Metal-on-polyethylene Total Hip
Replacement Devices
Polyethylene has been around since
the early 1960's. The way it differs now is the way you
process it makes a huge difference in how it wears. They
used to radiate them in air. They found out when they
did that it generated a lot of free radicals, and that
lowered its wear resistance. What they do now is they
radiate it in an inert atmosphere. The higher dosage of
radiation increases the cross-linking in the
polyethylene. If you take this polyethylene and do this
20 million times [moves it], that is about 20 years of
use for somebody who is pretty active, and then you try
to measure the wear. The wear is about 1/20th compared
to the polyethylene we were using more than a couple of
years ago. If you take the cup out after 20 million
cycles and weigh it, you can't even measure the amount
of wear. Sometimes you still see the machining marks
after 20 million cycles, and that is with body weight
load.
Bone grows into the stem, just as it
does into the cup. This is the titanium stem, this is
the rough coating, and that is where bone grows into
that. You wedge it into the bone. The femur is hollow in
the middle. The flutes [on the side of the stem] help to
stabilize it for that 6-week period while bone is
growing into it. In six weeks, if you try to take this
thing out, you would have to chisel the bone off to get
it out. So bone really adheres onto the surface.
When you radiate it, the wear
properties are much better, but the fracture toughness
is lowered by about 40 percent. The manufacturers don't
know if that is going to make a difference. Nobody has
fractured these yet. Engineers, however, are a little
worried that maybe in 10 years they may crack or
fracture. If that happens, it may be necessary to go in
and change the insert.
