ALMOST 10 years ago, Matthew Knights spent an entire pre-season doing ... not much.

The Richmond captain was one of the early victims of osteitis pubis, the mysterious groin complaint that limited his running, hurt when he kicked and soon became one of football's buzz terms. "OP" was the new scourge of footballers, and it still strikes them down.

Now there is a new threat. Femoroacetabular impingement is a hip condition that affects young and active people and a possible leading cause of many OP cases, not to mention chronic hip injuries, adductor problems and even back pain.

FAI has emerged and become understood only in the past five or six years, and one of Australia's leading hip experts, John O'Donnell, believes it will be the next big thing on the AFL injury scene.

"FAI," he said, "is the new OP."

FAI is caused by a small, bony bump on the "ball" part of the ball-and-socket hip joint - the head of the femur. Experts are still not sure whether the bumps - known as Ganz lesions or femoral neck bumps - exist from birth, but O'Donnell suspects they develop in early teenage years, as people become more active.

A lesion can sit there for years, possibly forever if a person isn't particularly active, and not do anything at all. But if it starts to grind into the cartilage lining either the ball or the socket, it will cause irreversible damage, and inevitable arthritis - be it one month down the track, two years or 10 years.

O'Donnell believes there would be four or five players on each AFL list with the condition; the demands of the sport - to run, twist and, most particularly, kick the ball across the body - promote that sort of movement and friction in the hip. The damage occurs only through that movement - you can't worsen someone's injury by jumping into them, and the condition can be picked up by watching how someone moves, before being confirmed on either an MRI or a CAT scan.

"There's all sorts of weird things people do," O'Donnell says. "Sometimes they walk with their feet turned out, rather than straight ahead, and you might see someone riding a bike and they have their knees a long way out from the cross bar; they can't get their knees in. All those things may be due to this."

Once detected, the lesions can be removed with a simple arthroscopy, which might mean eight to 12 weeks on the sidelines. O'Donnell deals with about 10 FAI cases a week, and has seen some people up and running in six weeks.

As more research emerges there is a belief amongst some specialists that removing the lesions early enough might see off some cases of OP before they start, or before they become chronic. Similarly, removing the lesion might spare players operation after operation to trim cartilage that will only get damaged again because the cause of it still sits there. That, in turn, could reduce the number of them who need hip replacements years after their careers end.

"People talk about chronic hip flexor problems, and groin strains - there's lots of different names for it, but I suspect they're all essentially the same thing," O'Donnell said.

"It's a good one to get early and hopefully prevent all those other things from occurring. I guess we'll find out in time what the impact is, but what we do know is that when we first started to trim off the bits of torn cartilage, a lot of those guys did fine for one or two years and then they started getting their problems back because they still had the bumps there, and they were still rubbing.

"Since we've been taking the bumps off, which is about six years, with very few exceptions they don't come back. At least so far. At the very least it gets them from two years to six years, and hopefully a whole lot longer. That's what we'll find out."

That, then, poses another question. If a player has a lesion, but is not symptomatic, what do you do? Carlton faced that question last November when they drafted Matthew Kreuzer knowing he has a lesion on his right hip but also knowing it had not begun to grind into his cartilage. He wasn't injured, but he had something that could cause injury.

Through the AFL's draft camp medical screening, the teenager consulted a specialist who recommended he continue to train and play but predicted he would need surgery in the first two years of his career. Knowing he has the condition at least means the Blues can monitor it - a better alternative than having their No.1 draft pick break down and lose years off his career.

O'Donnell would have made the same decision and kept Kreuzer out of the operating theatre. But his opinion could change in the next few years, as more becomes known about the long-term benefits of scraping a lesion away as soon as it is discovered.

"Right now, I'd wait until there are symptoms ...

But it's interesting. It will be interesting to see what more we find out about it."