Pressure ulcer prevention dressing design and biomechanical efficacy

Pressure ulcers (PUs), also termed pressure injuries in the US, Canada and Australia, compromise the quality of life (QoL) and increase the hospitalisation costs of patients.¹ In the US, annual expenditure on PU treatments has been estimated at about $27 billion.² On top of this, there are also fines for hospitals with relatively high PU incidence rates, costs of litigation and damages ordered by trial judges and an associated rise in medical liability insurance premia.^3,4,5 Generally, the authorities influencing this cost system, such as government health ministries, health maintenance organisations (HMOs), insurance corporates, judges and court juries tend to perceive PUs as mostly avoidable and thereby, often consider a hospital-acquired PU as a serious adverse event which is potentially punishable (in some countries, even by criminal law sanctions, including prison).⁶

PUs are triggered and driven by exposure to sustained cell and tissue deformations, which is the primary cause of cell death and tissue necrosis.^7,8,9 Once the skin breaks down in a forming PU, bacterial colonisation will likely follow (which is often compounded by the location of such injuries near the perineum, as in sacral PUs).

Biological processes which are essential for tissue repair, including angiogenesis, accelerated synthesis of growth factors, extracellular matrix deposition and contraction of the wound edges with re-epithelialisation are often impaired in PUs, which is characteristic of the chronicity of these wounds.¹⁰ PU chronicity has a significant negative impact on patients' QoL and incurs vast healthcare costs.

It is not surprising therefore that, against this background, PU prevention (PUP) strategies became critically important and are heavily weighted in healthcare quality ratings, patient safety evaluations, cost-effectiveness of hospital services and medicolegal liability. One of the emerging approaches in PUP is application of prophylactic dressings to protect specific anatomical sites which are considered to be at risk of PUs, typically the sacrum and heels, in certain patients who are assessed to be at an overall high PU risk. The objective of this educational article is to explain how the engineering considerations in the design of prophylactic dressings for PUP eventually determine clinical and cost-benefit outcomes related to PUs.

As recently described in the aetiology chapter of the 2019 European Pressure Ulcer Advisory Panel (EPUAP), National Pressure Injury Advisory Panel (NPIAP) and Pan Pacific Pressure Injury Alliance (PPPIA) Clinical Practice PU Guidelines⁸ and the International Consensus Document on Device-related PUs,⁹ unless stopped by a timely intervention, any forming PU progresses from deformation-inflicted cell damage, to localised inflammatory tissue damage and eventually, to development of ischaemic tissue damage. These three contributors to tissue damage have synergistic and cumulative effects and while each triggers at a different time point, damage spreads and develops in escalation when these factors join the injury spiral.^8,9

Accordingly, if a PU is allowed to progress in a person who is insensate or one who is unable to move independently and no prophylactic intervention is taken, a clinically significant wound rapidly forms (sometimes under intact skin which appears normal), within tens-of-minutes or within a couple of hours. It is important to understand though that such PUs onset at the death of just a few cells and progress from the micro-to the macro, until presenting themselves as skin breakdown or deeper tissue necrosis under intact skin.^8,9 Of note is that, in the context of the present article, the tissue damage cascade in PUs is triggered and driven by skin and underlying soft tissue exposure to sustained mechanical deformations and distortions.^7,8,9 This implies that, from a pathophysiology standpoint, alleviating the sustained exposure to tissue deformations is the most effective approach for primary PUP.^8,9 A good prophylactic dressing should therefore provide adequate alleviation of the sustained soft tissue deformations and distortions caused by bodyweight forces and the environment where they are applied. The sacral bodyweight forces deforming the soft tissues surrounding the sacrum of an individual and the properties of the specific mattress and bedsheets under the sacrum of that person, which make the immediate environment for the biomechanical body-support surface interaction, are one relevant example.

