Soft Tissue Response to External Load

A 3D Finite-Element (FE) Model for Evaluating Buttock Tissue Response under Sitting Load

                                                

Background

The critical pressure value which ceases blood circulation has been generally accepted as 4.27KPa (32 mmHg). However, clinical application of this value is difficult because the soft tissues (i.e. skin, fat, and muscle) in the sitting area have multi-layered structures and varying mechanical properties and metabolic demands. Moreover, in long time wheelchair sitting, the soft tissues, which are not naturally developed for weight-bearing, are subjected to a high compressive load which induces massive tissue deformation for a prolonged time.

A better understanding of soft tissue response to such an external compressive loading environment may help improve PU prevention and treatment strategies. Although stresses at the sitting area can be measured, a full-field experimental evaluation of the load transfer remains difficult. However, those difficulties associated with experimental measurements can be overcome by computational modeling, e.g. the finite element (FE) modeling.

The objective of this investigation is to explore, develop and optimize the methodology of using FE modeling to reliably simulate and predict how the buttocks response to external sitting loads.

Clinical Relevance

• Better understand buttock-thigh soft tissue response to sitting load
• Help find solutions to prevent/treat tissue breakdown in wheelchair users

Geometry of the buttock-thigh structure in a true sitting posture with and without sitting load was acquired using

an MRI

 

Skin initial strain in sitting joint configuration was defined as the strain when the subject moved from a standing posture to an upright sitting posture. The buttock skin initial strain in 80º hip and 90º knee flexion was measured from a healthy subject by images of an array of dots painted on the buttock skin. Green strain was calculated for an area of 15x15mm² as 43.6±10.4% and 11.2±0.7% for the axial and transverse directions, respectively.

Reconstruct 3D anatomical structure of buttock-thigh from MRI images:

3D geometry of the soft tissues and bones were reconstructed by image segmentation using SURFdriver (opens new window). Soft tissue and bones within the area of interest were reconstructed, i.e. half of the pelvis, half of the sacrum, coccyx, upper 1/3 of femur, hip muscles, fat and skin.

After segmentation, hip muscles were grouped based on their anatomical orientation.

Meshing each structure:

Each of the 3D models was meshed using both hexahedral (e.g., 8-node brick) and tetrahedral (e.g., 4-node modified tetrahedron) elements in HyperMesh (opens in new window).

To calibrate and validate the buttock-thigh/seat FE models with actual MRI recording under sitting load.

Location of Regions of Interests (ROIs) have been identified on the model and corresponding MRI images.

Displacement on the skin has been calculated from both MRI images and FE analysis. Right image is one of the slices in medial-lateral direction: Cartesian coordinate system is located on the center of the femoral head and locations of ROIs are described relative to this origin.