Review of the Evidence for Sub-Atmospheric Suspension System
Sub-Atmospheric Suspension Systems: Clinical Applications, Evidence & Reimbursement
American Academy of Orthotists & Prosthetists, Certificate Course, 10/10/09
Stefania Fatone, PhD, BPO(Hons)
Northwestern University, Chicago IL
Objective: The objective of this presentation is to review the current evidence for subatmosphericsuspension systems and identify gaps in the currently available evidence.
Introduction: Sub-atmospheric refers to the creation of an environment in which pressure falls below those observed at sea level (i.e., below atmospheric pressure or <1 atm or <100 kPa) . The terms suction and vacuum are synonyms for sub-atmospheric pressure. On an absolute pressure scale, zero is a condition where no air molecules exist (i.e., pure vacuum), on a relative pressure scale, zero equals 1 atmospheric pressure (1 atm or 100 kPa or 750 mm Hg). Suction suspension for prosthetic sockets first gained widespread use for transfemoral amputees in the 1930s, but it was not used as widely with transtibial amputees until the 1980s . Vacuumassisted suspension was introduced in the 1990s . Suspension is an important component of the fit and function of a prosthetic socket as it is believed to affect comfort, proprioception, performance of activities, and limb health. However, the evidence for use of sub-atmospheric vacuum-assisted suspension, has not been previously reviewed. The objective of this presentation is to review the literature on sub-atmospheric suspension systems for prosthetic sockets with the focus on quantitative evidence.
Methods: A search of Pubmed (National Library of Medicine) was conducted in August 2009 using the search term “artificial limbs AND (suction or vacuum).” This search identified 60 articles. Review of the abstracts resulted in 48 articles being excluded (1 on upper limb, 2 review articles, 9 descriptive articles, 9 not in English, 19 not on topic and 8 abstracts/journals not available). A search of Recal Legacy (University of Strathclyde) was also conducted in August 2009 using the search terms “vacuum”, “suction socket” and “limb volume”. A total of 320 articles were identified of which 262 were excluded based on review of article descriptions. Of the remaining 60 articles another 44 were excluded because they were duplicates of articles found in the Pubmed search or the journals were not available. Based on these searches, 28 articles were included in the review (7 on suction suspension, 8 on complications of suction suspension, 3 on vacuum-assisted suspension, and 10 on residual limb volume). Additionally 3 articles were identified and included based on a review of the references cited in the included articles (2 on vacuum-assisted uspension and 1 on suction suspension).
Experimental studies of suction suspension: Five papers were reviewed [4-8]. Patient preference for suspension was explored in a randomized cross-over trial of 8 persons with non-vascular transtibial amputations (TTA) comparing the silicone suction socket (3S) to the supracondylar patellar tendon bearing (PTB) socket. Only two subjects prefered the 3S system, with increased donning/doffing time and decreased comfort in the 3S offered as explanation for their preference . Radiographic analyses of skeletal displacement in 22 persons with TTA suggest better contact between the residual limb and the prosthesis in a PTB socket with suction compared to strap suspension, with significantly less vertical displacement of the tibia in the suction socket attributed to less tissue movement . A more recent case study of a traumatic TTA comparing a neoprene sleeve to 3S pin lock suspension suggested little difference in vertical tibial Sub-Atmospheric Suspension Systems: Clinical Applications, Evidence & Reimbursement American Academy of Orthotists & Prosthetists, Certificate Course, 10/10/09 displacement but less soft tissue movement in the 3S system . Unfortunately, the distal end of the socket was not sealed in the sleeve suspension, potentially compromising the effect of this condition. Socket pressures with sleeve suspension [5, 8] and pin suspension  have been investigated. Both studies demonstrated that negative pressure was present during swing, with negative pressure of -6.9 to -31.1 kPa* in the 9 persons with TTA wearing PTB sockets with rubber sleeve suspension, and peak negative pressure of -39.5±14.8 kPa with pin suspension and -26.1±7.0 kPa with sleeve suspension in 8 persons with TTA wearing total surface bearing (TSB) sockets . Both positive and negative pressures during swing were greater with pin suspension, which was also reported to cause proximal constriction of the residual limb during swing .
