Abstract:
A wheelchair cushion comprises a dual stiffness fluid. A cushion also comprises an ability to adjust fluid volume.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 13/932,218, filed Jul. 1, 2013, which claims the benefit of U.S. Provisional Application No. 61/666,319, filed Jun. 29, 2012, the disclosures of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates in general to cushions and more particularly to wheelchair cushions that are structured to redistribute load under tissues that support the bony prominences of the pelvis, such as the ischial tuberosities and the trochanters. 
     Wheelchair cushions that redistribute load are beneficial to users who are at risk of developing pressure ulcers due to prolonged sitting. In addition to redistributing pressure, wheelchair cushions should provide a stable base for sitting. This is particularly true for users who have compromised trunk stability due to neuromuscular deficits. 
     Wheelchair cushions use foam and other compressible materials to distribute pressure by compressing and therefore increasing the area over which the load is distributed. Many cushions have been developed that have varying stiffness of compressible materials, specifically foam. As an example, a cushion may be designed with softer material under the ischia and firmer material under the trochanters. The softer material compresses to allow the ischia to immerse, thereby allowing the trochanters to bear load. The firmer material does not compress as easily, so that when the user leans laterally, the material will resist compression and provide stability. 
     Another design of wheelchair cushions uses fluid, contained in a bladder, that is displaced under load. Fluid cushions are designed to allow the fluid to flow in response to loading. The natural movement of fluid results in flow from areas of high load to areas of lower load. Cushions use design elements to manage and control the flow in order to maintain a stable sitting base. For example, when a user leans to one side, the fluid would rapidly flow away from the increased load and create instability. The design of some fluid cushions is such that this flow is restricted. For example, a cushion that uses air as the fluid may restrict the flow of air between cells to increase the time required to flow from cell to cell. As a result, when a user leans to the left or right, the air does not immediately move away from the increased load, which would cause instability. Other cushions use specialty fluids that will flow gradually in response to pressure, but retain their shape and position in the absence of pressure. Again, by restricting flow over time, the cushions can improve stability. 
     In general, the market sees well-designed fluid cushions as superior to well-designed foam cushions. While foam cushions rely on deflection and compression to relieve pressure on the ischia, fluid cushions allow for fluid displacement and hydrostatic loading. This is advantageous for multiple reasons. First, in general, well-designed fluid cushions retain supportive properties over time, compared to foam cushions, which gradually break down. Second, there is less shear stress on the skin tissue while displacing fluid than while deflecting foam. Last, as mentioned previously, viscous, non-Newtonian fluids maintain their position in the absence of pressure and create more stability for the user. 
     SUMMARY OF THE INVENTION 
     This invention relates to a fluid-filled wheelchair cushion comprising a dual stiffness fluid. A cushion also comprises an ability to adjust fluid volume. 
     Various advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment, when read in light of the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an exploded view of a wheelchair cushion. 
         FIG. 2  is a diagrammatic sectional view of the cushion supporting a user. 
         FIG. 3  is a tool for adjusting fluid in the cushion. 
         FIG. 4  is a manner for adjusting fluid in a segment of the cushion. 
         FIG. 5  is a manner for adjusting fluid in a segment of the cushion. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings, there is illustrated in  FIGS. 1 and 2  a wheelchair cushion  10  comprising a base  12  and a bladder assembly  14 . The base  12  may be a structural foam base, which may be contoured (i.e., based on measurements of the user&#39;s anatomy). The bladder assembly  14  may be at least partially fluid filled, for example, by providing multiple bladders or envelops  16 ,  18  and  20  for receiving fluid. 
     The base  12  may be formed from rigid foam that is substantially incompressible (e.g., does not compress under the weight of the user), and which is substantially impervious to moisture (i.e., does not absorb moisture). An example of a suitable foam material would be polyolefin foam with a density of 300 lbs/ft 2 . One such foam is sold under the trademark OleTex Cross Linked Olefin Foams by Armacell. The structural base may be comprised of laminated foam with a well or recess  22 , formed by a dished out portion, or contour cut out of it. The well defines the pelvic loading area. The dimensions of the well are defined by pelvic anthropometry. The fluid-filled bladders  16 ,  18  and  20  fit into the well  22 , contained by perimeter walls  24 , and aid in the immersion and envelopment of the user&#39;s pelvis. 
     The bladders  16 ,  18  and  20  should prevent hammocking, a condition in which the bony prominences of the pelvis immerse into the bladder, but are not enveloped. This creates uneven pressure distribution and pulls the skin tissue into tension. The risk of hammocking can be reduced by providing a top surface  16 ′ and  20 ′ of the bladder  16  and  20  that is larger than the bottom surface  16 ″ and  20 ″ so excess material of the top surface  16 ′ and  20 ′ can conform to the curves of the user&#39;s body, or by using a stretchable material for the top surface  16 ′ and  20 ′ that will also conform. An example of an acceptable extensible (i.e., stretchable) material would be elastomeric polyurethane sheeting. A portion of the bladder assembly  14  which supports the ischia I may be split at  14 ′ into two bladders  16 , one for each ischial tuberosity. This permits fluid to be concentrated under the areas of high load, and also prevents fluid from flowing from underneath one ischium I to the other, leaving one to bottom out and the other with too much fluid. This split can also enable customization for users who may naturally exert more pressure on one ischium I than the other and would need different fill volumes in each area. Separate splits indicated at  14 ″ in the bladder assembly  14  may form a separate bladder  20  for the coccyx C. 
