Effect of prosthetic ankle units on roll-over shape characteristics during walking in persons with bilateral transtibial amputations.

INTRODUCTION The anatomical foot and ankle create a complex structure during walking, with the important functions of providing shock absorption and stability while contributing to the efficiency of walking. Three rockers of the foot and ankle [1]--the heel, ankle, and forefoot--act in a serial fashion during normal walking to advance the body forward over the stance leg. During the loading-response phase, the heel rocker sustains forward movement of the body without interruption and facilitates foot flat to ensure stability as the leg is progressively loaded, while simultaneously, restrained ankle plantarflexion provides shock absorption. Throughout midstance, the ankle rocker maintains forward progression of the body while the plantar surface of the foot remains in full contact with the ground. Finally, during terminal stance, the forefoot rocker enables the body to continue rolling over the foot as the ankle restrains dorsiflexion and the heel rises from the ground. These three discrete foot rocker mechanisms have an integrated effect during nondisabled walking to produce a functional rocker, or "roll-over shape" [2-3], which directly affects vertical motion of the body and is important for achieving smooth, efficient forward progression [4-5]. The ankle-foot roll-over shape can be calculated from quantitative gait data by transforming center of pressure data under the foot from a laboratory-based coordinate system into a shank-based coordinate system [2]. A circular arc is then fitted to the roll-over shape data, from which pertinent characteristics of the roll-over shape such as radius and arc length can easily be determined. The effective foot length is defined as the distance from the heel of the prosthetic foot to the most anterior portion of the roll-over shape [6]. The effective foot length ratio is the effective foot length divided by the total foot length. Scaled to an individual's height, the roll-over shapes that are created by nondisabled persons during walking have been found to be relatively invariant with walking speed [2-3], added weight [7], and changes in shoe heel height [8]. Presumably, nondisabled individuals maintain a particular roll-over shape to optimize walking biomechanics and efficiency. The concept of roll-over shape has also been applied to lower-limb prostheses for the evaluation and design of prosthetic feet [3,6,9] and has been used as a basis for understanding principles of prosthetic alignment [10-11]. Recent work indicates that fitting prosthesis users with prosthetic feet that have inappropriate rollover shapes can reduce the quality of gait and comfort of the user [12-13]. Some of the important functions associated with the foot-ankle rocker mechanisms are adversely affected or eliminated during the gait of prosthesis users because of reduced or absent ankle motion. However, these functions may possibly be restored or improved by fitting users with prosthetic ankle joints. Prosthetic components that increase prosthetic compliance and produce greater sagittal plane ankle motion are often prescribed to provide shock absorption during the loading-response phase [14-15]. Additionally, the compliance provided by prosthetic ankle units may allow the prosthetic foot to better accommodate uneven terrains, potentially improving stability during gait. For level walking, the prosthetic ankle units may also affect characteristics of the roll-over shape [10-11]. During nondisabled gait, a fairly well-defined and relatively consistent relationship exists between ankle joint moment and ankle motion during stance that is dependent on the walking speed [16], suggesting stiffness values for artificial mechanical systems that might replace the anatomical system. Appropriate consideration should be given for the net effect of the combined stiffness when pairing a prosthetic ankle unit with a prosthetic foot. Therefore, a prosthetic ankle unit that increases ankle motion during stance phase may produce different ankle-foot roll-over shape characteristics than the prosthetic foot alone. …