Effects of Alignment on the Roll-Over Shapes of Prosthetic Feet

Abstract

Recent work suggests that a prosthetic ankle-foot component's roll-over shape -the effective rocker it conforms to between initial contact and opposite initial contact (the 'roll-over' interval of walking) -is closely linked to its final alignment in the prosthesis (as determined by a skilled prosthetist using heuristic techniques). If true, this information may help to determine the appropriate alignment for a lower limb prosthesis before it is built, or a priori. Knowledge is needed for future models that will incorporate the roll-over shape including the relative effect of alignment on the roll-over shape's radius of curvature and arc length. The purpose of this study was to evaluate the hypotheses that: (i) Changes in prosthesis alignment alter the position and orientation of a foot's roll-over shape in prosthesis-based coordinates, and (ii) these changes occur without changing the radius of curvature or arc length of the roll-over shape. To examine the hypotheses, this study examined the effects of nine alignment settings on the roll-over shapes of two prosthetic feet. The idea that alignment changes move and rotate roll-over shapes of prosthetic feet in prosthesis coordinates is supported by this work, but the hypothesis that the radius of curvature and arc length do not change for different alignments is not strongly supported by the data. A revised approach is presented that explains some of the changes to the roll-over shape parameters due to changes in rotational alignment.

Stance phase mechanical characterization of transtibial prostheses distal to the socket: A review

“…Following selection for relevance and including those resulting from the secondary search, we identified 36 articles that satisfied the criteria [2][3][5][6][7][13][14][15][16][17][18][19][20][21][22]. In addition, five relevant review articles were identified [1,[8][9][11][12].…”

“…It is known that alignment can significantly affect the mechanical behavior of prostheses [3,24], effectively changing their AIPPs and, hence, introducing a confounding effect.…”

“…The roll-over shape is a spatial mapping of the center of pressure location along the plantar surface of the foot relative to a shank-based coordinate frame. The roll-over shape can be measured in an amputeeindependent manner from data generated using a test rig [2][3]16,18,[32][33][34], in which case it can be considered to be part of the AIPPs description of the prosthesis. By varying the applied loads, a family of roll-over shapes can be produced, which provide a representation of foot stiffness.…”

“…Interestingly, however, the study did not go on to characterize in detail the changes to the mechanical properties as a result of alignment. This is important, as another study demonstrated that through changes in sagittal plane alignment, different AIPPs can be obtained with the same prosthetic foot [3]; alignment determines the points of contact of the foot during stance. Although many of the constituent materials may have isotropic properties, the assembled prosthesis will have anisotropic properties because of the nonsymmetric geometry of the subcomponents.…”

“…In this context, AIPPs are defined to be the mechanical properties of the prosthesis that directly influence the comfort and performance of the amputee (i.e., the properties that the amputee experiences) and that are not based on in vivo measurements that may include confounding influences related to compensatory gait biomechanics. AIPPs describe the properties of the prosthesis as a whole and include the roll-over shape, alignment of the roll-over shape, elastic properties, damping properties, and mass properties, which all directly affect amputee gait [1][2][3][4]. Conversely, the underlying design details (e.g., subcomponent geometries, material densities, and Young's moduli) are not considered to be AIPPs; rather, they determine the AIPPs and therefore only indirectly affect amputee gait.…”