Background and Summary of the Invention

The present invention relates to a mattress, and particularly to mattresses for hospital beds and other devices upon which patients recline including, for example, stretchers and examination tables. More particularly, the present invention relates to a mattress for hospital beds including beds that can be manipulated to achieve both a conventional bed position having a horizontal sleeping surface and a sitting position having the feet of the person on or adjacent to the floor and the head and back of the person supported above a seat formed by the bed.

Beds and examination tables having mattresses that rest on adjustable articulating decks are known in the art. See, for example, U.S. Patent Nos. 5,077,843 to Foster, L. Dale et al. , 5,157,800 to Borders, and 5,129,177 to Celestina et al., all of which are assigned to the assignee of the present invention, and U.S. Patent Nos. 5,279,010 to Ferrand et al. , 4,183,109 to Howell, 4,411,035 to Fenwick, and 3,220,022 to Nelson, as well as German publication No. 716981. Each of these references discloses a bed or an examination table including a mattress having a top surface that articulates to adjust the position of the person on the surface. See also U.S. Patent Application

Serial No. 08/511,711 filed 04 August 1995 to eis iller et al. and titled "Chair Bed", the specification of which is herein incorporated by reference.

According to the present invention, a mattress for a bed is provided. The mattress includes a generally planar upwardly-facing support surface or sleeping surface for supporting a person and a stepped, downwardly-facing bottom surface. The bottom surface includes a peri etral portion extending downwardly from the support surface a first depth and a central projection extending downwardly

from the support surface a second depth which is greater than the first depth.

The illustrative bed on which such a mattress is used includes a base frame, an intermediate frame coupled to the base frame, a weigh frame coupled to the intermediate frame, and an articulating deck coupled to the weigh frame. The mattress of the present invention rests on the articulating deck.

The illustrative articulating deck has longitudinally spaced head, seat, thigh, and foot sections. The head, thigh, and foot sections are movable relative to each other and are movable relative to the seat section which is fixed relative to the weigh frame. The head, thigh, and foot sections are infinitely adjustable to allow the chair bed to attain any desired position within the range of movement of the head, thigh, and foot sections, thus accommodating changes of position of a person on the chair bed.

The mattress is suitable for use with such an articulating deck. The mattress includes a head portion, a seat portion, a thigh portion, and a foot portion. Each named mattress portion is associated respectively with the head, seat, thighs, and feet of the person resting on the sleeping surface of the bed as well as with the underlying head, seat, thigh, and foot sections of the deck.

The illustrative articulating deck carrying the mattress can be a longitudinal step deck that includes upper deck side portions and a central, longitudinally extending recess between the side portions. The recess is defined by a lower deck and side walls connecting the lower deck and the upper portion of the deck. The mattress includes an upwardly facing sleeping surface, a bottom surface, sides connecting the sleeping surface and the bottom surface, and a downwardly extending projection appended to the bottom surface and extending downwardly

therefrom. During use, the projection is received by the recess of the step deck. If desired, the projection can nest in the recess and engage at least a portion of the side wall of the deck. The central portion can thus cooperate with the side walls defining the recess to prevent movement of the mattress relative to the bed.

The step deck and a mattress configured for use with the step deck can be used independently of the bed and the articulating deck. For example, a step deck and companion mattress can be provided for a stretcher, gurney, examination table, or any device on which a patient rests. Such a stretcher, for example, can include a frame, a step deck mounted to the frame, the step deck having longitudinal upper side portions engaging the frame, and a mattress having a generally planar sleeping surface and a bottom surface including a projection configured so that the shape of the bottom surface generally conforms to the shape of the step deck. In the same manner, a step deck and associated mattress could be provided for a gurney. Such a gurney would be similar to the illustrative stretcher described above except that the frame would include wheels so that the gurney could be transported by rolling it from place to place.

In preferred embodiments of the mattress, the sleeping surface is generally planar and the projection is centrally located beneath the sleeping surface to form a thick centrally located portion of the mattress. The varied thickness of the mattress provides the mattress with "zones" including a thick body-support zone adjacent to the projection and a thin zone in areas away from the projection. The portion of the mattress adjacent to the upper deck portions form a thin perimetral zone engaging the upper deck portion.

