Development of an ultrasound system to measure in vivo dynamic cervical spine intervertebral disc mechanics
Shiuan, Kevin Shiue
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Neck and back pain have become increasingly prevalent in the military population, especially in those that experience variable frequency loading environments such as helicopter pilots. The number of cervical spine injuries is expected to increase with the advent of heavier head-mounted equipment with most problems attributed to intervertebral disc complications. Currently, CT and MRI are the gold standards for the evaluation ofthe cervical spine. However, those modalities are not suited to capture real-time biomechanical information in extreme environments. Clinical ultrasound, which is known for dynamic imaging, was explored as a valuable tool to image motion of the cervical spine, specifically vertebral motion and intervertebral disc deformation. Ultrasound imaging was first validated ex vivo and then utilized in vivo to demonstrate applicability. Compliances of FSU levels C4-C5 and C5-C6 were successfully calculated using static loads in compression and distraction. Dynamic analysis of intervertebral disc deformation (vertebral end plate displacements) from four cadaveric cervical spines (C2- C7) showed an overall uncertainty error of ±0.148 mm. The reliability of measurements was considered poor at vertebral loading frequencies higher than 4 Hz. A transfer function of intervertebral disc deformation in response to external translations was empirically derived through discrete Fourier transform analysis; however, the uncertainty of the measurements was too great to accurately describe the function. The capability of ultrasound to detect a difference in intervertebral disc responses in various conditions was explored on human subjects with the ultrasound probe mounted on a makeshift cervical collar at the C4-C5 functional spinal unit. A significant difference in response was detected when subjects wore an Army issued helmet with an attached weight to mimic night vision goggles. The results from these experiments will help develop ultrasound into an inexpensive, portable, and safe technique to evaluate cervical spine kinetics and kinematics. Furthermore, this expansion of ultrasound's capabilities beyond its current clinical indications can be applied within the civilian population to provide a low-cost, portable, and non-ionizing cervical spine imaging solution.
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