Controlled Coronal Stiffness Prosthetic Ankle for Improving Balance on Uneven Terrain

Thesis (Master's)--University of Washington, 2013Background For lower limb amputees, quality of life is directly related to the functionality of their prosthetic devices. Uneven terrain can cause significant disturbances to an individual's gait. Disturbances with significant inversion or eversion are more likely to cause imbalance than sagittal plane disturbances, yet currently marketed devices do not have controlled methods for reacting in the coronal plane. The objective of this project was to develop a prosthetic ankle that is capable of changing stiffness characteristics in the coronal plane with the objective of creating a more stable support for amputees. This device will be used in further research to characterize kinematics in the coronal plane and develop strategies of improving amputee balance while traversing uneven terrain. Methods A prosthetic ankle was designed, built and tested that is capable of controlling the rotational stiffness of the ankle for inversion and eversion. The effective length of two cantilever beam springs can be changed by a motor controlled through a battery powered on-board computer. A series of bench tests and human subject tests were performed to test the functionality of the device including stiffness characterization, comparison to available commercial devices, motor control characteristics, human subject standing balance center of pressure tracking, and walking on an inverted and everted plane. Results Characterization of the device and stiffness properties showed that over a series of springs it was capable of developing a range of rotational stiffness values from 23 Nm/rad to 510 Nm/rad. When placed in series with a stiff commercial foot, one particular set of springs proved to have a greater range of stiffness than that covered by a series of commercial feet. Human subject testing demonstrated methods and metrics that could be used to evaluate optimal stiffness values and profiles. Motor tests showed that the drive system did not perform well enough to quickly change stiffness values while fully weighted on an inverted or everted surface. Conclusions The range of stiffness characteristics over which the device is capable has proven to be sufficient. However, the drive system for dynamically changing the stiffness throughout the gait cycle requires a few design changes. The device has proven to be capable of testing the effects of amputee balance and kinematics while walking on uneven terrain. Further use of this device for research will help to develop the optimal coronal stiffness characteristics and controls for improved prosthetic ankles