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Dear All
Regarding to the discussion started by Marco Viceconti on the topic 1
"boundary conditions in bone stress analysis", we want to give our small
contribution.
The aim of a recent work of ours, submitted to Journal of Biomechanics, was
to investigate the effect of load simplification on the results predicted
by a finite element analysis of the bone-implant biomechanics during stair
climbing.
This investigation was made using a finite element model, fully validated
in a previous study, representing a human femur implanted with an
anatomical cementless stem.
Two load cases were compared: the reference load profile, derived from
literature (Bergmann, 1999), explicitly modelled the actions of the
principal muscles acting on the proximal femur during stair climbing; the
simplified load profile included only the hip joint reaction as measured in
vivo by an instrumented hip prosthesis (Bergmann, 2001).
In order to avoid boundary effects near the implant, the femur was modelled
down to the end of the diaphysis, which was assumed fully constrained.
The effects of the above load cases were evaluated by comparing:
bone-implant relative micromotion, bone-implant interface stresses (contact
pressure and frictional stress) and Von Mises equivalent stress (implant,
cancellous bone and cortical bone).
For each parameters the difference between peak values, the maximum
element-wise difference and the average (mean square) element-wise
difference were computed.
Results showed that under the action of the simplified load case peak
micromotion and peak Von Mises stress in the implant were substantially
unchanged (differences < 2%), while the peak Von Mises stresses in the
cortical bone increased of a 6%. The peak interface stresses increased of a
20%.
When compared element-wise, the relative micromotion differed of a 16% on
average. The average change for all three Von Mises stresses was 13% and
only 7% for interface stresses.
Data here obtained tend to confirm the acceptability of using only the hip
joint force, neglecting the muscle forces if the objective is to predict
the primary stability of a cementless stem, intended as the peak elastic
micromotion induced by a certain motor task.
On the contrary, if the study is aimed to investigate small differences in
the biomechanical behaviour of the bone-implant interface, with attention
to interface stresses or to the micromotion in very specific locations,
such load simplification may be used only to achieve a very gross estimate.
Because no methods are available to measure directly and non invasively the
force exerted by each muscle group during daily activities, from this point
of view the availability of a simplified model allows us to exploit from
literature the large data of hip contact force in-vivo measuraments and to
perform importan simulation studies.
Thus, we agree with all authors that stated that model complexity has to be
appropriate to the question investigated.
Best regards
*********************************
Dr. Ing. Alberto Pancanti, MSE
Laboratorio di Tecnologia Medica
Istituti Ortopedici Rizzoli
Via di Barbiano 1/10,
40136 - Bologna, Italy
tel: +39 51 6366554
fax: +39 51 6366863
e-mail: pancanti@tecno.ior.it
Home Page: http://www.ior.it/tecno/
*********************************
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For information and archives: http://isb.ri.ccf.org/biomch-l
---------------------------------------------------------------
Regarding to the discussion started by Marco Viceconti on the topic 1
"boundary conditions in bone stress analysis", we want to give our small
contribution.
The aim of a recent work of ours, submitted to Journal of Biomechanics, was
to investigate the effect of load simplification on the results predicted
by a finite element analysis of the bone-implant biomechanics during stair
climbing.
This investigation was made using a finite element model, fully validated
in a previous study, representing a human femur implanted with an
anatomical cementless stem.
Two load cases were compared: the reference load profile, derived from
literature (Bergmann, 1999), explicitly modelled the actions of the
principal muscles acting on the proximal femur during stair climbing; the
simplified load profile included only the hip joint reaction as measured in
vivo by an instrumented hip prosthesis (Bergmann, 2001).
In order to avoid boundary effects near the implant, the femur was modelled
down to the end of the diaphysis, which was assumed fully constrained.
The effects of the above load cases were evaluated by comparing:
bone-implant relative micromotion, bone-implant interface stresses (contact
pressure and frictional stress) and Von Mises equivalent stress (implant,
cancellous bone and cortical bone).
For each parameters the difference between peak values, the maximum
element-wise difference and the average (mean square) element-wise
difference were computed.
Results showed that under the action of the simplified load case peak
micromotion and peak Von Mises stress in the implant were substantially
unchanged (differences < 2%), while the peak Von Mises stresses in the
cortical bone increased of a 6%. The peak interface stresses increased of a
20%.
When compared element-wise, the relative micromotion differed of a 16% on
average. The average change for all three Von Mises stresses was 13% and
only 7% for interface stresses.
Data here obtained tend to confirm the acceptability of using only the hip
joint force, neglecting the muscle forces if the objective is to predict
the primary stability of a cementless stem, intended as the peak elastic
micromotion induced by a certain motor task.
On the contrary, if the study is aimed to investigate small differences in
the biomechanical behaviour of the bone-implant interface, with attention
to interface stresses or to the micromotion in very specific locations,
such load simplification may be used only to achieve a very gross estimate.
Because no methods are available to measure directly and non invasively the
force exerted by each muscle group during daily activities, from this point
of view the availability of a simplified model allows us to exploit from
literature the large data of hip contact force in-vivo measuraments and to
perform importan simulation studies.
Thus, we agree with all authors that stated that model complexity has to be
appropriate to the question investigated.
Best regards
*********************************
Dr. Ing. Alberto Pancanti, MSE
Laboratorio di Tecnologia Medica
Istituti Ortopedici Rizzoli
Via di Barbiano 1/10,
40136 - Bologna, Italy
tel: +39 51 6366554
fax: +39 51 6366863
e-mail: pancanti@tecno.ior.it
Home Page: http://www.ior.it/tecno/
*********************************
---------------------------------------------------------------
To unsubscribe send SIGNOFF BIOMCH-L to LISTSERV@nic.surfnet.nl
For information and archives: http://isb.ri.ccf.org/biomch-l
---------------------------------------------------------------