Assessment of Changes in Residual Limb Shape and Volume Over Time
Bioimpedance
Residual limb volume change is an important challenge in clinical prosthetics. Volume changes over the course of a day can alter prosthetic fit and detrimentally affect residual limb soft tissues. Strategies and devices have been developed to try to control residual limb volume change but no definitive effective method has emerged. The intent of this project is to provide the field with a means for in-socket assessment of residual limb volume change, a measurement tool that should not only allow current strategies and devices to be assessed but also may provide insight into the biophysical sources of volume change.
We are pursuing use of segmental bioimpedance to characterize residual limb volume change within the prosthetic socket. By assessing tissue electrical impedance over a wide range of frequencies, we can characterize the extracellular fluid volume change within a residual limb segment over time or between different conditions with accuracy sufficient for the clinical needs here. Preliminary testing on amputee subjects has shown significant changes for different postures and activities:
Results from bioimpedance testing - Extracellular fluid volume changes from testing on a trans-tibial amputee subject.
Next steps in this project are to use the tool to assess different volume control strategies in clinical use. We are also interested in using the bioimpedance data to help characterize the physiologic sources of volume change. We are seeking mechanical, electrical, and bioengineers interested in furthering the development of this device.
Optical Imaging
Results from our interface stress studies (see Publications Index) suggest that interface stress distributions change over both the short-term (diurnal) and the long-term (months). Shape alterations are known clinically to influence prosthetic fit; prosthesis users often add or remove socks over the course of a day to account for shape changes. However, results from our interface stress studies also indicate that interface stress changes are not uniformly distributed over the residual limb but are much more complex and not easily predicted. The purpose of this project is to measure and analytically characterize residual limb shape changes over time, and to correlate those changes with interface stress changes. Ultimately, new designs to overcome the detrimental effects of shape change on interface stress distributions will be pursued.
Accurate measurement of residual limb shape requires a fast and accurate device for shape assessment. We developed a digital silhouette scanner for this purpose, and it is shown below. Novel features of this custom scanner include means for tracking and correcting for subject movement during imaging, and a robust reconstruction algorithm to re-create the residual limb shape. The scanner images the residual limb in approximately 1 s with a resolution of ~0.24 mm.
Digital optical silhouette scanner - The scanner images the residual limb in approximately 1 s.
Results show that residual limb cross-sectional area changes down the length of the residual limb vary from subject to subject. Two examples after socket removal, one showing a relatively uniform expansion and the other more localized changes, are shown below.
Residual limb cross-sectional area change down the length of two residual limbs. Results from two different subjects, one showing relatively uniform limb enlargement over time (left panel), and one showing localized swelling (right panel). Times are intervals since doffing (prosthesis removal).
A next step in this project will be to use the scanner in clinical studies to characterize residual limb shape changes after use of different volume control strategies. We are seeking researchers with a clinical bioengineering interest to pursue these studies.
Assessment of Residual Limb Skin Quality Using Near Infrared Imaging
An important challenge in the use of a lower-limb prosthesis is to maintain healthy soft tissues in the residual limb. The purpose of this research is to pursue the use of near infrared imaging as a means to assess skin mechanical quality. The hypothesis tested is that the thermal recovery time (TRT), the time for the skin to reach a consistent temperature after being mechanical stressed for a short interval, is related to the tissue quality, i.e. the capability of the skin to withstand mechanical load. A long thermal recovery time indicates skin at high risk of injury.
To evaluate the utility of TRT measurement towards amputee skin assessment, we conducted TRT assessment on amputee subjects at regular intervals (once/month). The goal is to determine how soon beforehand TRT predicts an imminent skin breakdown event. Preliminary results from this study are encouraging in that prediction typically occurs 1 to 2 months before breakdown is clinically apparent.
Thermal imaging a subject. The camera provides an instantaneous thermal map of the lower limbs.
A next step in this project will be to extend use of TRT towards socket design and rectification. The intent is to use this tool to design sockets that will facilitate soft tissue load tolerance. We are seeking researchers with a clinical bioengineering interest to pursue these studies.
Assessment of the Quality of Sockets Made Using Central Fabrication Facilities
Central fabrication is a means for making prosthetic sockets that offers cost, time, and efficiency advantages over traditional methods. Using central fab, a prosthetist designs a prosthetic socket using computer software, then electronically sends the designed socket shape file to a commercial facility. The central fabrication facility uses computer-aided manufacturing equipment to carve a positive of the desired socket shape and then vacuum-forms onto that positive to create a socket. The socket is trimmed and then sent back to the prosthetist, typically within 1 to 3 days total time.
The purpose of this project is to evaluate the quality of central fab sockets and, if excessive inaccuracies occur, to identify sources of those limitations. A custom very accurate mechanical digitizer is used to measure socket shape. Testing of sockets from 10 central fab facilities (3 sockets per facility) showed that some centers made accurate sockets all of the time, some made inaccurate sockets all of the time, and some made accurate sockets some of the time.
Percentage difference in volume between fabricated socket and electronic shape data file. An absolute difference of less than 1.1% was considered acceptable. Results show that Companies 1, 2, 3, and 5 consistently made sockets within the 1.1% error range. Company #10 was consistently outside this range. The other companies made one or two of the three sockets within the range.
Next steps in this project will be to analyze steps in the fabrication process to better identify sources of shape error. Fabrication centers or individual prosthetists interested in having their socket fabrication equipment tested in this manner should contact us.
All Prosthetic Engineering research in our laboratory is funded by the National Institute of Biomedical Imaging and Bioengineering (NIBIB). We gratefully acknowledge their support.
BiomaterialsFibro-Porous Meshes for Biomedical Applications
Fibro-porous meshes made up of small-diameter polyurethane micro-fibers were fabricated using a custom electrospinning apparatus. Electrospinning was used since it allows fibers with these small dimensions to be made without "flocculation," i.e. aggregation of fibers. SEM analysis of micro-fibers showed that fibers were fused at the nodes, and that fiber diemensions were relatively consistent.
Left panel: Custom electrospinning system. Right panel: An electrospun mesh (bar is 20
mm).Electrospun meshes affixed to round Teflon frames were implanted in rat subcutaneous dorsum for 5 week intervals. Results were consistent with findings from the single fiber implant studies. Large fibers tended to be encapsulated with a large number of cells, while small fibers were not. More details are included in the Publications Index.
Results from a 5-wk implant. Bar is 50 mm.
Next steps in this project are to create electrospun meshes for particular clinical applications. We are currently working with the Ratner group to pursue degradable polyurethane meshes for tissue engineering bone-ligament interfaces.