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Synthesis of a cydoidal mechanism of the prosthetic ankle

Most prosthetic feet are designed to mimic shock absorption and push-off (Gitter et al., 1991). Such materials as tibreglass and carbon graphite in Seattle Foot, Flex-Foot, Carbon Copy II, so-called Energy Storing (ES) prosthetic feet, enable a greater portion of energy of the “falling” body to be accumulated and released before plantar flexion (Bartkus et al., 1994). The ES feet provide some amount of eversion/inversion as well in the Genesis Foot, Seattle-Light, and Dual Ankle Springs (DAS). 
 
Certain positive outcomes of using ES feet have been reported, e.g. improved ankle range of motion and gait symmetry (Wagner et al., 1987); a smaller number of skin problems like abrasions compared to the “conventional” SACH foot (Alaranta et al., 1994). The amputees preferred ES feet as transmitting Jess shock and having greater damping properties (Wirta et al., 1991). 
 
Nevertheless, ES feet have not shown sufficient improvement in overall performance (Childress et al., 1974; Torbum et al., 1990; Lehmann et al., 1993) in comparison with the conventional SACH foot (Goh et al., 1984). No significant differences in frequency of stump pain were observed (Aiaranta et al., 1994). No improvements have been found in such amputee gait characteristics such as the performance of the existing knee in trans-tibial patients (Edelstein, 1990). During the early stance, the patient's knee bends notably less than normal because the prosthetic foot, either conventional, or ES does not produce the controlled plantar flexion obtained naturally by eccentric contraction of dorsiflexors. Knee flexion is also less than normal during late stance. 
 
The author believes, that the reason for this is that both SACH and ES feet have a similar mechanical outcome in the dorsiflexion phase, namely, the moment of resistance to dorsiflexion, and this characteristic does not mimic the moment of resistance to dorsiflexion in normal gait. 
 
The moment of resistance (resistive curve) to dorsiflexion in a normal ankle typically has a concave downwards shape shown in Figure 1a (Scott and Winter, 1991). The beginning of dorsiflexion during regular level gait occurs with practically no resistance from plantar flexor muscles. Then, resistance slowly increases as the dorsiflexion progresses, while at the end of dorsiflexion the resistance rapidly increases nonlinearly. In contrast with this concave shape of resistive curve seen in the normal ankle, existing prostheses demonstrate a convex shape of their resistive curves (Fig. 1b). 
 
 
 
Fig. 1 
 
(a). Moment of resistance (resistive curve) to dorsiflexion in the ankle during normal walking (from Scott and Winter (1991), with kind permission from Elsevier Science Ltd., Kidlington OX5 1GB, UK. 
 
 
 
If one agrees that a concave resistive curve in the prosthetic ankle is beneficial for an amputee's gait and wants to build an initially compliant prosthesis, the conflict between compliance and durability must first be overcome. Mechanically there are two basic structures employed in the ES foot design. The first is an L-shaped leaf spring (Seattle Foot, Flex-Foot, Carbon Copy II). The second is a multi-bar linkage with elastic elements (Genesis Foot, DAS, College Park Foot). Both types of mechanisms have similar resistive curves (Fig. 1b) with at least non-concave shape (convex or linear). Both types of mechanisms provide an initial moment of resistance M0 which must be applied to a tibial component of the prosthesis to deviate from a vertical position and to articulate the ankle. As this analysis indicates, the initial resistive moment M0 cannot be made zero in both kinds of mechanisms. In a mechanism based on the L-shaped leaf spring, for example, the structure acts simultaneously as a resistor to deflection and as a feature for load bearing. If a designer intends to lower M0 he is at risk of losing durability. 
 
The purpose of this work was to synthesise a mechanism of a prosthetic foot with more natural moment of resistance in the dorsiflexion phase.