When Cassidy Posovsky left the Veterinary Teaching Hospital Tuesday morning, he’d done something he hadn’t done in at least three years.The German shepherd mix had walked on four legs — or at least three legs and a prosthetic peg that Denis Marcellin-Little, a professor of veterinary medicine and an orthopedic surgeon said was fashioned from aluminum, rubber and a spring.And the people who made that leg were the same ones who developed a new way to attach prosthetic limbs, one that eliminates risks of infection inherent to traditional prosthetic limbs that are attached to a piece of metal screwed into the remaining bone.Instead, Tim Horn and Jessica Springer have worked for about two with Ola Harrysson, associate professor of engineering, to develop a custom-made implant that fits perfectly into the end of a remainder of a bone. This process, compared to other methods, “saves bone and tissue because you’re able to leave most of the existing bone,” Springer, a graduate student in industrial, said. Surgeons, using this method, don’t have to reshape a bone to secure the implant.Their method also promotes osseointegration, or the bone fusing with an implant to allow animals to use their prosthetic limbs as “nature intended”: through their skeletal structures, not through skin.”This allows them to transfer loads — force or pounds — directly through the skeletal structure,” Horn said. A mesh-like ingrowth surface allows skin to grow around the implant, serving as a barrier between the implant and possible infection.Horn, a graduate student in industrial and systems engineering, said the turnaround for creating a custom-sized implant takes about 16 hours.”We identify a patient that the veterinarian thought was best for the procedure,” Horn said. “We take C AT scans of the patient, and from those CAT scans we can make 3-D computer models of the bone.”Horn held up a plastic pink bone with a rounded titanium implant on the end. The plastic bone, printed using three-dimensional images from CT scans and computer renderings of a yellow Labrador’s right hind leg, is a model of what’s left of Nubbie’s bone.He and Springer were able to design an implant that “fits the bone exactly,” Horn said. In about 10 hours, one of the three Electron Beam Melting machines located in Daniels Hall are capable of building titanium parts that fit the exact implant design.They then pass these two pieces — the model bone and the titanium implant — to the veterinarian slated to perform surgery.”The veterinarian can practice surgery because he’s got a real implant plastic model of the bone,” Horn said. “So when he does the surgery, he knows exactly what to look for.”Cassidy’s surgery went well, Marcellin-Little said in an interview Oct. 17. Horn said his bone had started osseointegrating with the implant — just six weeks after Cassidy’s initial surgery, he said, Cassidy was able to walk for a few minutes on his new leg.”The leg slipped right on and snapped into place,” Horn said. “Some bone is already growing into the implant.” Horn said it will be up to Cassidy’s owners, the Posovskys, to put the leg on their dog for a few minutes each day. This practice will both strengthen his bone allow him time to get used to walking carefully on four legs before he gets his permanent prosthetic leg — a carbon fiber leg with a longer, more flexible foot.”With this one, we’re overcoming that force of walking and, with dogs and cats, running,” Springer said of training animals to move safely with their new limbs. Horn said there is no set time when Cassidy will be given his permanent leg, which will have a mechanized fuse that, if stressed too much, will pop off the implant to prevent him from breaking a bone.”It depends on his situation and how it goes. This foot could last him a long, long, long time if it works and if he’s happy with it,” Horn said. “When the time is right, we’ll update the foot.”