Denis Marcellin-Little is an orthopedic surgeon at the College of Veterinary Medicine [4] at North Carolina State University. Over the last seven years, he has been pioneering a remarkable intervention for dogs with missing legs, giving them prosthetic limbs that are permanently attached to their bodies. In the procedure, known as transdermal osseointegration, Marcellin-Little implants one end of a titanium rod into whatever remains of a dog’s leg, attaching the metal directly to the bone. The other end of the metal implant attaches to an artificial foot. So far, he’s outfitted six dogs and two cats with these bionic limbs, and he’s currently preparing three more canines for the procedure. We caught up with Marcellin-Little to ask him what it takes to build a pooch a brand new leg.

Bark: How did you get interested in osseointegration?
Denis Marcellin-Little: I saw a very brief presentation, a few slides, on a cat that had received rod-like implants for both back legs in 2002 or 2003. It planted a seed in my mind. A little bit later on, in 2005, I had a cat that presented with no feet. Both back feet were missing. We worked with [a group of students in a biomodeling and biomanufacturing course] on designing a prosthetic leg that would actually be completely attached to the bone, and then we went on from there. We did the surgery. Over time, the process became much more sophisticated.

B: What kinds of injuries or ailments have your osseointegration patients had?
DML: They’re all over the place. Some of them are born without an extremity. Others have neonatal problems, like maybe the umbilical cord [was] wrapped around something or they have some injury during birth. Sometimes, patients get injured. Some of them have a wound that gets infected. And then you have tumors.

B: We know each case is a little different, but what’s the general procedure like? When a dog gets one of these implants, what happens?
DML: To be a candidate, the patient should have the right personality, the rest of his leg is working fairly well, he’s fairly docile, and he seems to be well loved and well taken care of. The patient and the owner have to be compatible.

The patient will get a CT scan, a good orthopedic exam, maybe some radiographs. And then we get to work. We make implants that are customized to the patient—what we call “freeform implants”—that are designed to merge with the bone. The next time we see the patient, we will be ready to do the surgery. By then, we have rehearsed that surgery, we know the implant is a perfect fit on the bone. And then we will do our surgery. We prepare the tissues, then we place the implant on the bone, and then we reconstruct the soft tissues.

B: How do you attach the implant?
DML: Our short-term strategies depend on the shape of the bone and the size. They rely on a combination of “press fit,” meaning the implant is firmly pushed in or on the bone. And if we feel that the press fit is not enough, then we will place screws or bolts inside the bone or on the surface of the bone. Our long-term strategy is “ingrowth.” The implants are made of [porous] titanium and the bone will grow into the titanium. Once you have ingrowth, you don’t really need your bolts anymore or your press fits. You have a very healthy, long-term, selfrenewing interface with the bone.

B: After the dog heals, you attach some sort of external foot to the prosthesis?
DML: There is a piece of metal that comes out of the skin and we usually put something round on it. The patients just start using these round little feet. We were absolutely amazed by our first patient—the cat that we did. We put a foot on him and he was running around like he was doing it forever. It was just amazing, the ability to recover. I realized very quickly that dogs adapt much more slowly than cats. They weigh more. The dogs need a little more rehabilitation, or a training period, where they start using their leg, they start trusting their leg. But once they trust it, they are very pragmatic and they will start to use it.

B: Some dogs get socket prostheses, which just slip on over their stumps. What’s the advantage of an osseointegrated prosthesis?
DML: Sometimes, we can’t use socket prostheses. In some specific locations they are not an option. They often are less energy efficient than the transdermal implant. They move out of alignment by an inch and suddenly, they don’t fit anymore. They become too loose or too tight or you have a lot of skin abrasions. If you’re going to go on a three- or four-mile walk, that would be very challenging. [With a socket prosthesis], there are very few dogs that can go on long walks. But with a transdermal implant, there is no limit to what you can do with your limb.

B: How do you think the procedure and approach will improve as time goes on?
DML: There are a number of things in transdermal osseointegration that are less than perfect and are continually evolving. The process is very different now than in 2005, and most likely will be very different seven years from now. It’s a complicated idea—there are a number of features, and they all can be refined and optimized.

One challenge would be to design a foot that is very ergonomic: it’s easy to put on and [take] off, it’s very stable when it’s on, it’s light, it’s strong, it’s wear-resistant, it has good traction. We could imagine that these things could become more sophisticated over time.

I foresee that the process will become more standardized; I also think that it will become more rapid. And, of course, we will know more about it. Right now, we don’t know much—it’s just one case here, four cases there. As you can imagine, the field can be greatly improved. I think it’s going to play a big role in medicine. I spend a lot of time and energy for a few patients that have very big problems, thinking that it will trickle down to other patients in the future. I would like people to know that there are options if a dog is missing a foot or two—or maybe three or four.