Nelson Mandela cannot shed tears because of damage to his tear glands caused while breaking rocks in the Robben Island penal colony, a surgeon said after an eye operation on the South African president.

Dr. Percy Amoils told reporters Wednesday the surgery to remove a cataract from Mandela's left eye had been more difficult than expected but went smoothly."Not only does the president have cataracts, he has severe eye damage to his tear glands, which started during his sojourn on Robben Island where his tear glands were burned out by the alkaline nature of the limestone," Amoils said.

"Therefore the president has no tears."

Mandela, 75, spent 18 years in the Robben Island penal colony as part of a life term handed down for his anti-apartheid activities.

Reporters who visited the island recently said the wind filled their eyes with grit during a tour of the limestone quarries where Mandela used to break rocks.

"The president has very little tear secretion, but he manages extremely well," Amoils said. "You only have to hear him reading speeches to know how well he manages."

He said the lack of tears was the reason Mandela suffered periodic bouts of red eyes.

"It was a very difficult operation," Amoils said, adding that Mandela was not experiencing much pain and was being treated with mild painkillers.

Mandela was wearing an eye patch after the operation.

Amoils said he had to make a larger cut than he had planned to implant an intraocular lens in the president's eye.

"Visibility during the operation was bad and I had to do a larger incision. Therefore the recuperation period will be longer, but the long-term effects will be just as good."

Amoils said the president first received treatment for a condition known as keratoconjunctivitis sicca five or six years ago.

Mandela was in an "extremely good state of health generally," Amoils said. "At a later stage, we will operate on the right eye."

Mandela spent a total of 27 years in prison before his predecessor, F.W. de Klerk, released him in February 1990 and began dismantling apartheid.