14

• 0.1 to 1.0 um

Question 15

What are the properties of UHWMPE which increase wear debris?

Answer 15

• Thinner <6mm
• Flatter
• Carbon fibre reinforced polyethylene
• Metal backed PE may minimise plastic deformation and loosening but reduce the effective thickness of the PE

Question 16

What factors cause catastrophic wear of Pe in TKR?

Answer 16

• VARUS knee alignment
• Thin PE < 8mm
• Flat , non conforming inserts- less contact area > stress
• Heat tx of inserts, sterilisation

Question 17

UHWMPE is Viscoelastic so what wear is it susceptible to?

Answer 17

• Abrasion wear

Question 18

Wear damage to UHWMPE articulating surfaces is mainly due to what type of wear?

Answer 18

• 3rd body wear

Question 19

Describe the characteristics of UHWMPE?

Answer 19

Tough Ductile Resilient Resistant to wear Low friction Thermoplastic- altered by high temps