WO2007010215A1

WO2007010215A1 - Pressure relieving cushion

Info

Publication number: WO2007010215A1
Application number: PCT/GB2006/002628
Authority: WO
Inventors: Jon Anthony Lewis; Ian Robert Mahoney; Howard L. Loewenthal
Original assignee: Invacare Uk Operations Limited
Priority date: 2005-07-19
Filing date: 2006-07-14
Publication date: 2007-01-25
Also published as: EP1912609A1

Abstract

There is provided a gas containing sac suitable for use in or as a pressure relieving cushion and including an inflatable structure having at least front and rear portions, and support regions for supporting the ischial tuberosites and coccyx of a user, gas in the sac being displaceable so that the action of a user sitting on the sac can cause (i) the front and rear portions to be inflated by displaced gas and (ii) the support regions to be suspended from the sac, wherein the inflated front portion causes the pelvis of the user to rotate rearwards and the inflated rear portion provides support behind the coccyx of the user.

Description

Pressure Relieving Cushion

This invention relates to pressure relieving cushions, sacs suitable for use in or as a pressure relieving cushion, and seats incorporating same. Particular, but by no means exclusive, reference is made to sacs and cushions which are suitable for wheelchair users.

It is known that people who spend substantial periods of time in a seated position are prone to develop pressure sores. Those who are unable to make even minor adjustments to their seated position are most vulnerable to the development of pressure sores: wheelchair users and other immobile persons can fall into this category. It is known to utilise specialist pressure relieving cushions in order to ameliorate the problem of pressure sores. A known class of pressure relieving cushions utilises a gel sac which seats beneath the user's ischial tuberosites (ITs). The utility of pressure relieving cushions can be examined in a quantitative manner by so called pressure mapping measurements in which a two dimensional map of the pressures exerted on a cushion when sat upon by a user is produced, the pressure map representing the distribution of pressures across the surface area of the cushion. The present inventors have determined that, whilst gel sac pressure relieving cushions of the type described above can provide an advantageous effect, it would be desirable to improve the pressure relieving characteristics still further. In particular, the present inventors have noted that "hot spots" appear in the pressure maps associated with prior art gel sac pressure relieving cushions. Also, the present inventors have determined that the gel sacs can sag over a period of time, leading to pressure mapping characteristics that deteriorate over a period of time.

According to a first aspect of the invention there is provided a gas containing sac suitable for use in or as a pressure relieving cushion and including an inflatable structure having at least front and rear portions, and support regions for supporting the ischial tuberosites and coccyx of a user, gas in the sac being displaceable so that the action of a user sitting on the sac can cause (i) the front and rear portions to be inflated by displaced gas and (ii) the support regions to be suspended from the sac, wherein the inflated front portion causes the pelvis of the user to rotate rearwards and the inflated rear portion provides support behind the coccyx of the user.

In this way, the action of the user sitting on the sac causes a relatively large surface of the sac to come into contact with the user, thereby reducing the pressures transmitted to the sac. The suspension of the support regions for the ischial tuberosites and coccyx can reduce pressure hot spots associated with these regions. Furthermore, the sac directs the user into a beneficial sitting posture and can provide a comfortable feel for the user. A further benefit is the possibility of conveniently folding the sac for storage.

It is preferred that the inflated structure further includes spaced apart side portions, wherein the action of a user sitting on the cushion can cause the side portions to be inflated by displaced gas so as to support the sides of the pelvic area of the user. This provides further stability and comfort for the user.

In preferred embodiments, the front portion includes a pair of leg portions. Advantageously, the rear support portion includes a protruding portion. This leg portion can act as an inflatable flap that provides support behind the coccyx of the user.

It is preferable for the sac to have a gas permeable material disposed therein in gas conducting communication with at least a portion of the inflatable structure to assist in the displacement of the gas to and/or from the inflatable structure when a user sits on the sac and/or when the user alights from the sac.

The gas permeable material may be a fibrous material and/or a foam.

The gas permeable material may surround the support region or regions for supporting the ischial tuberosites.

Preferably, the support regions include a first and second discrete region for supporting the ischial tuberosites and a third discrete region for supporting the coccyx. Most preferably, the positions of the first, second and third discrete regions substantially define the vertices of an isosceles triangle in which the side between the first and third discrete regions is congruent with the side between the second and third discrete regions.

