Sagittal plane motion in the human lumbar spine: comparison of the in vitro quasistatic neutral zone and dynamic motion parameters.

Background: Disabling low back pain is often attributed to clinical instability but defining instability is problematic. The most common parameter used to characterize instability in the lab is the neutral zone which is measured with a quasi-static technique. But, it cannot be measured from continuous motion data. Our goal was to describe the relationship between the quasi-static neutral zone and dynamic motion parameters that might reflect laxity about the neutral position. We also sought to determine if dynamic parameters were correlated with disc degeneration.

Methods: Fifteen cadaveric lumbar motion segments were tested with both quasi-static and dynamic (continuous load) methods. Quasi-static range of motion and neutral zone were compared with dynamic range of motion, hysteresis loop width, and two parameters derived from the hysteresis data: transitional zone size and slope. Degeneration was graded macroscopically.

Findings: Neutral zone size was moderately correlated with hysteresis loop width (r=0.69) and strongly correlated with the transitional zone slope (r=-0.80). Degenerative grade had a significant effect on dynamic range of motion and transitional zone size and slope with differences found between grade 1 (normal) discs and higher grades. Only transitional zone slope was different between grades 1 and 2.

Interpretation: The transitional zone slope (representing the neutral region stiffness) had the strongest correlation with neutral zone and could best detect lower grades of degeneration. The transitional zone slope might be a useful parameter in dynamic studies investigating the association between degeneration and motion segment behavior.