"Knee surgery is common. Young adults have their anterior cruciate ligament reconstructed after tearing it when playing sports. Middle-aged people have parts of their meniscus trimmed when they have pain and limited knee mobility. And older people have their knees replaced by metal and plastic when their cartilage has worn out. The idea that there is something mechanically wrong that needs to be fixed with orthopaedic surgery is a compelling idea – but is it right?