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Alison Mcconnell
07-08-2002, 05:51 AM
Hi there!

Thanks for all the wonderful replies and help I received from my posting seeking advice in deciding on loading rates for determining femoral stiffness. I'm sorry that I wasn't as prompt in replying as I had promised - once I got on the machine testing, we got rolling and couldn't stop!

In summary, it seemed a consensus that physiological loading was the ideal condition. That usually involves, during walking, full load being transferred in 0.1 to 1 second. Others suggested strain rates of 0.01 to 1 % strain per second. At the time of my original posting, I was still under the impression that weightbearing could be sustained nearly immediately post-op, but have since found out that it would be 6 wks to full weight bearing.

The desire for physiological loading must be balanced with the prevention of viscoelastic effects negatively affecting the repeatability of your experiment. We decided to go someone in between, and loaded at 8 mm/minute in axial loading, and 0.1 degree per second in torsional loading.

Thanks again for the wonderful feedback!
:)

Alison McConnell

Please find below my original posting and the replies I received.
ORIGINAL POSTING
*********************************************
Hi!
We are planning a study of the stiffness (load/deflection) of femoral
fracture fixation constructs in an cadaveric model.

I have been reviewing the literature in the area pertaining to femoral
biomechanics and so far haven't come across a justification for selection of
a particular loading rate. The range from 3 mm/minute (David SM et al, J
Ortho Trauma, 11, 1997) to 30 mm/minute (Koval KJ et al, J Ortho Trauma 10,
1996), to the displacement rate (i suspect quite fast) required to load at
10 N/s (Groebs A et al, ORS 2001).

During normal walking, Bergman et al (J Biomech 34, 2001 report reaching a
maximum vertical load of about 2.5 body weight in about 0.166s, or 15 BW/s
or 10 500N/s. I'm not sure what sort of displacement rate that would
correlate to, and i think the clinically relevant rate is the loading rate
anyhow.

Has anyone any suggestions in choosing a loading rate? As mentioned, we're
interested in characterizing the stiffness of the bone/implant construct.
Is chosing a slow loading rate going to give us a picture of the inherent
stiffness of the bone/implant construct, and take out the effect of
viscoelastic response (if that's an issue). Or is using a faster loading
rate more appropriate as it is closer to the clinical situation (ie.
walking)?

I look forward to your suggestions and will post a response promptly.

Thanks very much

Alison McConnell



REPLIES
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"Reed, Karen"
Alison,

I've done a bit of mechanical testing to determine bone material properties,
and have a few comments:

If you're trying to determine how the construct will behave in a physiologic
setting, use physiologic loading rates. Because of bone's viscoelasticity,
the stiffness is loading-rate dependent - most tests I've seen to determine
E load the bone from between 0.01 and 1.0 %strain/second.

Probably the safest thing to do would be to load at several (3?) different
rates so that you could comment about the effect of loading rate.

Another possibility would be to compare your construct stiffness to
identically tested whole bones (test the femurs, then fracture and fix, and
retest at the same strain rate). This would give you the best comparison.

If you want to comment on the stiffness of your bones compared to what's
already published, choosing a similar loading rate, or a series of rates
that span/include published rates would be best.

Good luck.

-Karen Reed
****************************************

i think physiologic loading is at 0.01 per second (strain rate) or less.

Alex

****************************************
Hi Alison,

We do sort of similar work here. We chose our variables
(compression/torsion) from physiologic data in the early post-operative
data. We did everything in load control also, as it is the more relevant
function.

Andrew

Andrew Mahar, MS
Orthopedic Biomechanics Research Center

Children's Hospital - San Diego
MC5054
3020 Children's Way
San Diego, CA
92123-4282
P: 858-495-4975 "For want of a nail,
F: 858-614-7494 the empire was lost."
amahar@chsd.org -proverb

****************************************

Dear Alison,

As usual, in the literature, you'll find a wide variety of loading rates.

The first question you should ask yourself is: what are you going to measure?
If you want to measure toughness in impact loading, you definitely
need a high rate. If you are going to measure strain in a
physiological condition, then your load will be pretty slow. While
toughness is significantly affected by loading rate, the strain
response is somewhat less sensitive to laoding rate (the elastic
component tends to predominate over the visco- phenomena).

It seems to me you are trying to replicate a physiological (i.e.
no-impact, no-trauma) condition. In my opinion, there are two
different criteria in defining the loading rate:


1) Physiological: you want to apply the load at a rate that is
similar to what occurs in vivo. If you look at the time-load curves
from telemetry (e.g. Davy, Bergman), you'll see that the time needed
to reach the laod peak value ranges between something like 0.1s and
1s, depending on the motor task and on the speed.


2) Technical: although viscous phenomena are less prominent for a
relatively slow loading, some creep always exists. In order to reduce
possible errors, in our protocol we apply the load in a relatively
slow ramp (maximum load is reached in 20s). Then the load is held for
3 minutes, to allow for settling of viscoelastic phenomena (creep
accounts for 2-5% of the total strain, in composite femora,
Cristofolini et al. (1996) "Mechanical validation of whole bone
composite femur models" J.Biomech 29(4): 525-535).

This way, you obtain a highly repeatable protocol and you overcome
possible problems related to:
- slightly different loading rates between specimens
- lack of synchronisation between strain gauge unit and material
testing machine
Additionally, this will increase reproducibility: if your test is
replciated by a different operator, with a different testing system,
it's easier to manage the test ina way that yields comparable results
if you rpoceed this way.

With this type of caution, we obtained a std between replicates on
the same specimen of less than 0.8%, and between specimens of less
than 11%.

Some references to this work:
Cristofolini L. (1997) "A critical analysis of stress shielding
evaluation of hip prostheses" Crit Reviews. Biomed. Eng. 25: 409-483
Cristofolini L. Viceconti M. (1999) "Towards the standardisation of
in-vitro load transfer investigations of hip rpostheses. A
reproducible protocol and its applciation" J. Strain Analysis Eng.
Des. 34: 1-15


In both case, I would not start focussing on the actuator speed
(although in the end that is what you will actally be setting), but
on the time scale.


Hope this helps. Best regards,

Luca

****************************************
giwan

Alison
Though the faster rate may more closely mimic walking a patient that just had surgery is not walking immediately on their leg. Weight bearing is initially minimized as is motion. I would expect the slower rate to be more realistic.
If you are interested in what is happening post initial healing then the faster rate would seem to apply. What type of implants are you looking at? Is this for comparison purposes or investigative?
****************************************






Alison McConnell
Research Engineer
MOBL- Martin Orthopaedic Biomechanics Laboratory
St Michael's Hospital - fully affiliated with the University of Toronto
W. Annex 1027
36 Shuter St
Toronto, ON
M5B 1A6

416 864-5579 (office)
416 864-5482 (lab)
416 359-1601 (fax)

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