Turning and Repositioning To Reduce Pressure Injuries

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Assessing evidence supporting redistribution of pressure for pressure ulcer prevention: A review

Stephen Sprigle, PhD, PT;* Sharon Sonenblum, PhD Rehabilitation Engineering and Applied Research Lab, Georgia Institute of Technology, Atlanta, GA

Abstract—The formation and underlying causes of p ressure ulcers (PUs) are quite complex, with multiple influencing fac- tors. However, by definition pressure ulcers cannot form with- out loading, or pressure, on tissue. Clinical interventions typically target the magnitude and/or duration of loading. Pres- sure magnitude is managed by the selection of support surfaces and postural supports as well as body posture on supporting surfaces. Duration is addressed via turning and weight shifting frequency as well as with th e use of dynamic s urfaces that actively redistribute pressure on the body surfaces. This article shows that preventative interventions must be targeted to both magnitude and duration and addresses the rationale behind sev- eral common clinical interventions—some with more scientific evidence than others.

Key words: body posture, clinical interventions, postural sup- ports, pressure magnitude, pressure ulcers, prevention inter- ventions, support surface, tissue loading, turning frequency, weight shifting frequency, wheelchair.

INTRODUCTION

The formation a nd underlying c auses of pressure ulcers (PUs) are quite complex, with multiple influencing factors. However, by definition PUs cannot form without forces, or press ure, on tissue. Because tissue loading is the defining characteristic of PU formation, i t naturally garners significant attention in research in PU prevention strategies.

Research has clearly demonstrated that the damaging effects of pressure are related to both its magnitude and

duration. Simply stated, tissues can withstand higher loads for shorter periods of time. Kosiak first demon - strated this characteristic 50 years ago by applying vary- ing loads to the trochanters and ischial tuberosities of dogs for varying periods of time [1]. High loads for short durations and low loads for long durations induced ulcers, with the time-at-pre ssure curve following an inverse parabola. Reswick and Rogers tried to extend this animal research into clinically relevant information, and using combinations of interviews and interface pressure measurements (IPMs), de termined a pressure-time rela - tionship that was similar to that of Kosiak [2].

Using the premise that both the magnitude and dura- tion of loading are important, we can diagram a simple model of PU development (Figure 1) that illustrates the reasoning behind certain clin ical interventions. Pressure magnitude is managed by the selection of support sur- faces and postural supports as well as body posture upon supporting surfaces. Duration is a ddressed via turning and weight shifting frequency as well as with the use of dynamic surfaces that a ctively redistribute pressure on the body surfaces.

Abbreviations: IPM = interface pressure measurement, Mobil- ity RERC = Rehabili tation Engineering Research Center on Wheeled Mobility, PU = pressure ulcer, SCI = spinal cord injury. *Address all correspondence to Stephen Sprigle, PhD, PT; Georgia Tech–Applied Physiology, 490 Tenth St NW, Atlanta, GA 30032-0156; 404-385-4302; fax: 404-894-9320. Email: [email protected] DOI:10.1682/JRRD.2010.05.0102

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This article reviews the evidence supporting clinical interventions that address the magnitude of pressure and the duration of that pressure. Within this article, “support surfaces” will refer to devices designed for horizont al (mattresses, overlays) and s eated (wheelchair cushions) postures. The term “pressure” will refer to the force or load exerted over an area of the body or on a localized area of the body surface.

CONTROLLED EXPERIMENTS OF TISSUE LOADING

A fairly extensive amount of re search has applied loads to tissues and monitored physiological outcomes. For obvious re asons, research with animal models uses controlled loading to create PUs or tissue necrosis, whereas human studies are limited to indirect measures, such as the effect of loading on blood flow.

Tissue Response to Loading in Animal Models As mentioned previously, Kosiak u ndertook seminal

research by applying load s to the trochan ters and isch ial tuberosities of dogs [1]. Load s ranged from 100 to 500 mmHg, and durations ranged from 1 to 1 2 hours.

Kosiak monitored animals for 14 days postischemia to determine the occurrence of PUs. Dinsdale applied pres- sures between 45 and 1,500 mmHg for 3 hours to swine with and without paraplegia [3]. Normal pressure was com- bined with friction in half the specimens. The results indi- cated that no necrosis occurred with normal pressures below 150 mmHg, but in combination with friction, tissue changes could be seen after loading with 45 mmHg. Daniel et al. also studied swine with and without paraplegia [4]. Using an indenter to apply load at the greater troch anter, they found that application of 200 mmHg for 15 hours did not induce a PU. Ulcers were obtained by applyin g 500 mmHg for 4 hours and 800 mmHg for 8 hours.

