Abstract
Background and Purpose:
The inability of intervertebral joints to resist perturbation due to laxity is traditionally measured in cadaveric specimens as their neutral zones (NZ). However in patients, quantitative fluoroscopic (QF) examinations substitute the Initial Attainment Rate for this. If these two measures correspond sufficiently, a clinical method for measuring segmental instability is possible. This study explored this by determining the criterion validity of the Initial Attainment Rate against the Dynamic NZ in an unloaded multilevel porcine spine.
Methods and Results:
A 5-segment porcine spine was prepared and mounted on a motorised horizontal motion platform fitted with a digital force gage. Left and right bending moments were calculated about each intervertebral joint for 10 repeated side bends using an inverse dynamics method. The Dynamic NZs and Initial Attainment Rates in the first 10° of platform motion at each level were correlated.
The Initial Attainment Rates were comparable to those found in vivo in healthy controls. Substantial and highly significant levels of correlation between these and Dynamic NZs were found for left (rho= 0.75, p=0.0002) and combined left-right bending (rho=0.72, p=0.0001) and moderate for right bending alone (rho=0.55, p=0.0012).
Conclusion:
This study found good correlation between the Initial Attainment Rate and the Dynamic NZ indicating the feasibility of assessing intervertebral laxity using QF in clinical studies. Confirmatory studies with multiple specimens adding sagittal and axial plane rotations are needed.
