NUPOC Launches the NU-FlexSIV Socket


The Northwestern University Prosthetics-Orthotics Center (NUPOC) has officially launched the Northwestern University Flexible Sub-Ischial Vacuum (NU-FlexSIV) Socket by teaching five workshops over the last year. NUPOC faculty, led by Ryan Caldwell, CP/L, FAAOP, and Stefania Fatone, PhD, BPO(Hons), taught the new socket technique to 60 prosthetists throughout the United States and Canada. The courses were underwritten by U.S. Department of Defense (DoD) research funds, with supplemental funding from medi USA, Whitsett, North Carolina, and Össur Americas, Foothill Ranch, California.


The NU-FlexSIV Socket requires a specific impression technique, rectifications, and socket shape. Photograph by R. J. Garrick, courtesy of NUPOC.

During the courses, prosthetists attended didactic overviews, demonstrations, and hands-on training in which they learned how to fabricate and fit the socket on their own patients who accompanied them. The closing module of the course discussed definitive socket fabrication options, and participants had the opportunity to review side-by-side comparative videos of the patients as they walked in their own sockets and in the NU-FlexSIV Sockets. Following the course, prosthetists had the opportunity to communicate in a private, online forum with the developers and exchange information with other course participants about their experiences implementing the socket in their clinical practices.

The NU-FlexSIV Socket has lower proximal trim lines that do not impinge on the pelvis, is flexible so muscles can move comfortably within the socket as they contract during activity and splay during sitting; and is held securely to the residual limb by vacuum as well as compression of an undersized liner and socket. The socket includes a highly compressive, cylindrical, fabric-covered silicone liner; a flexible inner socket; and a shorter, rigid outer socket with vacuum applied between the liner and inner socket. The undersized liner and socket are used to compress the residual limb, stiffening the soft tissue and decreasing relative motion of the limb within the socket.

"Prosthetists can easily fabricate the NU-FlexSIV Socket in their clinics, but it requires a specific socket impression technique and specific modifications to achieve accurate coupling of the limb/liner and socket," said Caldwell. Features of the socket technique include that it can be casted with the patient being in a seated position; during rectification, plaster is removed only from the positive model; and no special alignment of the socket is required-the particular alignment required by the prosthetic knee is used.

The NU-FlexSIV Socket can also be fabricated via central fabrication. To facilitate this, a CAD-CAM template of the socket was created by quantifying the rectifications. Early use suggests that the template provides a reasonable initial fit with only minor check socket adjustments.

The DoD funded the research and development of the NU-FlexSIV Socket-Development of Subischial Prosthetic Sockets with Vacuum-Assisted Suspension for Highly Active Persons with Transfemoral Amputations, Grant #W81XWH-10-0744-toward a goal of improving comfort and function following a transfemoral amputation. Evidence-based research is under way to evaluate if the socket delivers its objectives better than an ischial containment socket. Also funded by the DoD, this new clinical trial, Functional Performance Evaluation of the Northwestern University Flexible Sub-Ischial Vacuum (NU-FlexSIV) Socket for Persons with Transfemoral Amputation, Grant #W81XWH-15-1-0708, is currently recruiting volunteer subjects.

"All subjects will receive and be tested in both socket conditions for the same duration," said Fatone, principal investigator of both DoD-funded research projects. "Assessments will focus on socket comfort, functional performance, and patient-reported satisfaction. The clinical trial will be completed within three years, when we hope to add additional scientific data to existing anecdotal feedback and preliminary evidence of NU-FlexSIV Socket performance."