Critical load of the human cervical spine: an in vitro experimental study

OBJECTIVE: To determine the critical load of the osteoligamentous cervical spine in frontal plane. DESIGN: Whole human cervical spine specimens were loaded in axial compression with increasing force until the point of buckling. BACKGROUND: The osteoligamentous cervical spine and the surrounding muscles support the weight of the head and the external loads applied to it. Critical load is the maximum compressive force that the spinal column can sustain before buckling. Critical loads have been obtained for the osteoligamentous thoracolumbar spine (without the rib cage) and the lumbar spine. Critical load of the cervical spine has not yet been determined. METHODS: When a compressive force is applied to the cervical spine, it bends in the sagittal plane producing greater lordosis. The determination of critical load in Euler's sense requires blocking of this sagittal plane bending. A special apparatus was developed that constrained such bending in the sagittal plane, but allowed complete freedom of the spine motion in the frontal plane. Experiments were conducted to determine the axial force-lateral bending curves of whole cervical spine specimens. Critical load values were obtained from these curves. As an alternative to this method, bending stiffness in the frontal plane was experimentally determined and the critical load was computed using Euler's theory of columns. RESULTS: Based upon the study of seven spine specimens (CO-T1), the critical load for the human cervical spine was found to be 10.5 (3.8) N obtained by direct experimentation. The average critical load calculated with the Euler theory using bending stiffness data, was 11.9 (2.0), but there were large individual differences when compared with the experimental results. CONCLUSIONS: The critical load of the osteoligamentous human cervical spine is about one-fifth to one-quarter the weight of the average head.