Charnley showed his interest in the mechanics and pathology of the hip joint whilst still at medical school. He was driven by the desire to help patients.

"The hip joint, which is almost a perfect ball and socket, is frequently affected by types of arthritis which destroy the slippery surface and the geometry with the result that motion is restricted and pain develops."

Charnley concluded that "the only operation that could ever be universal would be an arthroplasty, because this is a reconstruction of a normal joint."

"The starting point… was the well known observation that after the Judet operation (replacement of the head of the femur with an acrylic prosthesis) the hip sometimes squeaks."

"A squeak indicates that frictional resistance to sliding is so high that the surfaces are seizing together."

PTFE shells – synthetic cartilage

"I have collected the evidence to show that the coefficient of friction of normal articulate cartilage is phenomenally low and in fact lower than anything encountered between solid substances in engineering practice"

"The idea behind the use of low-friction substance in arthroplasty was not merely an endeavour to eliminate wear. The LOW FRICTION concept would eliminate any twisting stresses being transmitted to the bond between the prosthesis and the living bone when walking."

"My initial experiments with PTFE (polytetrafluorethylene, TEFLON) used this substance as a 'synthetic cartilage'."

"It was hoped that the motion would take place preferably between two slippery PTFE shells rather than between one PTFE surface and the bone."

Cemented stem and PTFE cup

"It is probable that prosthetic replacement of the femoral head, articulating with a PTFE socket will prove more desirable than the double concentric shells."

Charnley's early attempts were to simulate nature. To achieve this aim he used PTFE as a "synthetic cartilage", first as interposition shells and then with a cemented stem. Between 1958 and 1960 the maximum diameter ball which anatomical considerations would permit (41.5mm) was chosen to replace the femoral head. This left only 5 mm as the maximum wall thickness of the plastic socket. The clinical success was measured in months with the result that the PFTE socket tended to move with the large steel ball and discomfort for the patient returned.

"Very good immediate post-operative results lasted about six months."