Volumetric wear

[Lopsided]

Lop, assuming your diameter is 50, you need to divide that by 2 in your calculation to get the radius.  That puts your result off by a factor of 4, so you'll have to start worrying in 50 years.  I hope you're prepared for that.   ;)

Aha! Yes Mike, you are right. I faffed that one up didn't I. Oh dear, only get fifty years.

Well I hope as Dr. De Smet says that wear should go down each year. I will be pretty upset if I need revision when I am ninety seven.

Lop-

I am sorry I have to flunk you on your math. 

First of all you used the diameter instead of the radius in the formula.  So using your femoral head size of 50 a hemisphere would have an area of 2 X 3.1416 X 25 X 25 = 3,927 square mm.

Now you complicated it with wanting to calculate the area of that actual implant which is not a complete hemisphere.  You provide the angle of measure as 170 degrees.  This means we are 5 degrees short of a hemisphere.  The depth of the device then is 25 - sin(5) X 25 = 22.82.

There for the area of your spherical cap is 2 X 3.1416 X 25 X 22.82 = 3,585.

I like your optimism that you will only lose the minimum of 3 cubic mm.  I think Mike was thinking even in the worse case of a loss of 18 cubic mm the reduction in thickness is minimal.

You cause me to think about several interesting questions.  Are the devices true spherical caps?  Is the wear even across the surface?  At what increased spacing due to wear does the device become problematic?

Dan

Yes, yes Dan, I did indeed mess that one up royally by using the diameter.

I did decide to use the fraction 170/360 instead of using sin((180-170)/2) (or whatever) as shallow angles tends towards a ratio anyway.

I most definitely did, however, make the assumption that wear should be even. The two surfaces must remain spherical. If the wear was not even, then they device would quickly fail - as does happen with edge loading.

[Dannywayoflife]

So realistically it looks promising to last a looooong time then! Hell as I've said before if I get 20 years out of one I'd be mega happy!

[ScubaDuck]

Danny-

Thanks for putting it back in perspective.  I can get all caught up in the math.  At the end of the day it appears that even at the highest levels of wear that they recorded it appears we would get several decades of use.  I can live with that.

Dan

[phillwad]

Great thread and thanks - makes me feel better and if I get past 100 yr old with these  - then old - bits of metal in both hips I will be smiling and thinking of this math - thanks

[Tin Soldier]

Dan - Units, units, units!

Also, again I'll reference De Smet's paper on comparison of the BHR and the C+.  I think these things are pretty darn spherical. You would get lots more unevenniss if they weren't.

Back the earlier part of the thread, I think a mass balance calculation using urine and blood analysis might not be very accurate due to metals being retained in the system for awhile.  Co seems to rid from the body pretty quickly, but Cr, I think is slightly lipophillic and tends to stick around in the body a little longer.

Thanks for the calcs everyone. 

[ScubaDuck]

Tin-

My comment about taking the mass of metals in the urine and blood was delivered with my tongue firmly planted in my cheek.  I am sorry I forgot to use the joking font.  :-). This was started by the comment about how they are able to measure the loss in live bodies which of course they can't.

Dan

[Lopsided]

Dan, I am looking at the icon image of you wearing the big yellow glasses, and I am wondering why your surgeon placed the resurfacing device over your mouth.

[ScubaDuck]

Lop-

You obviously haven't been around me much.  He used a spare one he had in his office to shut me up!  LOL

Dan

[Tin Soldier]

Oh, sorry Dan.  I didn't get it.  Twas lost in the technical world.  But I did find out that Co does leave the body pretty quick and Cr stays around a little longer.