Small Kentucky equine practice is at forefront of using stem cells to treat difficult fractures

WOODFORD Veterinary Clinic is a small practice near Lexington that caters to all types of four-footed creatures and some feathered ones, too.

Being located in the self-proclaimed Horse Capital of the World with two of the top equine clinics as their competitors, Woodford's large-animal practitioners Chris Johnson, D.V.M., and William Baker, D.V.M., want to stand out among their peers. The two surgeons have collaborated to adapt a relatively new human application of using stem cells to heal difficult fractures in horses.

Stem cells are called multipotential, because they have the potential to evolve into bone, tendon, ligament, skin, blood, and other cells. Stem cells learn what tissue to become when the demands placed on them cause them slowly to adapt to the job they are intended to perform. Stem cells can develop into different types of cell lines, depending on where they are needed and what stimulates them. Injected into tendons at the site of an injury, they form into the type of cells that comprise the damaged tissue and begin to reproduce, regenerating the tendon.

In human medicine, the debate over stem cell research centers around the use of embryonic stem cells, which are derived from human embryos. Adult stem cells are the most primitive cells in bone marrow from which all the various types of cells in the body are derived. In addition to stem cells, bone marrow contains fibrin, granulocytes, monocytes, neutrophils, and growth factors that enhance the healing process. Fat also contains stem cells.

Stem cell therapy, a technology still in its nascent stage, has been used in horses primarily to treat ligament and tendon injuries. Not only does the therapy promote regeneration of the tissue, but it also dramatically reduces inflammation and relieves pain, making the horse more comfortable during recovery.

During a procedure pioneered by Douglas Herthel, D.V.M., head of Alamo Pintado Equine Medical Center in Los Olivos, California, the horse is placed under general anesthesia and bone marrow is aspirated from the horse's sternum and then injected immediately into its injured ligament or tendon at the site of the lesion. Before bone marrow is harvested from the horse's sternum, the site of the injury is scrubbed and prepared for injection and the needles are guided into proper position using ultrasound to prevent the stem cells from degrading during the process. When everything is in place, the marrow is withdrawn from the sternum and instantly injected into the lesion.

Sesamoid fractures

Johnson and Baker followed Herthel's lead and began to treat horses with tendon and ligament injuries using stem cell therapy. While further researching the technology, Johnson came across cases in human medicine where the same therapy was used to heal nonunion fractures, those that had difficulty knitting back together.

"According to human literature, stem cells like to become bone," Johnson said. And that started him thinking about the possibility of using stem cell therapy to help heal difficult sesamoid fractures in horses.

"If you have a [fractured] sesamoid--either a track injury or sometimes foals will fracture their sesamoids--some of them aren't amenable to surgery or don't do very well with surgery," Johnson said. "That is, you could do surgery on them, but you don't gain a whole lot from all the work it takes to do those. So the idea was to see if there was anything better to do than just say, 'Oh, well, let it heal.' "

After studying what human surgeons had done, Johnson decided to try it with horses.

"I put a needle into the fracture lines of these sesamoid fractures and inject stem cells," he said. After a while, Johnson saw bony calluses begin to grow on the separated bones where previously little or no bone knitting had occurred.

"We have done a few cases of fractured sesamoids over the past year, and the jury is still out on how the horses will come back," Johnson said. "Radiographically, they are healing better than we thought they would."

Johnson and Baker have performed the procedure mostly on younger horses and on a few injured racehorses with badly fractured sesamoids and little hope.

"I don't know if any of the horses we've operated on are going to make it back [to an athletic career] or not," Johnson said. "My gut feeling is that they probably won't, but the sesamoids are healing much better radiographically than what is usually par for the course."

Johnson and Baker have not launched a formal research study of the procedure, but Johnson believes stem cell therapy eventually will have a vital place in healing difficult fractures in horses.

Bowed tendons

Most stem cell therapy performed by Woodford's equine surgeons is on bowed tendons. One case in particular is a son of Saint Ballado named Lord of the Game, who had injured his tendon as a two-year-old. After an appropriate layoff time to heal, Lord of the Game annihilated the competition in his career debut when he won a $10,000 claiming race at Hawthorne Race Course by 223/4 lengths.

