Minimum Vacuum Level Required to Maintain Limb Volume in Trans-tibial Amputees during Ambulation

Abstract

INTRODUCTION: Amputees often struggle to maintain a good fit throughout the day. For a good fit, the prosthesis should feel like an extension of the residual limb. In the morning when the limb is fully hydrated and placed in a slightly undersized, total-surface weight-bearing socket, the fit is good. After donning, the limb steadily loses volume and the fit is compromised. This volume loss can lead to poor spatial awareness, poor or slow limb responsiveness, irregular gait and increased pistoning of the residual limb (Board et al., 2001; Street, 2006). Use of a vacuum-suspension socket with a mechanical pump drawing -70kPa has helped eliminate volume loss. Developments in miniature electronic pumps have raised the following question: What is the minimum vacuum range needed to maintain limb volume? PURPOSE: To establish an accurate method of determining limb volume (+3g) to assist in identifying a vacuum-level that will maintain limb volume and assist engineers in designing lower limb prosthetics using miniature electronic pumps. METHODS: Two methods of determining limb volume were evaluated: Water displacement and force readings. Pilot work was completed on both techniques. The precision of water displacement was calculated using the radius of the limb and container. Sensitivity of an AMTI force platform was determined using changes in force readings after limb immersion. A 45.45 kg load cell was tested for precision by placing gram weights on the transducer and recording voltage output. RESULTS: The water displacement precision was found to be poor. A small change in meniscus height occurred with large volume changes, undetectable due to the coarse meniscus scale (0.3 cm = 17.40 cc). The AMTI force platform precision using a 12 bit AD converter was found to be 1.4N, exceeding the ideal 3g volume change. The 45.45 kg force transducer provided a greater precision ability to detect volume changes equivalent to 2-3 grams. CONCLUSION: Due to the lack of precision of water displacement and a standard force platform, changes in limb volume should be completed using a force transducer that can detect at least a 1g difference. Calculations based on these data would suggest a 4.54 kg load cell would allowed for a measure of .3 g or less making it precise enough to determine residual limb volume changes in amputees over the course of a day.