Bioengineer whose meticulous experimental work improved the reliability and strength of hip replacement joints
The biomedical engineer John Paul, who has died aged 86, played a crucial role in the development of reliable total hip replacement implants, which have improved the lives of great numbers of people. John produced the first realistic estimate of the forces that act on the hip joint during function; this information was essential for the design of safe implants and of machines to test them in the laboratory.
In the early 1960s John Charnley, an orthopaedic surgeon, introduced a metal and plastic artificial hip joint with an unnaturally small femoral head to reduce the effect of friction, ideas incorporated into the successful joints in current use. The first joints had a metal femoral component and a polytetrafluoroethylene (PTFE) socket fixed to the pelvis. Unfortunately the PTFE was unsatisfactory and wore out rapidly. Using ultra-high molecular weight polyethylene produced much less wear, but many of the joints broke, because they had been designed using unrealistic estimates of the forces that would act on them during function.
John set out to provide more reliable estimates of these forces and was able to do so because of his deep insight into the engineering principles needed to solve the problem and his meticulous experimental work.
The forces transmitted across the hip joint are produced by the weight of the body, the accelerations of the body and the power in the muscles that produce movement or stability. John designed and built an instrument to measure the forces produced by the body weight. He obtained the accelerations from recording moving volunteers using two cine cameras at right angles. These were digitised and then analysed.
John found it more difficult to determine the force in each muscle that affected the joint, which remains a problem, but was able to estimate the forces by grouping together muscles that had a similar effect. He showed that the forces on the hip were many times greater than the weight of the body. The magnitude of the joint forces and their variation during walking is known as the Paul Cycle, which was, and is, used in the design of hip implants.
He was born in Sunderland, where his father, William, was working as an engineering draughtsman. John grew up in Old Kilpatrick, in the west of Scotland, near the shipyards where his father worked, except for the second world war years, which he spent in Aberdeen with two elderly aunts. He was educated at Allan Glen's school in Glasgow, excelling both academically and on the rugby field. He studied mechanical engineering at the Royal College of Science and Technology (now the University of Strathclyde). The Royal College was unable to award degrees as it was not a university, so John had to take his BEng finals at Glasgow University. A query came back to the college from the examiners, who asked if there was a possibility of his having cheated, because they could not believe that an undergraduate could know so much.
In 1962 he was a founding member of the bioengineering unit at the University of Strathclyde with Robert Kenedi and Tom Gibson. I met John in 1966 when I studied for an MSc at the unit; he was one of the supervisors of my research thesis. In 1970 I joined the staff. John was head of the unit from 1977 until he retired from the university in 1992. His published research on hip joint forces, including his PhD thesis, are classic reference works. With colleagues at the unit he extended his work to the knee, ankle and elbow joints.
The bioengineering unit rapidly became an internationally famous centre of excellence and John began to travel the world to present his research and encourage the development of fledgling bioengineering centres. John was chairman of the international standard organisation committee on bone and joint replacement and the equivalent committees of the European and British standard organisations. He was president of the International Society of Biomechanics (1987-90) and a fellow of the Royal Academy of Engineering and the Royal Society of Edinburgh. He continued these activities until just before his death.
John's reputation is based on work that he initiated 50 years ago, but which remains relevant today. The Paul Cycle is the canonical data used for designing implants. These days implants very seldom break, but fail due to particles produced by wear, which can be investigated using joint simulators based on his work.
John's wife, Bette, whom he married in 1956, died in 2004. He is survived by his children, Gillian, Graham and Fiona, and five grandchildren.
• John Poskitt Paul, bioengineer, born 26 June 1927; died 13 November 2013