Linder-Ganz and Gefen exposed rat hind limbs to pressure magnitudes of 86, 262, and 525 mmHg for 2, 4, and 6 hours, respectively [5]. They used finite ele ment modeling to calculate internal stresses and concluded that tissue damage occurred with 13 kPa o f internal stress applied for 6 hours and 40 kPa of interna l stress applied for 2 ho urs. Both conditions represent an approximate stress application rate of 80 kPa/h.

While this is not a comprehensive list of animal PU etiology research, collectiv ely the studies illustrat e results obtained by applying different loads over different durations (Table). The use of different sizes and shapes of indenters, dif ferent loading parameters , and different animal models explains why a range of mag nitudes and durations are linked to PU development. Despite these differences, the evidence suggests that both magnitude and duration of loa ds must be considered in PU preven - tion and validates the simple intervention model in Figure 1.

Blood Flow Response to Loading in Humans While research has clearly shown a rela tionship

between pressure magnitude and duration and tissue damage, these studies have not de fined a critical ma gni- tude above whic h ischemia occurs. Many studies have used controlled experimental approaches for determining the pressure at whic h blood flow to tissue cea ses with significantly varying results. Lassen and Holste in found that the pressure required for vascular occlusion approxi- mated diastolic pressures when the measured skin approached heart level [6]. Holloway et al. loaded the forearm and found that blood flow decreased as external pressure approached mean arterial pressure and that occlusion was reached at ~120 mmHg [7]. Ek et al. found “weak positive correlations” between blood flow during

Figure 1. Rationale for redistribution of pressure.

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loading at the heel and systolic blood pressure [8]. Load- ing at the sac rum did not resu lt in the same relationship with blood pressure. Sangeorzan et al. de termined that 71 mmHg was need ed to occl ude flow over the tibialis anterior (a “soft” site) but only 42 mmHg occluded flow over the tibia (a “hard” site ) [9]. Bennett et al. measured occlusion pressure at the thenar eminences of nondis - abled subjects and found that 100 to 120 mmHg was nec- essary to occlude v essels in “low shear” conditions and 60 to 80 mmHg was needed in the pre sence of “high shear” conditions [10]. Bar re viewed the literature and concluded that a critical pressure is necessary to occlude blood flow and that while this threshold is related to ves- sel pressure, it appears to vary widely [11].

The animal and human studies contribute important information to the field of PU research by identifying tis- sue’s response to external loads. However, the results are very hard to apply clinically. Controlled loading at specific anatomical sites simply doe s not generalize to the person lying in bed or sitting in a wheelchair. For example, the magnitudes and durations of loading used to induce dam - age in animals greatly exceed those deemed a cceptable in clinical environments. This apparent discrepancy does not invalidate either the research or the clinical interpretation of the findings. Rather, these animal tests inform us about the mechanism of injury and the complex relationships between the variables involved when supporting the human body in sitting or lying positions.

To date, research has not identified a specific thresh- old at which loads can be deemed harmful across people or sites on the body. Tissue’s tolerance to load varies

according to the condition of the tissue and its location, age, hydration, and metabolism. All the factors common to PU risk assessment tools tend to influence how the tis- sue distributes the loading and its ability to wi thstand load.

EVIDENCE SUPPORTING CLINICAL INTERVENTIONS

Support Surfaces Support surfaces attempt to redistribute forces away

from bony prominences, thereby reducing the magnitude of loading at these at-risk sites. In general, creating suc - cessful support surfaces is challenging because of the dif- ferences in ind ividual risk factors, as well as the complicated nature by which force is distributed through- out tissue. For example, when a person sits on a cushion, normal loading works in combination with shear and fric- tional forces to induce complex tissue distortion. Conse- quently, myriad support surface designs ex ist that have benefit for some people, but for the most part, no single surface is optimal for all persons. Two very general cate- gories of support surfaces can be defined: reactive sur- faces that respond to the load placed upon them and active surfaces that dynamically a lter the body–support-surface interface. Although active surfaces serve as a duration intervention, their primary role as a support surface (thus affecting magnitude of loading) makes it natural to present them together with reactive support surfaces.

