Skin response to repetitive mechanical stress: A new experimental model in pig

Objectives: To develop a new animal model for investigating the relations between interface stresses at the skin, adaptation, and breakdown. There were two hypotheses. (1) In skin subjected to varying types of repetitive mechanical stress, the tissue response depends on the direction and magnitude of the load. As the shear stress increases, tissue breakdown occurs earlier, (2) In skin subjected to repetitive mechanical stress of longer duration, there will be evidence of tissue adaptation. Design: Multiple case control, single-blind. Interventions: Varying combinations of normal and shear mechanical loads are applied to pig's skin for short durations (breakdown studies) or longer durations (adaptation studies). Main Outcome Measures: Gross evidence of breakdown (visual inspection of skin) and microscopic changes (eg, histologic features of breakdown; thickness of epidermis and dermis; the length and shape of the basement membrane; concentration of inflammatory cells, mast cells, and fibroblasts; and quantity of elastin fibers). Results: The instrumentation was reliable and a significant improvement over past models in that shear forces were delivered and measured in a controlled manner. The animal model and tissue methodology provided consistent results, and it was found that skin breakdown occurred earlier as shear forces were increased. Evidence of tissue adaptation occurred in the long-term experiments, although corresponding morphologic changes have been difficult to elucidate. Conclusions: To address the problem of skin breakdown, new animal models are strongly needed to better understand basic biologic processes related to pressure ulcer development. (C) 1998 by the American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation.