how to make hoop stresses equal in the cylinder

[surgeonmunir]

I will be grateful for any help. We fix hip replacement parts to the bone using cement. This involves fixing a stainless steel component to the hollow of the bone using a grout which is cement. The stem is wedge shaped and has a smooth surface. The problem with this design is that with minimal trauma it splits the bone. I think this must happen because the hoop stresses are not equally directed in a cross section as well as along the length of the hollow of the bone because the ratio of the wedge taper is not proportional and there is quite a sudden decrease in the taper initially followed by a very slow decrease in the thickness.

I would like to know a method of proving this hypothesis and help in designing something that enables a more equal distribution of stresses. Pictures attached for understanding.

 

[rb1957]

if you're wedging the stem into the bone, i'm not surprised the bone can split ... I wouldn't've thought that bone was very strong in the transverse direction.

what if you supported the bone with something like a jubilee clip ?

btw, your attmt seemed to talk alot about hip joint replacements, but didn't (IMHO) illuminate your question any.

another day in paradise, or is paradise one day closer ?

 

[MikeHalloran]

You know those packs of tapered shims they sell in the home supply stores?
They're intended to fill the space between a door frame or window frame and the adjacent stud.
Installation goes as follows:
Gently push a pair of shims into the gap, at the same location, from opposite sides,
JUST FAR ENOUGH TO FILL THE GAP,
then drive a nail through the frame, the shims, and into the stud.
Repeat at several locations around the frame.
Then score the shims at the surface of the studs and the frame, and snap them off.
If you DRIVE the shims in, you will bend the frame, or the stud, or both; that's the wrong way to do it.

Given a small amount of axial force, the shims can exert a tremendous amount of radial force, because of their slim taper.

Similarly for the tapered stems of hip joint balls; hammer them into the cavity in the bone, and they will tend to split the bone.
So, avoid any trauma.
Patients should be dissuaded from parachute jumping or similar sports until the bone has bonded to the stem.
... or in the case of the Exeter stem, forever, because the claimed advantage of that polished stem is that it slowly drives itself further into the cavity, continuously, forever.

The article also vaguely and negatively references what I suspect were attempts at external reinforcement of the bone to prevent splitting, and asserts that the extant device is the perfect solution to all known problems associated with other similar devices.



Allow me to add a few words of caution.

I have worked with a few physicians, including a Nobel Prize winner. With one known exception, they have huge egos, bigger than you will see anywhere else. Maybe it's necessary to be successful at something as invasive as surgery. They tend to be _very_ defensive about their inventions. It goes beyond just putting their name on the device. Even the slightest hint of a suggestion that their work could be improved in _any_ way will be met with a _grossly_ disproportionate response. At a minimum, that response will include a demand that you be forced into the job market, and probably a huge temper tantrum of which you will remain unaware.

You are making a career decision.
Find something else to do.
Absent an M.D. and a Ph.D. in a related discipline, your suggestions will be ignored, or worse.






Mike Halloran
Pembroke Pines, FL, USA

 

[hendersdc]

I agree with the others that it isn't surprising the bone splits when a wedge is driven too far into it. As has been said, exterior reinforcement of the bone would be one option, you may also consider a more pliable cement or a two part solution consisting of an outer cylinder that is fixed to the bone (without an external taper) that has an internal taper that corresponds to the existing implant. This way any wedging/splitting motion is resisted by a presumably much stronger outer cylinder instead of the bone.

You could also add steps or undercuts to the sides of the existing taper. After cementing and healing these could transfer any axial force on the implant along the bone instead of radially into it. The wedging over time effect would be significantly reduced though which seems to be a selling point.

 

[hydtools]

This is like driving a wedge into bamboo, the bamboo will split. There needs to be a collar on the implant to transfer load to the end of the bone instead of driving the wedge deeper.

Ted

 

[Jughandle]

I think you are describing a bi-conic taper (rather than a 2-d wedge) that is being force fit into a cavity with matching internal tapers. And you want a uniform (or close to it) hoop stress thruout.

Disregarding any unfortunate and uncontrollable patient ailments (diabetes, osteoporosis, overweight) that will bias the results, the variables you have to work with are the remaining wall thickness of the bone, angles of the implanted tapers, lengths of the tapers, and the elusive coefficient of friction - a parametric study. Write the equations (probably simultaneous) with approximations then optimize variables for minimum hoop stress. Also supplement this with finite element models for a show ‘n tell. Intuitively, a structural fix would be to lengthen the implanted shaft and/or add a reinforcing band around the proximal femur as already mentioned.

To heed Mike’s warning above, you may want to pay special attention toward documenting everything in writing, depending on the work culture and the personalities of your organization.

Supposedly you have patient data that shows that the concept of seizing on a smooth taper is superior to one where bone growth is encouraged into a rough surface. I would have guessed the latter and like hender’s idea of undercuts. (Oh I hope I never need one of your products but if I do, I'm sure you will have perfected it by then.)