Effect of Model Design, Cushion Construction, and Interface Pressure Mats on Interface Pressure and Immersion

Article excerpt

INTRODUCTION

Wheelchair cushions have been designed to reduce and distribute the mechanical forces applied to the skin and to decrease the chances of skin breakdown. Common designs use elastic foam, viscoelastic foam, elastomer, air, viscous fluid, or some combination thereof. A proper cushion should distribute pressure, minimize peak pressure at bony prominences, and encourage proper posture, as well as meet users' preferences in maintenance, comfort, and aesthetics [1]. With all of these factors in mind, a seating specialist or clinician may measure interface pressures (IPs) as one tool for selecting the optimal cushion [2].

To perform the measurement, a clinician places a pressure mat made up of an array of sensels between the client and the support surface. The pressure mapping software displays numerical values (often represented on a color scale) of IP at each sensel site. Clinicians can use these values, combined with other factors, to select wheelchair cushions for their clients.

For pressure mapping to be a useful clinical tool, the limitations of each system must be understood [3-4]. Several studies have reported on accuracy, creep, and hysteresis [5-11]. Environmental factors such as temperature and humidity have been observed to affect the accuracy of the sensors [4,7,12]. Other studies have found poor repeatability of some variables used to analyze IP data, especially around curved surfaces [1,9,13]. Peak pressure in particular is not recommended for use in analysis because of its lack of stability [1].

Additionally, the presence of pressure sensors may affect the environment and the pressure distribution being measured. One influence is hammocking, or bridging, support surface contours. In 1993, Ferguson-Pell and Cardi reported hammocking tests performed on the Tekscan seat system (both covered and uncovered) (Tekscan Inc; South Boston, Massachusetts), the Talley Pressure Monitor 3 (Talley Medical USA; Lansing, Michigan), and the FSA seat mat (Vista Medical Ltd; Winnipeg, Manitoba, Canada) with four cushions [6]. They used a buttocks-shaped loader gauge instrumented with three sensors to compare IP measurements taken with sensing mats at the model-cushion interface to those taken with no mat present. They found significant differences in IP for all cushions with the FSA and covered Tekscan mats present [6-7].

Our motivation for this study was to better understand the influences, if any, resulting from introducing a mat at the buttocks-cushion interface. IP measurement is a potentially useful tool that clinicians use to select cushions and researchers and manufacturers use to document cushion performance. With a fuller understanding of the mat effect, they can inform patients of proper use of these devices. Therefore, the objectives of this study were to explore changes in IP and immersion caused by addition of an IP mat using two buttocks models. The project tested the following hypotheses:

1. The presence of an IP mat will affect the IP measured between a buttocks model and wheelchair cushion.

2. The presence of an IP mat will change the immersion of a buttocks model when loaded on a wheelchair cushion.

3. The mats' effects on the cushion interface will differ across buttocks model designs.

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METHODS

We used two buttocks models to collect data. These models were based on the anthropometry of a person with a 36 cm bitrochanteric breadth and 11 cm ischial spacing. These dimensions and the overall size and shape were consistent with the current International Organization for Standardization (ISO) buttocks models used for wheelchair testing [14]. One model was rigid and fabricated from wood Figure 1), and the other was Elastack (Sutton Technologies Inc; North East, Maryland) thermoplastic, or gel, with embedded rigid cylinders to represent the ischial tuberosities ITs) (Figure 2). …