In preferred embodiments, at least one support region is an indentation in the sac.

At least a portion of a support region might not be gas containing. For example, the gas containing sac may have upper and lower surfaces which, in at least a portion of a support region are bonded together. The surfaces may be bonded together by adhesive or by a suitable sac forming process, for example one that includes heat sealing. It has been found that it is undesirable for the sac to be filled with gas at a high pressure, since this can cause instability and reduce comfort. Generally, the sac is filled with gas at a pressure of less than 200 kPa, typically less than 150 kPa, preferably between 80 and 120 kPa, most preferably about 101 kPa. Conveniently, the sac is filled with air, although other gases (preferably chemically inert gases such as nitrogen or a noble gas) might be used.

The gas containing sac may further include means for allowing the pressure of the gas in the sac to be varied. This enables the user to fine-tune the characteristics of the gas containing sac by varying the pressure of the gas. In one embodiment, the means for allowing the pressure of the gas in the sac to be varied includes a valve communicating with an interior region of the sac allowing the sac to be inflated and/or deflated. The sac may be inflated by the user breathing into the sac through the valve, or alternatively a pump may be utilised, in which instance the pump may be incorporated in the gas containing sac.

In a preferred embodiment, the means for allowing the pressure of the gas in the sac to be varied includes means for altering the sac so as to reduce and/or increase the volume of the sac available to be filled with gas. Conveniently, the means for altering the sac includes a flexible portion that can be manipulated so as to reduce and/or increase the volume of the sac available to be filled with gas. The flexible portion may be rolled and unrolled, folded and unfolded or otherwise manipulated so as to reduce and/or increase the volume of the sac available to be filled with gas. Very conveniently, the flexible portion is a protuberance extending from the sac. Other means for altering the sac so as to reduce and/or increase the volume of the sac available to be filled with gas might be utilised, such as a plunger arrangement.

The inflatable structure consist essentially of a single chamber or a plurality of chambers in gas conducting communication with one another which are filled with gas.

Alternatively, the inflatable structure may include a plurality of chambers containing gas which are not in gas conducting communication with one another. With such embodiments, it is preferred that the gas containing sac further includes means for allowing the pressure of the gas in the sac to be varied thereby enabling the pressure of the gas in at least one of the chambers to be varied with respect to the pressure of the gas in the other chambers. In this way, the characteristics of selected portions of the gas containing sac can be fine-turned. The means for allowing the pressure of the gas in the sac to be varied may be as defined above. In a preferred embodiment, the gas containing sac includes two side chambers, each side chamber generally extending from the front to the rear of the sac, in which the pressure of the gas in the side chambers can be varied so as to compensate for pelvic obliquity.

The inflatable structure may include at least one lower chamber and at least one upper chamber, wherein the upper chamber is disposed over the lower chamber and the chambers are not in gas conducting communication with one another. In a preferred embodiment, the lower chambers include the side chambers defined above. Preferably, the gas containing sac has an upper chamber which includes the support regions for supporting the ischial tuberosites and coccyx, the front portion and the rear portion. Most preferably, a single upper chamber is provided.

Acco^rding to a second aspect of the invention there is provided a pressure relieving cushion including: a gas containing sac including an inflatable structure having at least front and rear portions, and support regions for supporting the ischial tuberosites and coccyx of a user, gas in the sac being displaceable so that the action of a user sitting on the cushion causes (i) the front and rear portions to be inflated by displaced gas and (ii) the support regions to be suspended from the gas containing sac, wherein the inflated front portion causes the pelvis of the user to rotate rearwards and the inflated rear portion provides support behind the coccyx of the user; and a base on which the gas containing sac is disposed and/or an overlay overlaying the gas containing sac. The gas containing sac can have any of the features discussed above in respect of the first aspect of the invention. Preferably, the base has a cavity formed in an upper surface thereof, and at least a region of the gas containing sac including the support regions is disposed in the cavity. In such embodiments it is advantageous that the base has shaped portions, each shaped portion communicating with the cavity and the upper surface, and the front and rear portions of the gas containing sac are disposed in the shaped portions. Preferably, the pressure relieving cushion further includes a fluid containing sac. Advantageously the fluid containing sac is a liquid or gel filled sac, although a suitable gas containing sac might be used.

