Volume Management: Smart Variable Geometry Socket (SVGS) Technology for Lower-Limb Prostheses

Smart variable geometry socket (SVGS) technology provides a method of dynamic volume management for lower limb amputees. Because amputees experience residuum volume changes during the day and over time, the fit of their sockets may vary. Currently, socks are often added or removed to maintain a stable and comfortable fit. SVGS technology automatically and continuously accommodates for residuum volume changes to maintain the fit of the socket. The system functions by adding and removing liquid from the intrasocket environment, regulated dynamically by intrasocket pressure. SVGS rationale, theory of operation, alternative approaches, and clinical relevance are described. (J Prosthet Orthot. 2003;15:107–112.) KEY INDEXING TERMS: Lower-limb prosthesis, volume management, socket fit, socket retention The lower-limb prosthetic socket is effectively a fixed volume into which the residual limb is placed. An increase or decrease in the volume of the residual limb results in a socket that is either too tight or too loose, respectively. Numerous factors contribute to prosthesis fit, including socket design, normal daily volume fluctuations, and long-term changes in limb volume. Poor socket fit remains the primary concern of lower limb amputees with regard to their prostheses. Daily variations in limb volume play a critical role in prosthesis fit. Of great importance is the maintenance of a precision socket fit. The precision fit of most suction sockets is lost within a short period. This occurs, in part, because of physiological volume changes of the residual limb resulting from normal ambulation and especially from the high compressive pressures exerted upon the limb. Volumetric changes occur hourly, daily, and monthly. There are limited data in the literature that quantify volume fluctuation that occurs in the residuum over the course of the day. Residual limb volume changes of between 11% and 7% support the argument that the socket fit changes throughout the day. A volume increase of 3% to 5% may cause significant difficulty for an amputee donning his/her prosthesis. The amount of daily volume fluctuation varies greatly among individual amputees and is a function of prosthesis fit, activity level, ambient conditions, body composition, dietary habits, and for women, monthly cycles. Current methods used to compensate for daily volume changes in the limb include multiple socks, pads, and inflatable air bladders. Socks and pads represent discrete levels of change in socket volume and do not continuously match volume changes occurring in the limb. Pneumatic systems, using inflatable bladders inserted in the socket to manage volume fluctuations, suffer from inadequate support of the limb during walking due to under-inflation, localized tissue compression due to over-inflation; high compliance from compression of the air inside the bladder causes excessive pumping of the residual limb in and out of the socket. The clinical use of pneumatic inserts is limited. A novel technology, dynamic variable geometry fitting system (patent pending) has been employed in a lower-limb suction socket application to compensate for daily limb volume fluctuations automatically and continuously over the course of the day. The smart variable geometry socket (SVGS) system developed for this application overcomes limitations of existing socket volume management methods by dynamically changing the volume of the socket to compensate for changes in volume of the residual limb. This technical note describes the theory of operation and method of use of the SVGS system for clinical use. SVGS SYSTEM: THEORY OF OPERATION Stability and comfort of the prosthesis are likely to be related to the quality of the fit at the interface between the limb and RICHARD M. GREENWALD, PhD, is affiliated with Simbex LLC, Lebanon, NH, and Thayer School of Engineering, Dartmouth College, Hanover, NH. ROBERT C. DEAN, ScD, is affiliated with Synergy Innovations, Inc., Lebanon, NH, Simbex LLC, Lebanon, NH, and Thayer School of Engineering, Dartmouth College, Hanover, NH. WAYNE J. BOARD, MS, is affiliated with Simbex LLC, Lebanon, NH. The research and development reported here were supported by Small Business Innovation Research (SBIR) awards 1R43-HD36154-01 and 7R44-HD36154-04 from the National Center for Medical Rehabilitation Research (NCMRR), National Institute of Child Health and Human Development (NICHD), National Institutes of Health, and ISI-9161011, ISI-9361417, ISI-9224137, DMI-9960955, and DMI-0091513 from the National Science Foundation. Copyright © 2003 American Academy of Orthotists and Prosthetists. Correspondence to: Richard M. Greenwald, PhD, Simbex, 10 Water Street, Suite 410, Lebanon, NH 03766; e-mail: rgreenwald@ simbex.com 107 Volume 15 • Number 3 • 2003 the socket. Volume changes in the lower limb over the course of the day result in a mismatch in the volume of the limb and the volume of the constant volume