A quantitative analysis of pressure and shear in the effectiveness of support surfaces.

PURPOSE: We compared pressure and shear among 3 support surfaces in a controlled laboratory setting.

METHODS: In this comparative study, interface pressure and shear were measured in 11 healthy subjects with use of 3 different Medicare-approved, group 2 therapeutic support surfaces. Each surface was placed on 1 of 3 identical platforms, allowing for a fixed, 45-degree head-of-bed (HOB) elevation. Multiple pressure readings were taken at each test site and each HOB position until a measurement that was determined to be reliable was obtained. A shear force measurement was obtained using a noninvasive instrument of proprietary design, and a formula linking the effects of shear force and pressure was calculated. Data were then compared using analysis of variance; specific differences were identified using the post hoc Duncan test.

FINDINGS: Pressure readings on the nonpowered fluid overlay device were lower at the trochanter than when compared with the powered, air-filled overlay (23.7 mm Hg vs 36.7 mm Hg; P = .017). Both the nonpowered fluid overlay and the powered air-filled mattress provided pressure readings at the sacrum with HOB elevation that were lower than those recorded on the powered air-filled overlay (16.2 mm Hg and 19.6 mm Hg vs 27.5 mm Hg, respectively; P < .001). Pressure measurements taken at the heels, supine sacrum, and scapula were not significantly different for any of the surfaces. Shear force readings were significantly lower on the nonpowered fluid overlay when compared with the powered air-filled overlay and powered air-filled mattress (79.3 g vs 257.82 g and 231.38 g, respectively; P = .015).

CONCLUSIONS: Measurements of shear obtained on the nonpowered fluid overlay support surface were significantly lower than those obtained on either of the powered support surfaces. Reduction in shear force might explain the accelerated healing observed in patients being treated with the nonpowered support surface. Randomized clinical trials to validate perceptions of enhanced healing rates are required.