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Prosthetic leg leads professor to innovation

A computer-generated rendering shows a scuba diving leg in walking and swimming mode. The leg was designed by Yvonne Ng and Troy Pongratz and a patent is pending.
A computer-generated rendering shows a scuba diving leg in walking and swimming mode. The leg was designed by Yvonne Ng and Troy Pongratz and a patent is pending.
Pongratz Engineering, LLC

For Yvonne Ng, a professor of computer science and engineering at St. Catherine University who walks with the help of a prosthetic leg, mechanical engineering is a day at the beach.

The professor not only holds degrees in mechanical engineering from both Princeton University and the University of Minnesota, but she also holds a reputation for innovation.

Ng shared her story with the St. Kate’s community in a lunch hour lecture she titled “Stories of Invention.” She recounted for the audience how her early adversity introduced her to concepts in engineering and eventually led her to apply for a patent on a prosthetic Scuba diving leg.

Discovering science at a young age
Ng was born with a birth defect where one leg was shorter than the other. Before Ng reached the age of 2, she began a long line of hospital stays and medical and surgical procedures that progressed from bone stimulation surgery to the amputation of her leg at age 4.
Ng said it was in those early days that she first discovered she wanted to work in the realm of science.

“I had been in the hospital for so long, I of course had my career goal figured out at the age of six. I wanted to either be a doctor or a cow girl,” said Ng. Her parents encouraged the former. “Being scientists themselves, they thought it was a good venue for me.”

Like many amputees, Ng’s amputated limb caused her to experience intense pain when walking. Ng went through several prosthesis combinations, trying to find one solution that would work well enough to relieve the pressure and pain.

Innovative solutions
Together with her father, Ng modified her prosthesis to suit her needs more closely, lessening the painful impact on her limb. They first experimented with buckles for skiing boots, asking a shoemaker to rivet the buckles onto Ng’s prosthesis. After incorporating wire clothes hangers into the design, the modifications to the artificial leg yielded positive results.

“It worked and life was suddenly better. My father was much happier and I was much happier,” said Ng. “He said you know maybe you should just be an engineer.”

In modifying her prosthesis, Ng used reasoning, bits of technology, and an understanding of shapes and spaces to achieve a desired result.

“I realized that engineering was a way of turning ideas into reality — using one, your intuitive understanding of the world, and two, scientific phenomenon,” said Ng.

A day at the beach
In her adult life, Ng has literally taken that understanding of engineering to the beach, especially since meeting her husband, Troy Pongratz.

Troy invited Ng to go to the beach with him and swim in the ocean off the shores of Hawaii.

“I said, ‘Let me walk you through what Yvonne’s day at the beach is like,’” said Ng. “I take off my leg. I crawl across the hot sand into the ocean, and let’s say I don’t get swept away with only one leg, and I actually have a lot of fun. Then I’m all wet, and then I crawl back over the beach to my leg, and pray that no one has stolen it because this has cost basically as much as a car. This is a $20,000 leg.”

Instead of resigning themselves to the difficulties of swimming, however, Ng and her husband went to the drawing board, and even on their honeymoon, they worked on an innovative prosthesis that would allow any amputee to walk to the water and take a swim.

After developing several models and working with students at St. Catherine University and the University of Minnesota to test the prostheses, the duo finally came up with a working model. They have applied for a patent on a scuba diving prosthesis and are awaiting the results.

Building on the Liberal Arts
Ng said much of her success in creating a new prosthesis is owed to a background in the liberal arts.

“One of the things we had to do to protect this intellectual property is to write a patent,” said Ng.

The process required them to draw pictures of the prosthesis, describe how everything was connected and write a claim for the patent. Ng tapped the talents of Amelia Hansa, one of her engineering students at St. Kate’s, to help in the patent process.

“I’m not a word person, but knowing how to process the words was really important to be able to get started, because it saved us money,” said Ng. “We didn’t have to pay a lawyer $400 an hour to write it, he only had to edit it.”

Ng and her husband, Troy, funded the production of the scuba diving leg and they began marketing the leg with the help of St. Kate’s alumna Stephanie Rosso ’02. The leg is a product of Pongratz Engineering, LLC, and the product is detailed on the company website at

The next step
“The thing I loved about the prosthesis is that I can be just like everybody else. That I think is the biggest part for me mentally,” said Ng.

To develop the prosthesis for others, such as war veterans, diabetic patients and other amputees, Ng assigned a team of mechanical engineering seniors at the University of Minnesota to test an early version of the model. The team tested and modified a scuba diving leg for a 200-pound person.

Anna Heithoff ’13, an intern from St. Catherine University is also working on a 350-pound version.

“A lot of the veterans are very large,” said Ng. “Actually, a lot of people who get amputations later in life come from diabetic backgrounds, so they’re very overweight. So it’s necessary to be able to handle that weight.”

Leading by example
Marcie Myers, a professor of biology at St. Kate’s, introduced Ng to her audience before the lecture in March. Like many others on campus — both students and faculty alike — Marcie was surprised to learn that Yvonne walks on a prosthetic leg. Marcie has studied gait as one of her research questions, and she said Ng’s gait was so normal she did not detect anything out of the ordinary.

“She is one of the most creative people that I know,” said Myers. “She is great with both ideas and implementation and I admire her greatly.”


April 7, 2011 by Melissa Kaelin

See also: Faculty, Leadership, STEM