Real-Time Finite Element Monitoring of Sub-Dermal Tissue Stresses in Individuals with Spinal Cord Injury: Toward Prevention of Pressure Ulcers

Abstract

Patients with a spinal cord injury (SCI) are susceptible to deep tissue injury (DTI), a necrosis in excessively deformed muscle tissue overlying bony prominences, which, in wheelchair users, typically occurs in the gluteus muscles under the ischial tuberosities. Recently, we developed a generic real-time, subject-specific finite element (FE) modeling method to provide monitoring of mechanical conditions in deep tissues deformed between bony prominences and external surfaces. We previously employed this method to study internal tissue loads in plantar tissues of the foot [Yarnitzky, G., Z. Yizhar, and A. Gefen. J. Biomech. 39:2673-2689, 2006] and in muscle flaps of residual limbs in patients who underwent transtibial amputation (Portnoy, S., G. Yarnitzky, Z. Yizhar, A. Kristal, U. Oppenheim, I. Siev-Ner, and A. Gefen. Ann. Biomed. Eng. 35:120-135, 2007). The goal of the present study was to adapt the method to study the time-dependent mechanical stresses in glutei of patients with SCI during wheelchair sitting, continuously in real-time, and to compare the trends of internal tissue load data with those of controls. Prior to human studies, the real-time FE model-adapted to study the buttocks during sitting-was validated by comparing its predictions to data from a physical phantom of a buttocks and to non-real-time, commercial FE software. Next, real-time, subject-specific, FE models were built for six participating subjects (3 patients with SCI, 3 controls) based on their individual anatomies from MRI scans. Subjects were asked to sit normally in a wheelchair, on a ROHO cushion, and to watch a 90 min movie. Continuous interface pressure measurements from a pressure mat were used as subject-specific boundary conditions for real-time FE analyses of deep muscle stresses. Highest peaks of compression, shear and von Mises stresses throughout the trial period, and averages of peaks of these stresses were recorded over the trial for each individual. These parameters generally had 3-times to 5-times greater values in patients with SCI compared with controls. Likewise, stress doses, defined as the integration of peak compression stress over time, were approximately 35-times and approximately 50-times greater in the subjects with SCI, the values referring to the highest of all peaks recorded throughout the trial, and to average of peaks over the trial, respectively. We believe that by allowing-for the first time-practical and continuous monitoring of internal tissue loads in patients with motosensory deficits, without any risk or interruption to their lifestyle, and either at the clinical setting or at home, the present method can make a substantial contribution to the prevention of severe pressure ulcers and DTI.