Reliability of assisted indentation in measuring lumbar spinal stiffness

The reliability of manual methods to assess spinal stiffness is modest at best. In response, instrumentation has been developed which may be reliable, but is often difficult to use in clinical settings. The purpose of this study was to determine the intra-rater reliability of assisted indentation (AI), a smaller, less automated technique of measuring spinal stiffness in vivo. Twenty-three asymptomatic subjects were included in the study. The AI device was placed over the 4th lumbar spinous process in each prone, resting subject. Ten indentations were performed at approximately 2-min intervals while load and displacement data were collected simultaneously. From these data, two outcome variables were calculated: Global Stiffness (GS; slope of the force-displacement data) and Mean Maximal Stiffness (MMS; peak force/peak displacement). Intra-class correlation coefficient values for 10 consecutive measures of GS and MMS were 0.93 and 0.91, respectively. A repeated measures analysis of variance (ANOVA) did not demonstrate significant differences between any indentation trials from the same subject. Measurement of spinal stiffness using AI demonstrated excellent intra-rater reliability. These data, in addition to specific features of AI (small, transportable, relatively low cost, ease of operation) suggest that AI may be of benefit within clinical environments.