Tokyo - Vets in Japan have carried out knee surgery on a rare white tiger cub, fixing a leg problem the animal had been born with.In what was being billed as the first such operation on an animal of its kind, surgeons fixed a congenitally displaced kneecap in its right hind leg.The nine-month-old male, named Sky, was under the knife for five-and-a-half hours on Tuesday at the Nihon University animal hospital in Fujisawa, south of Tokyo, the school said on its website.The animal was under general anaesthetic throughout the operation.The tiger, now weighing 56kg, was born in March at Tobu Zoo in a northern suburb of Tokyo.A team of veterinary surgeons began the surgery by cutting open the knee and lifting the displaced patella."It was a very difficult operation but we managed to complete it without any problems," said Kazuya Edamura, who led the operation."The bone and muscle there were more deformed than we expected. They were thicker and heavier, so we were forced to change the planned method of operation."But, he added, Sky has youth on his side and will make a full recovery.The white tiger is a rare variant of the Bengal tiger, born without the pigment that usually makes its fur orange.According to the website of the university's College of Bioresource Sciences, there are about 300 white tigers in captivity in the world, including some 30 in Japan.