Investigating Cartilage Stress

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Investigating Cartilage Stress

• Dennis Cody
• November 22, 2004

2
Outline

• History
• PTC Pro/Engineer
• Stanford
• VA Investigation of Stress in Cartilage
• Description of Patellofemoral Pain.
• Determine when bone can be assumed rigid.
• Understand apparent discrepancies in literature.

PTC
Stanford
VA
3
Parametric Technology Corporation

• 5 years in Quality Assurance Senior QAE
• Software Pro/Engineer, Pro/Mechanica,
  Pro/Intralink
• Transition to new defect tracking database
• Desire to enhance peoples lives and health

PTC
4
Stanford Design and Prototype Tools for
Surgical Procedure

• Implant pegs placement, depth, diameter, angle
• Method of creating peg holes
• Tools
• Cutting Block
• Depth Resection Gauge
• Drilling Template
• Template Impactor

• Posterior Peg Impactor
• Tibial Trial
• Tibial Spacer

PTC
Stanford
5
Outline

• History
• PTC Pro/Engineer
• Stanford
• VA Investigation of Stress in Cartilage
• Description of Patellofemoral Pain.
• Determine when bone can be assumed rigid.
• Understand apparent discrepancies in literature.

PTC
Stanford
VA
6
Outline

• When looking at stresses in cartilage

• Determine when bone can be assumed rigid.

• Understand apparent discrepancies in literature.

PTC
Stanford
VA
7
Motivation - PFPS

• What is PF Pain Syndrome?
• Anterior knee pain
• Associated with repetitive exercise
• Cause difficult to determine
• Muscle imbalance
• Attachment
• Bone shape, alignment

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Objective
VA

• Obtain PF joint data in young adult volunteers
  using non-invasive techniques.
• Kinematics
• Kinetics
• Contact areas
• Stresses
• Focus From static MR Images, create a finite
  element model that can be used for analyses

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Hypothesis
VA

• Subjects with PF pain will have elevated
  cartilage stresses (compared to age and activity
  matched subjects without PF Pain), either because
  of increased PF forces and/or decreased PF
  contact areas.
• Assumption to test When looking at patellar and
  femoral cartilage stresses due to physiological
  loads, the underlying bone can be treated as a
  rigid material.

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Background
VA

• Modeling and solving models with bone elements is
  expensive.
• Some studies consider bone as a rigid material.
  (Li et al., 2001, Zhang et al., 1999)
• Others consider the bone elements.
• (Beaupré et al., 2000, Brown et al., 1984)

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Background Previous Work
VA

• 3D Model of tibio-femoral joint (Donahue et al.,
  2002)
• Model with bone
• Model with rigid backing
• No difference of more than 2

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Background Previous Work
VA

• Ideal model with plug and indentor (Brown et al.,
  1984)
• Cancellous bone modulus value impacts effect of
  rigid implant in bone
• Impactor has small radius
• smaller than in PF joint?

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Background Previous Work
VA
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Methods
VA

• Contact formulation
• Plane Strain
• Cancellous modulus
• Subchondral bone thickness
• Cartilage bone interface radius

Figure modified from Beaupré et al., 2000.
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Model
VA

• Model hemisphere contacting a plate
  (axisymmetric)
• Allows curved and flat surface
• Two models
• Cartilage and bone elements

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Model
VA

• Model hemisphere contacting a plate
  (axisymmetric)
• Allows curved and flat surface
• Two models
• Cartilage and bone elements
• Cartilage with rigid backing

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Methods Plane Strain
VA

• Master-Slave surface

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Methods Plane Strain
VA
Remove ABAQUS Series

• Master-Slave surface
• Hertz contact

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Methods Plane Strain
VA

• Master-Slave surface
• Hertz contact
• Compare with results from Beauprés PE model

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Methods Plane Strain
VA

• Master-Slave surface
• Hertz contact
• Comparison with results from Beauprés PE model.
• PE vs.

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Methods Axisymmetric
VA

• Master-Slave surface
• Hertz contact
• Comparison with results from Beauprés PE model.
• PE vs. Axisymmetric

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?Stress With ?Radius
VA
r 20.5 mm 2r 40.5mm F 230N Cart thk 3.5
mm Subch bone 0.5 mm Canc modulus 600 MPa
2r
r
s 3-max 401 kPa
s1-max 432 kPa
s 4-max 407 kPa
s 2-max 396 kPa
2r
r
s5-max 449 kPa
s 7-max 414 kPa
s 8-max 420 kPa
s6-max 413 kPa
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?Stress With ?Load
VA
r 40.5 mm F 115N 2F 230N Cart thk 3.5
mm Subch bone 0.5 mm Canc modulus 600 MPa
s5-max 300 kPa
s6-max 294 kPa
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Stress Patterns
VA
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Trends in Results
VA
OI Osteogenic Index k sOctahedral Shear
sHydrostatic (k 0.35)
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Summary

• Contact model ran successfully in Abaqus
• Rigid assumption valid for healthy young
  subjects, probably not for osteoporotic subjects
• Model differences explain difference in results

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Thank You