Finite Element Model Finds Forty-Five Degree Lying Angle Balances Deep Tissue Stresses

A computer simulation investigating repositioning biomechanics has found that a 45-degree lateral-lying position produces lower and more evenly distributed deep-tissue stresses across major weight-bearing sites than other tested recumbent angles for an obese individual. Published in BMC Musculoskeletal Disorders, the investigation utilized a subject-specific three-dimensional finite element model to evaluate mechanical loading across the sacrococcygeal region, ischial tuberosity, and greater trochanter. While clinical practice frequently employs a 30-degree turn to offload the sacrum, the study demonstrated that posture-dependent stress redistribution behaves nonlinearly in the presence of increased soft-tissue volume, identifying distinct mechanical trade-offs between pelvic interfaces as the body tilts.

Pressure injuries represent a major complication among individuals experiencing prolonged immobility, making regular repositioning essential for offloading and redistributing cumulative mechanical forces. Although the conventional 30-degree lateral posture is widely utilized to protect the sacrococcygeal region, patients with obesity possess greater soft-tissue volume and altered regional tissue distributions that can substantially change load transmission during bed rest. Until now, the internal mechanical environment and deep-tissue responses at critical bony prominences have remained insufficiently quantified in this population, leaving clinicians with limited biomechanical guidance on the ideal recumbent angle.

To investigate these internal mechanics, the researchers recruited a 32-year-old healthy male volunteer with a height of 170 cm, body weight of 90 kg, and a body mass index of 31.1 kg/m². The volunteer underwent computed tomography and magnetic resonance imaging of the lumbosacral-gluteal region to capture precise anatomical structures.

To maintain consistent boundary conditions across all postures, the team employed an equivalent partial-body-weight loading method where controlled vertical displacement was applied through the pelvis and bilateral femurs until the vertical reaction force on the supporting cushion reached approximately 133 N. Numerical stability was confirmed through a mesh convergence test conducted on the 0-degree model using local element sizes of 4.0, 3.0, 2.0, and 1.5 mm. Changes in peak von Mises stress were limited to 2.42 percent between the 3.0 mm and 2.0 mm meshes, and 1.28 percent between the 2.0 mm and 1.5 mm meshes, leading the team to adopt the 2.0 mm mesh for all subsequent analyses.

In the baseline 0-degree supine position, contact pressure was concentrated predominantly in the sacrococcygeal and adjacent load-bearing tissues, yielding a predicted peak external contact pressure of 9.92 kPa, an arrangement that qualitatively matched previously reported human pressure-mapping patterns. Deep-tissue stress distributions varied markedly as the lateral angle changed.

The 45-degree lateral posture demonstrated the most favorable internal mechanical profile among the four tested configurations. At this intermediate angle, peak von Mises stresses at the bone-muscle interface were 0.26 kPa in the sacrococcygeal region, 0.60 kPa at the ischial tuberosity, and 0.27 kPa at the greater trochanter, reflecting relatively low and balanced stress across all monitored bony prominences. Conversely, tilting further to 60 degrees caused a sharp nonlinear rebound in sacrococcygeal peak von Mises stress, which surged to 68.60 kPa, indicating that excessive rotation can reintroduce severe localized deep-tissue strain.

The authors emphasized that their findings reflect relative mechanical trends within a single subject-specific model rather than absolute clinical predictions for all obese patients. Although external contact pressures demonstrated qualitative plausibility against published mapping data and mesh convergence confirmed numerical stability, the computational framework did not incorporate quantitative experimental measurements of internal deep-tissue stresses.

Beyond tracking peak von Mises stress at the bone–muscle boundary, the investigators specifically evaluated both von Mises stress and maximum shear stress at the muscle–adipose interface across all three regions of interest. Because the study analyzed simulated mechanical loading within a single subject-specific computational framework, the postural comparisons among the supine and tilted orientations were conducted descriptively without applying inferential statistical testing.

Under the study’s defined modeling and loading conditions, altering the recumbent angle substantially transformed how deep-tissue loads were distributed across the major weight-bearing zones. Although the 45-degree angle offered a potentially more favorable local mechanical environment by maintaining lower and more balanced interface stresses, the authors reiterated that the baseline contact evaluation verified only surface contact behavior rather than absolute internal stress magnitudes. In addition, because this work was an in silico computational simulation, clinical trial registration was not applicable.

Subject of Research: Finite element analysis of lateral-lying angles for pressure injury prevention in obesity

Article Title: Optimization of lateral lying angle for pressure injury prevention in obese patients based on finite element analysis

Article References: Tian, T., Yang, P., He, T., Lu, Q., He, M., Wang, Y., Bi, T., & Han, D. (2026). Optimization of lateral lying angle for pressure injury prevention in obese patients based on finite element analysis. BMC Musculoskeletal Disorders. https://doi.org/10.1186/s12891-026-10553-1

Image Credits: AI Generated

DOI: 10.1186/s12891-026-10553-1

Keywords: pressure injury prevention, finite element analysis, lateral lying angle, obesity biomechanics, deep tissue stress, sacrococcygeal region, greater trochanter, ischial tuberosity, bone-muscle interface, muscle-adipose interface, von Mises stress, patient repositioning, subject-specific modeling, contact pressure distribution, mesh convergence, biomechanical simulation, body mass index