In preferred embodiments, the fluid containing sac includes a fluid containing channel structure formed so as to adopt a state when a user sits on the fluid containing sac in which the flow of fluid to the front of the fluid containing sac is prevented or inhibited.

The channel structure may include a plurality of longitudinally inclined fluid containing tubes. In this way, the weight of the legs of a user acts to constrict the fluid containing tubes so as to prevent or inhibit the flow of fluid to the front of the fluid containing sac. This can be contrasted with prior art arrangements in which a plurality of fluid containing tubes are provided which extend longitudinally along the fluid containing sac and perpendicularly to the front of the fluid containing sac. The plurality of fluid containing tubes may be in the form of a generally herringbone or zig-zag arrangement.

Alternatively, the plurality of fluid containing tubes may be in the form of a serpentine or meandering arrangement.

Alternatively still, the channel structure may include a plurality of fluid containing tubes which extend transversely across the sac. The channel structure may include a plurality of fluid containing tubes which are tapered so as to prevent or inhibit the flow of fluid to the front of the fluid containing sac. Preferably, the plurality of fluid containing tubes taper towards the front of the fluid containing sac.

The plurality of fluid containing tubes may be generally conical. The channel structure may contain constrictions so as to prevent or inhibit the flow of fluid to the front of the fluid containing sac.

Alternatively, the channel structure may be a honeycomb structure.

The present inventors have found that, surprisingly, in embodiments in which the fluid containing sac overlays the gas containing sac, it is possible to obtain pressure mapping profiles that are superior to the profiles obtained when the fluid containing sac is employed without the gas containing sac.

In addition to the fluid containing sac overlaying the gas containing sac, the pressure filled cushion may additionally include the base on which the fluid containing sac is disposed. Such cushions are particularly useful in medical applications.

Alternatively, the pressure relieving cushion may consist essentially of the gas containing sac overlaid with the fluid containing sac. Such cushions have particularly utility in a variety of non-medical applications. Even more surprisingly, the present inventors have found that it is possible to obtain even better pressure mapping profiles when no fluid containing sac is employed. Accordingly, then, in further preferred embodiments the pressure relieving cushion consists essentially of the gas containing sac and the base. Such embodiments do not include a fluid containing sac, and have an upper surface formed at least in part by the gas containing sac.

A foam overlay may overlay the gas containing sac. In such embodiments, the pressure relieving cushion may further include the base on which the gas containing sac is disposed. The foam overlay can aid in maintaining the stability of the pressure relieving cushion and preventing inflation of the gas containing sac into regions of the pressure relieving cushion which the gas containing sac is not intended to occupy.

Preferably, the base is formed from a foam material, preferably a resilient foam material. Advantageously, the gas containing sac is attached to the base. This ensures thai the gas containing sac is correctly aligned with the base.

According to a third aspect of the invention there is provided a seat including a gas containing sac, said gas containing sac, including an inflatable structure having at least front and rear portions, and support regions for supporting the ischial tuberosites and coccyx of a user, gas in the sac being displaceable so that the action of a user sitting on the seat causes (i) the front and rear portions to be inflated by displaced gas and (ii) the support regions to be suspended from the gas containing sac, wherein the inflated front portion causes the pelvis of the user to rotate rearwards and the inflated rear portion provides support behind the coccyx of the user.

The seat may be present as part of a wheelchair. Alternatively, the seat may be an item of furniture, such as a chair, or a seat for an automobile, aeroplane or other vehicle.

