In a message delivered via his family, the operation is described as having gone "well and his condition is stable. Andrew is comfortable and has full range of movement. His family would like to thank the many well-wishers".
Nicholson fell after the last fence on the cross-country course, where he was in the lead. He was taken about an hour's drive to John Radcliffe Hospital for treatment.
The hospital is listed by Britain's National Health Service as "Oxfordshire's main accident and emergency site and provides acute medical and surgical services, trauma and intensive care".
The 54-years-old's pursuit of selection for a New Zealand record eighth Olympic Games will be in jeopardy but, at such potentially pivotal moments in one's life, such matters are trivial compared to maintaining wider health.
Nicholson's chances of participating in the Burghley Horse Trials, due to start September 3, are almost certain to be over. He is a five-time winner, including the last three.
Presently Nicholson has issues to negotiate with Equestrian Sports New Zealand which saw him opt out of the high performance squad. However, his hopes of being selected received a boost this month with his nomination to ESNZ's 'long list' of potential athletes submitted to the New Zealand Olympic Committee.
He is a winner of eight four-star titles, three Olympic Games team medals and three World Games' medals since first coming to base himself in England during 1980.