The pioneering research work of Clark,¹¹ reviewed in the book by Boateng,¹⁰ suggested some 30 years ago that dressings produced with an offloading region are able to redistribute the contact pressures at the site of application (the trochanters in Clark's work), pointing to the potential of prophylactic dressings in improving the biomechanical state of weight-bearing soft tissues. The concept of using dressings which were originally developed for wound treatment applications prophylactically, for PUP (as opposed to using dressings that are specifically designed for prophylaxis as in the Clark study¹¹), is relatively new but is gaining popularity (the number of peer-reviewed journal publications focusing on this topic appears to have doubled every three years since 2012). A meta-analysis of the relevant clinical trials to date (Fulbrook et al.¹²) concluded that prophylactic sacral dressings decrease the relative sacral PU risk by 70% in general wards and by 83% in intensive care units (ICUs); this has been confirmed in a large German trial which appeared soon afterwards (Hahnel et al.¹³).

At this point in time, about a decade after the first reported application of dressings (originally developed for wound treatment) to PUP, it is commonly accepted that dressings can reduce the risk of PU formation. The body of peer-reviewed published evidence was solid enough for the 2019 Clinical Practice PU Guideline (EPUAP)¹ to recommend use of a ‘…dressing to protect the skin for individuals at risk of pressure injuries (strength of evidence: B1)’, with reference to silicone-foam dressings which have been tested in the above referenced clinical trials. The guideline does not state which specific dressing should be used in the different possible clinical scenarios. That choice is left for clinicians to make, which puts heavy responsibility on their shoulders as dressings vary considerably in their shape, structure and engineering design, not to mention that all present commercial dressings were not developed primarily for PUP (but rather, for treatment of wounds).

As explained above, the prophylactic value in any dressing used for PUP is its ability to alleviate the sustained skin and subdermal soft tissue loads. Noteworthy is that a dressing which successfully alleviates sustained soft tissue loads at an anatomical site of application needs to continue to do so effectively over time, even when exposed to a changing environment, for example when a patient is incontinent and their bedsheets become wet. The recommended timeframes for continuously using silicone-foam dressings prophylactically vary between manufacturers and also depend on the facility and available nursing staff, the applied clinical protocols and the target cost-benefit outcomes. Typically, however, in PUP protocols, silicone-foam dressings which are the current market-dominant dressings in prophylactic use are changed once every 3–7 days.^13,14,15

As their name implies, silicone-foam dressings are made in a sandwich-like layered structure of different foams and silicone. Nevertheless, even silicone-foam dressings differ substantially in their shapes, thicknesses, thicknesses of the constituent layers and the specific types of materials in the dressing composition. As a result, these dressings present considerably different stiffness and conformability properties in laboratory testing¹⁶ and, thereby, variable effects on the loading states in the soft tissues they are aimed at protecting. A commonly reiterated fallacy in the PUP field is that all silicone-foam dressings are similar in their biomechanical performances; in fact, the specific dressing structure and ingredient determines the level of load redistribution in skin and deeper tissues.^{8,15,16,17,18,19}

To date, there are no reports of attempts to use dressing design concepts that do not fall under the silicone-foam dressing category for the purpose of PUP. Given the above, it follows therefore that the biomechanical efficacies of alternative dressing designs for alleviating sustained tissue loads cannot be extrapolated from the knowledge collected regarding silicone-foam dressings. For scientific and medical progress in PUP, it is important that other dressing design concepts are proposed, evaluated and discussed by the scientific and bioengineering communities as well as by practicing clinicians, so that alternative designs can be weighed and evaluated objectively, standardly and quantitatively. This will promote an evolution in prophylactic dressing technology, so that better-performing design concepts (across categories and within categories, for example, among silicone-foams) will eventually be adopted by the healthcare market. Such a technology evolution process requires development of dressing evaluation methodologies which facilitate comparisons of existing or future design concepts in a quantitative manner. The term ‘non-inferiority’ is important in this context; non-inferiority of a medical device (prophylactic dressings included) means that the device improves the condition of patients by no less than a certain amount with respect to the standard care. The theory for testing the non-inferiority of prophylactic dressings—developed by the author in multiple journal publications—has been compiled and made accessible to clinicians in this educational article.

