Nearly three years ago, Cassidy's fate was uncertain. Missing his right hind leg, virtually hairless, and 30 pounds (14 kilograms) underweight, the year-old old German shepherd mix was living in an animal shelter in the Bronx when Steve Posovsky saw him on a morning television show segment about pets. "There were tears in my eyes," recalls Posovsky, who lives with his wife, Susan, in Long Beach, N.Y. He contacted the shelter and shortly thereafter welcomed Cassidy as a member of his family. Although Cassidy has managed well on three legs, the Posovskys are now hoping a new type of prosthetic will let the revitalized 75-pound (35-kilogram) dog run and jump like others that have a full complement of limbs.

Cassidy isn't getting just any prosthetic leg, however. Having tried several versions of traditional prosthetics that attach to his truncated right hind leg via straps or sleeves—which Cassidy in every instance managed to kick off—the dog is now part of an experimental procedure to fit him with a new, carbon-fiber leg attached to his leg bone using a titanium fitting. [For firsthand observations and images from the operating room, visit our slideshow]

The first step in the procedure took place on a muggy July morning when veterinarians at North Carolina State University's Veterinary Teaching Hospital in Raleigh attached the titanium fitting. "We're excited but a little nervous," Posovsky admitted as he and his wife sat with their other dog, Bella, in the waiting room throughout the procedure.

For Cassidy and the Posovskys, a successful surgery means the bone will fuse to the titanium fitting through a process called osseointegration (the connection of living bone with an artificial implant) and, when the limb is connected to the fitting, Cassidy should have more control over it and thereby less discomfort than he experienced with other prosthetics.

For everyone else, success with the N.C. State team's custom-designed prosthetic could help improve the quality of life not only for animals and their owners, but for people seeking alternatives to the traditional human prostheses, says Denis Marcellin-Little, the school's veterinarian orthopedic surgeon who led Cassidy's surgery. An osseointegrated prosthetic will allow better limb movement, functionality and control, he says—benefits that could easily translate into better prosthetics, especially for the millions of war veterans seeking them after combat injuries.

Osseointegration surgery could also be a lifesaver for larger animals such as horses, says Gary Sod, assistant professor of farm animal medicine and surgery the Equine Health Studies Program at Louisiana State University's School of Veterinary Medicine. Sod, who has been experimenting with osseointegrated prosthetics for a few years in horses, also points out the risks. "If the bone starts to pull away from the implant, that implant starts to wobble," he says, and "that can cause the bone to fracture." The chances of a fracture, however, are fairly remote for a smaller animal like a dog, Sod notes, and although this would be uncomfortable, follow-up surgery could replace the fitting. With an animal the size of a horse that relies on a set of healthy legs to live, he says, a fracture "means the implant will fail, and you'll probably have to euthanize the animal."

Osseointegrated implants are also prone to infection, says Ola Harrysson, an assistant professor of industrial engineering at N.C. State who led the design of Cassidy's implant. Because infections can be difficult to treat once the implant is in place, Harrysson's graduate students are studying how skin and muscle cells would respond to different antibacterial coatings on titanium designed to prevent infection.


Researchers have been studying osseointegration since the 1950s, although much of that early work focused on dental implants. The first surgery to fit a human patient with an osseointegrated prosthetic leg was performed in Gothenburg, Sweden, in 1990. The design and attachment of weight-bearing limbs is still relatively new, particularly on people—only about 120 humans have undergone the procedure, many of whom live in Europe, Harrysson says.

This is the group's third animal surgery using the technique, and their first on a dog. The first two procedures were on cats, and the implant design evolved from the first to second, Harrysson says, but with exception of size, Cassidy's implant design closely resembles that of the one used on the second feline.

After almost four hours of surgery, the surgeons completed the tedious process of suturing Cassidy's soft tissue to the titanium by threading the needle through preset holes on the sides of the implant. From there, he was wheeled off to get x-rays that will help the vets determine the exact placement of the prosthetic.

Nearly one month after his surgery, which cost the family about $5,000, the Posovskys report that Cassidy has bounced back. In late October they will again drive to Raleigh, where the N.C. State team will finally unite Cassidy with his new limb. For now, Cassidy remains unfazed by his important roll in medical history. "He's hopping around like he was before," Posovsky says. "It's as if he never got anything done."

[For firsthand observations and images from the operating room, visit our slideshow]