Pressure ulcers (PUs), also known as pressure injuries (PIs), form in body sites of sustained soft tissue stress concentrations associated with either bodyweight‐induced or medical device‐related mechanical forces. ¹ The vicious cycle of PUs/PIs is triggered by sustained, localised tissue deformations which lead to the initial deformation‐induced cell‐scale damage, then causing secondary inflammatory damage and tertiary ischemic damage, and eventually, tissue death and the clinical presentation of these wounds. ¹ Hence, minimising the exposure to sustained or prolonged soft tissue stress concentrations is the most effective, scientifically proven approach to reducing the occurrence of PUs/PIs in clinical settings, as the 2019 International PU/PI Prevention and Treatment Guideline state. ¹

Advanced medical support surfaces are designed to reduce the exposure to localised interface pressures and sustained soft tissue deformations, and therefore, the risk for PUs/PIs. These devices are commonly categorised into two main types: (i) Support surfaces that redistribute pressures and thereby, disperse tissue stresses but do not affect the patient positioning, and which are termed reactive support surfaces. (ii) Support surfaces that periodically relieve interface pressures by influencing the patient positioning, called active support surfaces. More specifically, the reactive principle is based on immersion and envelopment to distribute the body weight of a user over a larger surface area, thus alleviating the peak interface pressures. The active support surface type is electrically powered (often with battery backup), and cyclically inflates and deflates regions under the patient by means of an air pump. Consequently, elevated interface pressures are transferred back and forth under different body regions to reduce the duration (but not necessarily the magnitude) of exposure to the interface pressures.

A hybrid support surface (HSS) marries the reactive and active features in a single device, and can therefore act at each mode. ² This combined ability is achieved by using high‐quality foam material inside the inflatable air cells in a technology termed “foam‐in‐air”. ² , ³ In a HSS, even if the air cells are all partially deflated while low constant pressure is kept by the pump for maximising the immersion (which is termed the “static mode” of operation), pressure redistribution occurs through envelopment, but when the active feature is turned on, periodic pressure reliefs ensue (which is termed the “dynamic mode”). A HSS is advantageous to use for at‐risk patients, particularly those who do not accommodate to, or do not clinically fit the classic “alternating pressure” mattress choice (eg, become nauseated or it affects their sleep quality). ³ The protective performance of support surfaces including HSS in mitigating the risk of PUs/PIs is commonly assessed using traditional interface pressure mapping that visualises and quantifies the contact pressures of users. However, this approach of merely measuring the interface pressures is limited as it does not directly address the fundamental cause of PUs/PIs, namely, the sustained exposure to soft tissue stress concentrations. ¹ Computer modelling and simulations (also termed “in silico experiments”) are more contemporary and relevant, and are highly effective at showing the tissue loading state within the body on a support surface ⁴ , ⁵ but such an approach was not employed for studying the effectiveness of a HSS yet. Accordingly, the goal of this study was to create a new computer modelling framework for quantitatively describing the mode of action (MOA) of a HSS, in terms of the effect of this technology on the internal tissue loading state at the sacral region of patients in a supine posture.

The distributions of the soft tissue strains and stresses at the sacral region are considerably different between the static and the alternating (cycles A and B) modes of the HSS, which is evident both visually (Figure 3) and quantitatively (Figure 4). Specifically, temporarily deflating the air cell directly under the coccyx (ie, cycle A of the alternating mode) reduces the magnitudes and dispersal of the deep tissue strain/stress concentrations around the coccyx with respect to the static mode. However, this deflation action in turn increases the strain/stress exposure under the sacral bone with respect to both the static mode and cycle B of the alternating mode (Figure 3; left panels versus right panels of the sagittal views). Alternating between cycles A and B in the alternating mode of the HSS thereby effectively shifts concentrated tissue loading from under the sacral bone (towards the sacral promontory) to the tip of the sacrum (coccyx) and vice versa (Figure 3).

The strain/stress histograms for the skin and subdermal tissues are all consistently demonstrating that the static mode of the HSS induces tissue loading levels that are between those of cycles A and B of the alternating mode, which is a further indication that the alternating mode of the HSS fluctuates the tissue strain/stress concentrations around the “baseline” static mode (Figure 4). With that said, there is considerable cyclic spatial shift of tissue loading sites as noted above between the coccyx and sacral promontory (Figure 3). Consistently with the results plotted in Figure 3, the %‐CVTEL data are all negative for cycle A, that is, the tissue strain/stress concentrations are generally alleviated with respect to the static mode of the HSS, and the %‐CVTEL data are all positive for cycle B which indicates that the tissue strain/stress concentration magnitudes overall increase for cycle B (Table 2). However, this latter effect is essential for achieving the spatial cyclic redistribution of concentrated tissue loading as demonstrated in Figure 3.

