_HEALTH MEDICINE Biotechnology

_A New Twist on Ankles

Sorin Siegler’s research corrects a flaw in the conventional understanding of ankle morphology and could result in better artificial ankles.

_Sorin Siegler

Siegler is a professor of mechanical engineering and mechanics in the College of Engineering.

…Foot bone connected to
the heel bone
Heel bone connected to the
ankle bone
Ankle bone connected to
the shin bone…
— “Dem Bones”

If only the human body, that complex masterpiece of bones, joints, tendons and organs, were as simple as the lyrics to the song known by all.

Sorin Siegler knows it’s not. The Drexel professor believes he’s made an important discovery about the shape of the ankle bone that upends 70 years of what was thought to be common knowledge on the subject.

A New Twist on Ankles


Every joint in the body has distinct biomechanical properties. What sets the ankle apart is that it must allow motion in all planes. “If you walk, it has to adapt to different kinds of terrain,” Siegler says. “It has to be able to take the initial load from the ground as it transfers to the body. Its structure is a little bit unique. It’s really two joints. There is one, which is between the heel bone and a small center bone in the middle called the talus, then the upper ankle joint.”

Siegler has discovered that the ankle is not, as textbooks taught for generations, the shape of a truncated cone that has its apex directed medially (toward the center of the body). It’s actually the exact opposite.

“It’s a truncated cone that has its apex laterally, away from the center of the body,” he says.

Siegler came to this conclusion after doing studies using medical imaging. “I took medical images [like an MRI] of 50 or so ankles, and I reconstructed them in the computer,” he explains. “I created three-dimensional renderings of those bones from an MRI or CT scan.”

When renowned orthopedist Verne T. Inman first mapped the shape and size of the ankle roughly seven decades ago, he did so without the benefit of a computer. His findings, which Siegler ultimately discovered were incorrect, became accepted knowledge.

“People looked at the ankle and said it’s acting like a hinge,” Siegler says. “Using that hinge assumption, he made measurements directly on the bone, but not straight — using the hinge that he thought was there — to determine the shape.”

Siegler measured the shape of the bones directly from their computer renderings. He believes that his accurate measurements could ultimately result in the design of a better artificial ankle.

Today, hips and knees are the two most commonly replaced joints, with surgical success rates north of 95 percent. Many surgeons, however, don’t even attempt ankle replacement surgeries, instead opting to fuse the joint. Why? Ankle replacement surgeries have an unacceptably high rate of failure, of around 70 percent, says Siegler.

“I believe one of the reasons for the failure rate is they don’t obey the shape characteristics of the joint,” says Siegler. “The replacements that are on the market today use either a cylindrical shape — not a truncated cone — or they use the cone, but the wrong cone, the cone that is based on Inman’s common knowledge.”

Siegler would like to change that. Using a $41,000 seed grant from the Wallace H. Coulter Foundation, Siegler is hoping to develop patents for new concepts on total ankle replacements. He plans to apply for a full grant in the fall.