Complications of suction suspension: Eight case reports of skin complications arising from use of suction suspension were identified, seven regarding persons with TFA [9-15] and one in a person with TTA . Two described cases of contact dermatitis that were resolved when the irritating material was removed from the suction socket [10, 16], while the remainder described cases of kaposi-like acroangiodermatitis [9, 11-15] resulting from poorly-fitted suction sockets [9, 12] and resolved with refit of suction socket [11, 12] or change of suspension [12, 14, 15].
Experimental studies of vacuum suspension:
Three studies were reviewed, all from the same group of authors comparing limb volume [17, 18] and interface pressure  in persons with traumatic TTA wearing TSB sockets without (knee sleeve alone) and with vacuum-assisted suspension of -78 kPa [17, 18] or -69 kPa  (converted from 52 to 233 mm Hg using http://www.unit-conversion.info/pressure.html). Pressures were reported to be significantly lower in stance phase and significantly higher in swing phase with vacuum . The authors suggested that this was the mechanism by which limb volume was maintained with vacuum-assisted suspension. They also reported a significant increase in residual limb volume after 30 minutes of walking, with significantly less pistoning of the tibia and liner, and ignificantly more stance phase and step length symmetry with use of vacuum . When comparing neutral, under- and over-sized sockets, they reported that average residual limb volumes were significantly larger than the volumes available in all sockets, yet none of the subjects reported discomfort or pain after 18 minutes of walking. Two additional non-peer reviewed studies were identified: 1) a study of proprioception in subjects with vascular and traumatic TTA comparing vacuum-assisted suspension to suction suspension with a one-way valve but finding little differences , and 2) a prospective study that compared pain and walking ability (using the Locomotor Capabilities Index) of two independent groups of subjects with dysvascular TTA wearing vacuum-assisted suspension or PTB sockets reporting that walking ability and pain were better in the vacuum group . Wound healing was also documented in the vacuum group.
Can we learn anything from Negative Pressure Wound Therapy?
Vacuum-assisted closure is an adjunct therapy using negative pressure to remove fluid from open wounds through a sealed dressing and tubing which is connected to a collection container . The VAC pump can be set at different levels of negative pressure (from 50 to 200 mm Hg) and in different modes of operation (continuous or intermittent) . The device covers the wound surface keeping it moist and insulated. It can pull the edges of deformable wounds together (macrodeformation), remove extracellular fluid and wound exudate, cause microdeformation at the foam-wound interface, and cause changes in blood flow, wound biochemistry, the systemic inflammatory response, and bacterial loads . The balance as to whether fluid-based or mechanical properties have greater influence on clinical efficacy of vacuum-assisted wound closure appears to be shifted to mechanical properties . It is proposed that applied mechanical forces deform tissues, resulting in deformation of cells, followed by stimulation of growth factor pathways, leading to increased mitosis and production of new tissue . It has been suggested that tensile stresses applied to the skin stimulate cellular proliferation while compressive forces lead to resorption of the underlying tissues . Unresolved issues include the optimal pressure level (VAC setting is typically 125 mm Hg negative pressure but research suggests -75 to -80 mmHg may reduce tissue damage) and duration of treatment (conventional VAC requires continuous use for 24-48 hours until dressing change but literature suggests use of 6-8 hours/day) .
Conclusion: Only a small number of experimental studies of suction or vacuum suspension for prosthetic sockets have been reported, limiting the evidence available regarding their use. Peer reviewed studies of vacuum-assisted suspension are limited to short-term analysis of healthy, traumatic TTA and used post-doffing measurements of residual limb volume which additional studies of limb volume measurement suggest may not be accurate [28-30]. Clinicians still need to know for whom vacuum-assisted suspension can safely and effectively be prescribed, the optimal time to provide vacuum-assisted suspension, the optimal pressure level, interaction between vacuum-assisted suspension and socket design, factors influencing acceptability and adherence to use, and effect on performance and limb health, especially for the long-term. Current evidence regarding vacuum-assisted suspension can not be generalized to persons with transfemoral amputation or those with diabetes or vascular problems. While direct association can not be made between use of negative pressure in wound therapy and prosthetic suspension because of the difference in mode of application (i.e., vacuum is not applied directly to the skin in prosthetic applications), current thinking regarding the role of tissue deformation in wound healing may help to explain the effects on tissue health that have been anecdotally reported in amputees using suction and vacuum-assisted suspension [21, 31, 32].
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