The R-zero line: a new kinematic concept of the lumbar spine.

A new kinematic concept is described that relates to the notion that the lumbar spine behaves as a complete structure. This concept is called the "R-zero line" and represents kinematically the neutral position of the lumbar spine between anteflexion and retroflexion. The clinical significance of this parameter is evaluated in normal individuals and in patients with a symptomatic herniated disc at L4-5, with a symptomatic herniated disc at L5-S1, and with spondylolysis/spondylolisthesis. The results of this study show that the R-zero line has a characteristic configuration in all four test groups.