Alterations in primary and coupled neck motions after facetectomy.

Normal neck motion is a complicated phenomenon. A primary motion results from an applied force or torque. Secondary coupled motions also result as a consequence of the primary motion. These resulting coupled motions were measured in isolated cervical motion segments using three-dimensional videophotogrammetry with a coordinate system fixed in space. A cross coupling algorithm provided error correction and conversion from local to true cartesian coordinates. The data were analyzed by computer, and the true coupled motions resulting from any applied force were obtained. A second group of specimens was facetectomized, and the experiments were repeated. After facetectomy, a moment about the anteroposterior axis resulted in marked reduction in lateral displacement, decrease in vertical displacement, and decrease in rotation about the vertical axis. Results were significant at P less than 0.01 and represent a significant decrease in coupled motions resulting from lateral bending. At P less than 0.05, a laterally applied force causes less lateral displacement and lateral flexion, but more longitudinal displacement. Flexion and extension movements are not significantly altered after facetectomy.