Charnley began discussing his theories with engineering friends at the
University of Manchester, and they all agreed that our joints were not
lubricated under the same principles as metal mechanical parts, which
used hydrodynamic lubrication, where a very thin film of fluid separates
the articulating surfaces and the parts move rapidly. Charnley and his
colleagues theorized that our joints, instead, used boundary lubrication
where the lubricant (synovial fluid) had an affinity for the joint surface
itself. To test his hypothesis, John Charnley and the engineers started to
build testing apparatuses to evaluate the “slipperiness” of cartilage.
The coefficient of friction (represented by the Greek letter mu, “μ”) is a
mathematical ratio that expresses the friction between two surfaces. If the
μ is very high, then a great deal of force is required to move an object
against another. Rough sandpaper or a rubber tire has a μ value higher than
- On the other hand, something very slippery, such as an ice skate sliding
on ice, has a coefficient of friction of only 0.03, and it seems implausible
that something could be more slippery than that. To determine the μ of
cartilage, Charnley and his engineering cohorts built an apparatus that
supported a platform, holding part of a human joint (a knee, and later, an
ankle) in place. The upper half of the joint was positioned above, with a
pendulum arm attached, which allowed the pioneering scientists to
calculate just how slippery healthy cartilage is. What they discovered was
astonishing. The coefficient of friction is 0.001, and is still the most
slippery solid surface ever tested. It is (mathematically expressed) five
hundred times more slippery than metal on bone and thirty times slicker
than that ice skate on ice.
Mr. Charnley published his biological studies in non-surgical scientific
publications; more important, he knew that the key to a good clinical
outcome would be to design implants that had a low coefficient of friction.
And he knew he could determine the μ with his testing apparatus while
tinkering with the shape and size of the implants. The race was on to
develop what he would always refer to as “low friction arthroplasty.”
Surgeons had been replacing arthritic and fractured femoral heads for
over a decade, oftentimes with acceptable results, but Charnley was
striving for better outcomes and longevity. To achieve true low friction
arthroplasty, a “slippery substance” was needed for the socket, and he
began querying the newly trained polymer scientists in England about
possible candidates. He was eventually guided to polytetrafluorethylene