The projection typically includes portions of the area of the bottom surface of the mattress in the head

portion, the seat portion, and the thigh portion. The thinner zone typically covers the foot portion of the mattress and a perimetral zone of the mattress along the sides of the thigh, seat, and head portions as well as the head end of the head portion of the mattress. As a result, the thick zone typically encompasses the majority of the portion of the mattress that will be supporting most of the weight of the person on the sleeping surface and the thin zone surrounds the thick zone. Additionally, the thin perimetral zone of the mattress adjacent to the upper deck portions of the articulating deck cooperates with the upper deck portions to provide "rammed edges" that increase firmness experienced by the person around the edges of the mattress. The mattress cooperates with the step deck to present a generally planar top surface while also providing the thin perimetral zone that results in the rammed edges. This increased firmness is advantageous when the person enters and exits the bed along the sides of the bed. The mattress may be provided in more than one piece, for example, a first mattress piece could fit into the recess and a second mattress piece could engage the upper deck portion and surround the first mattress piece, or a first mattress piece could fit into the recess and a second mattress piece could cover the first mattress piece and engage the upper deck portion.

Though there are many potential variations of step deck shapes and corresponding mattress shapes and numbers and types of mattress pieces that could be devised, any step deck having an upper deck portion and a recess defined by a bottom deck portion and walls connecting the bottom and the upper deck portions would achieve the desired results. Likewise, any mattress or combination of mattress pieces that provide a bottom surface generally

conforming to the shape of the step deck would achieve the desired results.

It is therefore an object of the present invention to provide a mattress for use on a patient-care bed. The patient care bed has a longitudinally extending patient-support deck provided with longitudinally extending raised upper deck portions extending along its sides. The mattress comprising a longitudinally extending central body portion to be placed on said support deck and lateral portions to be placed upon said upper deck portions. The central body portion has a depth greater than the depth of the lateral portions.

It is another object of the present invention to provide such a mattress having a deflatable foot portion. The mattress comprises a generally planar patient surface and head, seat, thigh, and foot portions. The foot portion is deflatable and has a first length when deflated and a second length when inflated such that the second length is greater than the first length. In addition, the foot portion can be configured to have a first height when deflated and a second height when inflated, the second height being higher than the firεt height.

In preferred embodiments, the foot portion of the mattress is inflated when the foot section is in the up position. When the foot portion of the mattress is inflated, the sleeping surface of the mattress adjacent to the foot section is generally planar with the sleeping surface adjacent to other sections of the deck. Illustratively, the foot portion of the mattress is automatically deflatable and inflatable and inflates and deflates as the foot section of the deck pivots between the up poεition and the down position.

Additional objects, featureε, and advantages of the invention will become apparent to those skilled in the art upon consideration of the following detailed

description of the preferred embodiments exemplifying the best mode of carrying out the invention as presently perceived.

Brief Description of the drawings

The detailed description particularly refers to the accompanying figures in which:

Fig. 1 is a perspective view of a chair bed in accordance with the present invention showing a side rail exploded away from the chair bed, head side rails and foot side rails positioned along longitudinal sides of the deck, and a swinging foot gate in a closed position;

Fig. 2 is a view εimilar to Fig. 1 showing the chair bed in the sitting poεition having a head section of an articulating deck moved upwardly to a back-support position, a thigh section of the deck inclined slightly upwardly, a foot section of the deck moved to a generally vertical downwardly extending down position, a foot portion of the mattress being deflated, and swinging gates moved to an open position with one swinging gate folded next to the chair bed;

Fig. 3 is a diagrammatic view of the chair bed of Fig. 1 showing the chair bed in a bed position including a mattress having an upwardly-facing sleeping surface held a predetermined first distance above the floor, the deck being in an initial poεition εupporting the εleeping surface in a generally planar configuration, and the foot section being a first length;

Fig. 4 is a diagrammatic view showing the chair bed in a low position;

Fig. 5 is a diagrammatic view showing the chair bed in a Trendelenburg position;

Fig. 6 is a diagrammatic view showing the chair bed in a reverse Trendelenburg position;

Fig. 7 is a diagrammatic view showing the chair bed in an intermediate position having a head end of a head section of the deck pivoted slightly upward from the initial position of the deck, a seat section positioned to lie in the horizontal plane defined by the seat section in the initial position of the deck, and the foot εection being inclined slightly so that the foot end of the foot section lies below the position of the foot section when the deck is in the initial position of the deck; Fig. 8 is a diagrammatic view showing the chair bed in a εitting or chair poεition with the head end of the head section pivoted upwardly away from the seat εection to a back-support position, the seat section lying generally horizontal aε in the initial deck poεition, the thigh section being raised upwardly, the foot section extending downwardly from the thigh εection and being a εecond εhorter length, and the portion of the mattreεs over the foot section being deflated;

