Abstract:
A therapeutic sling seat comprising a flexible material which is non-uniform in stiffness providing maximum contact surface area with the body of a person seated in the seat by spreading the forces away from high pressure areas using multiaxis tension forces. The seat material in tension adapts to the contour of the posterior of the seated person. By controlling the pressures on the ischial tuberosities, physical problems such as ulcers can be avoided.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent application is a divisional of co-pending application Ser. No. 08/965,268, filed Nov. 6, 1997 which application claims priority from provisional patent application Ser. No. 60/030,241, filed Nov. 8, 1996, which application is incorporated herein in its entirety by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to structures for supporting the human body while seated, and, in particular, to an improved cushioned sling seat structure which is designed to optimize the comfort and stability of the user. 
     2. Description of the Prior Art 
     There has been much research conducted over the years to attempt to improve the comfort of structures which serve to support the human body in a sitting position. It is well known that individuals who are required to sit for long periods are subject to discomfort, fatigue, pain, and various other afflictions associated with the lack of proper support of the torso of a person seated in an erect position over a sustained period of time. A common example of this problem occurs with individuals confined to wheelchairs. The lack of movement in this sitting position restricts blood flow, causing mechanical damage to body tissues and often leads to painful sores and ulcers. 
     Many attempts have been made to alleviate, if not prevent, these problems. These developments generally fall into several categories. 
     The most common types of cushions which have been used to address these problems are foam cushions. Foam cushions are popular, as they are lightweight, fairly durable, and inexpensive to manufacture. Foam pads with cut-out areas, convoluted surfaces, wedge-shaped slots and cored-out sections have been developed to minimize the adverse effects on the human body. Examples of these devices are taught in U.S. Pat. Nos. 4,042,987; 4,713,854; 3,111,689; 2,047,616; 3,337,884; and 3,222,694. There are several disadvantages, however, to the use of foams in cushions; foam generally has poor shear and tension properties, and also suffers from the inability to distribute loads or restoring forces evenly. 
     Another type of cushion which has been tried is a gel cushion. This type of cushion uses a viscous substance contained within some type of flexible structure capable of conforming with the body contours in contact with the cushion. While these cushions possess favorable tension and shear/friction properties, they are often heavy and bulky, and tend to deteriorate with heavy use. Examples of these cushions are taught in U.S. Pat. Nos. 3,858,379; 3,308,491; 3,663, 973; 3,689,948; 4,728, 551; 4,726,624; 5,336,708; 5,334,646; and 4,588,229. 
     Still another type of cushion which has been tried is a fluid-filled cushion. This type of cushion uses a fluid, such as gas or liquid, contained within a flexible envelope to allow the envelope to conform to the body contour. While gas-filled cushions are very light and inexpensive, the buoyancy is zero and the support of the body depends on a suitable hammock effect of the envelope plus the pressure of the air inside. Stability is generally greater with air filled cushions than that of liquid filled cushions because of increased tension in the envelope. Examples of the fluid filled cushions are shown in U.S. Pat. Nos. 2,823,394; 3,251,075; and 3,984,886. 
     Another type of cushion design which is available in the marketplace is the thixotropic, or “putty-filled” cushion. These cushions generally conform to the body in a suitable fashion as the material flows, can stiffen under fast loading to provide a solid and comfortable base for the body, and are designed to accurately control the flow of the material inside the cushion. However, this type of cushion tends to be fairly heavy, making it somewhat awkward to handle, can bottom out in some instances and tend to be relatively expensive compared to other cushions available. Examples of these cushions are taught in U.S. Pat. Nos. 4,726,624; 4,588,229; 4,728,551; and 5,018,790. 
     Finally, some cushion designs attempt to combine different features from the cushion types previously discussed to attempt to improve upon its performance. U.S. Pat. No. 2,819,712 combines a foam cushion with an air filled bladder; U.S. Pat. No. 5,524,971 uses a combination of fluid layers combined with foam layers; and U.S. Pat. No. 5,513,899 uses an envelope filled with a combination of petrolatum and hollow glass spheres. 
