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Surface Stress Profiling

Individuals with a spinal cord injury are susceptible to deep tissue injury, which is a pressure-related-necrosis that onsets in the gluteus muscles under the Ischial Tuberosities (IT). (Note: Ischial Tuberosities are protuberances at the ends of the lowest of the three major pelvic bones for the attachment of a muscle or tendon.) Early detection of this condition is currently not feasible because the injury starts and progresses under intact skin. In spinal cord injury patients, elevated stresses form around the IT. These stresses eventually result in muscle cell death.

An Israeli research group (Eran Linder-Ganz, Ziva Yizhar, Itzhak Siev-Ner, and Amit Gefen are researchers at Tel Aviv University and Chaim Sheba Medical Center) recently developed a real-time, patient-specific, finite element (FE) modeling method and experimental system to provide early detection of deep tissue injury. To allow real-time continuous stress calculations, a Tactilus pressure mat, which measures sitting interface pressures at 1Hz, performs Surface Force Profiling. These Tactilus data feed directly into the model as real-time pressure boundary conditions.

The symmetrical, two-dimensional, FE model of the IT and enveloping soft tissues was built for each individual based on an MRI scan of the subjects buttocks, taken while the subject was seated. After defining the geometry of the cross-section as shown in Fig. 1a, the subject was asked to sit normally in his own wheelchair on a wheelchair cushion and watch a for 90-min movie. Surface stress profiling measurements were acquired between the patients buttocks and the cushion using a Tactilus pressure mat. Pressure data were communicated to the PC laptop in real-time for the FE stress calculations, which are presented in Fig. 1b.

surface stress profiling soft tissue compression
Fig 1(a): Real-time finite element model of the buttocks of an individual with spinal cord injury Fig 1(b): A distribution of soft tissue compression stress intensities at a certain moment in time during the 90min sitting trial

The quantitative output of stress-time plots are shown in Fig. 2. This work found that muscle stresses under the left IT were substantially and consistently larger than respective measures under the right IT, despite the assumed symmetry of the buttocks anatomy. This finding, which was not observed in studies of control subjects, strongly indicates the unbalanced sitting posture of the spinal cord injury patient in his wheelchair. The surface stress profiling data provided by this work are useful to predict the unfortunate consequences of elevated localized muscle stresses, which may eventually cause the deep tissue injury. The system also demonstrated that the exposure time, to continuous, non-relieved, elevated stresses above 30kPa, was relatively high (i.e., longer than 30min). This is higher than observations in healthy individuals.

fig 2a fig 2b
Fig 2: Maximal stresses in gluteal muscle tissue of the buttocks under the right (a) and left (b) ischial tuberosities calculated in real-time during a 90min sitting trial

This research work was titled, Real-time Continuous Monitoring of Sub-dermal Tissue Stresses under the Ischial Tuberosities in Individuals with Spinal Cord Injury. It was presented at the June 2008 ASME Summer Bioengineering Conference in Marco Island, Florida.

Tactilus is a matrix-based tactile surface sensoressentially an electronic skinthat records and interprets pressure distribution and magnitude between any two contacting or mating surfaces and assimilates the collected data into a powerful Windows-based tool kit. Each Tactilus sensor is carefully assembled to exacting tolerances and is individually calibrated and serialized. The architectural philosophy of Tactilus is modular, allowing for portability, easy scalability, and simultaneous data collection from up to four discrete sensor pads. Tactilus employs sophisticated mathematical algorithms and advanced electronic shielding that maximizes the sensors immunity to noise, temperature, and humidity.