Mechanical axial compression experiments

When a force is applied to the posterosuperior quadrant of the head or when a crown impact is administered while the head is in flexion, the neck is subjected to a combined load of axial compression and forward bending. Anterior wedge frartures of vertebral bodies are commonly seen, but with increased load, burst fractures and fracture-dislocations of the facets can result. The latter two conditions are unstable and tend to disrupt or injure the spinal cord, and the extent of the injury depends on the penetration of the vertebral body or its fragments into the spinal canal. Recent experiments by Pin tar et al. [1989, 1990] indicate that burst fractures of lower cervical vertebrae can be reproduced in cadaveric specimens by a crown impact to a flexed cervical spine. A study by Nightingale et al. [1993] showed that fracture-dislocations of the cervical spine occur very early in the impact event (within the first 10 ms) and that the subsequent motion of the head or bending of the cervical spine cannot be used as a reliable indicator of the mechanism of injury. 

Instrument compliance is another cause of concern in making mechanical measurements, since the forces generated by the fluid in response to a deformation will tend to twist, bend or compress the rheometer components that also experience these forces. These include shear and normal force transducers as well as the frame of the instrument. A sophisticated approach to dealing with this problem is the use of a Force Rebalance Transducer (product of Rheometrics/TA Instruments described, for example, by Vermant et al. [102]) in which a feedback loop provides torsional and axial motions to compensate for the corresponding compliances and thus minimize their effects on data. In the case of torsional motion, there remains some compliance due to the twisting of the shafts supporting the fixtures [103]. 

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