Fig. 9 is a perspective view of a first embodiment of a step deck and a mattress in accordance with the present invention;

Fig. 10 is a sectional view taken along line 10- 10 of Fig. 9 showing the bottom of the step deck beneath the projection; Fig. 11 iε an exploded perεpective view of a εecond embodiment of a εtep deck and the mattress of the chair bed;

Fig. 12 is a εectional view taken along line 12- 12 of Fig. 11 of the εtep deck and the mattress and showing a C-arm (in phantom) for holding medical equipment such as fluoroscopic equipment;

Fig. 13 is an exploded perspective view of a third embodiment of the mattress and the deck showing the foot section of the deck and the foot portion of the attreεε in a minimized condition having the foot section

of the deck contracted and the foot portion of the mattress contracted longitudinally and deflated so that the foot portion of the mattress is thinner and shorter than when foot portion is inflated; and Fig. 14 is a diagrammatical view further illustrating how the surface foot section retracts or shortens and collapses or thins as the bed moves from the bed position to the chair position.

Detailed Description of the Illustrative and Preferred Embodiments

A chair bed 50 in accordance with the present invention having a head end 52, a foot end 54, and sideε 56, 58 is illustrated in Figs. 1 and 2. As used in this description, the phrase "head end 52" will be used to denote the end of any referred-to object that is positioned to lie nearest head end 52 of chair bed 50. Likewise, the phrase "foot end 54" will be used to denote the end of any referred-to object that is positioned to lie nearest foot end 54 of chair bed 50.

Chair bed 50 includes a base module 60 having a base frame 62 connected to an intermediate frame module 300 as shown in Fig. 1. An articulating deck/weigh frame module 400 is coupled to intermediate frame module 300. Side rail aεεemblieε 800, 802, 804, 806 and an extended frame module 610 having a εwinging foot gate 622 are coupled to articulating deck/weigh frame module 400. A mattress 550 is carried by articulating deck/weigh frame module 400 and provideε a sleeping surface or support surface 552 configured to receive a perεon (not εhown) .

Chair bed 50 can be manipulated by a caregiver or by a person (not shown) on sleeping surface 552 using hydraulic εyεtem module (not shown) εo that mattreεε 550, an intermediate frame 302 of intermediate frame module 300, and an articulating deck 402 of articulating deck/weigh

frame module 400 assume a variety of positionε, εeveral of which are εhown diagrammatically in Figε. 3-7.

Articulating deck 402 includeε a head section 404, a seat section 406, a thigh section 408, and a foot section 410. Mattresε 550 reεtε on deck 402 and includes a head portion 558, a seat portion 560, a thigh portion 562, and a foot portion 564, each of which generally corresponds to the like-named portions of deck 402, and each of which is generally asεociated with the head, seat, thighs, and feet of the person on sleeping surface 552. Details of deck 402 and mattreεε 550 will be explained hereinafter. Chair bed 50 can assume a bed position having deck 402 configured so that εleeping εurface 552 iε planar and horizontal, defining an initial position of deck 402 as shown in Fig. 1 and as shown diagrammatically in Fig. 3. In the bed position, εleeping εurface 552 is a predetermined first diεtance 566 above the floor. Chair bed 50 can alεo be manipulated to assume a low position shown diagrammatically in Fig. 4 having deck 402 in the initial position and having εleeping surface 552 a predetermined second diεtance 568 above the floor, the second distance 568 being smaller than first distance 566. The foot section 410 of the articulating deck 402 has a first length 465 when the deck 402 is in the initial position.

Chair bed 50 can be moved to a Trendelenburg poεition εhown diagrammatically in Fig. 5 having deck 402 in a planar configuration and tilted εo that head end 52 of εleeping εurface 552 iε poεitioned to lie cloεer to the floor than foot end 54 of εleeping surface 552. Chair bed 50 can also achieve a reverse Trendelenburg position shown diagrammatically in Fig. 6 having deck 402 in a planar configuration and tilted so that foot end 54 of sleeping surface 552 is poεitioned to lie cloεer to the floor than head end 52 of sleeping εurface 552.

