Re: Why chromium ions don't really worry me - allergies

[Guest guest]

Thanks for all the info - some food for thought. One point about

allergies. I was told (by Mr Treacy) that there is no known link

between surface allergies and deep allergies. I have a surface metal

allergy but he didn't hesitate to resurface my hip. He also told me

that numerous m/m THR have been implanted in people with surface

allergies. By this time I'd developed a kind of numbing trust in the

man so I didn't bother asking for the data!

Kathy

LBHR dysplasia June 2002

> Like everyone else here, from the moment I heard about resurfacing,

I

> also began to hear concerns about metal ion (and specifically

chromium)

> exposure due to wear of the metal-on-metal implant. I thought I'd

see

> if I could make a rough calculation of how severe this possible

exposure

> might be, based on publicly available information. This back-of-the

> envelope assessment follows.

>

> First, some background chemistry:

> Chromium ions come in two flavors: hexavalent (missing 6 electrons)

Chromium,

> which is extremely toxic, and trivalent (missing 3 electrons)

Chromium, which

> is comparatively benign stuff. All the ions shed by the implants

are

> trivalent chromium, to which the following data apply.

>

> Wear tests of the Corin Cormet 2000 resurfacing implant show the

following

> (see www.cormet.com/wear.asp):

>

> During the initial breakin period, the Cormet implant (the only one

for

> which I could locate detailed wear simulation data on the net)

> experiences an average volumetric wear rate of approximately 2.4

cubic

> mm per million cycles (steps), dropping to 0.4 cubic mm/Mcycle

> thereafter.

>

> According to a 1998 paper in the _Journal of Bone and Joint

> Surgery_(1), the average hip or knee replacement patient patient

puts

> 900,000 cycles/year on the hip/knee after surgery. Let's call it 1

> million cycles, just to simplify things later. I should note that

the

> most active subject in this study did 3.2 million cycles, and the

least

> active did a mere 70,000, so there's an awful lot of variability in

> this data.

>

> The alloy (F75) used for these implants is 66% Cobalt, 28% Chromium,

> and 6% Molybdenum so this wear debris is about 28% Chromium. This

isn't

> strictly accurate, since the alloy is 28% Chromium by weight, rather

> than by volume, but this is close enough for government work.

>

> The density of chromium is 7140 kg/cubic meter, or 7.14 mg/cu mm.

>

> So, by my calculation, the amount of chromium being dumped into your

> system is, roughly:

>

> 2.4 cu mm/megacycle * 7.14 mg/ cu mm * 1 megacycle/year = 17.136

mg/year =

> 46.95 micrograms/day.

>

> This drops to 7.83 micrograms/day after the breakin period.

>

> To put this in perspective, a Centrum® multivitamin tablet

contains

> 120 micrograms, which happens to be the RDA for chromium. This is

nearly

> 2.6 times the amount produced by the implant during the breakin

phase,

> and 15 times the dose delivered by the implant thereafter. This

isn't

> a fair comparison, because your gut doesn't absorb chromium real

> well, but I'll deal with that later.

>

> One measure of long-term toxicity is the " chronic reference dose " ,

> which is defined as " an estimate of a daily exposure level for the

> human population, including sensitive subpopulations, that is

likely to

> be without an appreciable risk of deleterious effects during a

> lifetime " (http://www.nbc-med.org/SiteContent/glossary.asp).

>

> According to the toxicity profile for chromium in the Oak Ridge

> National Laboratory Risk Assessment Information System

> (http://risk.lsd.ornl.gov/tox/profiles/Chromium_ragsa.shtml) the

> chronic reference dose for trivalent chromium is 1.5mg/kg-day,

which in

> my case (~75kg mass) is 112.5 mg/day, or roughly 2400 times the

amount

> shed by the implant during the breakin period, and 14377 times the

> amount produced thereafter. However, according to the same toxicity

> profile, only 0.5% of orally ingested trivalent chromium makes it

into

> your bloodstream. Correcting for this, a Cormet implant should give

me

> the equivalent of 8.3% of the chronic reference dose during the

breakin

> period and 1.4% thereafter if every last bit of chromium it produces

> dissolves and goes into my bloodstream (this isn't the case, but

it's the

> absoloute worst scenario). I should note that there's a factor of

> 10 uncertainty in the chronic reference dose, and that some people

> are allergic to chromium (so these figures don't apply to them -

> if you're allergic to metals, you probably shouldn't have any kind

> of metal-on-metal joint replacement).

>

> So, even though there's no long-term data on the safety of

resurfacing

> (and it's important to remember that there isn't), I'm personally

not

> real worried about the issue. I'm not going to take a multivitamin

> containing chromium after surgery, but I'm not going to worry about

it.

>

> Steve

>

> (1) Schmalzried, Szuszczewicz, Northfield, Akizuki, el,

Belcher,

> and Amstutz; " Quantitative Assessment of Walking Activity after

> Total Hip or knee Replacement " , JBJS 80:54-9(1998).