Entire limbs cannot be replaced, he said.
In Cassidy's case, it's taken 18 months between the surgery and receiving his permanent prosthetic leg in March.
"It's not simple, cheap or quick," Marcellin-Little said of the process.
That's because all equipment is designed and hand-built for each patient, and the experimental surgery must be rehearsed beforehand to make sure it goes smoothly.
It takes around three months of healing time before weight is put on the leg; the animal then must re-learn how to walk on all four feet.
For six weeks, Posovsky patiently helped Cassidy inch across the bedroom, putting one leg in front of the other. The hours upon hours of training finally paid off when one morning Cassidy began to walk on his own.
For most animal owners, externally attached prosthetics are a faster, less-expensive option.
OrthoPets, in Denver, manufactures prosthetics and braces for about 1,200 animals worldwide each year. Most are dogs, but they've also worked with cats, cows, birds, llamas, horses, even an orangutan.
"We joke that if it has an appendage and a heartbeat, we can usually help out," said Amy Kaufmann.
She started the business six years ago with her husband, Martin, who previously worked in the human prosthetics field.
The external prosthetics are built to "last a lifetime" by using tough industrial grade plastic, said Kaufmann. To combat chafing and irritation the devices are lined with special color-changing foam that turns black, alerting owners if a problem arises.
For an animal to be outfitted with an artificial limb, which is attached with straps, Kaufmann said front legs must still have the elbow joint and part of the radius a
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