Adjustable Sockets

Funded by the Department of Defense.

The purpose of this project is to develop a prosthetic socket for trans-tibial prosthesis users that automatically adjusts its size so as to maintain prosthetic fit and minimize residual limb fluid volume loss. The socket is potentially useful for active users who cannot be distracted to continually sense and adjust their socket size. The objective is to allow prosthesis users to continuously wear their prosthesis pain-free and distraction-free and operate at high levels of function.

A first step in the development of the automatically adjusting socket was to make a socket where the volume could be controlled and monitored. We developed a motor-driven, cabled-panel, adjustable-socket system. The motor is mounted beneath the socket and connects to the cable through tubes within the socket wall. Upon commands from a micro-controller, the motor moves in discrete steps (in either direction), which causes the three socket panels (anterior lateral, anterior medial, posterior midline) to displace radially. By keeping careful track of cable length, we are able to track socket volume changes induced by the radial movement of the panels.

The automatically-adjusting system operates using a collection of sensors in the prosthetic socket that monitors socket fit. When an unfavorable change in fit is detected by the sensors, the system adjusts socket size accordingly until fit is once again favorable. The sensors used are a collection of custom sensors developed in our lab.

To develop the automatic system, we created adjustable sockets for 12 participants and investigated how limb fluid volume changed when socket size was changed. Results showed that, on average, users tolerated a socket size increase 2.2% larger than their normal socket volume and tolerated a socket size decrease 3.0% smaller than their normal socket volume. User’s optimal size was, on average, 0.7% smaller in volume than their normal socket.

Fluid volume monitoring using bioimpedance analysis demonstrated that near the user’s optimal socket size, limb fluid volume changed in proportion to socket volume change. At larger socket volumes, however, limb fluid volume did not further increase, and the socket became unstable and uncomfortable.

Diagnostic Tool

Changes in residual limb volume adversely affect the prosthetic fit of people with limb loss. When volume change occurs, prosthetic fit can rapidly deteriorate from good to poor and put the user’s limb at risk of injury, precluding use of the prosthesis. Though there are a variety of clinical techniques available to try to accommodate residual limb volume fluctuations, practitioners face difficulties selecting an appropriate accommodation strategy for an individual prosthesis user because residual limb volume change patterns are quite variable and can be difficult for some users to detect prior to the onset of limb health issues. This study seeks to overcome this challenge in clinical care by presenting to the practitioner the fluid volume fluctuations their patient is experiencing, including where and when they occur and predicting how different accommodation treatments will affect each patient.

The objective is the translation of our portable bioimpedance analysis device created as part of a prior project into a meaningful and useful clinical diagnostic tool that facilitates diagnosis of limb fluid volume fluctuation and aids in selecting and fitting clinically-appropriate accommodation strategies to people with limb loss. Our bioimpedance analysis device allows us to quantify extracellular and intracellular fluid within living tissue (i.e., a prosthesis user’s residual limb) while an individual is wearing their prosthetic socket (see Residual Limb Volume Fluctuation and Accommodation in the Previous Research tab). We anticipate that the clinical research studies conducted here will provide key experience and knowledge towards a subsequent large-scale clinical trial.

Field Monitoring

We are developing a multitude of prosthetic monitoring devices that have a wide range of applications. Our ultimate goal is to develop instrumentation to be worn on the prosthesis with little notice by the user that will provide practitioners with valuable insights, enabling them to provide better care to their patients. Additionally, we develop devices for use as research tools, such as those that can be used to evaluate the effectiveness of different prosthetic components or new prosthetic technologies that we are developing.

In this work we develop devices to collect various biological signals of interest using sensors that we develop in our lab or existing sensors that we adapt for use in a prosthesis. We also develop methods for automatically analyzing the collected data to extract important clinical insights and then to visualize the information in manner that is clinically relevant and easy to understand.

We have developed or are currently developing methods to monitor variables including: socket fit, limb movement, interface pressures, activity tracking, daily socket use, and patient behavioral compliance.

Optimizing Liner Selection

A critical component in most prosthetic limb systems is the material used at the interface between the prosthetic socket and the residual limb. This material acts as a cushion between the socket and the limb and often helps create a mechanical connection. In lower-limb systems, this material comes in the form of an elastomeric prosthetic liner. Selecting the appropriate liner is an important decision; however, prior to this project, information quantifying liner material properties was lacking. The findings of this project are intended to enhance the practitioner's toolbox by providing information about how liners interact with residual limb soft tissues and how liners mechanically differ from one other. This information should help practitioners select appropriate liners for their patients.

In this work, we have created a suite of standardized mechanical tests to be performed on prosthetic liners. We have characterized several existing and emerging interface liner materials and have published our findings in a searchable tool on the website: http://www.linerassist.org/.

Sock Thickness and Use

Daily changes in the shape and size of the residual limb affect prosthetic socket fit. Prosthetic socks are often added or removed to manage changes in limb volume. The number and thickness of prosthetic socks required to maintain fit are highly variable. Manufacturers of prosthetic socks usually characterize the thickness of their products by a metric called sock “ply;” however, differences in materials, strain, and age of socks make the meaning of amputee sock ply confusing for both prosthesis users and prosthetists.

Using a custom research instrument, compressive stresses were applied to socks while sock thickness was measured, to reflect strain and loads experienced within the socket. We aimed to facilitate understanding of the meaning of sock ply towards clinical fit. An additional goal was to characterize how sock thickness changed under loading conditions experienced while a person with limb amputation walked in a prosthesis. Results from this investigation led us to propose a new nomenclature for sock thickness that may be more intuitive to practitioners and to the industry. We have also sought to better understand how persons with transtibial amputation use socks to manage diurnal changes in volume and comfort with a customized, self-report questionnaire. This work led to current projects involving the development of field monitoring devices to record sock changes and sock thickness during normal prosthesis use.

Adding and removing prosthetic socks is currently the most-used method for maintaining good socket fit when limb volume changes occur. Previous studies regarding sock thickness and sock use have shown that the volume accommodation provided cannot be reliably predicted by sock ply or age. Direct measurement of total sock thickness may provide more meaningful insight to quantify prosthetic users' socket fit and guide volume accommodation recommendations. We have developed a clinical tool for proper measurement of sock thickness in a clinical setting. We are designing it to be handheld and easy to use while providing accurate sock thickness measurements.

Tissue Health

A common problem among prosthesis users is skin breakdown caused by the repetitive mechanical stresses acting on the limb within the prosthesis. Skin breakdown (e.g. abrasions, skin tears, and pressure ulcers) can be painful and often results in decreased mobility for an individual. Fortunately, skin is also able to adapt to mechanical stress to enhance the tissue’s ability to bear the stress without suffering breakdown. The goal of this work is to develop techniques to prevent skin breakdown and to promote skin adaptation.

Previously we have investigated the phenomena of skin breakdown and adaptation through both animal and explant models, largely using histological techniques. Currently we are developing non-invasive imaging techniques to investigate the physiology of skin breakdown and adaptation in humans and to predict skin breakdown before it is visibly evident. Findings from this work could have broad-reaching applications from prosthetics to other areas where skin breakdown is a concern such as users of assistive devices and bedridden individuals.