Time-frequency analysis to detect bone fracture in impact biomechanics. Application to the thorax

Abstract

Experimental testing is a major source of data to quantify the tolerance of the human body to impact and to develop protection strategies. Correlating the time of rib fractures with the kinematics of the occupant and the action of safety systems would provide valuable data for assessing safety systems and developing injury risk functions. However, methods for determining rib fractures timing are not yet fully developed. Time-history analysis of data from multiple strain gauges mounted directly to ribs is commonly used for this purpose, but this method is not very sensitive and the time and cost required to instrument the rib cage with more than 100 strain gauges is prohibitive for many applications. In this study a new approach based on time-scale analysis of signals obtained from piezoelectric transducers (PZT) is reported. A post-mortem human subject was instrumented with four PZT on ribs 3 and 7 bilaterally and exposed to lateral blunt impacts to the shoulder and the chest. The fractures were documented after each test, and a criterion was developed to process the PZT signals. The criterion consists in detecting in the PZT signal the onset of a high frequency transient generated by the fracture of a rib using the continuous wavelet transform. Two thresholds were successfully determined to detect fractures that occurred (1) on an instrumented rib, and (2) on the adjacent rib. Further development of this method should allow the detection of all rib fractures using only a few PZT.

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“…Anatomical points that were evaluated by the PAS protocol: (1) tragus (anterior view); (2) acromion (anterior view); (3) anterior-superior iliac spine (ASIS); (4) spinal processes of the third thoracic (T3) vertebrae; (5) scapula (inferior angle); (6) greater trochanter; (7) tragus (lateral view); (8) spinal processes of the seventh cervical (C7) vertebrae; (9) acromion (lateral view); (10) posterior-superior iliac spine; (11) knee (articular line); and (12) lateral malleoli.…”

“…The biomechanics of the thoracic cage do not operate in isolation but are integrated with the body's global mechanics. Therefore, any change in the thoracic cage may cause repercussions in the overall biomechanics of the body (8-10). …”

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“…accelerometer The signals obtained from the PZT and the accelerometers were processed using the wavelet analysis methodology of Gabrielli and coworkers [7,8]. Time scale analysis calculations, performed using a Morlet wavelet, enabled the signals to be converted into three-dimensional scalograms where the three axes are time, frequency and amplitude.…”

“…Both the data from the PZTs and the axial femur accelerometer are analysed by wavelet techniques in order to identify any changes in principal frequency modes during the impact. Gabrielli and coworkers [8] established that PZT signals could be processed using a criterion based on continuous wavelet transform (CWT) to determine rib bone fracture times. The criterion consists in detecting in the PZT signal the onset of a high-frequency transient generated by the fracture of a rib.…”

“…The criterion consists in detecting in the PZT signal the onset of a high-frequency transient generated by the fracture of a rib. In the experiment performed by Gabrielli and coworkers [8], the ribcage was free to move as no displacement was constrained, and therefore the frequency modes due to the impact were low and the high-frequency fracture signals could be detected. Unfortunately, the stiffer system and more constrained boundary conditions associated with the current experiments resulted in wavelet data which could not be interpreted as the frequency modes generated by the impact and the bone fracture were both in the same bandwidth.…”