[philmont] Backpacking After THR

This is one I'll save just because it's great information and I'll probably need it in another 10 - 12 years if I keep backpacking at this pace...

Thanks Dr. Kirk!

Shane (from work)

---- kirk maes <kirkmaes@gmail.com> wrote:

=============
I am an orthopedic surgeon in Florida and I do a lot of hip
replacements as part of my day to day work. I converted to using
almost exclusively metal on metal bearing surfaces with all press fit
parts (no bone cement) about 7 years ago. This is probably the most
important change (upgrade) in my practice since I began my training
for orthopedics in 1991.

There are three issues to understand
1 stability
2 durability
3 longevity

Stability

As far as stability goes the metal on metal bearing parts are
remarkably predictable because they almost never dislocate (older
model parts were typically made of a 28mm metal ball in a 28mm plastic
socket and dislocated in about 2% to 5 % of patients) newer versions
which are usually metal on metal probably dislocate less then 1/300
and are typically a 36mm metal ball (at least) in a 36mm metal socket.

The natural hip ball in the typical male is anywhere from 48 to 60 mm
in diameter. The physics of the situation is that a large ball in a
large socket is a very stable arrangement and a small ball in a small
socket is more likely to jump out of place. I doubt that the concept
of having big balls representing greater toughness or strength comes
from this topic but it certainly does apply.

The socket of the average mans hip measures 53 mm in diameter. When I
ream it out I get it up to about to 55 mm and I press fit a 56 mm cup
shell into that socket. Usually it sticks in pretty good and I don't
need screws to secure it in place, hence the term press fit. Less than
10 % of the time I'm not happy with how secure it is stuck in the
socket so I either go up one size ( $ 1000.00 ) or secure it with a
couple screws ( $ 75.00 ea ). As long as the shell stays put for a
few months, the bone will grow into the rough backing of the shell and
will hold on to it forever. The screws provide temporary fixation
until the bone heals on to the shell. For that matter the press fit
shell with no screws is a temporary fixation until the bone heals on
to the shell.

Next I have to decide what liner I will place in the shell. These
types of shells are called modular shells and the three liner choices
are plastic, metal or ceramic. All of these products are better than
they where 10 years ago because we have learned a lot about this stuff
in the last decade. (Diamond surfaces are in the works but are
probably 10 years away) Plastic is the cheapest and probably lasts up
to 20 years now. Metal has been around for 25 years in Europe and has
not been proven to cause cancer or other major problems in the vast
majority of patients. Some reports of metal hypersensitivity are out
there but you are more likely to die in a car crash, or get struck by
lightning that get hypersensitivity. Then there is ceramic. Old
ceramics balls and liners would shatter on occasion but newer ceramics
probably wont.

Plastic has to be thick so it wont wear out too fast. Therefore the
thickness has to be greater which leaves less room for the desirable
large ball hence 28 and 32 mm balls go with the plastic liners, (and a
5% dislocation rate). When the plastic wears out the microscopic
particles causes a bad reaction in the surrounding bone and the bond
between the metal parts (or cemented parts) breaks down, they become
loose and need revision surgery.

Metal liners for that shell can be pretty thin because cobalt / chrome
is really strong stuff. The 56 mm shell we talked about above will
take a 42 mm liner and therefore a 42 mm ball. A 42 mm ball is hard to
dislocate probably 1/500. There are also metal cups available that
have the liner built right into the shell. This all-in one liner-shell
cup has no screw holes for back up security so the press fit has to be
reliable when it is put in the socket. These are non modular items so
they only come with a metal "liner". They are really cool products,
however, because the 56 mm shell has a built in 50 mm liner. Now let
me tell you something, a 50mm ball will not be coming out of that 50
mm socket! You could probably rock climb with that size hip ball!
Dislocation rate ???? probably 1/1000 or better, problem is no one
knows for sure.

Ceramic liners are a little brittle and may not have the strength of
the metal so they may need to be a little thicker, so the ceramic
liner would be about 36mm in diameter. Still a pretty good size to
prevent dislocation. Probably 1/300

Durability

All hip replacements are so durable now that I don't have much worry
about any version, plastic, ceramic or metal. I tell patients to not
run for exercise but it is ok to run through the parking to get to the
car if it is raining.

Longevity.

At least 20 to 25 years even with the newer plastics and in reality
for metal and ceramic probably for ever!

Politics

The fear about metal ions causing cancer is probably the ceramic
companies trying to bad mouth the metal companies to gain market
share. 25 years of experience in Europe is a good track record. The
hype about ceramic shattering and glass fragments poking out your butt
the rest of your life is probably the metal companies slapping the
ceramic guys around. The plastic wear issues are real and the newer
plastics are better but this issue is not dead yet and neither is the
50 year old guy who is going to philmont and will probably live to be
100.

Hip resurfacing is a bad idea. It is a bigger operation, with higher
complication rates, more risks for fracture, more nerve injury, takes
longer, and offers no advantages when compared with a metal on metal
standard press fit hip replacement.

Waves of the future

The press fit, metal, all-in-one liner-shell cup with no screw holes
(yes that's the non modular one in the example above…..56 mm shell and
50mm built in metal liner) with a ceramic 50mm ball secured onto a
press fit taper stem is going to be the gold standard in 5 to 10 years
because the ceramic ball in the metal cup will not produce the metal
ions that gives the metal a bad name and the ceramic ball wont really
shatter like the old ones did. If Beau Jackson had gotten one of these
babies he would have still been hitting home runs today!

Well, good luck, that's my two cents worth, but I usually charge
$300.00 for that consult!

Kirk Maes, MD
Orthopedic Surgeon

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Received on Mon Jun 2 06:27:11 2008