     While there have been many attempts to provide a superior cushion to satisfy a diverse range of users, each design has particular deficiencies which make it difficult to gain universal acceptance. The one feature which is present in all of the aforementioned cushions is the fact that they all are subject to compression forces. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a cushion which will provide maximum contact surface area along with uniform pressure on the body by spreading the forces away from high pressure areas using multiaxis tension forces. 
     It is also an object of the present invention to provide a seat which is inexpensive, lightweight, and durable, and offers stability. 
     It is a further object of the present invention to provide a cushion which can provide comfort and ventilation for a person seated for long periods of time without causing injury or fatigue. 
     These and other objects are accomplished in the present instance by a sling seat consisting of a thin and flexible material having non-uniform properties which allows the material in tension to adapt to the contour of the rear of the person seated. A person cannot sink into a compression type cushion more than the thickness of the cushion. In this tension type design, the ischial tuberosities of a person seated in the cushion are suspended, and will not “bottom out” in the cushion. By controlling the pressures on the ischial tuberosities and the coccyx of persons seated on a cushion manufactured according to the present invention, physical problems such as ulcers can be avoided in persons who must sit for long periods of time. Different embodiments of the invention are taught having features which enhance the desirable properties of the sling seat. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side elevation view showing a partial skeleton of a person in a sitting position; 
     FIG. 2 is an isometric perspective view showing a partial skeleton of a person in a sitting position; 
     FIG. 3 is a top view of a traditional wheel chair sling seat; 
     FIG. 4 is a top view of an alternative embodiment of a traditional sling seat having improved support of the back edge; 
     FIG. 5 is a top view of an alternative embodiment of the seat shown in FIG. 4; 
     FIG. 6 is a perspective view of another alternative embodiment of a seat according to the present invention; 
     FIG. 7 is a perspective view of a multi-layer design with stiffener, described by the technology of the present invention; 
     FIG. 8 is a top view of a molded sheet rubber design according the present invention; 
     FIG. 9 is a top view of an alternative embodiment of the seat shown in FIG. 8; 
     FIG. 10 is a cross-sectional view taken along lines  10 — 10  of FIG. 9; 
     FIG. 11 is a top view of another embodiment of a seat according to the present invention having additional pubic arch and ilium support; 
     FIG. 12 is a perspective view of another embodiment of a seat according to the present invention; 
     FIG. 13 is an exploded view of the components of an invisible frame design according to the present invention; 
     FIGS. 14 a  and  b  each show a perspective view of a cushion covering for use with the cushion taught in FIG. 13; and 
     FIG. 15 is a perspective view of a cosmetic cover for use with the cushion taught in FIG.  13 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to FIGS. 1 and 2, there is depicted a section of the torso of a person in a sitting position, with the skeletal structure shown in detail. In this position, the gluteus maximus  10  is supported by a horizontal frame member  12  such that the femur or thigh bones  14  are supported in a substantially horizontal position. Also shown in FIGS. 1 and 2 are the pelvic or hip bones  16 , the sacrum  17 , the coccyx  18 , the trochanter  19 , the lumbar vertebrae  20  of the spine, the ilium  21 , the ischial tuberosities  22 , the pubic arch  23 , the bicep muscles  24  and the proximal thigh regions  25 . 
     It is well known that one of the major causes of discomfort experienced by a sitting person is pressure. The weight of the upper body is concentrated at bony prominences or projections of the body, such as the ischial tuberosities  22  and coccyx  18 . Pressure exerted over a long period of time can often cause mechanical damage to the body tissue, causing ulcers and discomfort in the area of the gluteus maximus  10  and the bicep muscles  24  under the weight concentrated at the ischial tuberosities  22  and, to some extent, the femur  14 , depending on how the body is situated in a particular chair. Usually, a sitting person unconsciously adjusts his body position when discomfort is felt. However, for those handicapped persons who cannot feel pain or who are unable to adjust their body positions, tissue damage can very easily result. 