Table. Examples of animal pressure ulcer models highlighting different loading parameters.

Author Animal Model Loading Conditions Outcome Kosiak [1] Canine trochanter and 

ischial tuberosity 100–500 mmHg over 1–12 h Proposed inverse magnitude-duration

relationship. Dinsdale [2] Swine with and without

spinal injury 45–1,500 mmHg over 3 h with

and without friction Loading at 45 mmHg in the presence

of friction-induced damage. Daniel et al. [3] Swine with and without

spinal injury 200 mmHg for 15 h, 500 mmHg

for 4 h, 800 mmHg for 8 h No damage at 200 mmHg for 15 h,

but damage under other conditions. Linder-Ganz & Gefen [4] Rat hind limbs 86, 262, and 525 mmHg for 2, 4,

and 6 h, respectively Tissue damage occurred with loading

rate of 80 kPa/h. 1. Kosiak M. Etiology and pathology of ischemic ulcers. Arch Phys Med Rehabil. 1959;40(2):62–69. [PMID: 13618101] 2. Dinsdale SM. Decubitus ulcers in swine: Light and electron microscopy study of pathogenesis. Arch Phys Med Rehabil. 1973;54(2):51–56. [PMID: 4692634] 3. Daniel RK, Wheatley D, Priest D. Pressure sores and paraplegia: An experimental model. Ann Plast Surg. 1985;15(1):41–49. [PMID: 4083714]

DOI:10.1097/00000637-198507000-00005 4. Linder-Ganz E, Gefen A. Mechanical compression-induced pressure sores in rat hindlimb: Muscle stiffness, histology, and computational models. J Appl Phys-

iol. 2004;96(6):2034–49. [PMID: 14766784] DOI:10.1152/japplphysiol.00888.2003

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Judging the effectiveness of support surfaces is done with both direct and indirect methods. Indirect methods use physiological means such as blood flow , tissue oxy- genation, and interface pressure to judge performance. Direct methods follow a group of patients over time to determine PU occurrence. Direct methods are more valu- able but are harder to adminis ter and are limited in the number of interventions that can be inve stigated (i.e., types of surfaces).

In their systematic review focused on randomiz ed controlled trials with PU development as an outcome, Cullum et al. used the term “constant low-pressure sup- port surfaces” to describe the myriad foam, air , water, and elastomeric mattresses, overlays, and cushions [12]. Their review of the literature concluded that these sur- faces outperform standard hospital mattresses in prevent- ing PU formation. Comparisons between dif ferent constant low-pressure surfaces did not result in definitive outcomes. In othe r words, differences across the more common reactive surfaces have not been demonstrated in terms of PU outcomes.

Studies on wheelchair cushions are not as common as those on mattresses, but informative evidence is still available. Indirect measures, specifically interface pres- sures, comprise the bulk of studies on cushions [13–16]. Researchers have shown tha t high seated interface pres- sures were associated with PU occurrence [17–19]. Therefore, despite the limita tions in IPM as a less accu- rate representation of localized loading [5,20–22], it can be useful in selecting cushions.

Because active surfaces vary loading of pa rticular regions of the body, they intend to alter both the magni- tude and duration of loading. Active surfaces are avail- able for both mattresses and wheelchair cushions, with mattresses being use d and studie d more freque ntly. In part, this is the result of a funding decision in the United States by the Ce nters for Me dicare and Medic aid Ser- vices to not pay for powered wheelchair cushions for PU prevention. Evidence on commercially available a ctive cushions is limited to seco ndary outcomes [16,23]. Because the secondary measurements vary throughout the cycle of ac tive cushions, the results of such studies are hard to apply clinically.

Studies of active mattresses and overlays a re more common than those of cushions and have used both direct and indirect outcomes. Two recent systematic reviews do a very thorough job of covering the literature on alternat- ing pressure mattresses so the details will not be repeated

here [12,24]. Cullum et al. focused exclusively on direct outcomes (PU development), while Vanderwee et al. extended their review to include studies with indirect out- come measurements and a lternative study designs. But both groups reached the same conclusions: alternating pressure air mattres ses are better than standard hospital mattresses but their bene fit over constant low -pressure mattresses is unclear. Furthermore, differences across types of alternating pressure air mattresses were not dem- onstrated. Active surfaces also provide increased poten- tial for mechanical problems and user error compared with some alternatives. One major limitation of most of the reviewed studies, as pointed out by Cullum et al., was that turning schedules were not controlled. Therefore, it is possible that nurses made a point to turn patients on the standard mattresses more frequently than those on the active surfaces because of a perceived need for increased intervention. If true, than comparable outcomes could come with the benefit of re duced clinical intervention time for the active surfa ce, but research to evaluate this possibility is needed.

