World 400 metres hurdles champion Jana Pittman has been ruled out of the Olympic Games after tearing cartilage in her knee while warming up for the Zurich Golden League meeting.

Australian Pittman pulled out after suffering a knee injury before her race at the Letzigrund and a series of examinations by doctors revealed the 21-year-old had torn a part of the cartilage in her knee.

Zurich organisers said she would undergo arthroscopic surgery to repair the tear, and such injuries usually rule out any form of physical exercise or strain on the knee for at least two to three months.

"The injury Jana Pittman contracted when warming up has been diagnosed as a torn outer meniscus," a statement from the organisers said on Friday.

"Pittman will have to undergo arthroscopic surgery."

Pittman became 400 metres hurdles world champion when she beat American Sandra Glover in Paris last August but then failed to win another international race until the start of July in Croatia.