AMERICAN SOCIETY OF BIOMECHANICS
Presented at the Twenty-First Annual Meeting
of the American Society of Biomechanics
Clemson University, South Carolina
September 24-27, 1997
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| AMERICAN SOCIETY OF BIOMECHANICS
Presented at the Twenty-First Annual Meeting |
Seat interface pressure assessment is of interest to both researchers and clinicians since 25-85% of all spinal cord injured (SCI) develop pressure sores accounting for over 2.3 million Medicare hospital days in 1987 resulting in 1.1 billion dollars in direct insurance costs (Dinsdale, 1974; Staas & Cioschi, 1991). While both intrinsic and extrinsic factors are associated with the etiology of pressures sores, pressure is thought to be one of the key extrinsic factors in sore development (Reswick & Rogers, 1976). Currently, seat interface pressure distributions are measured statically in a clinical environment with the patient in a fixed position if measured at all. It is likely that the seat interface is loaded differently throughout the day with different activities of daily living (ADL's). One of the most common ADL skill utilized by paraplegics to move from one place to another is wheeling.
There is a general paucity of literature on the dynamic assessment of seat interface pressures. Few studies examined reaching tasks (Park, 1992; Swarts et al., 1988), the average pressures over time (Fisher & Patterson, 1983; Patterson & Fisher, 1986), and the average pressure with various cushions (Bar, 1991). The last three papers addressing the average pressures collapsed the discrete pressure values over a fixed time period causing the time dependent fluctuations in the pressure values to be lost. Currently, the only published study on assessing the dynamic pressures examined two able bodied participants on a specially designed wheelchair seat while wheeling (Eckrich & Patterson, 1991). The study was notable since it was the first to demonstrate the difference between static and dynamic seat interface pressures but lacks the external validity due to the participants selected for study and that no seat cushion was used as an interface. Eckrich & Patterson (1991) even claimed that " the effect of these dynamic pressure changes may act in a manner similar to a 'wheelchair pushup' supporting the vascular and lymphatic pumping mechanisms" (p. 121) based on their results. Due to this claim, indeed a further investigation of the dynamic assessment of seat interface pressures is warranted. Therefore, the purpose of the study was to investigate the peak pressure and pressure time integral during static seating in comparison to the ADL skill of wheeling of a group of SCI participants.
A convenient sample of fifteen patients which propelled a manual wheelchair for at least 5 hours per week over the past 6 months and functioned at a neurological level of T1 or below were used in this study. The Novel Pliance SystemTM which consisted of a flexible, 32 X 32 capacitive sensor mat (each sensor 1.5 cm2) interfaced with a PC was sampled at 10 Hz was used to measure seat interface pressures. The mat was calibrated by homogenous air pressure throughout the measurement range prior to collection. The participants were measured in their own wheelchair with a new Jay Active seat cushion prescribed for each participant. The order of measurement was randomized (static vs. dynamic) and the pressure sensitive mat was placed between the patient and the cushion . To identify a wheeling cycle from the data, a hand switch interfaced with an LED on the chair was placed in the participant's right hand. When the participant made contact with the push rim, the switch would close illuminating the LED. From the Pliance SystemTM a synch pulse was discharged from the analyzer illuminated a second LED on the chair. These LED's were captured on videotape with a camcorder (30 Hz) to determine the onset (right hand on push rim) and termination (right hand on push rim for the second time) of a wheeling cycle. The dependent measures were peak pressure (PP) in N/cm2, pressure time integrals (PTI) in N/cm2(s. The PP was the peak pressure value that occurred from any single sensor during the static and dynamic trials. This PP value occurred in the region of one of the ischial tuberosities in all trials. The PTI was calculated by integrating the peak pressure time curve for each trial enabling a cumulative effect of the peak pressure to be calculated over a specific time period. Three consecutive wheeling cycles from four wheeling trials at a speed of 1.2m/sec ((10%) were averaged and compared to the mean from two static trials. Use of a photoelectric timing system monitored wheeling speed. Figure 1 depicts the mean and 95% confidence interval from a single trial of three consecutive cycles. Note, that PP varies by 40% from the peak to the minimum throughout the cycle. The minimum PP value even drops below the PP during static loading. Measurement time for the static trials were truncated to the length of time required to complete an average dynamic wheeling cycle for that participant. To determine differences between static and dynamic variables a single factor repeated measures multivariate analysis of variance (RM MANOVA) was used. Follow up dependent t-tests using the Bonferoni alpha was used in the event that a significant multivariate effect was found.
Figure 1 - Mean and 95% confidence interval for peak pressure of three consecutive cycles from a single wheeling trial.
The results of the RM MANOVA showed a significant difference in the PP and PTI between the static and dynamic measurements (Wilk's = 0.00, p < .05). Follow up dependent t-tests yielded a difference in PP between the static and dynamic trials (t=5.40, p < 0.025) and no difference in the PTI between static and dynamic trials (t= 1.45, p > 0.025). Table 1 depicts the mean and standard deviation for static and dynamic trials for the two dependent variables.
| Variable | Static | Dynamic |
|---|---|---|
| PP (N/cm2) | 1.62 (0.50) | 2.03 (0.66)** |
| PTI (N/cm2(s) | 3.01 (0.93) | 3.62 (1.81) |
n=15, **p<0.025
Table 1 - Means and standard deviations for peak pressure (PP) and pressure time integral (PTI) for static and dynamic trials.
The PP during static seating was less than during dynamic seat interface pressures during wheeling. However, the PP varied throughout the wheeling cycle by 42% to a PP below the static loading condition. The differences between static and dynamic loading were results were similar to the results found by Eckrich and Patterson (1991). As is often the case in pressure measurement research, the pressure values cannot be directly compared to this study due to the instrumentation utilizing differing sensor sizes. Since PP changes throughout the wheeling cycle, is this as damaging to tissue as the static loading? To answer, this we must recognize Brand's (1980) warning that we do not know enough about the etiology of pressure sores to speculate as to the direct causes of tissue breakdown. On the other hand, does this fluctuation in PP facilitate a sort of "pumping mechanism" as suggested by Eckrich & Patterson (1991)? Since there are other therapeutic techniques utilize external manipulation of the skin to stimulate blood and lymphatic activity, it seems logical that this fluctuation in PP throughout the cycle could promote this activity. The PTI was calculated to determine the impulsive load of the PP. Between the static and dynamic conditions the impulsive loading was similar. Thus the cumulative effect of the loading was similar while the rate and range were quite different. Further research will address how these loading variables are influenced by the use of various seat cushions as a medium for altering pressures at the seat interface.
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