Tensile testing of the human cervical spine involves some unique instrumentation challenges. Because of the curvature of the neck, even simple tensile testing results in combined loading. In order to understand and characterize this loading, the test apparatus may need to be fairly complex. Historically, reported stiffnesses from load-displacement data have assumed that the compliances of the test frame and the casting do not intrude on the measured response. However, because of the relatively high stiffness of spine specimens and the complexity of the test apparatus, this is not necessarily the case.

In this study, 17 specimen preparations were tested in tension under a variety of end conditions. Tests were performed on whole spines and on individual motion segments. On average, the stiffness of the motion segment preparations was 257 N/mm, and the stiffness of the whole cervical spine was 48 N/mm. The test frame was found to have a stiffness of 933 N/mm. Assembling a whole spine from a series combination of 8 motion segments with a stiffness of 257 N/mm results in a whole spine stiffness of 32.1 N/mm (32% error). The actual motion segment stiffness can be calculated by assuming that the preparation stiffness is a series combination of the stiffnesses of the motion segment and the frame. Applying this correction to the segments in this study results in an actual motion segment stiffness of 356 N/mm. Taking the frame compliance into account, the actual whole spine stiffness is 51 N/mm. A series combination of 8 motion segments using the corrected stiffnesses results in a whole spine stiffness of 45.0 N/mm (12% error).

We conclude that the compliance of the frame and the fixation must be quantified in all tension studies of spinal segments. Further, reported stiffness should be adjusted to account for frame and fixation compliance. This may not be possible for existing spine data in the world literature due to an absence of data on individual test frame compliance.