     FIG. 3 shows a basic sling seat construction. A flexible membrane  30 , which may be fabric or any similar material, is suspended between a pair of supports  32  which are attached along opposing edges  33  of membrane  30 . When a person sits upon that type of sling seat, membrane  30  deflects to form a concave surface between a catenary and the form of the body of the person sitting in the seat. The strength of membrane  30  is a function of the mechanical properties of its material and the method of its manufacture. In a traditional sling seat, the material for membrane  30  is usually chosen such that mechanical properties such as elasticity and friction are uniform in any location and direction on the material. In the sling seat of the present invention, however, the material of membrane  30  is selected such that a large surface contact area is obtainable by use of a thin and stretchable material which is non-uniform in stiffness. Several suitable materials for use in the present invention are LYCRA and POLARTEC. These fabrics provide a strong, yet very flexible, membrane by which the body weight of a person is entirely supported by multi-axis tension forces in the material, and allow the seat to essentially conform to the shape of the body and provide maximum surface contact area. Thus, the area of the seat located under the ischial tuberosities  22 , where the pressure on the tissue is quite high in a traditional sling seat, conforms to the shape of the gluteus maximus  10 , thereby minimizing the pressure. 
     FIG. 4 shows an alternative embodiment of the basic sling seat construction of the present invention. The sling seat consists of a flexible membrane  34  having a forward edge  36  and a peripheral edge  38 . Edge  38  consists of a pair of substantially parallel sides  38 a connected by a curved side  38   b . Edges  38   a  on each side are connected by a rear edge  40 . Membrane  34 , which is constructed of a stretchable material non-uniform in stiffness such as LYCRA, is suspended along its peripheral edge  38 . In this embodiment, membrane  34  more completely conforms to the shape of the body of the person sitting in the seat than membrane  30  of the previous embodiment, as membrane  34  is supported on all sides except front edge  36 . 
     FIG. 5 shows an alternative embodiment of a sling seat manufactured using the principles of the seat shown in FIG.  4 . Membrane  50 , having an outer edge  51 , is constructed from the same material and in the same basic shape as membrane  34 . However, a peripheral membrane  52  is attached along the outer edge  51  of membrane  50 . Membranes  50  and  52  can be constructed from the same type of flexible material having non-uniform characteristics, or it may be desirable for membrane  50  to exhibit a different rigidity characteristic than the peripheral membrane  52 , so that the weight bearing zone of the seat, specifically the area under the ischial tuberosities, will conform more closely to the body and more evenly distribute the pressure. In this embodiment, the inner membrane  50  can be selected to more closely fit the desired characteristics of elasticity for different persons, while maintaining a different peripheral membrane  52  with physical characteristics most ideally suited for attachment to a support frame. 
     FIG. 6 shows a variable thickness molded rubber sling seat which is constructed according to the principles of the present invention. In this embodiment, cushion  60  consists of a one piece molded rubber form having a front edge  61  and a U-shaped channel  62  positioned about its remaining periphery. A series of notches  64  are present along the rear of the periphery to enhance the flexibility of cushion  60 . A cylindrical frame  66  formed from a pair of tubular sections  66   a  and  66   b  coupled together at a hinge  67  is sized to fit within channel  62  of cushion  60 . Hinge  67  allows frame  66  to be folded to a compact form. When cushion  60  is suspended from frame  66  in a suitable structure such as a wheelchair, cushion  60  deflects to conform to the shape of the body seated on it, as the rubber flexes in tension, accommodating the curvature of the body and minimizing the pressure by distributing the resultant forces evenly. 
     Referring now to FIG. 7, another alternative embodiment of a sling seat  69  similar to that of FIG. 3 is shown. A flexible membrane  70  has a forward edge  72  and rearward edge  74  and a pair of essentially parallel side edges  76   a  &amp;  76   b.  Membrane  70  is constructed of a flexible fabric which is non-uniform in stiffness, such as LYCRA or POLARTEC. A pad  78 , which is preferably constructed of a thin layer of foam, is positioned adjacent the bottom side of membrane  70 . Foam pad  78  can be manufactured using a range of thickness variations, along with various hole patterns, to further control the areas of contact with the seated person. A second membrane  80 , having a front edge  82  and rear edge  84  and a pair of essentially parallel side edges  86  and  86   b,  is positioned beneath pad  78 . Membrane  80  is also constructed from a flexible fabric which is non-uniform in stiffness. Beneath membrane  80  is a second pad  88  constructed from a thin layer of foam. Finally, a third flexible membrane  90  having a front edge  92 , and a rear edge  94 , and a pair of essentially parallel side edges  96   a  and  96   b  is positioned beneath pad  88 . The sling seat is formed by attaching side edges  76   a,    86   a,  and  96   a  together by conventional means, such as sewing, and also side edges  76   b,    86   b,  and  96   b  to form sling seat  69 . Seat  69  is then suspended between a pair of supports by attaching edges formed at  76   a,    86   a,    96   a  and  76   b,    86   b,    96   b.  By varying the physical properties of each of the layers, a variety of seats with desired characteristics can be obtained. In this arrangement, membranes  70 ,  80 , and  90  are always in tension, while the pads  78  and  88  held between the membranes are in compression. 
