Decubitus ulcers are major complications of tissue necrosis associated with spinal cord injury and the elderly who suffer from diminished pain sensation and /or diminished mobility. There is evidence in the literature that paraplegics have higher incidence of pressure ulcer than tetraplegics, and this prompted us to investigate the interface pressure redistribution during dynamic wheelchair propulsion. This study was composed of two parts. The first part aimed to investigate the movement of the ischial tuberosities (IT) and the redistribution of interface pressure during manual wheelchair propulsion. In the second part of the study, post-occlusive hyperaemia after three different loading regimes, namely (static) pressure only;​ combination of pressure and shear, and (dynamic) combination of pressure and cyclic shearing, were evaluated. Repeated measures ANOVA were used to compare the measured parameters in these experiments.

In the first part of the study, the three dimensional position of the pelvis was measured with an optical motion analysis system (Vicon 370, UK). Interface pressure was measured by a thin, flexible pressure mat (Tekscan, USA). Spatial resolution of the pressure mat was 3.2 sensors/cm2. The two systems were synchronized to capture data at 60 Hz. The participants were measured in a manual wheelchair with a rigid seat support. Propulsion cycles were performed on a stationary wheelchair ergometer at the subject’s maximum propulsion speed. The results indicated that during wheelchair sprinting, the predicted position of the ischial tuberosity based on the pelvic markers did not concur exactly with the corresponding peak pressure locations as measured by the pressure mat. The mean difference was 23 mm and 17 mm for the normal group (n=lO) and the wheelchair user group (n=12) respectively. Moreover, between-group differences of these location discrepancies were not statistically significant. The anterior-posterior rocking of the pelvis was found to be S.2 degrees for the wheelchair user group, whereas for the normal group, such pelvic rocking during propulsion was 11.2 degrees on average. In terms of average pressure over the ischial tuberosities, significant difference was found between the two subject groups during dynamic propulsion (P=0.04). It was also revealed that depending on the area selected for calculation, the average pressure values could vary significantly. In addition, two new pressure indices were introduced to reflect the interface pressure condition during static and dynamic sitting. These indices were the ratio of dynamic peak pressure to static peak pressure and dynamic average pressure to static average pressure values. These indices were shown to be significantly different between the two subject groups. The wheelchair user group showed indices closer to unity, indicating that dynamic movement could not provide much pressure relief for the affected tissues. The movements of the ischial bone and the cyclic loading imposed on the tissue underneath the “ischial tuberosities” during dynamic conditions may have implications to the etiology of decubitus ulcers.

In the second part of the study, tissue reperfusion after loading was measured using a laser Doppler perfusion imager (Moor Instrument, UK). Three different loading sites were evaluated including the forearm, the greater trochanter and the ischial tuberosity. Spatial hyperaemic changes in blood perfusion were characterized in terms of their magnitude, half-life, as well as the perfusion-time integral (total hyperaemia). The results indicated that resting skin blood flow at the trochanter area was found to be higher in the normal group as compared to the wheelchair user group. Experimental results obtained after loadings indicated that there were significant differences in the post-occlusive hyperaemic responses among the three different loading regimes. The peak and total hyperaemia after dynamic loading with pressure and cyclic shear in normal subjects were found to be significantly larger than those after the other loadings. The half-life of the hyperaemic response was found to be largest after dynamic loading. For active wheelchair users, the peak hyperaemic responses under the three loading conditions were found to be similar. However, both half-life and total hyperaemia were significantly increased when shear was included in the loading regimes. No significant difference in the post-occlusive hyperaemic responses could be found between the normal and the wheelchair user group.

Experimental results suggested that combination of pressure and shear stress could cause a higher distress to tissue than with pressure alone. It was also found that dynamic loading with cyclic shear could cause high distress to the involved tissue, which apparently required a prolonged period to recover.