Passive prosthetic ankle-foot mechanism for automatic adaptation to sloped surfaces.

INTRODUCTION When nondisabled persons walk on sloped or uneven surfaces, they make adjustments to the kinematics and kinetics of their gait patterns [1-4]. According to Hansen et al., the net result of kinematic changes can be viewed as a change in ankle alignment to account for the change in surface slope [1]. Furthermore, in nondisabled persons, the first step on a different surface is adapted to that surface [5-6]. But without direct neural control, persons with lower-limb amputations do not have the ability to adjust their prosthesis for every step. Failure to adapt to terrain can lead to increased peak loading in the socket, discomfort, and tissue damage on the residual limb [7] and may contribute to falls when an unexpected new surface is encountered. To realize a truly adaptive prosthesis, the alignment of the ankle must be set independently for each step based only on the mechanics of that step and the surface encountered. There are some nonadaptive lower-limb prostheses that are tolerant of terrain variance through the use of multiaxial deflecting elements or ranges of hydraulic damping placed in series with high stiffness foot plates or keels (e.g., Epirus and Echelon feet from Endolite; Miamisburg, Ohio). The physical stops at the end of the range of motion of these features still engage the high stiffness property at the same ankle angle regardless of surface slope, so the ankle torque-angle relationship for the high stiffness element remains at a single equilibrium point. Some microprocessor-controlled lower-limb prostheses are capable of changing alignment automatically, yet these systems often require incremental adjustments over several steps before adaptation is completed. Although these prostheses do adapt their alignment to surface slopes, they are not able to adjust for the first step on a new surface or for an uneven surface that is constantly changing slope. Some historical prosthetic foot-ankle systems were developed that provide first-step adaptation; however, these devices were not clinically viable. For example, Hans Mauch developed a passive mechanical prosthetic ankle that adapted to sloped surfaces [8]. The mechanism relied on gravity to hold a ball at the nadir of a track such that when the shank reached vertical the ball closed a port, essentially "locking" the ankle. This approach was well received by the test users; however, rotary hydraulic systems are difficult to seal properly and the device was discontinued because of hydraulic fluid leakage and the need for frequent maintenance. Hansen et al. have also developed two prototype foot-ankle systems that adapt on each step of walking based on nondisabled walking characteristics [9]. Williams et al. used a weight-activated knurled cam clutch to engage a high stiffness rubber bumper at foot flat [10]. The proof-of-concept prototype successfully demonstrated the plausibility of the concept, yet many improvements were needed, including refinement of the cam clutch mechanism and clutch engagement mechanism. Nickel et al. used a releasable wrap spring clutch mechanism to engage a high-stiffness rubber bumper at foot flat [11]. The prototype showed evidence of changing the equilibrium point in response to walking on sloped surfaces, but the clutch did not support sufficient torque. Our goal for the present work was to refine the design by Williams et al. and bring the concept closer to a clinically viable device. The refinements were aimed at (1) reducing engagement/disengagement noise by using a smooth cam surface instead of the knurled cam, (2) simplifying the engagement/disengagement mechanism by using a hinge instead of a telescoping slider, (3) redesigning the system to fit within a commercially available cosmetic foot shell, and (4) general design enhancements for structural strength, reduced weight, and reliable performance. METHODS Prototype Design The ankle-foot prototype of the present work (Figure 1) is designed to provide a step-specific equilibrium point, or set-point, setting the dorsiflexion-plantarflexion alignment (zero-torque angle) on each step based on the surface slope encountered. …