Chapter 51 - Biomechanics and Wear in Joint Arthroplasty Flashcards
what type of wear predominantly occurs in THA?
adhesive wear
what type(s) of wear predominantly occurs in TKA?
delamination, abrasion, adhesive wear
Peak contact force during walking in the hip is what percentage of body weight?
250%
what every day movement causes the highest joint reactive forces?
descending stairs
Peak contact force in the hip during push off phase of running is what percentage of body weight?
520%
Increased offset causes what effect of joint reactive forces?
increased offset = longer moment arm -> Decreased joint reactive forces
what does high levels of free radicals do to wear properties of ultrahigh molecular weight polyethylene?
results in oxidative degradation of the poly
increased wear rates, increased delamination, increased gross fracture
what levels of radiation were used for poly sterilization?
2.5-4 MRad
what levels of radiation are used for inducing crosslinking of PE?
5-10 MRad
What is remelting?
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
what is annealing?
poly heated below its melting point
more crystallinity is retained
less free radicals are removed
what is the result of cross linking compared to conventional poly on wear rates
highly cross linked polys have substantially lower wear rates than conventional poly
what poly wear rate is associated with very low risk of osteolysis
<0.1mm/yr
what does remelting do to wear characteristics?
reduced fracture toughness/fatigue toughness
what is the only type of cancer shown to have increased incidence in patients with metal on metal bearings?
Leukemia
pure zirconia heads demonstrated what wear characteristics compared to alumina?
significantly higher wear rates -> 0.17mm/yr (zirconia) compared to 0.07mm/yr(alumina)
what was the thought behind zirconia toughened alumina
excellent wear properties of alumina with decreased propensity for fracture by adding in the zirconia
risk factors for failure of alumina bearings
Both: obesity
Head:
- small head size - 28mm
- short neck
cup:
- excessive anteversion
following ceramic head fracture, what cant you do for the revision?
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
risk factors for trunnionosis
Increased BMI
Larger head size
smaller neck diameters
smaller neck tapers
longer neck lengths
lower rigidity
crevice corrosion
presence of small gaps allows fluid to enter and creates differences in oxygen tension
stainless steel most susceptible
titanium least susceptible
fretting corrosion
micromotion at the head neck junction
commonly implicated in trunnionosis
galvanic corrosion
two dissimilar metals in the presence of fluid causes electrochemical gradient
most commonly seen when titanium trunnion mated to cobalt chrome head
acl tears cause what pattern of knee pain/arthritis?
anterolateral pain
anterior on the lateral femoral condyle
Where does delamination occur in TKA wear?
along the poly surface as it articulates with the femur
where does abrasive and adhesive wear occur in TKA
debris formed by micromotion between poly and the tibial backplate, or between the post and the femur
pathophysiology of osteolysis
macrophage mediated response to debris -> releases cytokines -> activates osteoclasts to resorb bone
between ram extrusion and compression molding of tka poly what are the different wear properties?
similar wear rates
compression molded polys have a lower susceptibility to fatigue cracking and crack propagation