This article explains the essence of how bioengineers evaluate the biomechanical efficacies of prophylactic dressings. To illustrate the described concepts and methodologies, two fundamentally different dressing technologies are compared with regards to their tissue protection potential:

• A dressing with a core matrix made of cellulose fibres (Resposorb Silicone Border (RSB) Paul Hartmann AG, Germany, also sold by the name of Zetuvit Plus Silicone Border in some markets)
• A conventional silicone-foam sandwich structure of dressings by different manufacturers.

The bioengineering theory employed for this comparison is described, primarily to inform the clinical readership of the journal with regards to the criteria, processes and parameters used for evaluation of prophylactic dressings, so that clinicians can make their own informed dressing selection decisions, based on contemporary professional knowledge.

Computer modelling and simulations to determine the efficacy of dressings

From a medical statistics perspective, effective PUP is known to be substantially more difficult to demonstrate in clinical trials with respect to the successfulness of PU treatment interventions.²⁰ In order to establish a prophylactic clinical effect, patient group sizes in PUP studies need to be far greater than in any PU treatment studies. This is because some participants in PUP studies will not develop PUs at the endpoint of the study, but not as a consequence of the prophylactic intervention. The need to study large patient cohorts, typically in the orders of hundreds to thousands of patients for potentially reaching a statistically-powered evidence in clinical PUP research, involves vastly greater financial investments than in PU treatment studies, which is a substantial barrier for research funding from either governments or industry. Fortunately, bioengineering laboratory testing has developed considerably during the last decade and methods to thoroughly analyse the biomechanical efficacy of prophylactic dressings have emerged. In particular, computer modelling and simulations using up-to-date understandings of PU pathophysiology, the state of science in the field, and the principles of evidence-guided medical device evaluation (also promoted by the US Food & Drug Administration (FDA)) have opened an alternative route to the large clinical trials for testing the prophylactic capacities of dressings.^16,21,22

Computer modelling allows studies of tissue loading and mechanical stress concentration exposures at the weight-bearing skin and deeper tissues of patients, as a function of the specific engineering design of a tested dressing (for example, its geometry, ingredients and composition) and for the anatomical and physiological conditions of interest. Computer modelling coupled with experimental validation has already been established as the most powerful, cost-effective and robust bioengineering methodology currently available to inform product design and evaluation.^17,22 The modelling process, often conducted following the ‘finite element’ method, is based on dividing the complex structure of a body region with the interfacing dressing into virtual small ‘elements’, each having a simple geometry, such as bricks or pyramids, which are interconnected. The physical equations describing the bodyweight forces of the body position of a patient (for example, a supine position) are then analysed for each small (‘finite’) element. The information on the state of tissue loads is then transferred from each element to its neighbouring elements, in an iterative calculation process. This finite element calculation process results in the distribution of mechanical loads (also known as mechanical stresses) in the tissues and body site of interest. Further information on this modelling process is available in our previously published work.^{22,23,24,25,26,27,28,29}

A highly important advantage of this computer modelling process with respect to any clinical trial is that computer simulations are always deterministic, that is, they are not affected by biological and physiological variability. In other words, the results of computer simulations will never include the randomness or unpredictable factors which are inherent in clinical research. Certain input parameters fed into the modelling and simulations, for example, the stiffness behaviour and properties of the tissues and tested dressing, will always provide the same predictions of tissue loading states in a given patient anatomy, which facilitates effective, systematic dressing comparisons. The primary reason for the capacity of a computer model to eliminate biovariability is that a simulation represents the deterministic biomechanical interactions that describe the tissue responses to a definite set of bodyweight forces, in the presence of a specific dressing design.