     The fluid inside the bladder assembly  14  could be any fluid material. A viscous, thixotropic material is suitable. An example of an acceptable material would be a commercially available viscous fluid sold under the trademark Skwoosh by I-Tek Inc. The performance of Skwoosh is not easily altered by fluctuations in temperature, and it is approximately 75% lighter than the fluids most commonly used in commercially available fluid wheelchair cushions. 
     The cushion  10  may utilize a bladder assembly  14  with a plurality of bladders  16 ,  18  and  20 . These bladders  16 ,  18  and  20  may be divided into regions or zones that are filled with different viscosities of fluid. A first region, generally indicated at  26 , may encompass central bladders  16  and  18  located on the base of the well  22 . Three bladders  16  and  18  may be provided for supporting the ischia I and the coccyx C. The first region  26  may be comprised of a lower viscosity fluid to facilitate immersion and envelopment of the ischia I. A second region, generally indicated at  28 , is comprised of the two lateral bladders  20  located on the outside lateral sides of the bladder assembly  14 . The second region  28  is comprised of a higher viscosity fluid to provide more support to the greater trochanters T as they bear load. These two bladder regions  26 ,  28  support the bony prominences of the load-bearing pelvis. The first region  26  is designed to allow the ischia I to immerse, thereby allowing the more viscous second region  28  to support the trochanters T and redistribute the load laterally away from the ischia I, creating a substantially even distribution of pressure. 
     An exemplary cushion may comprise, for example, a Skwoosh fluid with a density of 0.24 g/cm 3  as a higher viscosity in the first region and a Skwoosh fluid with a density of 0.22 g/cm 3  as a lower viscosity fluid in the second region. The two viscosities can also be adjusted to be more or less viscous depending on the user&#39;s needs. 
     The dual density fluids allow for increased lateral stability for the user, without compromising the ability of the ischia to immerse easily. The ischia immerse in the less viscous fluid just enough to load the trochanters on the lateral bladders with the higher viscosity fluid. The trochanters are enveloped, but encounter higher resistance to movement, strengthening the stability of the user. 
     The cushion  10  is structured to permit adjustment in the fluid volume ischia support bladders  16  in the first region  26  of the bladder assembly  14 . Adjusting the volume of fluid in these bladders  16  may be desirable to accommodate the needs of different users. For example, some users have less soft tissue than others, creating areas of high concentrations of pressure in the buttock region. The amount of fluid the user sits on must be enough to immerse the pelvis, but not cause the user to bottom out. Changing the amount of fluid the user sits on changes the fluid pressure in the bladder so it can match the pressure exerted by the body and hydrostatically load the user. The optimal amount of fluid for a user depends on a variety of factors, including the user&#39;s body mass, pelvic structure and amount of soft tissue in the buttocks region. Current products approach fluid adjustability in multiple ways. 
     Fluid adjustment can be accomplished through the use of an external and/or internal reservoir. An external reservoir may be in the form of a tool that could transfer fluid between the reservoir and the first region of the bladder assembly  14 . This could be done in specific measured amounts. An example of an acceptable tool would be a large screw and pump syringe, such as the syringe  30  shown in  FIG. 3 . The bladders  16  may have a valve or seal that could accept fluid as well as keep the bladder  16  vacuum sealed. A vacuum seal cap, like those sold under the trademark Space Bag by Illinois Tool Works would be an example of an acceptable seal. The valve or seal used may be sufficiently large or gross (i.e., not fine). This permits fluids used in the bladders  16  that degrade under the high pressure (if squeezed through a small orifice) to be substantially unaffected. 
     An internal reservoir may be more logistically simple for the user and the manufacturer. In an exemplary cushion  10 , the bladders  16  may be subdivided into two segments. A first segment  16   a  may be a portion of the bladder  16  upon which the user sits. A second segment  16   b  of the bladder  16  is a reservoir that extends into channels  32  in the foam cushion base  12 , beneath the thigh area. Fluid adjustability is accomplished through controlling the fluid flow between the first and second segments  16   a  and  16   b.    
     The fluid volume in the first segment may be controlled in any suitable manner. For example, a variable sized reservoir may be created in the channels  32  in the foam base  12 . A suitable approach to varying the size of the reservoir would be to use extracted foam from channels  32  in the foam base  12  to create plugs  34  that can be reinserted into the channels  32 . The extent of the insertion of the plugs  34  (as shown in  FIG. 4 ) determines the length of the channels  32 , which in turn may control how much fluid can flow into the second segment  16   b . Another manner in which fluid volume may be controlled is by creating a seal between the first segment  16   a  and the second segment  16   b . As an example, a small pin  36  (shown in  FIG. 5 ) could be inserted in the side of the foam base  12 , perpendicular to the transition between the two segments  16   a  and  16   b . The pins  36  would pinch the bladders  16  so no fluid could flow between the different segments  16   a ,  16   b . Another manner in which fluid volume may be controlled is by using a reusable, re-sealable zipper or closure as used on storage bags sold under the tradename Ziplock by SC Johnson &amp; Son. The closure (not shown) could be situated between the two segments  16   a  and  16   b  so that it could be opened in order to transfer fluid, and closed to prevent the fluid from flowing. 
     In accordance with the provisions of the patent statutes, the principle and mode of operation of this invention have been explained and illustrated in an exemplary embodiment. However, it must be understood that this invention may be practiced otherwise than as specifically explained and illustrated without departing from its spirit or scope.