Support surfaces are the most important PU/PI prevention technology available to health care professionals for protecting their at‐risk patients. The preventative efficacy of the HSS investigated here was already clinically proven, with a reduction of 56% in the number of PUs/PIs achieved for the 6 months post‐deployment of this HSS in an acute hospital setting. ¹³ With that said, the MOA of this specific HSS has not been investigated before, and more generally, the literature is missing quantitative bioengineering methodologies for standardised evaluations of the protective efficacy of dynamic (alternating pressure) support surfaces. ³ Advanced computational modelling is powerful in both identifying the MOA and in providing the performance metrics for specific support surface technologies, particularly with regards to deep tissue load states and how these are affected by the support surface design features, ⁵ and potentially, its dynamic action.

Previous published work focused on reactive (static) support surfaces rather than on active or alternating pressure mattresses. ⁵ , ¹⁴ , ¹⁵ , ¹⁶ , ¹⁷ , ¹⁸ , ¹⁹ , ²⁰ The literature reporting computational modelling studies of advanced support surfaces is overall still sparse, ¹⁶ , ¹⁷ and none specifically refers to HSS. Hence the novel aspects in the current work are the focus on deep tissue load changes occurring when the support surface action changes from static to dynamic/alternating, and in particular, on identifying the MOA of the studied HSS in protecting patients at‐risk for PUs/PIs. We found that when used in its powered, alternating (active) mode, the HSS effectively shifts deep concentrated tissue loading from under the sacral bone (towards the sacral promontory) to the tip of the sacrum (coccyx), and vice versa. This aligns with the concept of internal load shifting in soft tissues by means of an alternating pressure mattress that periodically relieves localised tissue stresses, but for the first time in the literature, the current work visualises and quantifies this phenomenon for an HSS with the analyses encompassing both its static and dynamic modes of operation.

One of the core and unique features of the specific HSS technology studied here is that it is continuously monitoring for patient movements, and more specifically, for a prolonged period of non‐movement. When the “Auto Start” feature of this HSS is turned on, the HSS will automatically transition from its static to dynamic modes if three distinct and separate functional movements of the user are not detected within 1 h (‘functional’ is determined through four different weight categories). ² This is in fact an automated risk assessment followed by an immediate auto‐intervention when a PU/PI risk is detected because of the lack of movement. For the first time in the literature, the current work visualises and quantitatively demonstrates the effectiveness of such automated risk assessment and response to a detected risk, in the aspect of redistributing the deep tissue loads to mitigate that risk.

When evaluating the performance of a hybrid mattress, analysing the static mode in addition to the dynamic/alternating action is of great importance as well. The treatment plan for patients may not require continuous alternating pressure intervention, or the patient may at times feel uncomfortable or nauseated, or their sleep quality may be affected by the inflating and deflating cells, and there is also the unlikely scenario of a power failure. ²¹ An HSS is designed to be able to cope with all these aforementioned situations. The foam core included in each of the air cells ensures that the patient will always remain on a high‐quality foam surface when the cells are completely deflated (ie, if there is no power). ³ Our current results support the intended design of the foam core by showing that the foam cores adequately support the bodyweight of the patient when a cell is deflated, in which case the tissue loading levels are between those associated with cycles A and B of the alternating mode (Figure 4 and Table 2). The support of the foam cores, in the static and alternating modes alike, further improves the comfort of the patient on the HSS and assists in repositioning and performing independent movements in bed, which in turn contributes to improving the quality of life of the treated patient and ultimately, to better wound prevention and management outcomes. ¹⁸

The modelling is based on several assumptions, including that it does not represent the entire range of anatomical variations of at‐risk patients, nor the possible deviation in tissue mechanical properties and how these are affected by chronic conditions (eg, diabetes or scarring from a previous PU/PI that has healed). Likewise, COFs may vary depending on the individual skin characteristics, the bedlinen type and moisture at the body‐support interfaces. Finally, postures that vary from the pure supine position modelled here, such as a rotated trunk or suspended heels, may affect the currently reported results. Accordingly, interpretation of the data reported here should focus on the trends of changes of the tissue load states between the modes of the HSS rather than on specific tissue stress values. Future steps in the efficacy research of the studied HSS can address specific patient populations with distinct anatomical characteristics such as underweight or obese, or consider a case of an existing PU/PI which needs to be treated while the patient is supine (for example if that patient is ventilated or is connected to an extracorporeal membrane oxygenation ‐ ECMO machine). In addition, the protective effect of the HSS can be investigated for other anatomical regions (eg, the head, shoulders, or heels), or body postures, for example, for side or prone lying.

In conclusion, this study demonstrated a deep tissue offloading effect of the studied HSS, achieved by cyclically inflating and deflating the air cells, which shifts the internal tissue loads from under the sacral bone to the tip of the sacrum, and vice versa with the inflation/deflation cycles, thereby periodically relieving soft tissue sites at risk for a PU/PI.