Can a Biomimetic Ankle Joint with Proprioception Provide Feedback On Its Operating Environment?

In current prosthetic devices permitting rotation, one-degree-of-freedom pin-joint-based
ankle joints are primarily used, while two-degree-of-freedom devices remain in research;
however, it has been shown that they improve the balance of the user compared to
one degree of freedom. These research-based devices also predominantly make use of
pin joints, which are inherently less kinematically accurate than rolling contact joints of
the human anatomy. A two-degree-of-freedom rolling contact joint is multiplanar, and,
therefore, rigid links to constrain the joint cannot be used, as this could only operate in
one plane. Using soft multiplanar links analogous to ligaments would not only constrain
the joint but also allow for characterisation of the joints by measuring the displacement
of the links, which can be used to estimate joint angle for traditional control. A simplified
version of the tibiotalar and subtalar joints of the ankle was made, forming a joint complex
capable of motion in the coronal and sagittal planes. Silicone links were produced with a
silicone-based infrared waveguide embedded to provide proprioception of the joint. The
anterior talofibular, calcaneofibular, tibiocalcaneal, and superficial tibiotalar ligaments
of the ankle were manufactured as deformable waveguides and tested at 10+% strain to
verify linearity. The R2 value of all the ligaments when compared with a linear regression
model was above 0.996 between the operating range of 0-6% strain. Then the joint
complex as a whole was manipulated in the sagittal and coronal planes, providing evidence
that there were unique behaviours in the ligament stretch demonstrated in each discrete
plantar position, dorsiflexion, plantarflexion, inversion, and eversion. This could then
be used in a control scheme for the joint to identify the planar position of the joint,
which could then be used to drive and sense two planes of a prosthetic foot. This would
ultimately improve balance and stability by permitting the foot to adapt to the floor
in both planes with anatomical-like compliance, while allowing for the prediction of the
geometry for a control scheme with active control for balance and stability.