Hip Replacement 2

Question 1

What is bone cement made from?

Answer 1

PMMA - polymethyl methacrylate

Question 2

How is bone cement made?

Answer 2

monomer powder mixed with a volatile agent containing catalyst which makes a doughy substance

Question 3

What additives can be put into bone cement?

Answer 3

antibiotics

radio-opaque materials

Question 4

Is PMMA an adhesive?

Answer 4

No - a filler like grout

Question 5

What can be done to increase cement keying?

Answer 5

roughened prosthesis
beaded coatings
wire coating

Question 6

Name advantages of cemented prostheses

Answer 6

no need for exact fit as bone cement will fill the holes = less rasping and less bone loss

provides an even stress distribution as cement fills all holes

Question 7

Name some problems with cement

Answer 7

exothermic curing reaction can damage tissues

small fragments can cause intense inflammation

wear particles in the joint bearings rapidly increase wear

bone cement is weaker in tension and shear and such stress can cause loosening

Question 8

why is hydroxyapatite coating of interest?

Answer 8

could mean direct bonding between the bone and prostheses

Question 9

Why does a fibrous layer form between the bone and the prostheses?

Answer 9

micromotion - accurate fit and controlled weight bearing are important in prevention

Question 10

How can the longevity cemented protheses be increased?

Answer 10

techniques to increase keying of cement

coat metal components with PMMA which can then bond with inserted cement

combine PMMA coating with a cement that bone can bond to

remove cement altogether and allow an interference fit

Question 11

Why is the non-bonded interface a problem?

Answer 11

high amounts of motion releases wear particles which cause biological reaction

also increases wear at the joint bearings

fibrous layers can form

Question 12

Why can cement be removed from load calculations?

Answer 12

it takes such little load it can be ignored

Question 13

What is the advantage of an isoelastic stem?

Answer 13

they have the same stiffness as bone

Question 14

What is the main disadvantage of isoelastic stems?

Answer 14

high proximal load transfer leads to failure of the interlocking and chemical bonds at the interface

Question 15

What effect does cement have on load transfer?

Answer 15

allows good contact between the stem and bone therefore reducing stress concentrations

Question 16

What arguments are presented by Harris, Boston, USA?

Answer 16

proximal collar allows compressive load transfer. The collar prevents sinkage and the bone-cement interface is usually well interlocked so no-need to have slip

Question 17

Why does Ling disagree with Harris?

Answer 17

Ling says the collar-calc area acts as a pivot for the stem to rotate causing high stress concentrations distally and failure. Also debris produced by fretting at the collar. Cement metal interface is weak and therefore it should be free to slide

Question 18

Which of Harris and Ling have prevailed?

Answer 18

Neither - and both cement and cementless stems are used successfully

Question 19

What fit do cementless stems rely on?

Answer 19

press/interference fit

Question 20

What can be used to coat cementless stems?

Answer 20

HaP

Question 21

What causes thigh pain in cementless stems?

Answer 21

lack of distal contact

Question 22

Why does bone ingrowth into cementless stems sometimes not occur?

Answer 22

excessive movement - can cause growth of a fibrous layer

Question 23

Why is it useful to completely coat the stem in HaP?

Answer 23

helps bone ingrowth and potentially eliminates metal debris from bone- metal abrasion. It also gives the opportunity for the bone to bond to a larger area of the stem which lessens the chance of failure of the bond under subsequent loading.

Question 24

Why is the shape of the stem important in cementless stems?

Answer 24

there needs to be excellent bone-stem contact

Question 25

What effect does head offset have on load transfer?

Answer 25

reduces the bending moment and increases compression

Question 26

how can head offset be reduced?

Answer 26

decreasing neck length

increase offset angle

Question 27

What is the main problem with reducing femoral head offset?

Answer 27

increases the joint reaction force giving rise to greater wear and acetabular-bone-implant stesses.

Question 28

what equation explains the increase in J as a result of reduced femoral head offset?

Answer 28

J = W*d/x

Question 29

What two factors influence friction?

Answer 29

surface properties of the two materials in contact

the magnitude of the load pressing them together

Question 30

How many times less friction does a synovial joint have over a HDP-metal bearing?

Answer 30

3-100x

Question 31

Why are small femoral heads used?

Answer 31

smaller surface area = less friction = less shear force

Question 32

What is adhesive wear?

Answer 32

when two bearing surface stick to each other when they are pressed together, and, the softer one is torn off by the harder one

Question 33

What is abrasive wear?

Answer 33

occurs when surfaces are not perfectly smooth

Question 34

How can adhesive wear be reduced?

Answer 34

use bearing surfaces with low adhesion and provide lubricants

Question 35

How can abrasive wear be reduced?

Answer 35

good circulation of lubricant

highly polished surfaces

Question 36

What is the problem with HDP wear particles?

Answer 36

they cause intense inflammatory reactions

Question 37

What is the main cause of aseptic loosening?

Answer 37

HDP wear particles