A state-of-the-art computer model would further allow testing of how a tissue loading state in a certain patient may potentially change when the dressing conditions are altered. In particular, the tissue stress levels may change when some dressing materials alter their mechanical behaviour and stiffness properties, from the time point when the dressing is new (‘straight from the package’) and over the period of use, as the dressing ‘ages’ or accumulates wear and tear mechanical damage. Body movements, repositioning or the patient sliding in bed,³⁰ as well as moisture exposure, for example, due to wet bedsheet episodes, are known to cause such mechanical changes in dressings.^15,16,31

The core engineering design parameters required for a computer model to weigh and simulate these changes in the tissue loading state are listed in Fig 1, namely, the shape and size, the composition and the materials of the dressing. A change in the environment of the dressing once applied and specifically, exposure to moisture, may influence the biomechanical performances of the applied dressing and thereby, tip a desired balance between the ‘new’ and ‘used’ protective performances of the dressing. For example, the coefficient of friction (COF) of the dressing with the bedsheets is likely to change due to moisture (which creates more adhesion between the wet dressing and linen³²). Likewise, the thickness of a moist dressing may change as it absorbs moisture and, thereby, loses mechanical stiffness, resulting in ‘flattening’ of the dressing.³¹ Penetration of moisture from the bedsheets into the inner dressing structure may not only alter its stiffness or the stiffness of its individual components, but can also affect the thermal conductivity of the dressing (which would then affect its microclimate management capacity²⁹) or its water vapour transmission properties. Any such change in properties of a dressing or a combination of property changes will result in deviations in the mechanical and thermal states of the tissues protected by that prophylactic dressing, at the skin as well as in deeper tissues. Considerable changes in the tissue states will most likely affect clinical outcomes of PU incidence and prevalence and also the inter-related cost-benefit outcomes associated with the dressing of choice.

The theory of comparing biomechanical efficacies of dressing designs

To demonstrate the application of a dressing design evaluation process which considers the factors detailed in Fig 1, a computer model of a supine female patient—a ‘virtual patient’—to whom different sacral dressings have been applied, is discussed. The technical details regarding the development of this computer model and the associated validation experiments, as well as the laboratory tests conducted to acquire the dressing properties that were fed into the computer modelling are described elsewhere^33,34 and are summarised here briefly, for completeness. The modelling framework has been validated using the methodological validation process described in detail in our published work (and specifically illustrated in Fig 2³⁵). Specifically, in order to confirm that the internal body tissue deformations—predicted to be caused by the bodyweight forces using the computer model—are realistic, two magnetic resonance imaging (MRI) datasets are segmented. The first MRI dataset is acquired in a non-weightbearing supine position, where the scanned person (whose anatomical information is used for the computer modelling) lies on a custom-made, discontinuous support surface, designed not to load the buttocks region. The second MRI dataset is acquired from the same person, but on a continuous surface and during full weightbearing supine lying.³⁵ The computer modelling is developed based on the first, non-weightbearing MRI dataset (Fig 2(1)) and the weightbearing supine posture is then computationally simulated, by applying the bodyweight force fraction that acts on the pelvic region of the studied individual. The computer simulation data representing the deformed soft tissue structures (due to the bodyweight forces associated with the supine posture, without a dressing) are then compared with the second, weightbearing MRI dataset.³⁵ The criterion for satisfying the validation requirement is adequate fits between the tissue contours in the weightbearing computer simulation and the corresponding weightbearing MRI dataset (which describes the actual tissue deformations in the studied patient), as described in our previous publication.³⁶

Based on rigorous experimental measurements of the physical and mechanical behaviours and properties of multiple silicone-foam dressings, from several manufactures, versus the RSB superabsorbent cellulose dressing, we evaluated each dressing design concept. The two dressing types were considered in these computer simulations to be either at their new/dry conditions or, separately, at their used/moist conditions. After applying the different dressings on the sacral region of the above virtual patient, that virtual patient was positioned on various hospital mattresses (with stiffnesses of 10–30kPa, representing the available range³⁷). The results of the simulations gave the magnitudes and distributions of the soft tissue stresses around the sacral bone of the virtual patient, which then facilitated the computerised evaluation process, depicted in Fig 2.