Interventions for Reducing Duration of Loading The body’s motor and sensory systems are responsi-

ble for ensuring that we move periodically to change our posture. This may be in the form of discomfort eliciting movement or subconscious postural shifts or fidgeting. Many studies over the years have monitored movements in chairs a s metrics of co mfort and function [25–28], thereby establishing a base of knowledge about sitting as a dynamic activity. Many people at risk of dev eloping PUs are either unable to effectively reposition themselves or are not provided with the sensory feedback that elicits movements. Therefore, that loss of mobility and sensa- tion are identified as risk fa ctors within every PU risk assessment scale is not surprising.

We use this information to tar get movement as a means of redistributing pressure and altering the duration of loading on tissues. Cli nically, this includes turning schedules for patients who are in bed and weight shifting strategies for those who are seated.

Turning Frequency In a study on PU prevention interventions, Richard-

son et al. found that manual repositioning was the most commonly used intervention and that it was also the most expensive [29]. The idea of necessary repositioning has appeared throughout literature and textbooks since the

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1800s [30]. Evidence that some repositioning is neces- sary can be found across decades of literature.

In the United States, common practice requires that at-risk patients be repositioned at least every 2 hours if consistent with overall patient goals [31]. Despite efforts by a number of researchers to identify the origins of this practice, or at the very least identify evidence supporting the 2-hour turning practice, no strong scientific support exists [30,32–33]. In fact, earlier texts often included suggestions that the turning schedule depend on the mag- nitude of loading and condition of the patient.

Therefore, the s tandard practice of using the sa me turning schedules independent of support surface is not reflective of earlier work. Re cent evidence demonstrates the need to account for the support surface in determining the optimal turning schedule. Defloor et al. showed that 2- and 3-hour turning schedules resulted in the develop- ment of PUs in 14 to 24 percent of patients lying on standard mattresses. A 6-hour turning sc hedule for patients lying on a viscoelastic mattress resulted in simi - lar outcomes, but a 4-hour turning schedule for patie nts lying on a visc oelastic mattress signific antly reduced stage II PUs . Other research suggests that turning ma y need to occur more frequently than every 2 hours and that sufficient pressure reduction surfaces are needed in addi- tion to turning [32,34–36]. Recently, Vanderwee et al., using a pressure-reducing mattress, found no difference between repositioning patient s every 4 hours and alter - nating between 2 hours in late ral and 4 hours in supine [36]. In both interventions, more than 16 percent of parti- cipants developed a PU. Additionally, two studies of sec- ondary outcomes demonstrated that redness and oxygen reduction while lying in bed occurred in less than 2 hours [37]. Furthermore, in studies on turning, patients who are able will change posture between scheduled reposition- ings. As a result, these subjects are exposed to more posi- tion changes than offered by the intervention, which may mask a need for more frequent repositioning in those unable to reposition themselves [36]. The necessary repo- sitioning frequency may be so high that implementation is impractical for immobile patients [32].

Positioning Devices and Posture The entire premise behind turning is obviously to

reduce the amount of time di fferent body surfaces are exposed to loading. Operati onally, many facilities sequence between supine and two side-lying postures. The loading at specific body surfaces is highly dependent

on the resulting postures an d any positioning devices used. For example, side lying may expose a malleolus to damaging loading but proper positioning of the lower limbs and judicious use of positioning devices can effec- tively reduce loa ds from this bony prominence ( Figure 2(a)). Adopting a supine posture with the head of the bed elevated alters loading on the buttocks, which is why it is a controversial posture. Elevating only the head of the bed increases both the normal and frictional forces on the sacrum [38–39]. Mechanics suggests that as the head ele- vates, more of the upper-body weight will be transmitted through the buttocks to the supporting surface. In addi- tion, the tendency to slide is increased as the trunk sup- port is inclined. The complication is that it is a functional posture, adopted so people can converse with others , read, and eat, to name a fe w activities. Some of the fric - tional forces can be counteracted by raising the foot of

Figure 2. (a) Use of positioning devices to redistribute pressure and (b) raising foot of bed counteracts sliding tendency.