The seat may include a pressure relieving cushion in accordance with the second aspect of the invention. Alternatively, the gas containing sac may perform its function without being present as part of a pressure relieving cushion in accordance with the second aspect of the invention. For example, the seat may include a shaped seat portion in which the gas containing sac is disposed. Alternatively, the seat may include a seat portion having an aperture formed therein, the gas containing sac being disposed in the seat portion so that at least a region of the gas containing sac including the support regions is positioned in or above the aperture. In such embodiments, the gas containing sac may be secured to the seat portion. Embodiments of the invention will now be described with reference to the accompanying drawings, in which:-

Figure 1 is a plan view of a first embodiment of a gas containing sac of the invention;

Figure 2 is a cross sectional view along the line A - A of Figure 1 ; Figure 3 is a plan view of a gas distribution layer;

Figure 4 is a photograph of a base of a pressure relieving cushion;

Figure 5 is a cross sectional view of a first embodiment of a pressure relieving cushion;

Figure 6 is a cross sectional view of a second embodiment of a pressure relieving cushion;

Figure 7 shows a plan view of a second gas containing sac of the invention;

Figure 8 shows a third gas containing sac in (a) a plan view and (b) a cross sectional view along the line B-B of (a); Figure 9 is a cross sectional view of a third embodiment of a pressure relieving cushion; and

Figure 10 is a plan view of a gel sac.

Figures 1 and 2 depict a gas containing sac, shown generally at 10, of the present invention. The sac 10 has a pair of front legs 12, 14 and a rear portion/leg 16 extending therefrom. The sac 10 further includes a pair of lateral side regions 18, 20 and main chamber 22. The side regions 18, 20 are separated from the main chamber 22 by join lines 24 wherein the opposed walls of the sac 10 are joined together by a suitable technique. The join lines 24 define channels 26 allowing gas conduction between the side regions 18, 20 and the main chamber 22.

Disposed in the main chamber 22 are a pair of support regions 30,32 for the ischial tuberosites of a user and a further support region for the coccyx of a user. The support regions 30, 32, 34, which are preferably circular, each have regions 30a, 32a, 34a in the form of a concentric ring in which the surfaces of the sac 10 are joined together. It is possible that the surfaces of the sac 10 might be joined together over the entirety of the supports 30, 32, 34. The supports for the ischial tuberosites 30, 32 and support for the coccyx 34 together define the vertices of an isosceles triangle. It is noted that the relative spacings of the ischial tuberosites and coccyx do not vary substantially amongst adult populations, and therefore it is possible to provide a single sac design having general utility. In one embodiment, the separation between the centres of the supports for the ischial tuberosites is 12 cm, and the separation between the centre of the support for the coccyx and the centre of each support for the ischial tuberosites is 9 cm. Disposed within the main chamber 22 of the sac 10 is a gas distribution layer 38 of a gas permeable material. The gas permeable material may be of any convenient form, such as a fibrous material or a foam material. A relatively loosely layered fibrous material is particularly convenient for this purpose. The gas distribution layer 38 has a pair of apertures 40, 42 corresponding to the positions of the supports 30, 32 for the ischial tuberosites of the user. Additionally, the gas distribution layer 38 has a cut out portion 44 corresponding to the position of the support 34 for the coccyx of a user. Further still, the gas distribution layer 38 has a pair of short legs 46, 48 which, when the gas distribution layer 38 is positioned in the sac 10 protrude a short way into the front legs 12, 14 of the sac 10.

The sac 10 is a closed system, and operates by the redistribution of gas when the sac 10 is sat upon by a user. It is undesirable that overly high gas pressures are utilised in the sac when it is not in active use, and in fact a gas pressure at or around atmospheric pressure (101 kPa) is highly suitable. In use, a user sits on the air sac 10. In the configuration shown in Figures 1 and 2, the support regions 30, 32, 34 are somewhat indented in comparison to the main chamber 22, thereby assisting in the correct location of the ischial tuberosites and coccyx of the user in the corresponding support region. The action of the user sitting on the sac 10 causes a redistribution of the gas within the sac 10. In particular, gases are expelled from the main chamber 22 into the front legs 12, 14, rear leg 16, and side regions 18, 20, causing these regions of the sac 10 to inflate. A number of advantageous features are associated with this inflation process. The inflation of the front legs 12, 14 causes the front legs 12, 14 to act as flaps or wedges underneath the upper thighs of the user. This action causes the pelvis of the user to be rotated backwards somewhat, causing the user to adopt a more comfortable and medically desirable posture. The inflation of the rear leg 16 also causes the leg 16 to act somewhat in the manner of a flap, providing support for the user behind the coccyx, and additional comfort for the user in the sitting position. The inflation of the side regions 18, 20 causes these regions to rise somewhat, thereby bringing the side regions 18, 20 into contact with the sides of the user's pelvic region, thereby stabilising the user's sitting position and providing further comfort. The overall effect of the inflation of the front legs 12, 14, rear leg 16 and side regions 18, 20 is to increase the surface area of the sac 10 in contact with the user. As a result of this action, the pressure exerted by the user on the sac 10 is reduced. In this regard, a further highly advantageous feature of the invention is that when the user is sitting on the sac 10, causing redistribution of the gas in the sac 10 and inflation of certain regions of the sac, the support regions 30, 32 for the ischial tuberosites and the support region 34 for the coccyx are suspended from the sac 10. It has been found that this process can provide substantial reductions in the pressure transmitted to the sac through contact with a user's coccyx and, in particular, ischial tuberosites. This is advantageous since the present inventors have found that prior art pressure relieving cushions utilising gel sacs can have pressure hot spots associated with contact with the ischial tuberosites of a user.

