Assessment of residual-limb volume change using bioimpedence.

INTRODUCTION Residual-limb volume fluctuation creates challenges for lower-limb prosthesis users. Limb shrinkage can cause the socket to become loose and concentrate interface stress in soft tissues over bony prominences. Limb enlargement can induce excessive pressures and occlude blood flow, denying tissues of nutrients and restricting venous return. Both of these conditions can result in soft tissue injury. Residual-limb volume fluctuation has been recognized as a major challenge that should be a priority in prosthetics research [1]. Existing treatments for managing residual-limb volume change include stump socks, air-filled inserts (e.g., Pneu-Fit, Prosthetic Concepts, Little Rock, Arkansas; Pump-It-Up!, Love Associates Inc, Batavia, New York), fluid-filled inserts (e.g., Active Contact System, Simbex, Lebanon, New Hampshire; Volume Management Pads, Ohio Willow Wood, Mount Sterling, Ohio), and vacuum-assist devices (e.g., Harmony System, Otto Bock, Minneapolis, Minnesota). Although these treatments are used clinically, improvement in their performance is still needed. Important challenges for managing residual-limb volume change include selecting the proper sock thickness or insert volume, timing the ply changes correctly, and ensuring consistent performance. The treatment also needs to be matched to the individual patient--one treatment may be effective for one patient but not another. For progress to be made in treatment technologies and individual case prescription, a convenient means for accurate high-resolution assessment of residual-limb volume is needed. Most viable methods for measuring limb volume change suffer from important limitations in their usefulness to prosthetics research and practice. Radiological and magnetic resonance imaging (MRI) methods require large, expensive, specialized equipment not typically available to most prosthetists. Further, these methods may require supine positioning that can affect the measurement of interest. Laser and optical imaging methods do not provide sufficient resolution and/or take too long [2]. Perhaps the most important limitation of these methods with regard to the application of interest here is that they require the residual limb to be removed from the socket. This is a limitation because the volume of the residual limb while it is in the socket is the parameter of primary clinical interest. Residual-limb volume changes drastically once the prosthesis is doffed [3]. This research pursues a method for assessing residuallimb volume change while the residual limb is within the prosthetic socket. Our intent was to describe the modality, assess the instrument error, and then determine if volume changes measured on nondisabled and amputee subjects were outside of the instrument error. ELECTRICAL BIOIMPEDANCE Bioimpedance is a noninvasive means for assessing extracellular fluid (ECF) and intracellular fluid (ICF) within living tissue. Bioimpedance analysis has been used as a tool for body composition/body fat analysis [4-9] and in the assessment of fluid imbalance in hemodialysis patients [10-13]. It has not yet been applied to the prosthetics field. Biological conductivity occurs primarily through fat-free tissue and fluids [14]. Other elements, such as bone and adipose tissue, are essentially nonconducting. By applying a low current (