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

Pressure injury (PI) is a common complication in patients with prolonged immobility, and repositioning is an important strategy for pressure offloading and redistribution. The 30° lateral position is commonly used to reduce sacrococcygeal loading; however, in individuals with obesity, greater soft-tissue volume and differences in regional tissue distribution may alter load transmission during recumbency. The appropriate lateral-lying angle and the corresponding deep-tissue mechanical responses at major weight-bearing regions remain insufficiently quantified. This study aimed to compare deep-tissue stress distributions in the sacrococcygeal region, ischial tuberosity, and greater trochanter at 0°, 30°, 45°, and 60° using a subject-specific finite element model. A healthy adult male volunteer(age 32 years, height 170 cm, body weight 90 kg, BMI 31.1 kg/m²) underwent CT and MRI of the lumbosacral–gluteal region. A subject-specific three-dimensional finite element model comprising bone, an equivalent continuous muscle compartment, adipose tissue, and skin was constructed, and models were established for the 0° supine and 30°, 45°, and 60° left lateral-lying positions. An equivalent partial-body-weight loading approach was used, whereby controlled displacement was applied through the pelvis/bilateral femurs until the total vertical reaction force on the support cushion reached approximately 133 N; the same target load was maintained across all postures. Von Mises stress and maximum shear stress at the muscle–adipose interface and peak von Mises stress at the bone–muscle interface were evaluated at the three regions of interest. Mesh convergence was assessed using the 0° model with local element sizes of 4.0, 3.0, 2.0, and 1.5 mm by comparing peak von Mises stress and spatial stress distributions. Baseline biomechanical plausibility was assessed by qualitatively comparing the predicted buttock–support contact-pressure distribution at 0° with previously reported human pressure-mapping data. Postural results were compared descriptively without inferential statistical testing. The relative changes in peak von Mises stress from MESH2 to MESH3 and from MESH3 to MESH4 were 2.42% and 1.28%, respectively, while the principal stress-concentration locations and spatial distribution patterns remained generally consistent among MESH2 ~ MESH4. MESH3 was therefore retained for subsequent analyses. In the 0° position, contact pressure was concentrated predominantly in the sacrococcygeal and adjacent weight-bearing regions, with a predicted peak contact pressure of 9.92 kPa; the overall distribution pattern was qualitatively consistent with previously reported human pressure-mapping data. Deep-tissue stresses showed posture-dependent redistribution. At 30°, stress at the muscle–adipose interface was concentrated primarily around the left greater trochanter. At 45°, peak von Mises stresses at the bone–muscle interface were 0.26, 0.60, and 0.27 kPa in the sacrococcygeal, ischial tuberosity, and greater trochanter regions, respectively, representing relatively low and more balanced stresses among the tested postures. At 60°, the sacrococcygeal peak von Mises stress increased to 68.60 kPa, indicating a nonlinear stress rebound at the larger lateral-lying angle. Under the defined modeling and loading conditions, lateral-lying angle substantially altered the distribution of deep-tissue stresses across major weight-bearing regions. Among the four tested postures, the 45° lateral position was associated with relatively lower and more balanced bone–muscle interface stresses, suggesting a potentially more favorable local mechanical environment for the obese individual represented by this model. Mesh convergence supported the numerical stability of the adopted spatial discretization, whereas the baseline contact-pressure comparison supported the biomechanical plausibility of the external contact behavior but did not constitute quantitative experimental validation of absolute deep-tissue stresses. The findings should therefore be interpreted primarily as relative mechanical trends within this subject-specific model and require further validation in multi-subject, experimental, and clinical studies. Not applicable.