After use, the user alights from the air sac 10, whereupon gas flows from the inflated regions into the main chamber. The gas distribution layer 38 is particularly convenient in this regard since it tends to assume its original shape once the pressure applied by the user is removed. The expansion of the gas distribution layer 38 on removal of the pressure applied by the user acts rather like a pump, assisting in the redistribution of the gas within the sac 10. The overall effect of the configuration adopted by the sac 10 in use is to increase the surface area in contact with the user and to reduce the pressure exerted by the user on the sac, which thereby reduces the likelihood of pressure sores being developed.

Figure 7 depicts a second embodiment of a gas containing sac, showing generally at 70. The second embodiment of a gas containing sac 70 shares many of the elements of the first embodiment shown in Figures 1 and 2, and identical numerals are used to depict such shared elements. The gas containing sac 70 further comprises a protuberance 72 positioned adjacent the rear portion 16. The protuberance 72 enables the gas pressure in the sac 70 to be adjusted by a user. The gas pressure can be increased by folding or rolling up the protuberance 72, which can be unrolled or unfolded in order to reduce the gas pressure. Advantageously, the gas distribution layer 38 extends into the protuberance 72. Other ways of increasing and decreasing the gas pressure in the gas containing sac can be utilised, such as a valve which might be inflated by a pump or by a user's breath. Figure 8 shows a third embodiment of a gas containing sac, shown generally at 80. The third embodiment of a gas containing sac 80 shares a number of the elements of the embodiment shown in Figures 1 and 2, and identical reference numerals are used to denote such shared elements. The gas containing sac 80 permits selective inflation/deflation of discrete areas of the sac. The gas containing sac 80 is divided into upper and lower sections by an internal wall 82. The lower section comprises first and second lower chambers 84, 86. The first and second lower chambers 84, 86 are positioned side by side, and are separated by a seal line 88 which extends longitudinally along the gas containing sac 80. The seal line 88 is depicted by a broken line in Figure 8a, although it is understood that the seal is not part of the upper region of the gas containing sac. The first and second lower chambers 84, 86 are each in communication with a protuberance 90, 92 disposed adjacent to the rear portion 16. The upper portion of the gas containing sac 80 is essentially equivalent to the sac 10 shown in Figures 1 and 2. The first and second lower chambers 84,

86 each have a gas distribution layer 94, 96. Each gas distribution layer 94, 96 extends into the corresponding protuberance 90, 92 provided in the relevant lower chamber. Each protuberance 90, 92 can be rolled up or folded in a manner akin to the protuberance of Figure 7 in order to fine-tune the gas pressure in each lower chamber 84, 86. In this way, the gas containing sac can be inflated so as to compensate for pelvic obliquity. Numerous variations are possible; for example, the first and second lower chambers may not extend to regions corresponding to any or all of the front legs 12, 14, rear portion 16 and lateral side regions 18, 20. Alternatively, a gas containing sac might not possess upper and lower regions separated by an internal wall, but rather comprise two side by side main chambers separated a barrier such as a wall or a seal line. Each of the side by side main chambers could be provided with a separate inflation/deflation device such as a rollable or foldable protuberance.