Specifically, the process of evaluating the biomechanical protective efficacy of dressings in prophylactic use is composed of three primary consecutive steps which are illustrated in Fig 2. First, the computer model of the pelvic region of the virtual patient is solved by the algorithm of the finite element software to determine the stress levels in the soft tissues around the sacral bone under the tested prophylactic dressing, as well as for the same patient without a dressing. The state of tissue stresses with the dressing in place is calculated for both the new dressing (in its ‘straight from the package’ condition) and for the same dressing in its used condition, i.e., considering the effects of moisture on the frictional and stiffness properties of the components of the tested dressing (as measured a-priori in designated laboratory tests).

Second, tissue stress histograms, graphically describing the distribution of stress values in the soft tissues near the sacrum, are plotted to quantitatively describe how much of the tissue volume (if any) benefits from a reduction in the stress levels, as a result of application of the tested dressing. The areas bounded under the above ‘tissue stress curves’ are further calculated, to reflect the stress exposures with the dry/new dressing (A_dry) and used/moist dressing (A_moist) against the no-dressing (A_no-dressing) case, using corresponding single numerical (scalar) values. A hypothetical new dressing which does not result in any reduction in the sacral tissue stress levels will have the same stress curve as the no-dressing case and so, the difference between the A_no-dressing and A_dry for that dressing will be zero (or near zero), indicating no improvement in tissue protection with respect to a bare skin condition.

The opposite hypothetical example would be of an excellent prophylactic dressing design for which the stress curve drops substantially with respect to the no-dressing curve, i.e., the A_no-dressing is considerably higher than the A_dry value. A separate issue is whether the well-performing dressing can maintain its good performances after being exposed to moisture, and this is reflected in the difference between the A_dry and A_moist values. Theoretically, the A_moist value of a tested dressing should be higher than its A_dry value, i.e. the tissue stress exposure will likely increase when the dressing absorbs moisture and its materials degrade. Therefore, a relatively small A_moist–A_dry difference indicates adequate endurance of the tested dressing under moisture conditions, i.e., the tissue stresses do not rise substantially over time in the presence of moisture within or in the vicinity of the dressing. If hypothetically, the A_moist–A_dry difference is positive and considerately above zero, i.e., the tissue stress curve induced by a certain dressing is substantially higher when the dressing is moist with respect to its dry tissue stress curve, then this implies that the dressing loses its protective effect over the time of use. Such a potential dressing case highlights the point that any dressing design should be evaluated not only for its protective performances when new, but also after a period of use, when its components have absorbed moisture and perhaps also experienced other mechanical damage.

In the third and last step of the algorithm, depicted in Fig 2, three biomechanical efficacy indices can be calculated, based on the A_dry, A_moist and A_no-dressing values derived from the tissue stress curves, as follows. A protective efficacy index (PEI) defined in our published work^27,28 is first calculated for each tested dressing design, separately at the dry and moist dressing conditions, using:(Equation 1a)PEIdry[%]=100×Ano-dressing−AdryAno-dressing(Equation 1b)PEImoist[%]=100×Ano-dressing−AmoistAno-dressing

As explained above, an ineffective dressing would have low PEI values which would reflect lack of reduction in the tissue stress state when the dressing is applied. In other words, the more biomechanically effective a certain dressing design is in reducing the stress exposure in soft tissues, the lower its stress curve would be (with respect to the no-dressing curve), resulting in greater PEI values (Equations 1a and 1b).

