In the medical literature and clinical guidelines,1 pressure ulcers (PUs) are defined as localized injury to the skin and/or underlying tissues that develop as a result of excessive and sustained pressure and/or shear, usually under a weight-bearing bony prominence. After more than a decade of rigorous research work, it is now well established that severe PUs are caused primarily due to exposure to sustained large tissue deformations over critical time periods that compromise the integrity and homeostasis of cells and the viability of tissues.1-6
PUs are categorized with respect to either their depth or the types of tissues involved. Superficial (skin) PUs are commonly associated with frictional forces, shear loads, and microclimate factors, whereas deep tissue injuries (DTIs) are caused by sustained deformations and localized forces in muscle and fat.1,7,8 PUs are generally associated with a number of contributing or confounding factors, primarily impaired mobility and sensory capacities, as well as compromised perfusion, abnormal body mass index (BMI), and type 2 diabetes, to name a few.9,10 Populations at general risk are the elderly and frail; patients who sustained a spinal cord injury (SCI); individuals with neurological diseases, brain trauma or stroke, and neuromuscular diseases that restrict mobility; and surgical patients.9 All of these individuals are more likely to spend prolonged time periods in a static position in a bed or a wheelchair.
Bariatric patients, who are less mobile as well, are known to be at risk for DTIs, particularly when undergoing a surgery that results in prolonged immobility.11,12 According to obesity.org, obesity is highly correlated with type 2 diabetes; nearly 90% of people living with type 2 diabetes are overweight or obese. This increases the risk of neuropathy/sensory impairment; it is common to see not only bariatric tissue changes, but also diabetes-related tissue changes in these patients, a syndrome often termed diabesity.
Sitting-acquired PUs and DTIs are a common and life-endangering complication for individuals who chronically sit or depend on a wheelchair for mobility. As described in the prospective, inception cohort study by Allman et al13 and the phenomenological pilot study of Hopkins et al,14 the onset of sitting-acquired DTIs can lead to septicemia, osteomyelitis, renal failure, organ system failure, and serious infection, hindering functional recovery, causing pain, and reducing the quality of life for both patients and caregivers. Reddy et al’s15 systematic review demonstrated the burden on health care systems: the management of a single full-thickness PU can cost up to $70,000, and annual PU treatment costs are estimated at $11 billion in the United States alone. Therefore, prevention should be the primary strategy for minimizing the impact of sitting-acquired DTIs.
Tremendous effort is being invested to thoroughly understand DTI etiology, with the aim of facilitating more efficient risk assessment and revision of prevention strategies targeting population-specific risk factors. In particular, the purpose is to minimize internal tissue deformations and localized forces, now recognized in the scientific literature16-18 as well as in the current (2014) International Guidelines for Pressure Ulcer Prevention and Treatment1 as the most important factors causing the injury.
Many studies, both clinical and computational,16,18-21 recently have shown persons with obesity (defined by the World Health Organization22 [WHO] to be a BMI >30) and/or diabetes are at an increased risk for PUs. For individuals who are obese and especially morbidly obese according to the WHO classification (BMI >40), greater body-weight loads are transferred to the soft tissues of the buttocks through the ischial tuberosities (ITs) during sitting. Previous computer simulation studies16,18 have shown increased body-weight loads cause increased internal tissue loads, which are quantified by means of the mechanical strains (dimensionless deformations, measured by means of magnetic resonance imaging [MRI] and/or through biomechanical modeling) and stresses (forces per unit area of tissue evaluated, again, using biomechanical modeling) in the muscle and fat tissues of the buttocks. Per the clinical study of Cox et al,23 for example, individuals with a SCI are expected to gain 1.3–1.8 kg per week during their rehabilitation phase due to a lower level of physical activity. Additionally, as mentioned previously, obesity is commonly associated with diabetes, which increases the risk for PU development due to impaired perfusion, which can be amplified by vascular disease, ischemic heart disease, or congestive heart failure.24 Moreover, diabetes inflicts abnormal biomechanical changes to tissue stiffness. As shown in both animal studies and human cadaveric measurements,25-27 type 2 diabetes is associated with stiffening of collagen-rich connective tissues such as skin and subcutaneous fat. These changes to the biomechanical properties of tissues, where the affected tissues cannot adequately deform to dissipate body-weight loads, may add to the overall risk of injury. Furthermore, as suggested by Gefen,28 in the case of type 2 diabetes, peripheral sensory neuropathy may prevent patients from detecting the onset and progression of tissue damage.
Given the knowledge that sustained excessive and localized strains and stresses in soft tissues may jeopardize cell and tissue viability, the most important principle in preventing sitting-acquired PUs and DTIs is to minimize exposure to these strains and stresses in tissues. For this purpose, clinicians are normally guided to prescribe a soft, thick cushion on the wheelchair to better redistribute the buttocks-support contact pressures as well as the internal tissue loads.1 However, despite the known increased risk of PUs for persons with obesity and diabetes, no specific recommendations of a preferred support surface type (a cushion in particular) are available for these populations.
Finite element (FE) computational modeling is a powerful tool in PU research. It facilitates determination of internal mechanical loads (eg, deformations, strains, and stresses measured in Pascals) in tissues of weight-bearing body parts such as the heels and buttocks. Based on these data, FE modeling further facilitates isolation of the influence of specific intrinsic and extrinsic biomechanical risk factors for PUs and DTIs.10, 29-32 In practice, the sophisticated 3-dimensional (3D) geometry of body organs, typically acquired from medical imaging (such as MRI), is used to build an anatomically realistic reconstruction in the computer, which then is divided into numerous small elements (“finite” elements), each with a simple geometry (eg, bricks or pyramids). Then, the equations that describe the biophysical mechanical interactions between the weight-bearing tissues and the support surface are solved for each element with respect to its neighboring elements to ultimately form diagrams of the transfer of mechanical loads within the entire studied organ. This method allows researchers to artificially manipulate the anatomy and biophysical properties of the tissues (through changes in the geometrical features or mechanical properties assigned to the tissues) in order to identify the influence of different biomechanical factors (eg, muscle atrophy, presence of scars, and so on) on the resulting loads and hence their affect on PU and DTI risk.
The authors have been investigating the biomechanical efficacy of flat foam cushions, contoured foam cushions, and air cell-based (ACB) cushions for several years, using state-of-the-art, imaging-based FE modeling.30,31,33,34 The effect of increased BMI or fat mass on internal tissue loads in the buttocks also has been studied.16,18,33 However, work regarding these specific patient populations was limited to interactions of the seated buttocks with uniform flat or contoured foam cushions; more sophisticated cushion designs were not studied, although such data are needed to strengthen the volume of evidence in the field, particularly in light of certain findings. ACB technology was found to be superior to foams,31,34 and ACB cushions were noted to be used extensively on bariatric wheelchairs in clinical practice.
Accordingly, a computer modeling study was conducted using ACB cushions (provided by the manufacturer, ROHO Inc, Belleville, IL) to integrate previous modeling concepts regarding the ACB technology with documented pathoanatomical and biomechanical tissue changes that result from obesity and diabetes. The purpose of this biomechanical modeling study, which had a theoretical (computer simulation) study design, was to determine the trends of changes in internal tissue loads (strains and stresses) in individuals seated on an ACB cushion if obesity and diabetes occurs and evolves to assess the efficacy of the ACB technology in protecting these individuals if these diseases are present.