The gas containing sac of the invention can be used as a pressure relieving cushion or as part of a pressure relieving cushion. Conveniently, a pressure relieving cushion further includes a suitable base on which the gas containing sac is disposed. Figure 4 shows an embodiment of a suitable base, shown generally at 50. The base 50 has a cavity 52 formed in its upper surface, and shaped regions 54, 56, 58 leading between the upper surface of the base 50 and the cavity 52. The shaped region 54 communicates with a rear edge of the base 50. The shaped regions 54, 56, 58 provide smooth, curved support surfaces. In use, the gas containing sac is disposed upon the base 50 so that the main chamber and ischial tuberosites support regions and coccyx support region of the sac is disposed in the cavity 50, the rear leg of the sac is disposed in the shaped region 54 and the front legs of the sac are disposed in the shaped regions 56, 58. A suitable material for the base is a foam material. A resilient foam is preferred.

Figure 5 shows an embodiment of the invention in which an air sac 60 is disposed in the base 50 and is overlaid with a gel sac 62. Pressure mapping data were acquired for the configuration shown in Figure 5. The results indicated excellent pressure mapping characteristics over a period of time. Figure 6 shows a second embodiment of the invention in which the gas containing sac 60 is disposed on a base 50 with no overlaying gel sac. Surprisingly, pressure mapping of this configuration provided data which are superior to that obtained with the first embodiment of the type shown in Figure 5. Other configurations might be envisaged. For example, it may be possible to incorporate the gas containing sac directly into a seat. A seat having an aperture formed therein might be utilised, the gas containing sac being disposed over the aperture and appropriately attached to the seat, for example in the manner of a sling.

A known class of pressure relieving cushions utilises a gel sac which seats beneath the user's ischial tuberosites (ITs), Typically, such gel sacs comprise a portion on which the user's ITs rest, and a frontward portion having a plurality of longitudinal tubes which run parallel to one another and extend to the front of the gel sac. The present inventors have identified a problem with such gel sacs, namely that in use the gel in the frontward section tends to migrate to the very front of the gel sac. The present invention also provides fluid filled sacs suitable for use in or as a pressure relieving cushion in which the flow of fluid to the front of the fluid filled sac is prevented or inhibited. Figure 10 shows a gel sac 100 which has a top surface 102 and a bottom surface (not shown) connected by front edge 104, a rear edge 106 and side edges 108. The gel sac 100 has a rearward region 110 on which the ITs and coccyx of a user are positioned during use. In front of the rearward region 110 and extending to the front edge 104 is a channel structure 112 defined by the side edges 108 and a plurality of interdigitated walls 114 extending from respective side edges 108. Thus, the channel structure 112 defines a serpentine fluid pathway. In prior art arrangements having longitudinally aligned channel structures, fluid in the sac tends to be displaced towards the front during use owing to a longitudinal pressure gradient. Relatively high pressure is exerted by the posterior of the user, with the pressure exerted by the leg tapering off progressively towards the front of the cushion. With the serpentine fluid flowpath provided by the embodiment as shown in Figure 10, there is no progressively reducing pressure gradient along the flowpath and so gel is not displaced to the front of the sac. In fact, the user's legs may act to pinch the fluid pathway and hence inhibit the forward flow of gel. Conveniently the interdigitated wall 114 are seal lines between the top and bottom surfaces of the sac, although the use of wall extending between the top and bottom surfaces might be envisaged, as indeed might other arrangements.

A further configuration is shown in Figure 9, which depicts an arrangement in which an air sac 60 of the invention is disposed in the base 50 and is overlaid with a foam layer 99. This arrangement aids in maintaining the stability of the pressure relieving cushion. Furthermore, an appropriately formed foam layer 99 prevents the gas containing sac from inflating into regions of the pressure relieving cushion in which it is not intended that the gas containing sac should occupy. The foam layer 99 can be separate to the base 50 or alternatively, might be integral therewith, with an aperture of sufficient dimension being provided to enable the gas containing sac to be located in the base 50.

Gas containing sacs of the present invention can be conveniently manufactured from plastics materials using well-known techniques. Whilst the invention is particularly applicable to specialist medical applications such as wheelchairs, the invention might be utilised in non-medical applications, where the comfort afforded by the present invention and the ability to settle a user into the correct sitting posture are beneficial.

