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2 screws orthogonal to each other -> compression

  1. Jun 26, 2010 #1

    I just fnished med school and I have been conduction biomechanical research for the past 3 years now. I am currently working on a project plan and I am encountering the following problem:

    We would like to measure compression across a joint, using partially threaded screws. For the standard procedure, 3 screws, more or less orthogonal to each other, are used. From a mechanical point of view, a second screw, orthogonal t the first one, should not add any any compression as the energy is absorbed by the first screw.

    I would like to ask you 2 questions:
    1) which law / mechanical rule has to be applied in the case mentioned above
    2) i would like to illustrate this in a presentation ... any ideas?

    Thank you for your help in advance!

    Kind regards,


    Attached Files:

  2. jcsd
  3. Jun 26, 2010 #2
    Google Poisson's ratio, that might help.

    How do you measure the compression? A load cell?
  4. Jun 26, 2010 #3
    thank you for your reply. We are actually using pressure sensitive film (fuji) (which, in this case, is more suitable then Tekscan)

    Regarding Poisson's ratio: looked through the wikipedia article, but I did not really get it. I thought it was rather the screw itself than the deformation of the material.

    I would very much appreciate it, if you could give me some more assistance!


  5. Jun 26, 2010 #4
    Sebastian, I don't really understand what you are doing, what is the purpose of the device, what do the screws do? Does it involve bones?
  6. Jun 27, 2010 #5
    the goal is the fusion of 2 bones. Independent of the bones, one believes that compression (contact area * force) is one of the depending factors for a successful fusion.
    Usually partially threaded screws are used. Depending on the side of fusion, 2 or 3 screws are used which a inserted from different direction, but more or less orthogonal to each other.

    I was told, that once one screws is inserted and therefore compresses the two bones, a 2nd or 3rd screw, orthogonal to the 1st one, only adds stability (rotation, flexion) but is not able to produce greater compression.

    I am now wondering whether this is true (even though I do believe so), which mechanical law has been applied and whether there is a nice way of illustrating this for a presentation.


  7. Jun 27, 2010 #6
    I'm not sure either. The screws can apply compression if, when installed they, are put into tension, that should occur along the narrower, unthreaded part of the screw presumably nearest the screw head. The screw head has to seat such that the compressing force is applied at that point (where the screw head bears). The threads should only be active in the furthest piece, the the threaded part of the screw crosses the joint it will tend to resist compressing the joint.

    If I were doing this with wood (because I wouldn't want to be the one who stripped the threads out of someones bone), I'd put all three screws in and then start tightening (tensioning) them a bit at a time like lug nuts on a wheel.

    And yes, the orthogonal screws can all contribute to the compression force. Look into vector addition.
  8. Jun 27, 2010 #7
    what about shear forces?
  9. Jun 27, 2010 #8
    What about them? There is friction (ouch).
  10. Jun 27, 2010 #9
    The tension in the second screw will result in pressure somewhere. So the answer could go various ways depending upon some details.

    Referring to you drawing, how are the screws involved with the upper bone: do the screws thread through both bones or is the upper bone drilled with thread clearance?
  11. Jun 28, 2010 #10
    I am no engineer, therefore pls excuse silly answers.

    The screws are partially threaded and the threads only engage with the lower bone
  12. Jun 28, 2010 #11
    OK. In this case the pressure will be increased by a second screw. However, if there is any thread engagement at all in the upper bone there won't be a tendency for the screw to pull the two parts together, but only hold them in proximity.

    The mechanics is not much different than working with wood, just a bit wetter.
  13. Jun 28, 2010 #12
    I concur with that phrak. Sebastian, you might google free body diagram too, there is also free software for simple animation but I've not used it.

    The sum of the pressure times the area of contact (not constant, increases closer to each screw) equals the sum of the force from the three screws along the same axis.
  14. Jun 29, 2010 #13
    Sardonicus, by same axis, I hope you should mean the normal vector to the area over which the pressure acts, give or take a minus sign. But the area where bones contact is not planar, nor does the bone likely meet over a continuous surface, so there are more approximations.
  15. Jun 29, 2010 #14
    Yes, the force vectors are parallel, colinear preferably. Which bones are you screwing together Seb?

    I know little about bones just what I gleen from gnawing on them (not human bones), and from pictures and TV. Seems they are often hollow, or at least the marrow is soft. My dog (incidently named Bones) can lick the marrow out of bones. So generally the best part of the bone to screw into is the hard outer wall or shell correct?

    Seems like it would be difficult to put screws in effectively at least in a regular tubular bone. Screwing in nomal to the surface through a hole in a metal plate would provide better anchorage maybe.
  16. Jul 19, 2010 #15
    I think the answer could go various ways depending upon some details.
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