     This seat forms a concave surface which surrounds the contour of the person seated upon it, as the membrane and foam layers are pliable. However, sometimes it is desirable to limit the deflection of the seat for support purposes. This is accomplished by the use of a stiffener layer  98  which may be sandwiched between the layers. Stiffener  98 , which may be constructed from a polycarbonate plastic sheet such as LEXAN having a thickness of approximately 0.125 inches, contains a cutout section  99  which comfortably accommodates the ischial tuberosities and sacrum of the body, while stiffener  98  provides rigid support beneath the femurs to assist the person when rising from the seat. Stiffener  98  also strengthens the seat for durability and handling. 
     Referring now to FIG. 8, another alternative embodiment of a sling seat  100  similar to that of FIG. 4 is shown. A flexible membrane  102  has a forward edge  104  and a peripheral edge  106 . Edge  106  consists of a pair of essentially parallel sides  106   a  connected by a curved side  106   b.  In this embodiment, membrane  102  is constructed of a suitable sheet rubber, such as neoprene, which is non-uniform in stiffness. Membrane  102  is suspended along its peripheral edge  106  to form sling seat  100 . 
     Within the interior of membrane  102 , a pair of apertures  108  are positioned to approximate the positions of the ischial tuberosities of a person sitting in seat  100 . Extending radially from each of apertures  108  is found a series of small oval perforations  110 . The area  112  of membrane  102 , between apertures  108  serves to provide support for the pubic arch of the person seated in this seat, as the pubic arch is a sensitive part of the body and pressure on it must be carefully controlled by sufficient deflection downwardly to avoid distress. The essence of the design of the present invention is to achieve gentle contact without pressing with excessive force against the sensitive pubic arch. 
     In the embodiment shown, perforations  110  are arranged in a series of concentric circles extending outwardly from apertures  108 . Within each circle, perforations  110  are arranged in a sunburst pattern. Perforations  110  are staggered from one circle to another. The sunburst pattern increases flexibility in the circumferential direction, thus enhancing radial expansion of membrane  102 . Apertures  108  and perforations  110  allow membrane  102  to more comfortably accommodate the ischial tuberosities by more efficiently distributing the body weight. Different patterns of perforations  110  can be employed to accomplish this distribution. 
     FIG. 9 shows an alternative embodiment of the seat shown in FIG.  8 . Seat  120  consists of a flexible membrane  122  having a front edge  124  and a peripheral edge  126 . Edge  126  consists of a pair of essentially parallel sides  126   a  connected by a curved side  126   b.  Membrane  122 , which is constructed from a suitable sheet rubber such as neoprene, contains a pair of recessed sections  128  which have been molded into membrane  122  (see FIG.  10 ). The area  132  of membrane  122  between sections  128  acts as a support for the pubic arch. 
     Recessed sections  128  each contain a series of perforations  130  which are arranged in a series of concentric circles. Sections  128 , like apertures  108  shown in FIG. 8, allow for the accommodation of the ischial tuberosities of a person seated in seat  120 . The concentric circular pattern can also be used in the embodiment shown in FIG. 8, as it increases the vertical drop of membrane  102 , thus allowing membrane  102  to form cups to more comfortably support the ischial tuberosities. 