After calculating the PEI_dry and PEI_moist for a tested dressing using Equations 1a and 1b, respectively, a protective endurance (PEN) index can further be calculated, as:²⁷(Equation 2)PEN[%]=100×PEImoistPEIdry

As noted above, an ideal (but likely unrealistic) dressing design would perform identically when being either dry or moist, i.e. its PEI_dry and PEI_moist values will be equal and so, its PEN will be 100% or near that value (Equation 2). Another (extreme) example is of a certain hypothetical dressing that, when becoming moist, allows tissue stress levels to increase to the same level as if no dressing is applied, i.e., it has a PEI_moist which is near zero; in this case, as per Equation 2, the PEN will also reduce to (near) zero, even if the dressing performs well when it is new/dry (i.e., has a relatively high PEI_dry value).

An ideal prophylactic dressing should clearly have both high PEI and PEN values,^27,28 affirming that it not only provides effective protection when new (as reflected by a relatively high PEI_dry value), but can also deliver a similar extent of tissue protection over a period of use (manifested in a relatively high PEN value). However, as already noted, a certain dressing can theoretically present excellent protective performances when it is new/dry, but poor performances when it becomes used/moist. Alternatively, a dressing can start with mediocre protective performances when new/dry and retain those performances reasonably well over the time of use, but since performances were already inferior when it was new from the package, the delivered tissue protection would still be at a relatively low level overall. This implies that an additional performance parameter is required, to determine the quality of the balance between the PEI of the new dressing (i.e., PEI_dry) and the PEN level of the same dressing. This is the reasoning for defining the third performance parameter, the prophylactic trade-off design parameter (PTODP), as:(Equation 3)PTODP[%]=PEIdry×PEImoist

The above PTODP parameter is, in fact, an interaction term between the new/dry protective performances and the used/moist performances of a tested dressing design. The greater the PTODP value (Equation 3), the better the trade-off point between delivery of tissue protection when the dressing is new and its ongoing protective efficacy when that dressing absorbs moisture over time. In other words, based on the present bioengineering algorithm and the quantitative measures used therein, an ideal dressing design should have a relatively high PEI_dry value, reflecting its prophylactic performances when it is new, as well as a relatively high PEN, demonstrating its ability to maintain the initial adequate performances while in use on the body of a patient. Both the PEI_dry and PEI_moist would therefore contribute to a high PTODP (Equation 3).

The above three performance indices: PEI, PEN and PTODP (defined in Equations 1, 2 and 3, respectively) form an effective dataset which objectively and quantitatively characterises the protective biomechanical performances of any prophylactic dressing design of interest. These parameters capture the protective quality when the tested dressing is new (‘straight from the package’, PEI), in its used condition after absorbing moisture (PEN) and also, when combining these two conditions in the normal lifecycle of a dressing in prophylactic use (PTODP). The PTODP index is of particular importance, as it specifically evaluates the balance between the ‘new’ and ‘used’ performances of the same dressing design (Fig 1), which facilitates the comparison of different dressing design concepts. Of note is that the true strength of the above three indices is in comparative evaluations, where different dressing design concepts or product alternatives can be rated by these parameters for the purpose of decision-making.

Also noteworthy is that it is possible to define the PTODP using other formulations that are based on combining the PEI_dry and PEI_moist performance indices, including, for example, assignment of different weights for PEI_dry and PEI_moist (such as PEI_dry+PEI_moist where α and β are the respective, relative weights for PEI_dry and PEI_moist, based on their expected importance in the life cycle of a prophylactic dressing and α+β=1). Nevertheless, for simplicity and to avoid arbitrary assignment of weight values, the PTODP has been defined here following the classic definition of an interaction term in statistics, which is an explicit multiplication of the two relevant variables, PEI_dry and PEI_moist (while effectively assigning an equal weight for each).

