Compression in material due to a tapered variable pitch headless screw

[rogueMech]

I had a surgeon I work with ask me if I could give him the mechanical details behind how surgical compression screws for bone work (ie

http://www.acumed.net/acutrak-2-mini).

Now the main features of these types of screws are:
- Variable pitch; screw tip has a coarse thread while the head has a fine thread. The relative displacement of the bone fragments creates compression (the coarse section displaces further with each turn relative to the fine section)
- Tapered shaft; allows the later threads to dig into new material as the screw is screwed.

Visualizing all this is easy, but I can't seem to wrap my head around the math/FEA to prove it. Essentially, I'd like to get the numbers behind the compression on the bone the screw creates...

Any ideas?

Cheers,

 

[dhengr]

You seem to kinda have your head around how it works physically. So, just pulling some numbers out of the air for this discussion, say the course threaded tip advances at a rate of 4mm/rev. (a function of the thread pitch) and the fine threaded back portion advances at a rate of 2mm/rev. On each revolution the tip will pull the bone in the back, fine thread, 2mm closer to the bone the tip is embedded in, thus closing the bone fracture separation. And, rather than the exact compressive force, it would seem to me that the Dr. would be most interested in knowing that he/she brought the two pieces of bone back into contact with some minimal force, and the correct orientation and fit. And, I assume they do that by experience and x-ray.

The exact compressive force or stress is a whole different animal. First, you need to know a number of the mechanical properties of the bone, and the dimensions and shapes of the bones. Then every shape of bone or orientation of the screws will give you a completely different structural configuration to deal with. In its simplest form you might take a 3" long piece of 1" dia. steel pipe and rip it in half lengthwise. Looking at an end view, the saw joints are at 3 and 9 o’clock. Then run the screw through the two halves of the pipe, at 12 and 6 o’clock, and study the forces and stresses produced with each 1mm that the screw pulls/draws the two halves together. This is akin to two point loads on two arches, and their stresses and deflections, but don’t forget that there are lateral thrusts from each arch at the sawed joint, tending to spread the bone laterally. So, you don’t want too much force or you might displace the joint fit-up. If the joints or the fracture are at 10 and 1 o’clock, or it’s a skewed fracture, you have a completely different structure to deal with. That’s a fairly complex problem in the real world. I would wonder that the Dr. might be more interested in some feeling for or value of torque for a given screw size vs. a given bone size for a snug compression fit-up.

 

[tbuelna]

rogueMech-

The mechanics behind that variable pitch/variable diameter screw thread are indeed quite complex. First of all, the variable pitch and variable diameter of the screw threads can definitely produce a compressive preload within the bone substrate engaged by the threads. And this compressive preload along the screw threads can be beneficial if the bone material is not subject to creep. But performing an accurate FEA would seem to be problematic due to the inherent variables presented by human bone material.

The concept of using variable pitch screw threads with fasteners in weaker materials is not new. This is commonly done with steel studs in aluminum parts.

 

[rogueMech]

Thanks dhengr & tbuelna. We have already characterized the relative displacement of the two bone pieces, but trying to characterize the compression created between two bone fragments is where we'd like to go. Most papers on the subject simplify the bone structure to two "bone bricks" being sandwiched together. Essentially, this means the problem is reduced to the situation where the screw enters perpendicular to the fracture plane (with the fracture plane being a smooth surface/interface).

Time to hit up the matlab & machine design theory.....

 

[paddingtongreen]

Why does everyone want to use FEA, it is useless unless you understand the problem.
Think of a bolt with two diameters, the narrow end threaded at say 8tpi and the thicker end threaded at 16tpi. Imagine that the bone fragments have been tapped appropriately. You put the bolt in and the narrow end engages. You turn the screw and it advances until it engages the thick part. Now, when you turn it more, say one turn, the thick part advances 1/16" but the narrow end advances 1/8". The bolt moves 1/16" so the narrow end bone must move 1/16" closer.
The difference between this and your cases is negligible, your tapered screw cuts its own threads but the action is the same.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[hydtools]

There is a video on this site that may be helpful:

http://www.synthes.com/sites/NA/Pro...Systems/Pages/Headless-Compression-Screw.aspx

Ted

 

[dvd]

Look at the patents on those devices.