Evaluation of a Two Degree of Freedom Passive Ankle Prosthesis With Powered Push-Off1 (2015)

Ankle prostheses are limb replacement solutions that provide functional performance for transtibial (below-knee) amputees. An estimated 623,000 major transtibial amputations were performed in the U.S. in 2005 [1]. Having a functional limb replacement that the amputee feels physically and socially comfortable wearing significantly improves his or her quality of life [2]. Most commercially available ankle prostheses are passive devices that do not have actuators or batteries to provide powered push-off at the ankle. Without push-off at the ankle, the amputee relies primarily on his or her intact limb for propulsion. The natural ankle performs net rotational work, but the passive prosthesis is characterized by energy dissipation. This contrasting behavior produces asymmetric gait (walking pattern). Active prostheses with motors and batteries can produce powered push-off, but tend to be bulky and expensive. Collins and Kuo [3] developed a passive prosthesis that demonstrates push-off by storing energy from heel-strike and releasing it after heel-off at a time determined by a microcontroller. This paper presents the performance results of an alternative passive prosthesis (CamWalk) that uses a mechanical cam to time the storage and release of energy for powered push-off.The novel design concept for CamWalk is the compliant coupling between two degrees of freedom. Translational work is performed by leg force, F, causing a deflection, r, along the leg axis. Rotational work is performed by ankle torque, T, causing a foot rotation, θ. The mapping of translational work to rotational work via the coupling spring is shown in Eq. (1), where kc is the stiffness and δ is the deformation of the coupling spring.(1)∫F·dr ⇒12kcδ2 ⇒ ∫T·dθThe torque increase from the coupling spring must occur at the peak ankle angle to achieve optimal powered push-off. The peak ankle angle occurs at roughly the same time that the heel lifts off the ground (heel-off). A cam mechanism is used to enforce this timing condition. The cam disengages a support spring, Ss, to increase leg deflection and energy stored in the coupling spring, Sc. Leg compression, or drop, at heel-off is a potential source of discomfort and was therefore limited to a maximum deflection of 5, 10, or 15 mm. Figure 1 shows CamWalk’s configuration during key stance positions. More details of the design are provided in Ref. [4].The spring network of CamWalk was designed to closely match the torque-angle curve for a natural ankle, especially the counterclockwise path of the curve that indicates net rotational work by the ankle. A quasi-static model, with the ankle angle profile θ(t) and the leg force profile F(t) as inputs, was used to calculate the torque profile T(t) for generating the ankle torque-angle curve. The design parameters of the spring network were optimized for producing the closest torque-angle match to that of a natural ankle of an average healthy walker characterized by gait data published by Winter [5].A proof-of-concept prototype was fabricated for the purpose of performing gait analysis for human subjects walking with the device. The prototype had a mass of 2.2 kg with a height of 170 mm from the bottom of the foot to the top of the pyramid adapter. A Vicon© imaging system and force plates were used to capture the kinematics and kinetics of the subject’s gait. The gait analysis was performed for the amputee with their original conventional prosthesis, then using CamWalk for each deflection setting followed by a few survey questions to gauge their subjective perception of the performance of the device.The fourth human subject showed the most rotational push-off at the ankle when using CamWalk. CamWalk’s torque-angle performance for the 15 mm leg deflection setting is shown along with the conventional passive prosthesis torque-angle curve in Fig. 2. Using his original prosthesis, the amputee walked at 1.9 m/s; the natural ankle performed 21.8 J of work per step; and the prosthesis dissipated 8.42 J of energy per step. Using CamWalk, the amputee walked at 1.2 m/s; the natural ankle performed 16.1 J of work per step; and CamWalk performed 7.73 J of rotational work per step. The amputee’s prosthesis work relative to his natural ankle work went from −38.6% with his conventional prosthesis to 48% with CamWalk.CamWalk’s performance for first three amputees showed minimal or no positive rotational work. The predominant reason for the unexpectedly low work was that the cam was not adjusted properly for the other amputees during testing. Figure 3 shows an example torque-angle curve for CamWalk at the 15 mm leg deflection setting with the cam allowing the drop to occur too early in the step. The ankle torque increases prior to heel-off which resists rather than aids walking. Using the θ(t) and F(t) profiles and the observed drop timing as inputs into the quasi-static model, the simulated T*(t) profile shows a similar clockwise loop on the high ankle angle portion of the torque-angle curve in Fig. 3.CamWalk demonstrated its ability to produce net rotational work about the ankle as shown in Fig 2. However, the rotational push-off is sensitive to the drop timing controlled by the cam. Overall, CamWalk performed as designed, and the quasi-static model was proven to be applicable to this problem. The trade-off between rotational power and discomfort from the drop was the most critical information gained from the survey questions. Of the deflection settings, only one amputee preferred the 5 mm setting, the others preferred the 10 mm setting. Although the fourth subject preferred the 10 mm over the 5 mm deflection setting, he preferred the feel of the device when there was no drop. It seems the drop near heel-off removes much of the appeal from the powered push-off benefit of CamWalk. The next generation design will therefore include an energy storage function so that the drop can occur at heel-strike, when the foot impacts the ground and the energy will be released at heel-off.This research was supported by the U.S. Department of Education/National Institute on Disability and Rehabilitation Research Award (H133G120256-13).