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jtchang70
09-22-2004, 06:22 AM
This is a question about study design ...

We are studying the mechanical characteristics of various external joint fixators meant to help support joints after injury/surgery. For our first phase, we would like to study the devices in isolation -- i.e., not an anatomical cadaver study, but a purely mechanical study. To do this, we are mounting the devices on rods/pipes -- although they could be called "bones", the rods/pipes are really just used to load the devices in a realistic way (e.g., load elbow fixator at wrist).

Our difficulty lies in deciding on simple but effective endpoints to report. How exactly do we define "good" and "bad"? I believe mechanically, what we want to find out is "stiffness" ... which is simple with a straight uniform beam with one load, but not so simple with a complex device that twists and bends. We are considering measuring the physical "endpoints" of the device, where it enters "bone" ... e.g., see how the ulnar pin location moves relative to the humeral pin. However, we would still need to translate that data to something more useful ... a surgeon looking at ulnar pin position/orientation changes can't make much out of that data.

One thought is to report the ulnar end movements in individual anatomical directions -- e.g., lateral translation/rotation, etc. However, splitting up the complex distortion into those parts may hide what's really happening at the joint. For example, a lateral ulnar translation plus a lateral rotation may actually mean the proximal end of the ulna (at the elbow joint) has moved medially. Nonetheless, on a purely mechanical level, is it useful to describe the ulnar end movements in this way?

Another thought is to define a hinge point, and use that as the elbow proxy ... then report translational/rotational changes of that point. But how do you pick that hinge point then? No matter how you do it, it's pretty arbitrary, because different fixators (or even the same fixator) can be pinned in a number of ways. For example, if you say "the intersection of the axes of the humeral rod/pipe and the ulnar rod/pipe" ... that assumes a certain location of those axes relative to the device.

Any thoughts would be greatly appreciated ... thanks!
johnny chang