Leonardo Da Vinci left the Mona Lisa unfinished because he gravely injured his arm while fainting, according to a study.
The artist's disability in later life is well-documented, but its cause has been debated by art historians for centuries. In recent years partial paralysis as a result of a stroke has emerged as the dominant theory. Proponents have pointed to Leonardo's vegetarianism as a clue, arguing that the high-dairy diet he is assumed to have eaten would have made a stroke more likely.
However, two senior Italian doctors now claim to have solved the mystery, having studied a drawing of Leonardo by an obscure Lombard artist.
The blood-red chalk picture by Giovan Ambrogio Figino depicts an elderly Leonardo with his right arm at a right-angle to his body, swaddled in the folds of his clothes as if in a sling.
His thumb and first two fingers are extended, his ring and little fingers contracted.
Writing in the Journal of the Royal Society of Medicine, Dr Davide Lazzeri, a plastic surgeon, and Dr Carlo Rossi, a neurologist, argue that if Leonardo had suffered a stroke, it would be far more likely his entire fist would have been clenched.
They also point out there is no evidence the artist suffered any facial or neurological impairment, which is common following a stroke.Instead, the medics say the hand position, sometimes known as "claw hand", is far more likely to have been ulnar palsy, the kind of traumatic nerve damage elderly people often sustain following a fall while fainting or suffering a dizzy spell.
"This may explain why he left numerous paintings incomplete, including the Mona Lisa, during the last five years of his career as a painter," said Dr Lazzeri. Leonardo was unusual because although he was left-handed - drawings attributed to the artist show the shading and hatching sloping from upper left to lower right that leaves this in little doubt - it is widely agreed that he used his right hand to paint.
According to the new study, this explains why the artist remained lucid and active - which he might not have been following a stroke - but not able to paint in the latter years of his career.
The research quotes an entry in the diary of Antonio de Beatis, a cardinal's assistant, following a visit to Leonardo in 1517, which states: "One cannot indeed expect any more good work from him, as a certain paralysis has crippled his right hand... And although Messer Leonardo can no longer paint with the sweetness which was peculiar to him, he can still design and instruct others."
The Mona Lisa is thought to have been started in 1503 or 1504 in Florence, and Leonardo is said to have "lingered over it" for years before leaving it unfinished.
Before the injury, thought to have taken place in 1505, Leonardo was reputed to have been immensely strong. A passage from a book by fellow painter Giorgio Vasari read: "He was physically so strong that he could withstand violence and with his right hand he could bend the ring of an iron door knocker or a horseshoe as if they were lead."
