These have been bad days for Barbaro.

The Kentucky Derby winner 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 yesterday. “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% 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,” Mr. Richardson said.

The colt was doing well yesterday, according to Mr. 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.

Based on Mr. 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,” Mr. 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% or 90%.

“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 yesterday 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.”