Abstract
Abstract
Introduction
Altered mechanical loading is a contributing factor to low back pain, a condition affecting 80% of the population at some point in life. A plethora of in-vitro studies exist focusing on 6 degree of freedom (dof) testing of functional spinal units (FSU) to obtain a specimen stiffness matrix. Due to differences in the performance of test apparatus and in the technique used to manipulate raw data it is difficult to compare results from different groups.
Objectives
Our primary objective was to develop a standardised technique to benchmark the performance of testing apparatus; a secondary objective was to standardise the data manipulation technique.
Methods
6 tests each at 250N and 500N preload were performed on synthetic FSU specimens using the Bath spine simulator, with a further 3 tests performed on porcine specimens. Three techniques were used to evaluate stiffness: first the slope of the entire load-displacement curve, inclusive of loading and unloading portions, was considered, secondly zonal stiffnesses were defined by dividing the load displacement curve into elastic and neutral zones, finally stiffness was calculated only for the loading portion of the elastic and neutral zone. The standard error of the residuals was used to compare results.
Results
The stiffness matrix principal elements of the synthetic FSU showed repeatability of 3.2% and reflected specimen symmetry in the x and y (8% error). The stiffness calculation techniques including both loading and unloading portions of the curve were affected by hysteresis, an issue that did not arise when only the loading portion was used in calculations.
Conclusions
It is recommended that testing apparatus performance is evaluated with synthetic specimens, to allow benchmarking against different set-ups. Furthermore, it is recommended that stiffness calculations are performed only on the loading part of the curve to eliminate the influence of hysteresis on the results.
Declaration of Interest
(b) declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research reported:I declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research project.
