# A medic needs some physics help from you smart people...

#1
Hi everyone

I'd like to apologise in advance for my lack of any physics knowledge. I basically know nothing in physics and will need your help to get my head around a simple problem.

I have to write a report on what factors determine the stiffness/rigidity of external bone fixation (see the picture). When someone gets a fracture their bone is fixed like that with screws and a rod, to bring the bone is closer together helping healing.

We used plastic bone models and changed variables like how close the pins are to the fracture site, how many rods there are and how far are these rods are from the skin. We then used a machine to compress the bone to measure stiffness/rigidity (see picture for machine).

This then generated some data where I have: time (s), load (n) and extension (mm). I did some reading and it seems that stiffness = change in force / change in displacement. So stiffness (n/mm) = load/extension.

So essentially I'm plotting a graph of load against extension and then drawing a line of best fit? Does the gradient of the line of best fit = stiffness?

What else can I do with this data?

I am going to use the value I get for stiffness as the output and compare this for different models (i.e. comparing ones with pins close to fracture/pins far away from fracture).

Am I on the right track?
0
6 years ago
#2
Yep by A level knowledge stiffness is = force/displacement (extension). It's in metres not mm so convert that. Graph is a bit weird since you would normally put the variable you change (force) in the X axis which would make the gradient 1/stiffness.
0
#3
(Original post by Vikingninja)
Yep by A level knowledge stiffness is = force/displacement (extension). It's in metres not mm so convert that. Graph is a bit weird since you would normally put the variable you change (force) in the X axis which would make the gradient 1/stiffness.
Thank you I will change extension to meters.
would you recommend doing load on the x axis or extension then? does it actually make a difference? all the example graphs I've seen have plotted extension on x axis.

btw: this is for an undergrad assignment, I thought the question I was asking was probably too simple for that.
0
6 years ago
#4
(Original post by elephantalkali)
Thank you I will change extension to meters.
would you recommend doing load on the x axis or extension then? does it actually make a difference? all the example graphs I've seen have plotted extension on x axis.

btw: this is for an undergrad assignment, I thought the question I was asking was probably too simple for that.
I'd go with the conventions used in medical literature, lab handouts or whatever.

the usual rule for experimental results it's the dependent variable going on the y axis and the independent variable on the x axis but force/extension graphs are often an exception...

http://www.antonine-education.co.uk/...als_page_2.htm
1
#5
(Original post by Joinedup)
I'd go with the conventions used in medical literature, lab handouts or whatever.

the usual rule for experimental results it's the dependent variable going on the y axis and the independent variable on the x axis but force/extension graphs are often an exception...

http://www.antonine-education.co.uk/...als_page_2.htm

Thank you
Now how do I compare the results I get?
So if I get something like:
Bone 1 = 10
Bone 2 = 15
Bone 3 = 50

Is there any statistical test to see if the difference is significant?
0
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