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the bed, but this will not reduce the normal forces on the buttocks [38] (Figure 2(b)).

The seated posture also affects how loads are redistrib- uted. Sitting on a sling seat with a pelvic obliquity induces asymmetric loading on the isch ial tuberosities, not to men- tion contributing to postural instability (Figure 3(a)). A slouched, kyphotic posture is typ ified by p osterior pelvic tilt, a posture that loads the sacrum and coccyx while seated (Figure 3(b)) [40–41].

In summary, body posture and positioning have a direct relationship to loads on specific body sites, which is why posture must be co nsidered when devising PU prevention strategies.

Weight Shifting Wheelchair users are often at high risk of developing

sitting-acquired PUs. Persons with absent or diminished sensation and/or mobility are always at high risk of PUs [42–43]. A variety of maneuvers to shift body weight off the buttocks are taught to wheelchair users at risk of PUs. They can push down on the seat or armrests to lift the buttocks off the cushion s urface (Figure 4(a)), lean for- ward to rest t heir trunk upon the lower limbs (Figure 4(b)), or lean to one side and then lean to the opposite side (Figure 4(c)). Persons who use power wheelchairs and cannot independently perform these maneuvers are sometimes prescribed variable position wheelchairs that incorporate powered tilt and/or recline to redistribute weight off the buttock area (Figure 5).

Most guidelines that suggest weight shift or pressure relief frequency have been developed for p ersons with spinal cord injury (SCI) because of the effect of SCI on sensation and mobility. For the SCI po pulation, recom- mendations for weight shift frequency have typically ranged from 15 to 30 seconds every 15 to 30 minutes to 60 seconds every hour [44–47]. Based on the wide range of these guidelines, one can infer that they were based on a combination of clinical experience, clinical insight, and research findings.

In addition to weight shift frequency, one must also consider the duration for which a weight shift is held . In other words, not only do wheelchair users have to perform weight shifts regularly, they must attend to the duration of these maneuvers. The ability to sustain a weight shift is dependent on myriad factors, including functional ability, strength, flexibility, and postural control [46]. A 2003 study measured tissue perfusion to investigate the length of time required for tissue to reperfuse in an SCI cohort (n = 46)

[48]. The mean duration of weight shift required to return transcutaneous partial pressure of oxygen to unloaded lev- els following upright sitting was 1 minu te 51 seconds (range = 42–210 seconds). This fin ding suggests that the

Figure 3. (a) Pelvic obliquity from sitting on sling seat and (b) posterior pelvic tilt loads sacrum and coccyx.

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duration of weight shifts currently recommended (i.e., 15– 30 seconds) is inadequ ate. Further, this suggests that th e common practice of sitting push-ups is not sustainable for many to achieve reperfusio n. Consequently, the authors supported the use o f alternate, sustainab le methods of weight shift, namely fo rward lean, lateral lean, and rear - ward tilt. Partial weight shifts may also allow for better sus- tainability by persons with SCI.

Figure 4. (a) Push-up weight shift, (b) forward-lean weight shift, and (c) side- lean weight shift.

Figure 5. (a) Manual Tilt-in-Space wheelchair and (b) Power T ilt-in-Space wheelchair. Images used with permission. ©Invacare Corporation.

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Three recent studies investigating PU prevalence in an SCI cohort considered weight shift behavior as a potential risk factor [49–51]. None of the st udies found weight shift behavior or frequency of weight shifts to be associated with PU occ urrence. However, each of the studies used self-report to measure weight shift practices. Further objective analyses ar e needed to determine the role of weight shifts in PU prevention.

CONCLUSIONS

The review of res earch corroborated the clinical interventions commonly used for load redistribution but also identified areas of uncertainty. As with all means of prevention, some interventions are better supported than others and some interventions have a legacy quality to them and little el se. Nonetheless, several clinically ori- ented suggestions can be made.

