Alex Zanardi has had a third operation to treat severe head injuries sustained in a hand cycle accident last month.
The 53-year-old Italian, a former Formula 1 driver turned four-time Paralympic gold medallist, had a five-hour operation to reconstruct and stabilise his injuries.
Zanardi collided with a lorry during a race in Pienza, Italy, on 19 June.
His neurological condition remained "severe" and he is sedated and ventilated in intensive care.
Zanardi's cardio-respiratory and metabolic condition is "stable", a hospital statement said.
The statement described the cranial and facial fractures Zanardi had suffered as "complex".
The operation "required careful programming that made use of computerised, digital and three-dimensional technologies, made to measure for the patient," according to Professor Paolo Gennaro, director of maxillofacial surgery at Siena University Hospital.
The statement added that the next update on Zanardi would be "issued as soon as there are significant changes in the athlete's clinical picture".
Zanardi has become a global sporting icon for the strength of will and determination he has shown in overcoming adversity in his career, after he lost both legs in an accident while racing in a Champ Car event in Germany in 2001.
He drove in F1 from 1991-4 and in 1999, for the Jordan, Minardi, Lotus and Williams teams, scoring a best result of sixth place.
He raced in the US-based Champ Car series - a form of Indycar - from 1996-98, winning the title in 1997 and '98.
Following his accident at the Lausitzring in 2001, Zanardi underwent extensive rehabilitation, and returned to motorsport in 2005, spending four years racing for BMW in the World Touring Car Championship, and winning four races.
He then turned to hand-cycling, in which he has won 12 world championships in addition to his four Paralympic gold medals, as well as the New York City marathon in 2011.