Anthropomorphic Feet May Reduce Risk to OI Implants

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A study conducted by Mark Pitkin, PhD, and Laurent Frossard, PhD, focused on the loading effects of prosthetic feet on osseointegrated (OI) implants. Their results showed that a foot with anthropomorphic characteristics (i.e., the ability to mimic the function and stiffness of a natural ankle), tended to decrease maximal loads at the bone-implant interface and may minimize risks to the interface. The information can help prosthetists make evidence-based decisions about the best options when fitting patients who have bone-anchored prostheses, such as the selection of components and alignment of the prosthesis.


Frossard said that in discussions about OI, the role of the prosthetist as the "primary point of contact for consumers and responsible for prosthetic loading" has been overlooked. "We often forget that prosthetists play a key role in the prevention of load-related adverse events when fitting bone-anchored bionics prostheses," he said.

A typical stiffness curve of a sound ankle and prosthetic ankle with non-anthropomorphic design.

Images courtesy of Laurent Frossard and Mark Pitkin.

OI implants carry risks such as early loosening, mechanical failure of percutaneous and medullar parts, periprosthetic issues, and infections, the authors noted. Underloading can lead to early loosening and infection, but overloading might compromise the bone-implant interface. Therefore, the researchers sought a "Goldilocks" loading regimen (loading that is neither too much nor too little) for safe and efficient development of osseointegration around the implant during rehabilitation and beyond.

The authors' hypothesis was that ideal loading could be achieved with a prosthetic ankle that showed a moment-angle relationship similar to a sound ankle. The Free-Flow foot was selected for its anthropomorphicity demonstrated in previous studies with amputees using traditional socket attachments.

"The Free Flow foot was initially designed by WillowWood to be anthropomorphic. So it made sense to test the anthropomorphic characteristics of this particular foot and to compare it to other feet with non-anthropomorphic designs," Frossard said.

Pitkin and Frossard calculated the slope of the moment-angle curve (stiffness) twice for each of the four able-bodied participants in experiment 1: for the first half and for the second half of the moment-angle curve. The difference of stiffnesses (those at the second half minus at the first half) was called the index of anthropomorphicity (IA).

The ankle angle of dorsiflexion and bending moment for the Free Flow foot (HC: heel contact, TC: toe contact, HO: heel off, TO: toe off).

In experiment 2, the same recordings and calculations were performed for three participants fitted with transtibial osseointegrated fixation during walking with their usual prosthetic feet and the Free-Flow Foot.

The IA was 5.88 ± 0.93 for the able-bodied participants, indicating that the stiffness during the first part of the dorsiflexion phase was substantially less than during the second parts, as the calf muscles resisted to angulation in the ankle substantially less than during the second part of dorsiflexion phase.

For amputees fitted with the Free-Flow foot, IA was 2.68 ± 1.09 and −2.97 ± 2.37 for the same amputees fitted with their usual feet.

"Understanding the missing links between anthropomorphicity of bionic feet and developments of osseointegration is key for an evidence-based fitting of bone-anchored bionics prostheses," Froussard said.

The open-access study, "Loading Effect of Prosthetic Feet's Anthropomorphicity on Transtibial Osseointegrated Implant," was published in Military Medicine.