Charnley changed his approach to replacement of the hip joint. Rather than simulating nature Charnley decided to seek an engineering solution to a biological problem.

"It was pointed out to me that the best engineering practice would be to use the smallest diameter ball which would cope with the expected load."

"It is possible to circumvent the coefficient of friction by concentrating on the low frictional torque in the assembled unit by reducing the diameter of the ball."

"Resistance to movement of the head in the socket is greatly reduced by reducing the radius of the ball and, therefore, reducing the 'moment' of the frictional force. If at the same time the radius of the exterior of the PTFE socket is made as large as possible, the 'moment' of the frictional force between the socket and the bone will be increased, and this will lessen the tendency for the socket to rotate against the bone."

"Since there was no way of estimating rigours of service in the human hip joint over a period of years, we had to proceed by trial and error... from the starting size of 41.5mm diameter... this was first reduced to 28.5mm then 25.25mm and finally to 22mm"

It is the fundamental change of concept from low-friction the property of the materials to low-frictional torque the principle of the design, that marks the turning point in the evolution of the low-frictional torque arthroplasty henceforth referred to as the Charnley LFA.

The design of the components, instrumentation and the surgical technique of the operation were in place, but unfortunately it had become clear to Charnley that Teflon was not a suitable material for the socket.

Cemented stem and High Molecular Weight Polyethylene.

"I am very optimistic about the properties of the new plastic 'High Molecular Weight Polyethylene' - The coefficient of friction is not particularly low but by mechanical design, the frictional resistance of the unit can be made very low."

It had become clear that Teflon was not suitable for hip replacement because of its poor wear properties and the tissue reaction to the wear debris.

The problem was overcome with the availability of a new plastic - High Molecular Weight Polyethylene (HMWPE). The new plastic was tested in the Biomechanical Laboratory at Wrightington Hospital by Harry Craven - Charnley's Engineer – and since November 1962 the material has been used for the cup.