Application of the theory to evaluation of different dressing design concepts

To demonstrate the utility of the above theory and indices in the evaluation of dressing design concepts, we apply it here for comparison of the RSB dressing technology with the mainstream silicone-foam dressings. The silicone-foam ‘sandwich’ design concept is the market-dominant for PUP, however, using (treatment) dressings prophylactically is a relatively new innovation; unlike the science of support surfaces for instance, the field of prophylactic dressings has not evolved yet to explore any alternative design concepts. In this context, no manufacturer currently claims that their dressings have been designed specifically for prophylaxis or that their dressings are recommended for just this purpose. On the contrary, some manufacturers recommend dressings developed originally for treatment to now be considered as dual-use, for prevention as well as treatment,^12,38,39,40 which is advantageous in some aspects, such as allocation of manufacturing resources, logistics and storage (at both the manufacturer and client ends). Under these circumstances, however, it is yet unclear if certain dressing design concepts would perform better in prevention than in treatment or vice versa, and this requires laborious bioengineering research to study. Narrowing the discussion to PUP per se, a good prophylactic outcome depends on multiple design features that should work synchronously, as illustrated in Fig 1. In this regard, a prophylactic dressing is similar to a support surface, such as a mattress or wheelchair cushion. The pattern and extent of redistribution of tissue stresses and the amount of reduction of tissue stress concentrations, especially near bony prominences, depend on the shape, size and materials of the support surface.^37,41 Any moisture accumulated between the body and support surface may immediately translate to an increase in the COF at the interface, compromised material behaviours and, as a result, increased tissue stresses (which are in turn coupled with the microclimate conditions⁴²). The durability of the materials composing the support surface to moisture would therefore be critical for stable protective performances, as is indeed the case regarding prophylactic dressings (Fig 1).

The PEI, PEN and PTODP comparisons that were conducted between the silicone-foam products and the RSB technology, through the algorithm described in the previous section and in Fig 2, are reported in the charts in Fig 3. As could be foreseen, the different dressing design concepts result in dissimilarities in the corresponding biomechanical protective performances. Even if focusing on just the silicone-foam dressing family, here comprising four different commercial dressings, the PEI of the tested silicone-foam dressing products pooled together exhibited considerable variabilities in their PEI values (for both the dry/new and used/moist conditions; Fig 3a). This underpins that the protective performances of any dressing depends on the specific structure and materials selected in the design process. In other words, each dressing performs differently, even if it falls under the same design concept, for example, of ‘silicone-foam’, depending on its specific constituents and construction. This infers that the clinical choice is not just between technology categories or design concepts but also, for a certain dressing design concept, between specific products that deliver potentially different protective efficacies and at different costings, depending on their ingredients.^18,43

In the pharmaceutical world, each ‘generic’ medicament has its specific and unique chemical formula. As long as different products, by different manufactures, adhere to the same formula and manufacturing process, the products would be equivalent, hence generic. Dressings in contrast, can never be considered ‘generic’ and would always perform according to their specific structure and composition, which differ substantially across manufacturers, including within a given design category such as ‘silicone-foams’. In other words, the main difference between a pharmaceutical generic drug and a prophylactic dressing is that there is only one way of producing a generic drug but numerous possible ways for an engineering design of a dressing. Different dressings should therefore be assumed to provide different prophylactic outcomes and clinicians should therefore request manufacturers and distributors to provide the evidence specific to the product under consideration.

The results of the protective efficacy analyses depicted in Fig 3 demonstrate that the RSB dressing has greater PEI values with respect to the silicone-foam dressings (for which the PEI data were pooled together from the different manufacturers), but its PEN is slightly lower than those of the silicone-foam dressings. However, when the two latter indices, the PEI and PEN, are weighed together by means of the PTODP measure, the RSB dressing emerges as having a better balance between its ‘new’ and ‘used’ performances. In fact, the RSB dressing was able to achieve that PTODP advantage since it already opens with a relatively high PEI (1.6-times and 1.3-times greater than that of silicone-foams dressings for the new/dry and used/moist conditions, respectively; Fig 3a). Accordingly, despite the PEN of the RSB dressing being 0.8-times that of silicone-foam dressings (Fig 3b), its superior starting point in delivery of a protective effect to sacral tissues (when it is new/dry) has enabled its higher PTODP value (being 1.9-times greater than that of silicone-foam dressings; Fig 3c).

