Meet Derby, a shelter dog born with deformed front legs who recently did something he was never expected to do: run.

Tara Anderson came across Derby's story on animal rescue website Peace And Paws and says she knew she had to do something.

"I kept looking at his photo and reading his story, and I cried literally every time," Anderson, said in a viral video that's been viewed more than 4 million times. "I had to try to help this dog."

Anderson agreed to foster Derby, and she got him a cart so he could move around.

However, the bulky cart still limited his mobility, so Anderson, an employee of 3D Systems in Rock Hill, South Carolina, presented the problem to her team at work and they decided to build the dog a set of prosthetic legs.

They wanted Derby to be able to run, so they designed loop-shaped legs to prevent the dog from sinking into the ground, and they set the legs low, so wearing the prosthetics wouldn't be too drastic of a change for Derby.

Once the legs were designed, Anderson's team used a 3-D printer to make the legs.

While office printers apply ink to paper, 3-D printers "print" with plastic, creating thin layers that meld together into a complete, three-dimensional object.

The devices have been used to create everything from guns to homes, and there are even 3-D food printers that create entire meals.

Recently, 3-D printing has been explored as a potential way to cheaply manufacture prosthetic limbs. Scans of a patient's limb can be saved in a computer, preventing them from having to get a new mold as they grow.

Derby took to his new legs quickly, which amazed Anderson and Derby's new adoptive parents, Sherry and Dom Portanova.

"I don't become impressed very quickly, but when I saw him sprinting like that, it was amazing," Dom said.

Today, Derby is running 2 to 3 miles with his owners every day, and Dom says he's even faster than they are.

You can watch Derby run in the video above.

Related on MNN: