British doctors have helped to create a surgical robot that will revolutionise treatment for the 160,000 people a year who are given a new knee or hip.

The Sculptor robot enables surgeons to install replacement joints in exactly the right place and removes the risk of them not fitting properly. Ill-fitting joints can cause patients pain and force them to undergo corrective operations. Sensors in the positioning arm of the Sculptor tell the computer where the surgeon is cutting away bone.

The machine stops surgeons from making a mistake while they are removing the old, worn bone in an arthritic knee by disabling their mechanised cutting tool if they stray outside the area shown in a model of the knee in its computer. It is the first orthopaedic surgical tool to use this 'actively constraining' technology, which has been developed after 15 years of research by a team of engineers, computer scientists and doctors at Imperial College London. The team was led by Professor Justin Cobb and Professor Brian Davies.

The robot has been used at Charing Cross Hospital in London in three knee-replacement operations. Its precision helps to ensure a cleaner, smoother match between the healthy bone that is left and the two new pieces, one metal and the other plastic, that are inserted into the knee. Currently, as many as 30 per cent of people who have knee surgery feel some restriction that may be related to the accuracy with which the operation was performed. In some it fails altogether.