     FIG. 11 shows another alternative embodiment of a sling seat manufactured according to the present invention. Seat  140  is similar to the embodiments shown in FIGS. 4 and 5. However, seat  140  includes a membrane  142  located beneath the pubic arch of the seated person for the purpose of creating pressures and to also provide vertical lift to support the person&#39;s weight. Membrane  142  is supported along the periphery  144  at the rear of seat  140  and is affixed at an area  146  in the central region of seat  140 . Preferably, membrane  142  is Y-shaped and oriented such that the coccyx and ilium of the seated person is located in an area  148  between arms  142   a  and  142   b  of membrane  142 . Also located on seat  140  are a pair of wedges  150 . Wedges  150 , which are constructed of a stiff material, are located along periphery  144  on either side of membrane  142  and are constructed such that they are angled at preferably a 45° angle from periphery  144  toward the central region of seat  140 . Wedges  150  are used to properly position the hip bones of the seated person forward and also to keep the sacrum away from the back edge of seat  140 , aiding in positioning the body in the seat for optimum comfort. Arms  142   a  and  142   b  are anchored at the inner central edges of wedges  150 . 
     FIG. 12 shows an embodiment of the seat of FIG. 3 in which no frame is used to support the sling seat. Membrane  170  is preferably constructed from a flat sheet of a flexible fabric of a non-uniform stiffness, as is disclosed in FIG. 3, having a pair of parallel edges  172 . Edges  172  are attached to a cover  174 . Cover  174  is preferably a canvas bag made from a fabric such as CORDURA. Edges  172  are sewn along upper edges  176  of cover  174  such that membrane  170  is stretched across the top surface  178  of cover  174 . Cover  174  is then stretched over a core  180 . To ensure proper tensioning of membrane  170 , elastic straps  181  are used to wrap under core  180 . Core  180  is preferably composed of a layer of a semi-rigid composite foam such as L-380XLPE foam. Core  180  may also contain a series of perforations  182  to control its stiffness. 
     This combination forms an “invisible” frame which helps to avoid injury if a person is not properly fitted or seated in the seat. The bony points of the body such as the coccyx, trochanter, and sacrum may come into contact with a rigid frame, thus causing possible severe injury. The seat taught in FIG. 12 can avoid this possibility, which can be a serious problem with wheelchair patients. 
     FIG. 13 shows an embodiment of a seat manufactured according to the principles of the present invention. Seat  200  consists of a top layer  202  having a curved front end  204  along with a pair of apertures  206  which allow for comfortable accommodation of the ischial tuberosities of the user. In addition, a plurality of circular apertures  208  may be located along the central area of layer  202  between apertures  206  and front end  204  to provide ventilation. Layer  202  is preferably constructed from a soft foam, such as an open cell polyurethane. 
     Adjacent layer  202  in seat  200  is a layer  210 . Layer  210 , which is essentially planar, contains a pair of apertures  212  which correspond to apertures  206  in layer  202 . Surfaces  212   a  of apertures  212  may be tapered inwardly, as can be seen in FIG. 13, to assist in proper location of the ischial tuberosities. Layer  210 , which is preferably constructed from a semi-rigid foam such as VOLTEK L200 cross-linked polyethylene, also contains a series of apertures  214 , similar to apertures  208  in layer  202 , which assist in providing ventilation for seat  200 . Finally, layer  210  also contains a plurality of small perforations  216  which provide stiffness control for layer  210  at the proximal thigh and trochanter areas. 
     A sheet rubber layer  218  is positioned adjacent layer  210  away from top layer  202  within seat  200 . Layer  218 , which is preferably neoprene rubber, contains a plurality of apertures  220  corresponding to apertures  212  and  206  of layers  210  and  202 , respectively. Layer  218  also contains a cutout section  222  located along each outer edge to accommodate the trochanters of the user of seat  200 . In addition, layer  218  contains a pair of extensions  224   a  and  224   b,  which, together with a support member  226 , which separates apertures  220 , form a Y-shaped support, which is generally indicated at  228 . Support  228  provides tension support and enhances weight distribution for the user of seat  200  by properly positioning the coccyx and ilium. Layer  218  also contains a plurality of perforations  230  which provide stiffness reduction at the proximal thigh. 
     Situated below layer  210  is a median divider device  232 , preferably constructed from VOLTEK L-200, which provides stability for the knees of the user of seat  200  by correctly positioning the legs of the user. Device  232  is preferably adhesively affixed to the underside of layer  210  centered along its front edge. 
