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Joutsen et al. Physical modeling studies of vibration transmittance to assess healing after sternotomy, SSRCTS Geilo 2015

Physical modeling studies of vibration transmittance to assess healing after sternotomy
A Joutsen1,2, J Hautalahti1, A Paldanius1,2, J Hyttinen2, J Laurikka1
Heart Center, Tampere University Hospital, Tampere, Finland. 2Department of Electronics and
communications engineering, Tampere University of Technology, Tampere, Finland
Objectives: Sternal instability following a sternotomy is a risk factor in the early postoperative
period for mediastinitis causing morbidity and mortality. A device measuring vibration
transmittance has been developed to assess sternal healing. To learn how the measuring geometry
affects the transmittance, a bench test using artificial physical models was conducted.
Materials and methods: The developed device includes two units, an actuator and a sensor, that are
placed in contact with the measured object. The actuator emits a 3 s long 20 Hz – 2 kHz vibration
stimulus and an accelerometer inside the sensor measures the transmittance of the vibration. Three
simple block models and one anatomical model based on CT images were built to simulate sternal
anatomy. Synthetic ballistic gel, 3D printed polylactic acid and polyurethane rubber modeled
thoracic soft tissue, bone and cartilage respectively.
Results: The three block models simulated intact (A), split (B) and steel wire bound (C) sternums.
The actuator – sensor distance was 6 cm. Ten repeated measurements were made on the block
models to assess the vibration transmittance. The measured power of the transmitted vibration was
A > C > B. All the comparisons between the conditions were statistically significant (p<0.001).
The anatomical model simulated intact sternum (D), intact sternum with soft tissue incision (E),
split sternum (F), loose closure (G) and tight closure (H) using steel sutures. The actuator and
sensor were placed bilaterally on the 2nd, 3rd, 4th and 5th costal cartilages, 6 cm apart. 20 repeated
measurements were made on each costal level 2-5 and condition (D-H). The results showed very
high variance between the costal levels and conditions with no trend, contrary to what was found in
the block model conditions A-C.
Discussion: Our results show that in the simple block models the vibration transmittance is
behaving as expected: intact > bound > split. However, the more elaborate anatomical model failed
to give consistent results, possibly due to gel separation from the 3D printed frame during the model
manipulation, surface unevenness below the measuring device and manual holding of the device
during a long measurement session. The device may be useful in assessing sternal healing, but
further work is needed to discover the most suitable mode of operation.