It further follows from the above analyses that the best prophylactic use of RSB dressings is to either apply them for limited time intervals where tissue protection is needed, such as during surgeries which are typically limited to several hours, or have them replaced frequently to maximise their protective performance when they are new/dry, as reflected by their PEI values (Fig 3). For durations of anaesthetic sessions and surgeries above which a considerable PU risk is indicated (typically >2 hours^33,34) and where the support surface is expected to remain mostly dry (for example, examinations under anaesthesia, precision surgery, endoscopies and in general, where there are no major vascular procedures), the PEI benefits offered by the RSB dressing technology may be maximised.

Of note is that application of prophylactic dressings, as good as those may be, does not reduce the need for regular repositioning of patients which is a well-established, fundamental intervention for PUP.^44,45 Regular repositioning reduces the duration of cell and tissue exposure to bodyweight forces transferred through the same body region. Repositioning is strongly recommended in the International Clinical Practice Guideline for Prevention and Treatment of Pressure Ulcers/Injuries,¹ as well as in numerous national and specific care setting guidelines.⁴⁶ Application of an adequately performing prophylactic dressing, for example, to the sacral area, provides additional tissue protection for that region during the time period of sustained tissue loading in a supine position, but without movement, cell and tissue damage will eventually occur, even with the best possible dressing. Therefore, it should be clarified that prophylactic dressings do not and cannot replace an effective repositioning practice.

Summary and conclusions

The parameter which describes the biomechanical efficacy of a new/dry dressing used for PUP is the PEI.^27,28 Its important counterpart, PEN, describes the capacity of the said dressing to continue delivering a similar protective effect once it has absorbed moisture during a period of prophylactic use.²⁷ Based on these two indices, a third index is calculated, namely the PTODP, to consider the interaction of the PEI and PEN parameters, i.e., the quality of the balance between biomechanical performances of the same dressing in its new and used conditions (Fig 1). A good dressing design is characterised by relatively high PEI as well as PEN values, resulting in a high PTODP score. That is, the PTODP criterion ‘punishes’ either a dressing design which performs poorly when new/dry, or which is compromised when being used/moist, by rating it lower.

The above three indices, the PEI, PEN and PTODP, become most useful in methodological comparisons of different dressing design concepts (Fig 2), as they highlight important aspects and the specific pros and cons in the selection of a preventative dressing. For example, these criteria can inform which prophylactic dressing technology would be more appropriate for short-term use, for example, for application during surgery or an examination involving anaesthesia. In this paper, we have demonstrated the application of this evaluation process, employing the PEI, PEN and PTODP parameters, with a comparison between two inherently different dressing technologies: the market-dominant silicone-foam dressing design concept (represented through multiple dressing types) versus the fundamentally different RSB dressing design, which has a superabsorbent cellulose core. Noteworthy is that the present comparison only reflects the preventative, but not the treatment capacitates of these two different dressing technologies. Moreover, with respect to prophylaxis only, the focus has been on alleviating the sustained soft tissue loads, whereas the heat release aspects (discussed elsewhere in our published work^29,47) were not addressed here. That said, it was demonstrated that the RSB dressing design had a better equilibrium point between its performances at ‘new’ versus ‘used’ conditions (Fig 1), i.e., a greater PTODP score than those of silicone-foam dressing designs (Fig 3). Finally, quantitative bioengineering criteria should be defined for non-inferiority of dressing designs considered for prophylactic use, and the PEI, PEN and PTODP indices can serve as the base for such non-inferiority criteria.

Reflective questions

• What are the fundamental bioengineering considerations in the design of prophylactic dressings?
• Why are preventative dressings from different manufacturers never equal in their performances?
• Why is a good balance required between the performances of a new (straight-from-the-package) dressing and the same dressing that has been in use for a certain period of time?