cement

· methyl methacrylate

· self-curing acrylic cement

· formed upon mixing of a powder with a liquid preparation

presentation

Powder

· 40g packet

· polymethyl methacrylate

· powdered polymer granules

· benzoyl peroxide

· initiator

· initiates polymerisation of monomer

· barium sulphate

· radiopacifier

Liquid

· 20ml phial

· methyl methracylate

· liquid monomer

· dimethyl toluidine

· activator

· promotes polymerization

· hydroquinone

· stabiliser

· free radical scavenger

· prevents spontaneous polymerisation

· ascorbic acid

· inhibitor of spontaneous polymerisation of monomer

· colouring

· green chlorophyll in Palacos

Mixing

· powder and fluid mixed together

· add liquid to powder except for Palacos

· should mix slowly for 90 sec.

· produces dough

· monomer takes polymer into solution

· activator (toluidine) cleaves initiator (peroxide)

· reaction releases free radicals

· viscosity of dough increases

· heat given off (exothermic reaction)

· dough changes from fluid to solid

· variable temperature-sensitive time

· time increased with decreased temperature and increased humidity

· pressurisation increases strength

· cement mass is an aggregate of previously polymerized granules stuck together with the newly polymerized monomer

properties

Mechanical properties

· amorphous polymer which is

· weak in tension

· strong in compression

· elastic modulus between that of cancellous and cortical bone

· no adhesive properties

· strength affected by

· laminations

· entrapped blood

· attempts to improve tensile strength have been attempted

· vacuum mixing and centrifugation

· increased laboratory tensile strength

· unclear effect on longevity of cemented hip arthroplasty

· has viscoelastic properties at body temperature

· this confers on it a number of properties

· load spreading

· shock-absorbing

· decoupling

Volumetric changes at polymerisation

· monomer contracts when it polymerises

· contracts 20%

· monomer makes up 1/3 of cement

· leads to 6-7% contraction of cement

· rise in temperature causes thermal expansion

· of cement itself

· of air or vapour in voids in dough

· overall effect is small overall shrinkage

Porosity

· varies from 10-30%

· increased porosity causes decreased fatigue life

· porosity can be decreased with

· vacuum mixing

· centrifugation

EFfect

Local

· produces heat

· reaches 40^o in femur and 90^o in acetabulum (Charnley)

· causes thermal necrosis of 0.5 mm of surrounding bone

General

· monomer enters circulation on pressurisation

· cleared by lungs

· metabolised to methacrylic acid and then to carbon dioxide

· can cause hypotension

· through peripheral vasodilatation and myocardial depression

· pressurisation can cause fat emboli

Antibiotic loaded cement

rationale

· antibiotic released in such a way that

· local concentrations much higher than can be achieved by parenteral therapy

· systemic concentrations negligible

· advantages are

· local concentrations vastly higher than MIC

· no systemic complications

antibiotics

Requirements

· antibiotic must be heat-resistant

· exothermic reaction

· antibiotic must be in powder form

· liquid inhibits polymerisation

· organism must be sensitive to antibiotic

· broad-spectrum antibiotics often used

Amount

· addition of small amounts of powder does not significantly alter tensile or compressive strength (but some effect on fatigue strength)

· up to 4.5g can be added without significantly altering strength of cement

· 0.5g to 2g usually added to 40g powder

Release

· elusion highest in first few hours

· decays with time

· may be present for 6 weeks

· elution rate varies with

· type of antibiotic

· concentration of antibiotic

· porosity of cement

uses

Primary THR

· antibiotic-loaded cement used to secure component

· used instead of systemic antibiotics

Advantages

· decreases periprosthetic infection

· does not increase rate of loosening

Disadvantages

· increases rate of superficial infection

· causes emergence of resistant strains

Conclusion

· should not be used routinely

Revision THR

Cement

· antibiotic-loaded cement used to secure component

· no excellent studies

· seems that is superior

Beads

· cement beads used to fill space

· can use greater amounts of cement

· drug delivery system

· no structural strength required

Spacer

· antibiotic-loaded temporary cement spacer

· complexity varies

· all-cement Thompson-type spacer

· stainless-steel endoskeleton coated with cement

· prevents shortening and may allow mobilisation

· results as good as or better than without

· success in 93.5% of cases