Mechanical Properties of the Human Achilles Tendon

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Mechanical Properties of the Human Achilles Tendon
Clinical Biomechanics 16 (2001)

• author Tishya A.L
• Advisor???
• presenter???

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abstract

• To determine whether the human Achilles tendon
  has higher material properties than other tendons
  and to test for strain rate sensitivity of the
  tendon.
• While the human Achilles tendon appears to
  experience higher in vivo stresses then other
  tendons.It is not known how the Achilles tendon's
  material properties compare with the properties
  of other tendons.

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• Modulus,failure stress, and failure strain were
  measured for excised human Achilles tendon loaded
  at strain rates of 1/s and 10/s.
• Failure stress and failure strain were higher at
  the faster strain rate, but no significant
  difference in modulus was observed.
• The material properties of the human Achilles
  tendon measured in this study are similar to the
  properties of other tendon reported in the
  literature despite higher stress imposed to the
  Achilles tendon in vivo.

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• The human Achilles tendon does not adapt to high
  in vivo stress by developing correspondingly high
  material properties. This leaves the tendon at an
  increased risk of injury and may help to explain
  the high incidence of Achilles tendon injuries.

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Introduction

• The Achilles tendon is one of the most frequently
  injured tendons in the human body.

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• It has been suggested that the Achilles tendon
  experience higher in vivo stress than most other
  tendons.
• Ker et al. used cross-sectional area ratios and
  estimated that while most tendons experience peak
  stresses below 30 MPa, the Achilles tendon
  experiences peak stresses around 67 MPa.
• Komi et al. reported peak stresses of 59 MPa for
  walking and 111 MPa for running.

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• Exercise can lead to increases in tendon modulus
  and strength while immobilization or stress
  shielding can lead to decreases in these
  properties.
• Material properties for many tendons
• modulus values 500-1850 MPa
• failure stress 50-125 MPa
• failure strain 13-32 for bone-tendon
• 5-16 for tendon

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• Most studies find higher failure stresses and
  failure strains at faster strain rates although
  changes in modulus are often not observed.
• Thermann et al. reported mechanical properties
  for two displacement rates equivalent to strain
  rates of approximately 3/s and 30/s.

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• for the faster rate
• mean failure stresses 41.4 MPa
• mean failure strains 44.3
• for the slower rate
• mean failure stresses 38.4 MPa
• mean failure strains 49.2

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• Based on the results of Thermann et al., it would
  appear that Achilles tendon has a lower modulus
  and strength than other tendons and that its
  properties are not sensitive to strain rate.
• In this study, try to determin whether the
  Achilles tendon has low modulus and strength, or
  high modulus and strength, as predicted by
  functional adaptation.

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Method

• Eleven pairs of fresh frozen Achilles
  tendons were procured from human donors aged
  35-82yr (mean 56.8, SD 13.3).

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• The tendon cross-sectional area was measured
  using ultrasound images.

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• Schematic of the testing setup.

10 cm
1.5 cm
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• Two strain rates were used.
• One specimen in each pair was tested at
  1 mm/s, and the contralateral specimen was
  tested at 10 mm/s.
• These rates correspond with approximate strain
  rates of 1/s and 10/s.
• The slower rate is considered quasi-static.
• The faster rate is representative of physio-logic
  activities such as walking.

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• A computer recorded force (F) and crosshead
  displacement (d) while a CCD camera recorded the
  movements.
• Stresses were defined as sF/Amin
• Strain were defined as ed/L0
• The failure load was defined as the maximum force
  recorded during each test.
• The displacement of failure was defined as the
  displacement at which the maximum load was
  achieved.

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• The failure stress was calculated as
• sfailFfail / Amin
• the failure strain for bone-tendon complex
• efail_complexdfail / L0
• the fail strain for tendon substance
• efail_tendon(lf l0) / l0

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Results

• There was no clear relationship between failure
  mode and age.
• The avulsed specimen had a mean age of 57.0 yr.
• The failed within tendon substance had a mean
  age of 56.8 yr.

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• Average properties were determined from grouped
  data at each for all specimens excluding those
  that failure by avulsion.

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• These properties are at the lower end of the
  ranges reported in the literature for other
  tendons.

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• The greatest strains occurred below the bottom
  marker.
• Strains between the bottom marker and the top of
  the PMMA block were usually in excess of 20 .
• This accounted for the higher failure strains of
  the bone-tendon complex compared with the failure
  strains of the tendon substance.

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Discussion

• We found that the modulus did not differ between
  the two rates, while the failure stress and
  failure strain increased approximately 15 at the
  faster rate.

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• Lewis and Shaw found that the modulus was higher
  at 100/s than at 10/s while failure stress and
  failure strain did not differ between rates.
• Embalming may have altered the mechanical
  behavior of the tendon.
• Differences in the strain rates used may have
  lead to differences between their results and
  ours.

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• This study considered two different measurements
  of failure strain.
• the entire bone-tendon complex
• tendon substance
• Consistent with previous studies, strains of the
  bone-tendon complex were higher then strains of
  the tendon substance.
• large part to large deformations near the
  calcaneal insertion below the bottom marker

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• Thermann et al. found that
• mean failure loads
• 4635 N (SD, 923) for strain rate of 3/s
  
• 4977 N (SD, 1168) for strain rate of
  30/s
• Authors results
• mean failure loads
• 4617 N (SD, 1107) for strain rate of
  1/s
• 5579 N (SD,1143) for strain rate of
  10/s

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• The failure stresses we measured were much higher
  then those reported by Thermann. This was due to
  higher cross-sectional area measurements in their
  study.
• Thermann
• mean area 127 mm2
• failure stress 41 MPa for the faster rate
• 38 MPa for
  the slower rate
• Author
• mean area 67 mm2
• failure stress 86 MPa for the faster rate
• 71 MPA for the slower
  rate

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• for failure strain
• Thermann
• Mean failure strains 44.3 for faster rate
• 49.2
  for slower rate
• Author
• Mean faulure strain 16.1 for faster rate
• 12.8
  for slower rate
• Thermann used approximately half the length of
  tendon that author used.

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• Ker estimated that the Achilles tendon
  experiences peak in vivo stress around 67 MPa.
• Komi reported a peak Achilles tendon stress of 59
  MPa during walking and 111 MPa during running.

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• Form literatures, Temperature effects are also
  expected to minimal.
• Human Achilles tendon does not possess high
  material properties even though it seems to
  experience high in vivo stresses.

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• THANK YOU!