Claims

1. A gas containing sac suitable for use in or as a pressure relieving cushion and including an inflatable structure having at least front and rear portions, and support regions for supporting the ischial tuberosites and coccyx of a user, gas in the sac being displaceable so that the action of a user sitting on the sac can cause (i) the front and rear portions to be inflated by displaced gas and (ii) the support regions to be suspended from the sac, wherein the inflated front portion causes the pelvis of the user to rotate rearwards and the inflated rear portion provides support behind the coccyx of the user.

2. A gas containing sac in which the inflatable structure according to claim 1 further includes spaced apart side portions, wherein the action of a user sitting on the cushion can cause the side portions to be inflated by displaced gas so as to support the sides of the pelvic area of the user.

3. A gas containing sac according to claim 1 or claim 2 in which the front portion includes a pair of leg portions.

4. A gas containing sac according to any one of claims 1 to 3 in which the rear support portion includes a leg portion.

5. A gas containing sac according to any previous claims having a gas permeable material disposed therein in gas conducting communication with at least a portion of the inflatable structure to assist in the displacement of the gas to and/or from the inflatable structure when a user sits on the sac and/or when the user alights from the sac.

6. A gas containing sac according to claim 5 in which the gas permeable material is a fibrous material.

7. A gas containing sac according to claim 5 in which the gas permeable material is a foam.

8. A gas containing sac according to any one of claims 5 to 7 in which the gas permeable material surrounds the support region or regions for supporting the ischial tuberosites.

9. A gas containing sac according to any previous claim in which the support regions include a first and second discrete regions for supporting the ischial tuberosites and a third discrete region for supporting the coccyx.

10. A gas containing sac according to claim 9 in which the positions of the first, second and third regions discrete substantially define the vertices of an isosceles triangle in which the side between the first and third discrete regions is congruent with the side between the second and third discrete regions.

11. A gas containing sac according to any previous claim in which at least one support region is an indentation in the sac.

12. A gas containing sac according to any previous claim having upper and lower surfaces which, in at least a port of a support region, are bonded together.

13. A gas containing sac according to any previous claim in which the sac is filled with gas at a pressure of less than 150 kPa, preferably between 80 and

120 kPa, most preferably about 101 kPa.

14. A gas containing sac according to any previous claim further including means for allowing the pressure of the gas in the sac to be varied.

15. A gas containing sac according to claim 14 in which the means for allowing the pressure of the gas in the sac to be varied includes a valve communicating with an interior region of the sac allowing the sac to be inflated and/or deflated.

16. A gas containing sac according to claim 14 in which the means for allowing the pressure of the gas of the sac to be varied includes means for reducing and/or increase the volume of the sac available to be filled with gas.

17. A gas containing sac according to claim 16 in which the means for altering the sac includes a flexible portion that can be manipulated so as to reduce and/or increase the volume of the sac available to be filled with gas.

18. A gas containing sac according to claim 17 in which the flexible portion is a protuberance extending from the sac.

19. A gas containing sac according to any previous claim in which the inflatable structure consists essentially of a single chamber or a plurality of chambers in gas conducting communication with one another which contain gas.

20. A gas containing sac according to any one of claims 1 to 18 in which the inflatable structure includes a plurality of chambers containing gas which are not in gas conducting communication with one another.

21. A gas containing sac according to claim 20 when dependent on any one of claims 14 to 19 in which the means for allowing the pressure of the gas in the sac to be varied enables the pressure of the gas in at least one of the chambers to be varied with respect to the pressure of the gas in the other chambers.

22. A gas containing sac according to claim 21 including two side chambers, in which the pressure of the gas in the side chambers can be varied so as to compensate for pelvic obliquity.

23. A gas containing sac according to any one of claims 20 to 22 in which the inflatable structure includes at least one lower chamber and at least one upper chamber, wherein the upper chamber is disposed over the lower chamber and the chambers are not in gas conducting communication with one another.

24. A gas containing sac according to claim 23 when dependent on claim 22 in which the lower chambers include the side chambers.

25. A gas containing sac according to claim 24 having an upper chamber which includes support regions for supporting the ischial tuberosites and coccyx, the front portion and the rear portion.