Support Surface Assessment Selections of mattre sses, overlays, and cushions

should be based upon assessment. Research is clear that individual factors can contribute to PU susceptibility, and all the PU risk assessment scales are based upon indi - vidualized evaluation. Research has also shown that indi- vidualized evaluation improves the selection of mattress [52] and wheelchair cus hions [53]. Long-standing evi- dence supports the use of seating clinics to select and pre- scribe wheelchair cushions [54]. One of the benefits of this type of individuali zed evaluation is its educational aspect in informing patients and clients about skin health and proper equipment use.

Interface Pressure Interface pressure can be used to identify a reas of

unacceptably high pressures and to ensure a s ite is ade- quately off-loaded during posture changes or a weight shift. We advocate for use of pressure mapping to rule out products rather than as a sole means to prescribe a par- ticular product [21]. For exa mple, if the interface pres- sure under the ischial tuberosity is deemed too high for a particular person by a clinician, then the clinician should deem that p roduct unacceptable. That said, one cannot infer that published IPM values will generalize to other clients or patients. Another useful role for IPM is assess- ing how posture or position changes influence loading on tissue. Repositioning in bed or while seated is necessary

to unweight different parts of the body. IPM can offer visual feedback to clinicians, patients, and clients as they sequence through different postures.

Weight Shift and Turning Frequency Periodic repositioning is an important preventative

measure. Patients and clients who can independently redistribute pressure should be educated to do so and taught strategies to ensure compliance. Persons who can- not reposition must rely on others to set and follow a rou- tine. Evidence on how often a weight shi ft should be performed and evidence behind turning schedules is lim- ited. The odds are that repositioning frequency is not the same for all people and surfaces. This can be inferred by the wealth of evidence in dicating the individualized nature of PU ris k and supports the approach that reposi- tioning frequency should reflect the person, his or her equipment, and the environment of use. • Standard hospital beds are poor support surfaces.

Ample evidence has show n that standard ma ttresses are inadequate to prevent PU s. Even relati vely “low tech” mattresses and overlays offer better prevention [12].

• Increasing activity has many health benefits, includ- ing tissue health. In a study of more than 600 persons with SCI with and without a history of recurrent PUs, Krause and Broderick identif ied behaviors that were shown to be protective [50]. These behaviors included a healthy lifestyle, fitness, and exercise. Putting peo- ple into equipment and postures that permit functional activity addresses the key PU risk fac tor of immobil- ity. We should promote reaching, leaning, and moving as a means of promoting functional independence and maintaining skin integrity.

• The European and U.S. National Pressure Ulcer Advi- sory panels have recently released their joint Interna- tional Pressure Ulcer Guidelines for Prevention and Treatment. The document addresses both PU preven- tion and PU trea tment by assessing many clinical interventions.

• When reviewing conflicting literature, pay close atten- tion to external validity. Literature regarding pressure redistribution and support surfaces is o ften equivocal and may be contradictory. This can occur because of differences in methods, measurements, and subjects. When reviewing literature, pay attention to how the studies reflect your clinical situation. P erhaps some studies better reflect your patient mix or techniques.

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ACKNOWLEDGMENTS

Author Contributions: Study concept and design: S. Sprigle, S. Sonenblum. Analysis and interpretation of data: S. Sprigle, S. Sonenblum. Drafting of manuscript: S. Sprigle, S. Sonenblum. Critical revision of manuscript for important intellectual content: S. Sprigle, S. Sonenblum. Administrative, technical, or material support: S. Sprigle, S. Sonenblum. Financial Disclosures: The authors have declared that no competing interests exist. Funding/Support: This material was based on work supported by the Rehabilitation Engineering Research Center on Wheeled Mobility (Mobility RERC) and the Georgia Institute of Technology. The Mobil- ity RERC is funded by the National Institute on Disability and Reha- bilitation Research of the U.S. Department of Education (grant H133E080003). Additional Contributions: We thank Dr. Kath Bogie for her guid- ance in outlining the manuscript and offering important feedback. Disclaimer: The opinions contained in this article are those of the authors and do not necessarily reflect those of the U.S. Department of Education or the Georgia Institute of Technology.

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Submitted for publication May 26, 2010. Accepted in revised form August 16, 2010.

This article and any supplementary material should be cited as follows: Sprigle S, Sonenblum S. As sessing evidence supporting redistribution of pressure for pressure ulcer prevention: A review. J Rehabil Res Dev. 2011;48(3):203–14. DOI:10.1682/JRRD.2010.05.0102

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