Comparative effects of posture on pressure and shear at the body-seat interface

Abstract

Abstract-This study considers the effects of seated posture and body orientation on the pressure-distribution and surface shear (tangential) forces acting at the bodyseat interface. Nine postures typically assumed by wheelchair users were studied. Comparisons were made within and between two study groups, made up of 12 subjects with spinal cord injuries (SCI) and 10 nondisabled subjects. Both interface pressure and the surface shear were measured simultaneously in each of nine reproducible, seated postures. The same seat cushion was used for all trials. The Oxford Pressure Monitor, a pneumatic cell device, was used to measure and record the interface pressures. Instrumentation for measuring and recording the surface shear force was constructed specifically for the study. Analysis consisted of statistically comparing changes in pressure values and shear forces derived from eight sitting postures with reference to values recorded in a defined neutral sitting posture. The pressure-distribution findings suggest that in the postures studied SCl subjects have maximum pressures that are higher than nondisabled subjects in all postures, ranging from 6% to 46% depending on the posture. Maximum pressures can be reduced by postural changes: forward flexion to 50°, -9%; backrest recline to 120°, -12%; and, full body tilt, -11 %. On average, the §GI group members have peak pressure gradients (PPG) that are 1.5 to 2.5 greater than the nondisabled group. The maximum reduction in PPG occurred at backrest recline of 120°, -18%. only recline of 20" causes a 25% increase in the surface shear force. These results suggest that caution must be taken when using nondisabled subjects as surrogates for people with SCI because of the inherent differences between the groups. Also, researchers and clinicians should recognize that posture and body orientation in space are additional variables that can have a profound effect on the interaction between a seated person and his or her supporting surface.

A survey of wheelchair use by paraplegic individuals in Japan. Part 1: Characteristics of wheelchair cushions

“…Figure 1 shows characteristic con®gurations of these three ready-made wheelchair cushions. The polyurethane foam cushion had a medium density 10 and a¯at sitting surface. The silicone gel¯otation consisted of a 5 cm thick silicone gel pad sandwiched between polyurethane foam pads.…”

The QA Pressure Measurement System: an accuracy and reliability study

“…These data were tested for (intra-observer) reliability with an ICC (3,1) . With the exception of the 2 kg weight (ICC (3,1) =0.56 [95% CI=0.47 ± 0.64]), all reliabilities were good (ICC (3,1) 40.75), mean ICC (3,1) =0.85 (95% CI=0.74 ± 0.95), range: 0.56 ± 0.93 (95% CI=0.47 ± 0.95). The mean of all mean-pressures was 7.5 (+4.9) mmHg and the mean of all maximum (peak-) pressures was 20.6 (+7.9) mmHg (see Table 3).…”

“…31 We performed ICC (3,1) calculations as measure of intrarater reliability for the calibration measurements and ICC (2,1) calculations as measure of inter-rater reliability for both studied groups. Portney and Watkins 32 de®ned an ICC (2,1) 40.75 to be good and an ICC (2,1) 50.75 to be poor/moderate.…”

“…Hobson 3 used an Oxford Pressure Monitor to study the e ects of di erent sitting positions on the pressures at the body-seat interface of normal subjects and SCI patients. In accordance with the results of our study, Hobson found that, whilst the measured average pressures varied little between both groups, the SCI patients had signi®cantly higher maximum pressures than non-disabled subjects.…”

“…2 ± 6 Shear-force is the second etiologic factor. 2,3 Other secondary etiologic factors in the formation of decutibus ulcers are; tissue properties including temperature, moisture, tissue viability, age and hygiene, 4,5 nutritional status, 7,8 general health/clinical status, activity level, posture and deformities 7 and psychological factors. 7,9 Additional contributing factors in the pathogenesis of decutibus ulcers are; decreased or absent sensibility, 4,10,11 spastic or¯accid paralysis, 4,10 coma and/or the (over)use of sedative drugs and edema.…”