Aε deεcribed above, chair bed 50 iε convertible to a sitting position shown in Fig. 2 and shown diagrammatically in Fig 8. In the sitting position, head end 52 of head section 404 of deck 402 iε pivoted upwardly away from intermediate frame 302 to a back-εupport position providing a pivotable backrest so that head section 404 and intermediate frame 302 form an angle 512 generally between 55 and 90 degrees. Seat section 406 of deck 402 is positioned to lie generally horizontally as in the initial position, foot end 54 of thigh section 408 iε slightly upwardly inclined, and foot section 410 of deck 402 extends generally vertically downwardly from thigh section 408 and has a length 464 that is εhorter than when deck 402 iε in the initial position. Foot portion 564 of mattress 550 is inflatable and is in a deflated condition when chair bed 50 is in the sitting position. Foot portion 564 of mattress 550 is thinner and shorter when deflated than when inflated.

Chair bed 50 iε capable of assuming positionε in which head, thigh, and foot εectionε 404, 408, 410 of deck 402 are in positions intermediate to those shown in Figs. 3 and 8. For example, chair bed 50 can assume an intermediate position shown diagrammatically in Fig. 7 having head end 52 of head section 404 of deck 402 pivoted slightly upwardly from the initial poεition, εeat εection 406 positioned to lie in the same generally horizontal plane as in the initial position, foot end 54 of thigh section 408 raised slightly upwardly from the initial position, and foot section 410 being inclined so that foot end 54 of foot section 410 lies below head end 52 of foot section 410.

Additionally, articulating deck 402 of chair bed 50 is configured as a εtep deck 412 as εhown illuεtratively along with illuεtrative εtep mattress 550 in Figs. 9, 10, and 11-13. The step deck and mattress of Figs. 11-13 are

those illustrated in Figε. 3-8. Step deck 412 includeε an upper deck 414 and a central, longitudinally extending receεε 456 defined by a lower deck 430 of step deck 412 and a wall 438 surrounding recess 456 and connecting lower deck 430 to upper deck 414. Upper deck 414 includes longitudinally extending upper deck side portionε 417, a head end upper deck end portion 416, and a foot end upper deck end portion 460.

Mattreεε 550 includeε a generally upwardly-facing sleeping surface 552 and a bottom surface 586 that is generally parallel to εleeping εurface 552 and that iε positioned to lie beneath sleeping εurface 552. A perimetral side 578 connects sleeping surface 552 and bottom surface 586. A projection 576 iε appended to bottom surface 586 and extends downwardly therefrom. Preferably, projection 576 is spaced-apart from sideε 578 of mattreεε 550 and neεtε in recess 456. Projection 576 may engage wall 438 of step deck 412 to prevent movement of mattress 550 relative to step deck 412 and to maintain the generally central position of mattress 550 on deck 412.

Preferably, mattresε 550 iε provided with a thick zone 582 adjacent to receεε 456 and projection 576, and a thin zone 580 engaging upper deck 414 aε εhown in Fig. 10. For example, thick zone 582 can be one and one-half times the thickness of thin zone 580. In one preferred embodiment, the thick zone is approximately 7 1/2 inches (19 cm) thick and the thin zone is 5 inches (12.7 cm) thick. Thick zone 582 is positioned to carry the majority of the weight of a person (shown in phantom) εupported on sleeping surface 552 to maximize the comfort of the perεon. In contraεt, perimetral thin zone 580 provideε a perimetral portion of mattreεε 550 that appears to the person on sleeping surface 552 to be firmer than thick zone 582, facilitating entry onto and exit from sleeping surface 552 along sides 578 of mattresε 550.

Aε can be εeen, εtep deck 414 and mattress 550 can be used in many applications requiring a εupport surface for supporting a person. For example, step deck 414 and mattress 500 can be configured for use as a stretcher to be carried by caregivers and as a gurney having step deck 414 mounted on a frame with wheels for transporting the person εupported by the gurney.

Articulating deck 402 iε the εurface upon which the mattreεε 550 reεtε as εhown in Figε. 11-13. Deck 402 iε illuεtratively segmented into head, seat, thigh, and foot sectionε 404, 406, 408, 410, three of which, head section 404, thigh section 408, and foot section 410, may be rotated to change the angle of inclination of the back, thighs, and lower legε of the person (not shown) with reεpect to seat section 408. Seat εection 406 of deck 402 remainε horizontal and the head, thigh, and foot εectionε 404, 408, 410 of deck 402 can move relative to the εeat section 406 and relative to each other, thereby moving the head, thigh, and foot portions 558, 562, 564 of mattresε 550 relative to εeat portion 560 of mattresε 550 and relative to each other.

