Three years ago, Drexel mechanical engineering and mechanics professor Sorin Siegler stumbled upon a significant discovery that changes how we understand the ankle.
“I was approached by a company that wanted to develop a resurfacing procedure for the dome of the talus [the pivot bone of the ankle] and I was researching the geometry of this bone using 3-D models obtained from MRI and CTs from various individuals,” Siegler wrote in an email.
The biomechanics company wanted the team to create an implant that would mimic the talar dome, which is the top of the ankle bone that connects with the leg. “Sometimes that cartilage can get damaged,” Jason Toy who studied with Siegler during his doctoral studies and worked on the project with him, said.
“We started using some [computer-assisted design]-type techniques to look at the bones,” Toy continued. “That’s when we realized some of the bones were different than had been described in the past.”
Using a CT scan, Siegler noticed that the inner “slice,” or medial side of the ankle was larger than the outer, or lateral, slice. When he rendered the ankle in 3-D, he arranged the slices vertically. According to the Drexel News Blog, from there he discovered that the talus forms “a truncated cone with its apex on the lateral side.”
a professor at the University of California, San Francisco, published “The Joints of the Ankle.” He also created ankle slices, but these came from a cadaver. Measuring the slices of the talus, he hypothesized that the ankle rotated on a fixed axis. From there, he proposed that the talus possesses an overall conical shape with an apex on the medial side of the foot.
(vertical) plane of the talar dome has an indentation shaped like a saddle. Because of this, as stated in the Drexel News Blog, “the leg-to-ankle connection allows the foot to rotate on a flat plane” and land on the outside or inside of the foot when walking. Taking this into consideration, redoing Inman’s measurements without considering the fixed axis indicates that the apex of the talus bone cone is on the lateral side. This matches Siegler’s conclusion and contradicts Inman’s supposition.
“[O]nce a concept becomes entrenched and it becomes common knowledge, it moves beyond the point of being subject to challenge and it is being challenged only once a discovery shows it to be incompatible with reality such as happened in this case,” Siegler said. He added that he did not set out to disprove Inman; his results were incompatible with Inman’s theory.
“Ankle replacements haven’t [been] met with much success [perhaps because of the incorrect geometry],” Toy said.
For their work, Siegler’s team won the 2013 Clinical Biomechanics Award from the International Society of Biomechanics.
Siegler is currently on sabbatical in Bologna, Italy. He and his team are focused on making an ankle implant that geometrically matches his hypothesis. “Rather than using incorrect concepts such as Inman’s or cylindrical surfaces, this implant will capitalize on the new discoveries and use surfaces that best represent the anatomical surfaces of the joint that is to be replaced,” he said.
“The ankle complex is made up of two articulations [joints]. One is between the talus and the tibia [shin bone] and fibula [calf bone] and this is the ankle joint proper, sometimes referred to as the upper ankle joint,” Siegler explained. “Then below it the same bone, the talus, articulates with the heel bone, the calaceneus, to form what is known as the subtalar joint or the lower ankle joint.”
According to the Drexel News Blog, the team intends to compare a 3-D printed replica of a “state-of-the-art prosthesis” against a prototype designed by Siegler in a variety of biomechanical tests. One such test will be to implant the devices in the ankles of a cadaver to see which “produces a more natural ankle movement.”