Ultrasound elastography reveals the relation between body posture and soft-tissue stiffness which is relevant to the etiology of sitting-acquired pressure ulcers

Abstract

Objective: Sitting-acquired pressure ulcers (PUs) are common in wheelchair users. These PUs are often serious and may involve deep tissue injury (DTI). Investigating the mechanical properties of the tissues susceptible to DTI may help in guiding the prevention and early detection of PUs. In this study, shear wave elastography (SWE) was used to measure the normative mechanical properties of the soft tissues of the buttocks, i.e. skeletal muscle and subcutaneous fat, under the ischial tuberosities, in a convenient sample of healthy adults without weight bearing and with weight bearing of different times. Approach: We compared the stiffness properties of these soft tissues between the lying prone and sitting postures, to determine whether there are detectable property changes that may be associated with the type of posture. We hypothesized that muscle contractions and 3D tissue configurations associated with the posture may influence the measured tissue stiffnesses. Main results: Our results have shown that indeed, SWE values differed significantly across postures, but not over time in a specific posture or for the right versus left sides of the body. Significance: We have therefore demonstrated that soft-tissue stiffness increases when sitting with weight bearing and may contribute to increasing the potential PU risk in sitting compared to lying prone, given the stiffer behavior of tissues observed in sitting postures.

Our contemporary understanding of the aetiology of pressure ulcers/pressure injuries

“…Relevant biophysical and mechanical tissue properties are the density and composition, water contents, stiffness, strength, and diffusion coefficients. These properties shape the magnitudes and distributions of the mechanical forces that are applied to the tissues at the regions of contact with a support surface, object or medical device, and also, within the tissue structures 16 . In addition, the morphology, mechanical properties, and tissue tolerance to the loading can all change over time because of ageing, lifestyle, chronic injury, or disease 16‐18 .…”

“…These properties shape the magnitudes and distributions of the mechanical forces that are applied to the tissues at the regions of contact with a support surface, object or medical device, and also, within the tissue structures 16 . In addition, the morphology, mechanical properties, and tissue tolerance to the loading can all change over time because of ageing, lifestyle, chronic injury, or disease 16‐18 . In general, externally applied forces, even of a uniform nature, will lead to highly irregular internal tissue strain/stress responses, that is, different loading intensities at different tissue locations, depending on the internal anatomy, body posture, and properties of tissues of the individual 19‐22 .…”

“…For example, while an individual is sitting in a chair, it is common that internal tissue strain levels in muscle can reach values of 50% and above 19,20,23 . Furthermore, soft tissue stiffness increases under weight‐bearing sitting and may contribute to increasing the potential PU/PI risk in sitting, compared with lying prone, given the stiffer behaviour of tissues observed in sitting postures 16 …”

“…16 In addition, the morphology, mechanical properties, and tissue tolerance to the loading can all change over time because of ageing, lifestyle, chronic injury, or disease. [16][17][18] In general, externally applied forces, even of a uniform nature, will lead to highly irregular internal tissue strain/stress responses, that is, different loading intensities at different tissue locations, depending on the internal anatomy, body posture, and properties of tissues of the individual. [19][20][21][22] This can also be referred to as heterogeneous or nonhomogeneous tissue responses to the applied loads.…”

“…19,20,23 Furthermore, soft tissue stiffness increases under weight-bearing sitting and may contribute to increasing the potential PU/PI risk in sitting, compared with lying prone, given the stiffer behaviour of tissues observed in sitting postures. 16 Techniques available for assessment of internal tissue deformations are magnetic resonance imaging, elastography, and ultrasound. 16,24 These imaging modalities can be used in combination with subject-specific theoretical computational models such as finite element (FE) models; FE modelling is a method of solving (bio) mechanical problems by means of a capable computer workstation and dedicated software, to estimate deformations, strains, and stresses throughout cell and tissue structures, and thereby predict the risk of cell and tissue damage.…”