The head, εeat, thigh, and foot εections 404, 406, 408, 410 of articulating deck 402 cooperate to define step deck 412 aε εhown in Figs. 11-13. Step deck 412 includes an upper deck 414 having a head end upper deck portion 416 appended to head end 52 of head section 404, side upper deck portions 418, 420, 422, 424, 426, 428 appended to sideε of the head, εeat, and thigh εectionε 404, 406, 408, and a foot end upper deck portion 460 appended to foot end 54 of weigh frame 506 adjacent to thigh εection 408. The upper deck portionε 416, 418, 420, 422, 424, 426, 428, 460 and a top εurface 411 of foot εection 410 are coplanar when articulating deck 402 iε in the initial poεition and cooperate to form upper deck 414 which is generally parallel to weigh frame 506.

Step deck 412 alεo includeε a lower deck 430 having a head εlat 432, a seat slat 434, and a thigh slat 436. Head, εeat, and thigh εlats 432, 434, 436, are coplanar when articulating deck 402 is in the initial poεition and they cooperate to form lower deck 430 which iε generally parallel to weigh frame 506 and to upper deck 414 when articulating deck 402 iε in the initial position.

Lower deck 430 is connected to upper deck 414 by a wall 438 including a head end wall 440 connecting head slat 432 to head end upper deck portion 416, side wallε 442, 444, 446, 448, 450, 452 connecting head, εeat, and thigh εlats 432, 434, 436 to side upper deck portions 418, 420, 422, 424, 426, 428, and a foot end wall 454 connecting thigh εlat 436 to foot end upper deck portion 460 aε εhown in Fig. 11. Step deck 412, then, compriεeε upper deck 414 and is formed to include a central, longitudinally extending recess 456 defined by lower deck 430 and by wall 438 connecting lower deck 430 to upper deck 414. In the preferred embodiment, foot section 410 of εtep deck 412 iε diεplaced from receεε 456 and formε part of upper deck 414, as shown in Figs. 11 and 13.

Mattresε 550 iε received by articulating deck 402 and includeε a projection 576 sized to be received by recesε 456 aε εhown in Figε. 11 and 12. Conεequently, mattress 550 is thinner along εideε 580 of mattreεε 550 where mattress 550 engages upper deck 414 of step deck 412. Conversely, mattress 550 is thicker in portions adjacent to projection 576. Preferably, projection 576 iε positioned directly beneath portions of mattreεε 550 carrying a majority of the weight of the person on sleeping surface 552. The thick portion of mattreεs 550 including the thickness of mattress 550 between sleeping surface 552 and a bottom surface 586 engaging upper deck 414 plus the thickneεε of projection 576 provideε greater comfort for the perεon on εleeping εurface 552. Mattress 550, then.

has a thinner perimetral zone 580 and a thicker body- support zone 582 adjacent to projection 576. Preferably, body support zone is 1% times the thicknesε of perimetral zone 580. For example, perimetral zone can be 5 incheε (12.7 cm) thick and body-εupport zone 582 can be 7% inches (19 cm) thick.

Thinner perimetral zone 580 and upper deck side portions 417 cooperate to define "rammed" edges that provide greater firmnesε around the edges of sleeping surface 552 as the result of sleeping surface 552 being in closer proximity to upper deck 414. This increased firmness is advantageous when the perεon enters and exits the bed along the sides of the bed.

Projection 576 includes a side wall 584 that can be configured to engage at least portions of the wall 438 of step deck 412 as shown in Fig. 12, thereby preventing lateral and longitudinal εliding of mattreεε 550 relative to step deck 412. Also, mattress 550 includes sideε 578 connecting εleeping surface 552 and bottom εurface 586. Mattress 550 and step deck 412 are configured εo that sideε 578 of mattreεs 550 are expoεed above deck 402 as shown in Figs. 11 and 12 providing the caregiver greater and easier access to mattress 550, rather than engaging a portion of a frame or upstanding walls of a deck as is found with conventional mattress and deck systems.

