Examples of Shaped Seat Surface Designs

Designs for specific clinical requirements can now be developed using the principles described in the sections above. The work in the previous chapters could lead us to suggest that there is a unique optimum seat design for each subject, depending on his or her seating requirements and body characteristics. However shaped seat cushioning is aiso produced in standard shapes with contours and 'cut outs' over regions vulnerable to pressure sores. Such cushions are probabiy better than flat foam cushions for the large group of chairbound people who require comfort and postural support from their cushion and who are not at high risk of pressure sores. Inevitabiy the envelopment of the buttocks and the extent of lateral support provided by standardised cushions are less than that which could be provided by individualised designs. Computer-aided manufacture is still a quick, automated method of producing contoured surfaces for such cushions. To be abie to adjust the shape and size of the cushions to fit an individuai, computer aided design wouid be required.

The shape design of ‘off-the-shelf seat cushions requires detaiied information of the anthropometry and weight of the chair-bound population. Data is available on the width of the hips, distance from the back of the buttock to the back of the knee and distance apart of the tips of the ischial tuberosities whilst sitting (Diffrient et al., 1974; Chung & Weimar, 1988; Harris & Mayfield, 1983). The information has been tabulated in Appendix 15.

The distance between the back of the seat and the back of the buttocks varies over several centimeters depending on the seating circumstances of an individual. With ‘standard’ shaped seating a strip could be cut off the back of the standard surfaces to achieve the necessary dimensions. Examples of customised and ‘off-the-shelf designs are described and sketched in Figure 40. Typical dimensions for the surfaces are shown in Appendix 16.

Shaped Supporting Surface to Improve Comfort and to Control Pelvic Position

This supporting surface design would be useful for people who have poor control of the position of their peivis on the seat or who are uncomfortable during sitting due to excessive loading of the ischiai or sacrai regions of the buttock. This design would be effective if it were available in a range of standard sizes and shapes. To ailow for body motion during sitting, the contours wouid be quite shaliow (for exampie, 40 mm deep) and ‘open’. It would not be idea for peopie who are at high risk of pressure sores or who have a pronounced postural asymmetry during sitting that could not be reduced via a seating intervention.

Shaped Supporting Surface to Reduce the Risk of Pressure Sores around the Pelvis

This supporting surface design would be useful to someone who has developed a pressure sores over the left buttock, as a result of an oblique pelvis causing weight bearing through the left buttock and who is not able to move around in their seat. It is designed to contain the soft tissues in the gluteal region of the buttock and to minimise the load supported through the left ischium. The pelvic obliquity is neither increased or corrected by the seat. This design would need to be customised to the body characteristics of each client. The surface geometry in Figure 40 is based on the shape of the able-bodied female who was measured in the early chapters of this thesis.

limited posterior support to prevent the pelvis being pushed anteriori

slope to prevent the body slipping fonvard

localised 'cut-out' at region which is known to be at risk of pressure sores

eg. left ischial tuberosity

thigh supports for w eight bearing

'cut-out' to m inim ise w eight bearing through sacrum

allow lateral contours to allow for variations in the lateral position of the pelvis on the seat

highly contoured aroi ischial 'cut out' to preve a change in pelvic orientatidi

'cut-out' to m inim ise w eight bearing through

sacrum

S i

00 Seat Surface to Prevent Sliding Off the Front of the Seat and a Postero-lnferior Rotation of the Dorsal Surface of the Sacrum

postero-lateral support for soft tissue co ntainm ent an d w eight bearing in gluteal region

support in hip region to

p revent discomfort and pressure sores

Individualised Seat Surface for Pressure Sore Risk Reduction

localised 'cut outs' in regions of pain or pressure

so re risk.

planar surface to allow body to m ove freely on

the seat

shallow 'open' contours to allow for m ovem ent and for som e

variations in body size

thigh supports for weight bearing an d control of

position of th e thighs

'cut-out' to m inim ise weight bearing throu

sacrum

Seat Surface for Localised Reductions in Weight Bearing through the Bony Prominences and to Allow Full Motion of the Body on the Seat

FIGURE 40

jo stero -late ra l support for soft tissue co ntainm en t, w eight bearing an d position

Standard Seat Surface for control

Shaped Supporting Surface to prevent the Body Moving Forward in the Seat and /or an Anterior Pelvic Rotation

This supporting surface design would be useful to people who have a pronounced posterior pelvic tiit which is causing them to slip forward on the seat. It would be used in conjunction with a sacral pad, so that correcting forces would be supplied through the proximal thigh and sacral pad to force the hips into flexion and the pelvis into a position where its dorsai surface has iess of a postero-inferior orientation. Also such designs would have to be well tested to ensure the client would not slip forward onto the thigh support.

Shaped Supporting Surface to Allow Full Motion of the Body on the Seat

This design would be useful to someone who needs to and is able to move around in their seat, but is uncomfortable due to load bearing through one or both ischial tuberosities or through the sacrum. It could be designed with ‘cut outs' over one or more of the the bony prominences. By keeping the ‘cut-outs' shallow and of small area, there is a minimal reduction in contact area and there is unlikeiy to be a change in pelvic orientation due to the ‘cut-outs'. This design would be most effective if it were available in severai sizes.

The designs to meet particular clinical needs could be produced as prototypes. It would then be necessary to test them extensively.