"I'm upset, worried, not sleeping well," chief surgeon Dr. Dean Richardson said Sunday in a phone interview with The Associated Press. "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."

The surgery was performed Saturday, after Barbaro developed a "deep abscess" in the right hind foot last week. The abscess could not be treated without removing a recently applied cast, which was replaced with a protective brace "in order to provide the foot with a chance to heal."

Richardson said in a statement issued Sunday by the University of Pennsylvania's New Bolton Center in Kennett Square, Pa., that the pins were placed transversely through his right hind cannon bone -- one of the three bones healed after being shattered eight months ago in the Preakness.

He said the pins are connected to "external sidebars that in turn are connected to a lightweight alloy foot plate." This results, Richardson said, "in the horse eliminating all weight bearing from the foot. The horse's weight is borne through the pins across his cannon bone."

If the bone were to break again, Richardson said: "I think we'll quit."

After months of improvement, Barbaro has endured several setbacks the past three weeks. This one could be the most critical, since other problems could arise.

With the back legs hurting, Barbaro's front feet likely will bear more weight, making them susceptible to laminitis, the often fatal hoof disease the colt has had in his left rear since July.

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

Allowing the pins across the cannon bone to bear weight carries "significant risk," but Richardson said "we believed it was our only option given the worsening of the right hind foot problem. Unfortunately, we felt we needed to take this risk because this approach offered our only hope of keeping Barbaro acceptably comfortable."

When laminitis was discovered in his left hind foot in July, Richardson had to remove 80 percent of the hoof. Other issues have followed. Earlier this month, Barbaro had his first "significant setback" with Richardson removing damaged tissue from the colt's left hind leg.

Richardson said in his statement on Sunday: "We will continue to treat Barbaro aggressively as long as he remains bright, alert and eating. This is another significant setback that exemplifies how complex his medical situation remains because both hind limbs have major problems."

Richardson further explained that Barbaro had been uncomfortable on his right hind foot because of the development of a "deep subsolar abscess secondary to bruising when he went through 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. "We attempted to manage the right hind foot in a cast and then in a custom fabricated brace, but it was impossible to have access to the foot for treatment as well as acceptable stability and comfort."

On Saturday, owner Gretchen Jackson told The Philadelphia Inquirer that Barbaro was again facing tough odds.