26. A pressure relieving cushion including: a gas containing sac including an inflatable structure having at least front end rear portions, and support regions for supporting the ischial tuberosites and coccyx of a user, gas in the sac being displaceable so that the action of a user sitting on the cushion causes (i) the front and rear portions to be inflated by displaced gas and (ii) the support regions to be suspended from the gas containing sac, wherein the inflated front portion causes the pelvis of the user to rotate rearwards and the inflated rear portion provides support behind the coccyx of the user; and a base on which the gas containing sac is disposed and/or an overlay overlaying the gas containing sac.

27. A pressure relieving cushion according to claim 26 in which the base has a cavity formed in an upper surface thereof, and wherein at least a region of the gas containing sac including the indented regions is disposed in the cavity.

28. A pressure relieving cushion according to claim 27 in which the base has shaped portions each communicating with the cavity and the upper surface, and the front and rear portions of the gas containing sac are disposed in the shaped portions.

29. A pressure relieving cushion according to any one of claims 26 to 28 including a fluid containing sac, preferably a liquid or gel filled sac.

30. A pressure relieving cushion according to claim 29 in which the fluid containing sac includes a fluid containing channel structure formed so as to adopt a state when a user sits on the fluid containing sac in which the flow of the fluid to be front of the fluid containing sac is prevented or inhibited.

31. A pressure relieving cushion according to claim 30 in which the channel structure includes a plurality of longitudinally inclined fluid containing tubes.

32. A pressure relieving cushion according to claim 31 in which the plurality of fluid containing tubes are in the form of a generally herringbone or zig-zag arrangement.

33. A pressure relieving cushion according to claim 30 in which the channel structure is in the form of a serpentine or meandering arrangement.

34. A pressure relieving cushion according to claim 30 in which the channel structure includes a plurality of fluid containing tubes which are tapered so as to prevent or inhibit the flow of fluid to the front of the fluid containing sac.

35. A pressure relieving cushion according to claim 34 in which the plurality of fluid containing tubes taper towards the front of the fluid containing sac.

36. A pressure relieving cushion according to claim 34 or claim 35 in which the plurality of fluid containing tubes are generally conical.

37. A pressure relieving cushion according to claim 30 in which the channel structure contains constrictions so as to prevent or inhibit the flow of fluid to the front of the fluid containing sac.

38. A pressure relieving cushion according to claim 30 in which the channel structure is a honeycomb structure.

39. A pressure relieving cushion according to any one of claims 29 to 38 in which the fluid containing sac overlays the gas containing sac.

40. A pressure relieving cushion according to claim 39 also including the base on which the fluid containing sac is disposed.

41. A pressure relieving cushion according to claim 39 consisting essentially of the gas containing sac overlaid with the fluid containing sac.

42. A pressure relieving cushion according to any one of claims 26 to 28 consisting essentially of the gas containing sac and the base.

43. A pressure relieving cushion according to any one of claims 26 to 29 in which a foam overlay overlays the gas containing sac.

44. A pressure relieving cushion according to claim 43 further including the base on which the gas containing sac is disposed.

45. A pressure relieving cushion according to any one of claims 26 to 31 or claims 33 to 35 in which the base is formed from a foam material, preferably a resilient foam material.

46. A pressure relieving cushion according to any one of claims 26 to 31 or claims 33 to 45 in which the gas containing sac is attached to the base.

47. A seat including a gas containing sac, said gas containing sac including an inflatable structure having at least front and rear portions, and support regions for supporting the ischial tuberosites and coccyx of a user, gas in the sac being displaceable so that the action of a user sitting on the seat causes (i) the front and rear portions to be inflated by displaced gas and (ii) the support regions to be suspended from the gas containing sac, wherein the inflated front portion causes the pelvis of the user to rotate rearwards and the inflated rear portion provides support behind the coccyx of the user.

48. A seat according to claim 47 present as part of a wheelchair.

49. A seat according to claim 47 or claim 48 including a pressure relieving cushion according to any one of claims 26 to 46.

50. A gas containing sac, pressure relieving cushion or seat substantially as herein described with reference to the accompanying drawings.