In preferred embodiments, sleeping surface 550 is generally planar and projection 576 is centrally located beneath sleeping surface 550 to form thick body εupport zone 582 of mattreεs 550 surrounded by perimetral zone 580 engaging upper deck 414. Mattress 550 may be provided in more than one piece, for example, mattress 550 may comprise a first mattresε piece fit into recess 456 and a second mattresε piece εurrounding and abutting εideε of the first mattresε piece and engaging upper deck 414, or a firεt mattress piece could fit into recess 456 and a second

mattreεε piece having a planar bottom εurface could fit over the first mattreεs piece so that the bottom of the second mattresε piece engageε the first mattress piece and upper deck 414. However, a one-piece mattress 550 including both body-support zone 582 and perimetral zone 580 is preferred.

Additionally, mattresε 550 can include an inflatable portion 574 that can aεεume both an inflated poεition and a deflated poεition. Preferably, inflatable portion 574 is positioned to lie in foot portion 564 as shown in Fig. 13 so that inflatable portion 574 can be inflated to serve as sleeping surface 552 when foot section 410 of deck 402 is in the up position and so that inflatable portion 574 can be deflated and inclined downwardly when the foot εection 410 iε lowered to the down position to provide room for the lower legs of the person when chair bed 50 iε in the sitting poεition. Foot portion 564 is thinner and shorter when deflated than when foot portion 564 is inflated. Foot portion 564 of mattreεε 550 and foot εection

410 of articulating deck 402 cooperate to minimize the length of the foot of chair bed 50 as shown in Fig. 13. Foot section 410 and foot portion 564 are a first length 465 when foot section 410 is in the up position and a second length 464 when foot εection 410 is in the down position, first length 465 being greater than second length 464. Also, foot portion 564 is a firεt thickness 608 when foot section 410 is in the up position and a second thickness 609 when foot section 410 iε in the down position, first thickneεε 608 being greater than second thicknesε 609.

In addition, the width 604 of foot portion 564 of mattress 550 is leεε than the width 606 of head portion 558 of mattreεε 550, the width 606 of head portion 558 typically being a standard mattress width as shown in Figs.

11 and 13. This difference between the widths 604, 606 permits a standard fitted εheet (not εhown) to be tightly installed onto mattresε 550 while remaining looεe adjacent to foot portion 564 εo that pressure relief can be maintained in the section of foot portion 564 receiving the heels (not shown) of the person (not εhown) εupported on sleeping εurface 552.

In preferred embodimentε, deflatable foot portion 564 includes a first set of air bladders 1618 and a second set of air bladders 1620 alternately poεitioned with air bladderε 1618 aε εhown in Fig. 14. Air bladders 1618 and 1620 are configured to collapse to a near zero dimension when air is withdrawn from the bladderε 1618 and 1620. The firεt εet of bladderε 1618 are oriented to collapεe in a first direction which is generally parallel to the foot εection 410 of deck 402 as illustrated by double headed arrow 1622. The second set of bladders 1620 are configured to collapse in a second direction generally perpendicular to the foot deck section 410 as illustrated by double headed arrow 1624. This orientation of bladderε 1618 and 1620 in foot εection 1502 advantageouεly causes the foot section 1502 to contract or shorten and to collapse or thin as the bladders 1618 and 1620 are deflated aε foot εection 410 moves from the up position to the down position. In the εitting position, foot section 410 and foot portion 564 of mattress 550 move from a generally horizontal position to a generally vertical, downwardly extending position.

Foot portion 564 deflates aε it moveε from the up position to the down position in the direction of arrow 1626 as shown in Fig. 14. In the up position, foot portion 564 has a length of about 27 inches (68.6 cm) and a thicknesε of about 5 inches (12.7 cm) when the bladderε 1618 and 1620 are fully inflated. When in the down poεition illustrated at location 1628 in Fig. 14, foot portion 564 is fully deflated and has a length of about 14

incheε (35.6 cm) and a thickneεε of preferably leεε than one inch (2.54 cm). The length of the εurface foot section is preferably reduced by at least 40% and the thickneεε of the εurface foot section is preferably reduced by at least 80% as foot portion 564 moves to the down poεition. The width of the surface foot portion 564 remainε εubεtantially the same in both the up poεition and the down poεition. Although the invention haε been deεcribed in detail with reference to preferred embodimentε, variations and modifications exist within the εcope and εpirit of the invention aε described and defined in the following claims.