f Modelling Modelling and gait evaluation of asymmetrical-keel foot prosthesis P. Allard 1~ F. Trudeau 1"3 F. Prince 1~ J. Dansereau 3 H. Labelle TM M. Duhaime TM 1 Laboratoire d'Etude du Mouvement, Centre de RechercheP6diatrique, H6pital Sainte-Justine, 3175 C6te Ste-Catherine, Montrdal, PQ, H3T 1C5, Canada 2 Ddpartement d'Education Physique, Universit6 de Montreal, CP 6128, succursale A, Montr6al, PQ, H3C 3J7, Canada 3 Departement de M6canique, Ecole Polytechnique de Montr6al, Montr6al, PQ, Canada 4 D6partement de Chirurgie, Service d'Orthop6die, H6pital Sainte-Justine, 3175 C6te Ste-Catherine, Montreal, PQ, H3T 1C5, Canada Abstract--The paper documents a new concept in prosthetic foot design. It is based on the capacity of a flexible keel to allow a greater medio-lateral function than previously available. The heel has a complex curvature consisting of a medially concave shape that joins the mid-foot. There a hump acting as a leaf-spring ends at the metatarsal break, with an inwardly curved toe extremity. These curvatures contribute to increased medio-lateral control at heel-strike and propulsion for weight transfer and push-off. Results from finite-element modelling indicate that the asymmetrically shaped keel is at least twice as active in storing energy compared with a completely symmetrical one. A preliminary gait study is carried out for a 24- year-old below-knee amputee fitted with the new design, the SPACE foot and a dynamic elastic response foot with a symmetrical keel. With the SPACE foot, there is a 14% increase in walking speed combined with a reduction in the phasic asymmetries. The absolute difference between the initial and terminal double support is 1.4% for the asymmetrical keel design compared with 4.4% for the symmetrical keel foot prosthesis. The peak ankle power generation burst indicates that the SPACE foot behaves as a dynamic elastic response foot. Keywords--Biomechanics, Feet, Flexible keel, Finite element modeling, Gait analysis, Inverse dynamic, Prosthetics Med. & Biol. Eng. & Comput., 1995, 33, 2-7 1 Introduction PROSTHETIC FEET can be grouped differently according to the function emphasised. Considering joint mobility or motion, for example, some feet are articulated at the ankle, such as the single-axis type; others form a rigid segment (solid ankle cushioned heel (SACH)) with the shank segment. These can be considered as the first generation of prosthetic feet where increased stability and joint mobility were essentially sought. With the use of polymers and composite materials came the dynamic elastic response (DER) foot. The energy of deformation stored in the structural element, the keel, during heel-strike could be recovered in the push-off period to drive forward the deficient limb. With this original and innovative concept, the second generation of prosthetic feet emerged quickly. The energy storage and release capacities of these DER prostheses have been well demonstrated in laboratory tests Correspondenceshould be addressedto Dr. Paul A/lard, PhD, PEng First received4 June and in final form 1 December1993 9 IFMBE: 1995 (C ONTOYANNIS,1987). However, Prince et al. (PRINCE et al., 1991) have shown that, during running gait, amputees fitted with the SACH foot developed about the same forward and vertical impulsions as those with the flexible keel foot prostheses. Torburn et al. (ToRBURN et aL, 1990) also found that, during free or fast walking, there are no clinical advantages to any of the five energy-storing feet (SACH, STEN, Carbon Copy, Seattle and Flex) tested. Part of this limitation may be due to inadequate medio-lateral propulsion by the amputee and the minimal contribution of the present flexible keel foot prosthesis in providing sufficient medio-lateral propulsion necessary during weight transfer. There have been attempts to include this function into the prosthetic foot by differential keel thickness (SABoLICn, 1989) or extended midfoot portion (PALFRAY, 1991). This paper presents a new concept in foot prostheses design. It is based on the capacity of the flexible keel to allow a greater medio-lateral function than previously reported, while storing and releasing energy in all three planes. Finite-element modelling and gait analysis techni- ques are used .t0 illustrate the potential of this third-generation flexible keel foot prosthesis or asymme- trical keel design. 2 Medical & Biological Engineering & Computing January 1995