That victory began a five-race winning streak for the gelding. Now, after nine career starts, he has seven wins, including the Prairie Meadows Cornhusker Breeders' Cup Handicap (G2) at Prairie Meadows Racetrack and the Hanshin Cup Handicap (G3) at Arlington Park. He also ran second in the Claiming Crown Jewel Stakes at Canterbury Park and third in the National Jockey Club Handicap (G3) at Hawthorne.

In explaining how stem cell therapy helps tendons to heal better, Johnson said, "Normally, when a horse bows a tendon and has a significant core lesion, basically the central fibers of the tendon are disrupted, torn apart. Normal tendon fibers have a set pattern that determines their elasticity, and if you let a tendon heal naturally after injury, you have a haphazard pattern of scar tissue, so it has little elasticity to it."

When stem cells are injected into the lesion, they cause regeneration of more tendon fibers, which are elastic, and they hinder the formation of scar tissue, which does not "give" under pressure but tears instead, causing reinjury of the tendon.

"Our goal is quality of healing, not decreasing the time," Johnson said. "Most of the badly bowed horses we do are still off six months to a year before they ever go back to training. I've tried putting them back sooner because sonographically everything looked good, but when we started putting some pressure on that tendon, I've had it rebow, even with the stem cell therapy. That's why we give them more time.

"Most of these tendons physically look good after stem cell therapy. Even badly bowed tendons decrease in size and look very good cosmetically and on ultrasound, but some of them still need time before you put pressure on them."

Johnson advocates rehabilitation of horses using an underwater treadmill during healing.

"That takes a lot of their body weight off the tendons, but it still is not passive motion like you would have in a swimming pool. I want some pressure on that tendon because form follows function," he said, referring to the adaptation of stem cells to the job they are intended to perform. "If you are loading that tendon during the initial healing stages, I feel that those fibers are going to line up and be parallel and a lot more elastic.

"It's a balancing act, because you don't want the horse doing too much, obviously, because it will redamage the tendon."

Alternate sources of stem cells

The quickest, easiest, and cheapest way to obtain stem cells for transplant is to use the bone marrow from the horse's sternum. The entire procedure of harvesting the bone marrow and injecting it into an area where the surgeon wants to achieve better healing costs about $1,000.

"The problem is that you only get about two to four stem cells per 100,000 bone marrow cells," Johnson said. "So the volume of stem cells is much lower with direct bone marrow injections."

For clients who can afford to spend twice as much money and have horses that can wait three days for the procedure, Johnson and Baker harvest fat from the horse's tail head and send it to Vet-Stem, a laboratory based in Poway, California, that processes the fat sample to concentrate the stem cells. Vet-Stem places the stem cells in ready-to-use, injectable syringes and ships them to the veterinarian via overnight carrier.

According to Johnson, despite the time lapse while the stem cells are in transit, degradation is not an issue because of the large volume of cells that can be derived from just two teaspoons of fat.

"If you have several hundred thousand stem cells per cc [cubic centimeter], then 1% or 2% degradation is not that big of a deal," he said.

One problem, however, is harvesting fat from very lean, athletic horses. Johnson posed the problem to Robert Harman, D.V.M., Vet-Stem's founder, who researched it and found that leaner horses had a higher concentration of stem cells in their fat.

"The actual amount of fat is smaller, but the concentration of stem cells is supposed to be greater," Johnson said. "So they can get the same amount of stem cells out of a smaller amount of fat on a fit, young racehorse.

"We've done between six and a dozen Vet-Stem procedures, but we've probably done 75 to 100 using bone-marrow injections," he added. "Both groups of horses are doing well."

Although the technology is unavailable in the United States, laboratories in Europe provide another source of stem cells by culturing them from bone marrow. "The problem is that it takes about two to three weeks to get that done," Johnson said. "So that is a bit of a problem as far as treating things in a timely fashion, but I'm sure they are working on decreasing the time it takes to do that."

He believes that once perfected, growing stem cells from bone marrow will be the ideal way to obtain them for transplant.

Meanwhile, Johnson is optimistic about the future of stem cell therapy, but says that science still has a lot to learn.

"Stem cell therapy certainly is not a panacea, but I'm cautiously optimistic that we are heading in the right direction," he said. "The science has to be developed and refined, and it's probably going to take several more years before we get it down to a fine science."