Plastic cups

LIFETIME HIP & KNEE

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#082: Plastic cups

The search for the cup material..

For the other half of the hip replacement was..

And still is, to some degree, more challenging…

And interesting.

The cup requires the same fundamental properties as the stem. It needs to:

Be stable in + compatible with the salty human body
Fix to the pelvic bone + stay fixed over millions of cycles
Perform as the other half of a moving hip replacement.

‘Teflon'

Charnley and his team knew that a ‘low friction arthroplasty’ was..

Largely dependent on the inherent friction of the material.

Non-stick ‘Teflon’ has one of the lowest coefficients of friction of any solid.

Discovered by accident in 1938

By an engineer at DuPoint in New Jersey..

‘Teflon’ (the trademark), or
Polytetrafluoroethylene (PTFE) is a
Carbon-based fluorine-long chain polymer..

Each carbon atom has 4x outer shell electrons

They bond with 4x other atoms

Each dashed line above is one bond

Or 2x paired electrons.

Electrons in each carbon-carbon bond

Are shared equally in the middle.

Not pulled to one end or the other.

So, there’s no +ve or -ve end, so..

The bonds are very stable, and..
Can form long chains

Meanwhile:

That strong C-C bond does not react, and
Electronegative Fluorine (F) repels other atoms.

That makes it very slippery

So slippery in fact that NOT even a gecko can't stick to it.

Not even a Gecko can't stick to Teflon

Anouki/Shutterstock

Non-stick Teflon did look like a good first choice.

There was just one problem..

Unfortunately, it didn’t work.

Early ‘Teflon’ cups performed well in lab

So Charnley tried them out in 300 patients..

Most failed within 1 to 2 years.

What works in the lab..

Doesn’t necessarily work well in real life.

That's a recurrent theme in joint replacement history.

So, while ‘Teflon’ now coats frying pans..

And insulates electrical wires in aerospace and computing.

‘Teflon’ is not used to replace hip joints.

Meanwhile, Charnley's machinist was shown a new material..

Polyethylene

Polyethylene is another long chain carbon-carbon polymer

Now the world’s most common plastic.

But back then.. it was unheard of.

Polyethylene was another happy accident..

Discovered by the German chemist

Hans von Pechmann in 1898.

It took until 1935 before an ICI chemist..

Came up with a reliable way to make it.

The first polyethylene cups were implanted in 1962.

And given the ‘Teflon’ failures, Charnley waited 5 years..

Before announcing the results to the world.

And the data was convincing..

By the 1970s, the Charnley hip was:

One-piece monoblock stainless steel stem
Supporting a small femoral head (7/8 inch or 22.2 mm)
Moving against a polyethylene cup.

The small head had a low surface area.

Less surface means less overall friction.

Less friction means less wear.

Think thin ice skating blade on ice

=> Low friction

Wide car tyre

=> More grip

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