Conformity and Contact Stresses

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Conformity and Contact Stresses
Load
R1
R2
Conformity is determined by the radial clearance
between articulating components.The contact area
influences the magnitude of stresses experienced
by both components of total joint
replacements.The hip joint is more conforming
than the knee joint.
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Contact Stress Hips Vs. Knees
Worse
Knees
PeakStress
Hips
Better
0
5
10
15
20
Thickness (mm))
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Different damage modes occur in different devices
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Stresses in Total Knee Replacements
Contact Pressure
Max. Principal Stress
Greenwald et al., AAOS, 1997
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Uniaxial Yield Strength of UHMWPE
LCS rp
Ortholoc III
Miller/Galante I
LCS mb
Natural knee
105? Flexion 4 x Body Weight
Kinemax
Insall/Burstein II
Miller/Galante II
Kinematic
PCA I
PCA II
0
10
20
30
40
50
Maximum Contact Pressure (Mpa)
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CoCr Knees Roughen

• Thirteen consecutively retrieved CoCr clinical
  components
• All condyles had scratches, some oblique to
  articulation
• (Levesque et al. ORS, 1998)

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UHMWPE in Total Knees Clean vs. Abrasive
Conditions. Widding K, Scott M, Jani S, Good V.
ORS 2003

• Cross linked and conventional polyethylene
• Knee simulator testing
• Roughened femoral components

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Abrasive Tumbling
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Knee RoughnessLevesque et al, ORS, 1998
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Clean and Abraded Knee Wear
84
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Device Longevity Issues

• Design plays a role in conformity and contact
  pressures
• Materials selection dictates elastic modulus,
  Poisson ratio, yield stress (as well as fatigue,
  fracture, wear and corrosion properties)
• Manufacturing- surface finish, sterilization,
  etc.
• Patient activity also determines magnitude of
  stresses and loading cycles on device