     A foam cushion  234  is positioned adjacent sheet layer  218  on the side opposite layer  210 . Foam cushion  234  is preferably a composite device, constructed from different foam materials having different physical properties which are selected to match the vertical deflection of seat  200  required for the different body parts of the user. In the embodiment shown in FIG. 13, cushion  234  is composed of separate foam sections  234   a,    234   b,    234   c  and  234   d.  Section  234   a  supports the ilium, section  234   b  supports the mid-back, section  234   c  supports the proximal thigh and trochanter regions, and section  234   d  supports the legs of the user. Cushion  234  contains an elliptical cutout opening on region  236  which preferably contains a surface  236   a  which tapers outwardly from a lower surface  236   b  in the direction of layer  218 . 
     Cushion  234  also contains a recessed area  238  along its rear edge which comfortably accommodates the coccyx of the user of seat  200 . Finally, cushion  234  contains a plurality of apertures  240  similar to apertures  208 ,  214  of layers  202 ,  210  respectively which help to provide ventilation for seat  200 . 
     Adjacent cushion  234  is a stiffener plate  242 . Plate  242  provides support which allows seat  200  to act as a sling seat when it is placed on an existing chair or support such as a wheelchair. Plate  242 , which is constructed from a relatively thin rigid material such as plywood, a hard plastic, or a high impact polystyrene sheet, contains an elliptical opening  244  comparable to cutout region  236  in foam cushion  234 . Plate  242  insures that cushion  234  stays flat and also supports the back section under the coccyx of the user of seat  200 . Plate  242  also includes a series of apertures  246  which provide ventilation for seat  200 . 
     In some instances where seat  200  will not fit properly into a wheelchair, a booster device  248  can often be employed to raise the hips of the user above the side rails of the wheelchair. Device  248 , which is preferably constructed from a rigid material such as VOLTEK L200, contains an elliptical opening  250  which is comparable to openings  244 ,  236  in plate  242  and cushion  234  respectively, which tapers in the same manner as cutout region  236  in cushion  234 . 
     Finally, booster device  248  may contain a plurality of raised protrusions  252  which fit within apertures  246  of plate  242  to ensure accurate positioning of booster device  248  in relation to seat  200 . 
     FIG. 14 a  and  b  illustrate a removable cover system for use with the seat of FIG.  13 . Referring now to FIG. 14 a,  a removable protective cushion bag  260  is shown. Bag  260 , which is preferably constructed from a water resistant and low-friction material such as CORDURA, contains a zipper  262  which is affixed along its rear end such that seat  200  can be easily inserted into bag  260  to protect it from any debris or liquids which may tend to soil seat  200  if unprotected. Bag  260  also contains a series of apertures  264  to allow for air circulation and ventilation. Affixed on the side of bag  260  is an indicator  266 , which when aligned with a mark on the wheelchair, enables seat  200  to be placed in exactly the same position on the wheelchair each time after it has been removed. 
     FIG. 14 b  shows a variation of cushion bag  260 , designated as a slip cover  260   a,  which can be used with the seat of the present invention. Cover  260   a  has a front flap  268  which is folded over seat  200  after it is inserted into cover  260   a  and fastened to the underside using a removable attachment means such as VELCRO (not shown). Cover  260   a  also contains a rubber sheet  270  attached to the underside of the top of cover  260   a  for added strength and durability. Sheet  270 , which is preferably manufactured from neoprene, may also contain apertures  272  to accommodate the ischial tuberosities or any irregular bony points of the user of seat  200 . 
     FIG. 15 shows a separate cosmetic cover  274  which may be used as an additional cover over bag  260  or cover  260   a  as further protection from the elements of the environment of use of seat  200 . Cover  274 , which preferably manufactured from a strong, flexible, washable fabric such as LYCRA or POLARTEC, is stretched over seat  200  and bag  260  or cover  260   a  and is held in place by an elastic cord  276 , which holds cover  274  tightly in place about seat  200 . 
     While this invention has been shown and described in terms of several preferred embodiments thereof, it will be understood that this invention is not limited to any particular embodiment and that many changes and modifications may be made without departing from the true scope and spirit of the invention as defined in the appended claims.