socket. Volume loss results in a pistoning or pumping motion between the tissue and the socket, which can decrease retention security during ambulation and cause skin lesions and, with suction sockets, loss of suction. To prevent pistoning, a method of maintaining a consistent and comfortable level of socket fit is required that is independent of individual intrinsic factors (hormone levels, water retention, short-term weight change) and extrinsic factors (air temperature, activity level). On the other hand, skin tissue cannot be subjected to constant levels of high pressure ( 8 kPa or 1 psig) for any sustained period of time without tissue necrosis. The desired level of socket tightness (i.e., comfortable and stable fit) during various activities is usually a personal preference for the amputee; it may change for different activities, such as office work and athletics. The SVGS system (Figure 1) has three main components: a fluid reservoir, a mechanical control circuit (Figure 2), and multiple discrete bladders located inside the socket. Power for the system is provided entirely by the amputee—no external power source is needed. During normal ambulation, a natural pumping cycle occurs within a lower-limb suction socket: suction is created during the swing phase followed by compression during the stance phase (Figure 3). Fluid, typically water, is drawn into the bladders from the reservoir during the swing phase, then distributed among the bladders during stance. Gravity and the dynamic forces on the swinging prosthesis control the level of suction, so no vacuum Figure 1. Rendering of SVGS applied to a transfemoral suction socket. The placement and size of the bladders and the placement of the reservoir below the socket are for illustrative purposes only and represent one possible combination for use. Figure 2. Prototype control system photo (left) and schematic (right). CV, check valve; F, particle filter; NV, night valve; PRI, ischemic pressure regulator; PRS, stance pressure regulator; R, flow resistor; RES, reservoir; FP, fluid fill port; B, bladder. Greenwald et al. JPO Journal of Prosthetics and Orthotics 108 Volume 15 • Number 3 • 2003 control is required. Various check valves (CV) control the flow direction within the system. The bladders inflate until they reach a maximum stance pressure, set by the prosthetist using the adjustable stance pressure regulator (PRS) to a level that is comfortable for the amputee within the limits of the pressure regulator, between 28 and 105 kPa (4 and 15 psig). The volume of fluid drawn from the reservoir and pumped into the bladders to achieve the maximum stance pressure is a function of the instantaneous volume mismatch between the socket and the residuum. Various sizes, shapes, and placement of the bladders within the socket are possible. The prosthetist selects the number, location, and size of bladders that are considered to best match the limb geometry, musculature, and bony prominences. It is important to note that the tissues are not subjected to a continuous and highpressure level during ambulation because the pressure in the socket is continuously varying between suction and the maximum pressure set by PRS. Ischemic relief for the residuum tissues is integrated into the system to relieve bladder pressure during stasis below the tissue ischemic limit of 8 kPa ( 1 psig) above ambient. A resistor (R) continuously bleeds liquid from the bladders back into the reservoir at approximately 1/10 of the liquid circulating rate. Because the liquid is essentially incompressible and the socket frame is relatively stiff, a few milliliters of liquid bleeding from the bladders reduces the stance pressure to below the ischemic limit. To prevent loss of suction when not walking, the system drains only enough liquid to drop the pressure below the ischemic limit. This is accomplished by an ischemic pressure regulator (PRI) that maintains residual bladder pressure at approximately 3.5 kPa (0.5 psig). By retaining most of the liquid required for volume management during walking, a relaxed fit of the socket is created while still maintaining a close-fitting socket; only a few steps are required, when walking resumes, to automatically return stance pressure to its set point and the socket fit to its dynamic walking tightness. A manually operated night-valve (NV) bypasses PRI and R to release all of the fluid from the bladders back into the reservoir to provide maximum socket volume for donning and doffing. The rationale for using liquid fill rather than air fill is presented below. The intrasocket environment in lower-limb, total-contact suction socket prostheses is characterized by dynamic changes in normal and shear forces applied to the tissues. Leg motion, muscle contraction, and ambulation create changes in the pressure as the tissue shifts within the socket. The tissue, composed primarily of water, reacts to changes in pressure with a fluid transport mechanism that forces fluid either into or out of the limb. It is norma