Consider the three-legged dog. Maybe you own one, or have seen them in the park or in any of the billions of Dodo videos about them. Lopsided yet resilient, they evoke a kind of fawning sympathy from us humans that’s unmatched by the typical quadruped canine.

“People are drawn to specially abled pets,” says Rene Agredano, a cofounder of the pet-amputation support website Tripawds. “I think the attraction is that we just want to help them. We just want to make sure that they are given an equal chance at having a happy life.”

More and more, that desire to help manifests as prosthetics, especially in cases where the animal has lost more than one limb. Pets with artificial limbs have become a whole genre of feel-good videos of their own. A cat with bionic hind legs. A tortoise with wheels. The clips make the rounds on Facebook, where they add a smidge of feel-good optimism to the otherwise dour deluge that is your newsfeed. 3D printing has propelled the industry forward. Printed prosthetics can be lightweight, affordable-ish, and infinitely customizable. Doctors craft beaks for birds. High school students build artificial dog legs in their free time.

But not all pet prosthetics are created equal. And some veterinarians and people in the tripod community worry that the proliferation of easily craftable attachments could lead to unintended consequences for the critters that wear them.

Walking Distance

Dogs can lose a limb for a variety of reasons. Maybe they were born with an abnormal limb, or were hit by a car, or developed a cancerous growth that necessitated an amputation. A common quip among tripod lovers is that “dogs have three legs and a spare.” It’s true, to a point. Dogs adapt remarkably well to losing a limb, says Theresa Wendland, who specializes in animal sports medicine and rehabilitation at the Colorado Veterinary Specialist Group. But complications can arise as the animal compensates for what’s missing. In older dogs and dogs with arthritis or other mobility issues, putting extra weight on the remaining limbs can be a big problem.

Photograph: Dive Design
Photograph: Dive Design

“It really impacts their spinal mobility,” Wendland says. “They have to change the range of motion in their other limbs. They have to pull themselves forward in ways that are very unnatural.”

Prosthetics, if made right, can restore that range of motion. But as heart melting as it is to see a three-legged dog return to running on four legs, it’s not easy to build a proper puppy peg leg. Wendland, who works with the orthopedic and prosthetic company OrthoPets to help dogs adapt to their new limbs, says that it’s an involved process that takes time and technical know-how.

As with human prosthetics, an animal prosthetic has to be individually tailored to the build of the wearer. That means factoring in the animal’s size, weight, height, stance, and gait. (A kit for a doberman won’t fit on a dachshund.) To do that, orthopedists have to study the animal’s movements and try to mold a limb that will sync up with the others. While techniques vary, a standard process is to make a plaster cast, design the prosthetic based on photos and video, then build it out with durable thermoplastics and metal. From there, they tweak the finer details by hand until it works with the animal. The process can take weeks.