Landis, second overall with just under two weeks of racing left, said on Monday on the Tour's first of two rest days, that he broke his right hip in a crash on a training ride near his home in California in January 2003.
He said the break completely severed the blood supply to the bone. Without it, the ball of Landis' hip joint has been gradually dying and collapsing, causing gnawing pain, said his doctor, Brent Kay.
The bone damage is "as bad as it can get," Kay said. "Everything is pretty much worn down."
The 30-year-old Landis also carried the injury for the Tours of 2003 and 2004 - when he rode on Armstrong's squad - and in 2005, when he finished ninth riding for his current team, Phonak.
"Using it doesn't in any way increase the chance that it will be unusable later. It is already ruined," Landis said. "Whenever the pain gets too bad I will have it replaced, probably sooner than later."
Phonak team doctor Denise Demir said Landis could have surgery in August, after the Tour finishes on July 23, if they settle on a suitable choice of surgeons by then.
"He's tired of the pain," she said. But he refuses to take pain medication, she added. "He doesn't want it. He says it makes him tough."
Kay said Landis' pain threshold is "off the chart." But he also said the discomfort keeps Landis from sleeping and that it has been hard to help him "because he rarely says very much."
"I can't say that it has any effect on the way I race," Landis said.
"It's not easy to give it a number and say this is how much it hurts," he said. "Whatever happens, I do my best to try to focus on the race itself rather than my hip. And the race, in a way, is therapy for my hip because it consumes everything I think about."
The team doctor said Landis received two cortisone injections into the hip this year to fight swelling.
"They've been somewhat successful. I can't say they take away the pain altogether and they don't really help with the arthritic pain but with the inflammation," Landis said.
Armstrong, who retired last year, first won the Tour in 1999 after surviving testicular cancer that had spread to his lungs and brain and required surgery and rounds of debilitating chemotherapy. He retired from cycling after his seventh win last year.
Landis had pins inserted for the fracture and has since had two more surgical procedures to try to restore blood to the bone - the last in November 2004 while still on Armstrong's team.
Patients are not meant to walk for six to eight weeks after such surgery, he said, "but at that point I didn't have that amount of time, and so after a few days I went back to using it."
Landis said he kept his condition secret to all but about 10 people before announcing it on Monday, and did not tell Phonak when he first joined. "My mom found out last week," he said.
Team manager John Lelangue said he was told at the start of this season but added that the condition has not dented his confidence in the cyclist's abilities. Demir, the team doctor, said she knew last year, but that she and Landis kept it secret.
"It wasn't my intention to cover something up, it's just that there was so much going on that I was overwhelmed," Landis said. But he also said he had planned to go public at some point because "sooner or later it would be a story."
Demir said Landis' right leg is two centimeters (nearly an inch) shorter than his left leg since the crash, when the ball at the top of the hip bone snapped off, affecting the balance of his muscular structure. She said Landis cannot push the right pedal with the force that he can with the left.
Landis said the pain is worse in time trials - which forces him to change from a usual riding position - and climbing steep hills. Those disciplines are key to winning the Tour, but he has shown that even with the injury he can excel at both.
"I've proved that I can win some of the hardest races, so whatever happens here will be an outcome of the race and not based on anything having to do with my hip," he said.
The race resumes on Tuesday with a flat stage from Bordeaux to Dax in southwest France. Landis is currently one minute behind leader Serhiy Honchar of Ukraine. Wednesday brings the first steep climbs in the Pyrenees.
With a replacement hip made of titanium, steel, ceramic or other materials, Landis will venture into uncharted territory for a Tour cyclist. But Kay said they have talked to leading amateur cyclists with prosthetics and they "are doing very well."
Landis said he hopes his career will not be over.
"I love racing my bicycle," he said. But he added: "I'm prepared for whatever happens next."