In vitro assessment of shear force buffering in sacral dressings to alleviate pressure injuries in bed-based patients

To characterise and compare the effectiveness of sacral dressings at alleviating the transfer of shear forces to the skin and, therefore, to lower the risk of and prevent pressure injury (PI) development in bed-based patients.

Dressings were evaluated against a ratio of applied to transferred shear and peak shear force observed on the skin. The evaluation was undertaken using a custom benchtop in vitro experimental setup replicating the skin-bedsheet interface. Forces were applied using an automated test rig within which a pair of load cells (ATI Nano-17; ATI Industrial Automation Inc., US) were embedded, tracking the applied normal and shear forces. Normal loading regimes of 11.1N (6kPa) and 14.8N (8kPa) were applied, to match expected clinically relevant sacral pressures. Pressures were applied with an error of 2.3% and 0.6%, respectively. No significant difference was found in the loading of dressings (p>0.05).

The performances of three dressing designs were assessed, with six samples taken for each. All dressings were found to improve significantly (p<0.001) in performance relative to the control dataset for the dynamic coefficient of friction (DCoF), peak shear at skin (Fτ) and shear ratio (τratio). The Allevyn Life dressing (AllLi; Smith+Nephew, UK) showed the lowest DCoF (0.372±0.052), which was significantly lower than all other dressings (p<0.001). There was no significant difference between Mepilex Border (MepB; Mölnlyke Health Care, Sweden) and Avarus Border Foam (AvBF; Medtrade Products Ltd, UK). AllLi recorded the lowest peak shear at the skin (1.01±0.07N), which was significantly lower (p<0.001) than that measured for MepB and AvBF. With regard to the shear ratio, AllLi (0.233±0.007 and 0.283±0.013) and MepB (0.255±0.011 and 0.275±0.013) exhibited a significantly (p<0.001) higher shear ratio at both pressures, respectively, than AvBF.

Although significant differences were identified between dressing types, DCoF was not indicative of dressing performance. Dressing compression, rather than thickness, was indicative of peak shear buffering. All dressings reduced the shear observed at the skin with respect to the control. The reduction in shear ratio between the control and all dressings tested was significant, ranging from 0.528-0.567 at 6kPa and 0.510-0.560 at 8kPa. Prophylactic use of these types of dressings to reduce risk of PI (widely specified within hospital and care setting protocols) confirms that this benefit is seen in the clinical setting. However, while there was a significant difference between the three dressing types (p=0.039 at 6kPa and p=0.050 at 8kPa), further work is required to explore how this translates to a clinical setting. Shear buffering is a complex process and performance is dependent on the compound response of the constituent dressing elements, rather than being dominated by a single component.