Kentucky Derby winner Barbaro suffered another significant setback over the weekend, and his fight for survival may have reached a critical point.

After Barbaro developed a deep abscess in his right hind foot, surgery was performed Saturday to insert two steel pins in a bone, one that was shattered but now healthy, to eliminate all weight bearing on the ailing foot.

The procedure is a risky one, because it transfers more weight to the leg. If the bone were to break again, chief surgeon Dr. Dean Richardson said: "I think we'll quit.

"When things start to go bad, it's like a house of cards," he said Sunday in a telephone interview with The Associated Press. "If one thing fails, that puts more stress on another part. And if that fails, then you're stuck with managing two problems. That's why these are difficult cases."

The right rear leg was on the mend until recently. It's the one Barbaro shattered at the start of the Preakness Stakes eight months ago, and the three broken bones had completely healed.

Now this. The surgery, in which a cast was removed and replaced by an external brace known as a skeletal fixation device, addresses one problem but could create others.

Barbaro likely will have to bear more weight on his front feet because of his two ailing back legs, making him more susceptible to laminitis, a painful and often fatal disease caused by uneven weight distribution. Laminitis already struck Barbaro's left rear foot in July, and 80 percent of the hoof was removed.

"It's something that we are watching closely, and that could also be a thing that could lead to us quitting," Richardson said.

The colt was doing well Sunday, according to Richardson, and "we will continue to treat Barbaro aggressively as long as he remains bright, alert and eating," he said in an update sent out by the University of Pennsylvania's New Bolton Center in Kennett Square, Pa.

Based on Richardson's advice, owners Roy and Gretchen Jackson have been making the decisions concerning Barbaro. Their major concern from the start has been to keep Barbaro comfortable.

"No one is interested in putting the horse through any type of misery," Richardson said. "We're going to treat him the best way we can as long as he stays comfortable. And we're going to stick with that no matter if his chances are 1 percent or 90 percent.

"If he gets to the point where we just don't think it's reasonable to go on, we will not go on."

Gretchen Jackson spent time with Barbaro on Sunday, and said her colt is "still bright-eyed and still eating.

"It's not over 'til it's over," she said. "I'd say he's comfortable and being treated very well. As long as he's comfortable ... Dean knows our feelings. We trust him."

After months of upbeat progress reports, Barbaro has endured several setbacks the past three weeks.

On Jan. 9, Barbaro had a cast placed on his left rear leg to help realign a bone. The next day, Richardson removed damaged tissue from the colt's left rear hoof, and Barbaro was placed in a sling to help him keep weight off his feet.

On Jan. 13, another section of his left rear hoof was removed, and a cast was placed back on his right hind leg for additional support. He was gradually improving, but last Thursday, Barbaro's left rear cast was replaced and a custom-made plastic and steel brace was applied to his right hind leg. The leg also was fitted with a special orthotic brace for more support.

In the latest setback, the right hind leg is again at risk.

The pins in the right rear leg are connected to an external brace, which is connected to a lightweight alloy foot plate. This results "in the horse eliminating all weight bearing from the foot," Richardson said Sunday in a statement. "The horse's weight is borne through the pins across his cannon bone."

Allowing the pins to bear weight carries "significant risk."

"We believed it was our only option given the worsening of the right hind foot problem," Richardson said in a statement. "Unfortunately, we felt we needed to take this risk because this approach offered our only hope of keeping Barbaro acceptably comfortable."

He explained Barbaro had been uncomfortable on his right hind foot because of an abscess that developed when the horse had a "period of discomfort" on the left hind foot.

"It is not laminitis, but the undermining of the sole and part of the lateral heel region are potentially just as serious," Richardson said.

Sunday, Richardson sounded as serious as he did on May 21, the day after the Preakness, when he delivered the news that Barbaro's chance of survival was a "coin toss."

"I'm upset, worried, not sleeping well," he said. "A lot of people are very, very committed and spent a huge amount of emotional sources on this horse. So it's very upsetting when things go badly."