Contribution of prosthetic knee and ankle mechanisms to swing-phase foot clearance

Individuals who ambulate with transfemoral prostheses fall more frequently than users of transtibial prostheses or able-bodied persons. One factor contributing to these falls may be reduced toe clearance during swing phase of the prosthesis. Quantitative gait data were acquired from ten persons with transfemoral amputations who were fitted with either single-axis or four-bar knees, in a cross-over experiment. Toe clearance during prosthetic swing phase was measured from these trials. To differentiate between effects on toe clearance attributed to knee mechanisms and those produced by compensatory gait deviations used by subjects, 3-D swing-leg simulations, incorporating models of the knee mechanisms, were applied to these empirical data. Additionally, two currently available prosthetic ankle mechanisms that dorsiflex to facilitate toe clearance during swing phase were modeled. Four-bar mechanisms had an empirical average of 2.2 cm more minimum toe clearance than single-axis knee mechanisms (p < 0.01). In the simulation study, single-axis knees provided the least clearance (4.6 cm) during prosthetic swing phase, followed by four-bar knees (5.8 cm), and the two ankle mechanisms (6.4 and 8.7 cm). Four-bar knees provide greater foot clearance during prosthetic swing phase than single-axis knees, but prosthetic ankle mechanisms that dorsiflex during swing phase provide substantially more.