Is the clinical concept of spinal stiffness multidimensional?

This study investigated whether the poor reliability of judgments of posteroanterior (PA) spinal stiffness is due to rater bias or is a consequence of raters each having individual concepts of PA stiffness.

Three pairs of manipulative physical therapists with a minimum of 5 years of experience took part in the study.

The raters were required to make stiffness judgments of a series of metal springs, and their performance at this task was compared with that obtained when they rated the PA stiffness of patients with low back pain. A range of reliability indices were calculated and evaluated to establish whether rater bias contributed to poor reliability in either task. The relationship between each rater's estimates of the magnitude of the stimuli and the measured stiffness of the springs was also assessed using the Pearson Product-Moment Correlation Coefficient.

The average intraclass correlation coefficient (2, 1) for rating spring stiffness was found to be .60, whereas for human spines it was found to be .19. There was no evidence of rater bias contributing to poor reliability for rating stiffness of human spines. The average correlation between the rater's estimates of the magnitude of the stimuli and the measured stiffness of the stimuli was .80.

Physical therapists demonstrated much better ability to judge spring stiffness than the PA stiffness of human spines. This difference in performance implies that mechanical stiffness is not equivalent to the clinical concept of PA stiffness. Posteroanterior stiffness may have more than one dimension, and individual interpretation of stiffness as a construct may lead to rater disagreement in the clinic. The reliability of judgments of PA spinal stiffness may be enhanced in the future if its dimensions can be identified, defined, and taken into account during clinical procedures.