Both posterior and anterior procedures of stabilization are used for operative immobilization of unstable functional units of the cervical spine The primary stabilizing effect of each procedure was examined and the two were compared in an experimental study To this end the functional units C-5 and C-6 were removed from ten fresh cervical spines, the discoligamentous structures being preserved, and C-6 was embedded in methacrylate As a result of a tensile force in a vertical direction applied to the base of the spinous process of C-5, a flexion bending load was introduced into the unit, the main component of which was measured with the aid of one vertical and two horizontal-displacement transducers The respective tilting angle a and the translation were calculated on the basis of these values Each individual functional unit was measured with and without the discoligamentous lesion This posterior instability was then stabilized with an H-plate, a hook plate, sublaminar wiring, and various combinations of these. Our results lead to the following clinically relevant conclusions: 1 With isolated posterior instability, posterior fixation with the hook plate appears to bring about exercise stability. 2 With complete discoligamentous instability, the combined procedures certainly produce exercise stability, from a biomechanical point of view, the posterior hook, plate alone being capable of guaranteeing secure fixation.