As wheelchair and seating clinicians, we're no strangers to the risks of pressure injuries that can arise when soft tissue remains compressed against a surface for too long. We invest significant time and effort into preventing such injuries, with a particular focus on the ischial tuberosities (ITs). These bony prominences, which bear the brunt of our body weight when we sit in a wheelchair, are prone to tissue damage due to the concentrated pressure they endure over time. For years, we believed that the Gluteus Maximus muscle played a crucial role in protecting the ITs, and that the loss of tissue mass was primarily a consequence of decreased muscle mass in the buttocks. However, recent studies have shed new light on the intricate anatomy of the buttocks and provided unexpected insights into the importance of adipose tissue, rather than the Gluteus Maximus, in safeguarding against pressure injuries.
Until recently, there had been limited research on seated buttock anatomy. Earlier studies conducted in 1985 used clay immersion to examine the shape and immersion of the ischial region, revealing sharper prominences and greater depths in individuals with a history of pressure ulcers. However, knowledge about the tissue composition beneath the ischium during a seated posture remained scarce. Prevailing beliefs among pressure injury researchers suggested that muscle dominated this region. But recent investigations have challenged this assumption.
Over the past few years, researchers like Sharon Sonenblum from Georgia Tech University embarked on a mission to answer a seemingly simple question: What sets one backside apart from another? To tackle this query, they subjected numerous bottoms to MRI scans, generating an unprecedented wealth of data about buttock composition. The results were surprising, as they unveiled the fact that humans predominantly rest on fat rather than muscle. This discovery has redirected research efforts towards understanding how the natural padding in our buttocks changes over time, particularly in individuals who spend prolonged periods in seated or lying positions.
The study yielded two significant findings. Firstly, it was observed that the majority of individuals do not rely on their Gluteus Maximus to support their ischial tuberosity while seated. In fact, most participants showed little to no Gluteus Maximus coverage over their ITs. More than 90% of subjects had less than 25% of their ischial tuberosity covered with Gluteus Maximus. Instead, the Gluteus Maximus was typically found posterior and lateral to the highest point of the IT, rather than beneath it.
Additionally, the study revealed that the thickness of tissue covering the seated ischium varied considerably, ranging from 5.6mm to 32.1mm. In most cases, adipose tissue—fat—constituted approximately 80.5% of the tissue under the ischium, while other tissues like tendons, ligaments, and muscles accounted for an average of 19.5% of the thickness.
Interestingly, it's not just the lack of muscle coverage on the ITs that affects an individual's risk of pressure injuries. The qualities of the adipose tissue itself play a role too. Adipose tissue exists in different parts of the body, such as visceral, intramuscular, and subcutaneous regions, each with distinct characteristics depending on its location. Changes in adipose tissue can occur due to factors like obesity, exercise, and hypoxia, with particular attention given to subcutaneous fat in the buttocks. The mechanical loading experienced by wheelchair users, who spend extended periods sitting, has the potential to alter the characteristics of subcutaneous adipose tissue. Recent MRI-based research comparing subcutaneous adipose tissue under the ischium to the surrounding adipose tissue has shown a correlation between these characteristics and the biomechanical risk when seated.
This research has highlighted the pivotal role of adipose tissue in protecting against pressure injuries and has led to a hypothesis that changes in adipose tissue under the ischial tuberosity are associated with an increased biomechanical risk for such injuries. Hopefully, further investigations into this hypothesis will enhance our understanding of pressure injury causes and lead to clinically useful diagnostic techniques that can identify changes in adipose tissue and biomechanical risk. This knowledge could inform early preventative interventions and potentially reduce the incidence of pressure injuries.
Whether you're a wheelchair user seeking optimal pressure distribution and postural support, or a clinician looking to enhance the wellbeing and function of your clients, our Assistive Technology Consultants possess the knowledge and expertise to address your concerns effectively.
References:
Sonenblum, S.E., D. Seol, S.H. Sprigle, and J.M. Cathcart (2020). Seated buttocks anatomy and its impact on biomechanical risk. Journal of Tissue Viability, 29(2), 69-75.
Sonenblum, S. E., Measel, M., Sprigle, S. H., Greenhalgh, J., & Cathcart, J. M. (2021). An Exploratory Analysis of the Role of Adipose Characteristics in Fulltime Wheelchair Users’ Pressure Injury History. Frontiers in Bioengineering and Biotechnology, 9. https://doi.org/10.3389/fbioe.2021.753897
Feltman, R. (2023, May 2). A scientific exploration of big juicy butts. Popular Science. https://www.popsci.com/science/butt-science/