John Tracey Scales was a distinguished professor of biomechanical engineering at the Institute of Orthopaedics, University of London, who pioneered the use of biologically inert plastic materials in orthopaedic surgery. He was born an only child, in Colchester, on 2 July 1920. His family later moved to Stanmore, and he was educated a Haberdasher’s Aske’s School. He then went on to King’s College, London, before proceeding to Charing Cross Hospital for his clinical studies. He held junior appointments at Charing Cross Hospital and the Royal National Orthopaedic Hospital, before spending two years in the National Service as a captain in the RAMC. He then held further junior posts in London. He managed to convince H J Seddon, director of the Institute of Orthopaedics, of the need to develop biologically inert plastic for use in orthopaedic surgery, and a department of plastics was established under his direction. In November 1954 a knee prothesis made of stainless steel and acrylic polymer was successfully used to replace the diseased joint of a 20-year-old woman, the first operation of its kind in the world. Scales went on to develop the first Stanmore total hip replacement, made in collaboration with J N Wilson. With Alan Lettin he developed replacements for the knee, elbow and shoulder. In 1974 the department became the first university department of biomedical engineering in Britain, with Scales as its first professor. He also developed porous wound dressings, and created a low air loss mattress for use in the treatment of severe burns and severe pressure sores. This work led to his appointment as honorary director of research at the RAFT Institute for Plastic Surgery at Mount Vernon Hospital, Northwood. He continued his research work at the RAFT Institute after his retirement. From 1997 to 1998 he was a visiting professor at Cranfield University. Scales contributed 175 articles to professional journals and books. He was a member of various committees and professional bodies, including the European Society of Biomechanics and the Society for Tissue Viability. In 1986 he was awarded the OBE for his work, and was made a freeman of the City of London in 1995. He died in a nursing home on 30 January 2004. His wife died in 1992. They had two daughters. He is survived by his daughters and his partner, Phyllis Hampson.