Chapter 51 - Biomechanics and Wear in Joint Arthroplasty

Question 1

what type of wear predominantly occurs in THA?

Answer 1

adhesive wear

Question 2

what type(s) of wear predominantly occurs in TKA?

Answer 2

delamination, abrasion, adhesive wear

Question 3

Peak contact force during walking in the hip is what percentage of body weight?

Answer 3

250%

Question 4

what every day movement causes the highest joint reactive forces?

Answer 4

descending stairs

Question 5

Peak contact force in the hip during push off phase of running is what percentage of body weight?

Answer 5

520%

Question 6

Increased offset causes what effect of joint reactive forces?

Answer 6

increased offset = longer moment arm -> Decreased joint reactive forces

Question 7

what does high levels of free radicals do to wear properties of ultrahigh molecular weight polyethylene?

Answer 7

results in oxidative degradation of the poly

increased wear rates, increased delamination, increased gross fracture

Question 8

what levels of radiation were used for poly sterilization?

Answer 8

2.5-4 MRad

Question 9

what levels of radiation are used for inducing crosslinking of PE?

Answer 9

5-10 MRad

Question 10

What is remelting?

Answer 10

process to decrease free radicals

bringing a poly beyond it’s melting point, changing the poly from its partial crystalline state to its amorphous state

this DECREASES wear properties of the poly

Question 11

what is annealing?

Answer 11

poly heated below its melting point

more crystallinity is retained

less free radicals are removed

Question 12

what is the result of cross linking compared to conventional poly on wear rates

Answer 12

highly cross linked polys have substantially lower wear rates than conventional poly

Question 13

what poly wear rate is associated with very low risk of osteolysis

Answer 13

<0.1mm/yr

Question 14

what does remelting do to wear characteristics?

Answer 14

reduced fracture toughness/fatigue toughness

Question 15

what is the only type of cancer shown to have increased incidence in patients with metal on metal bearings?

Answer 15

Leukemia

Question 16

pure zirconia heads demonstrated what wear characteristics compared to alumina?

Answer 16

significantly higher wear rates -> 0.17mm/yr (zirconia) compared to 0.07mm/yr(alumina)

Question 17

what was the thought behind zirconia toughened alumina

Answer 17

excellent wear properties of alumina with decreased propensity for fracture by adding in the zirconia

Question 18

risk factors for failure of alumina bearings

Answer 18

Both: obesity

Head:
- small head size - 28mm
- short neck

cup:
- excessive anteversion

Question 19

following ceramic head fracture, what cant you do for the revision?

Answer 19

you cannot convert to a metal on poly bearing, you MUST do either a ceramic on poly or ceramic on ceramic to prevent third body wear and head erosion

Question 20

risk factors for trunnionosis

Answer 20

Increased BMI
Larger head size
smaller neck diameters
smaller neck tapers
longer neck lengths
lower rigidity

Question 21

crevice corrosion

Answer 21

presence of small gaps allows fluid to enter and creates differences in oxygen tension

stainless steel most susceptible
titanium least susceptible

Question 22

fretting corrosion

Answer 22

micromotion at the head neck junction
commonly implicated in trunnionosis

Question 23

galvanic corrosion

Answer 23

two dissimilar metals in the presence of fluid causes electrochemical gradient

most commonly seen when titanium trunnion mated to cobalt chrome head

Question 24

acl tears cause what pattern of knee pain/arthritis?

Answer 24

anterolateral pain

anterior on the lateral femoral condyle

Question 25

Where does delamination occur in TKA wear?

Answer 25

along the poly surface as it articulates with the femur

Question 26

where does abrasive and adhesive wear occur in TKA

Answer 26

debris formed by micromotion between poly and the tibial backplate, or between the post and the femur

Question 27

pathophysiology of osteolysis

Answer 27

macrophage mediated response to debris -> releases cytokines -> activates osteoclasts to resorb bone

Question 28

between ram extrusion and compression molding of tka poly what are the different wear properties?

Answer 28

similar wear rates

compression molded polys have a lower susceptibility to fatigue cracking and crack propagation