CRITICAL LOAD OF THE HUMAN CERVICAL-SPINE - AN IN-VITRO EXPERIMENTAL-STUDY

Objective. To determine the critical load of the osteoligamentous cerv ical spine in frontal plane. Design. Whole human cervical spine specim ens were loaded in axial compression with increasing force until the p oint of buckling. Background. The osteoligamentous cervical spine and the surrounding muscles support the weight of the head and the externa l loads applied to it. Critical load is the maximum compressive force that the spinal column can sustain before buckling. Critical loads hav e been obtained for the osteoligamentous thoracolumbar spine (without the rib cage) and the lumbar spine. Critical load of the cervical spin e has not yet been determined. Methods. When a compressive force is ap plied 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. Ex periments were conducted to determine the axial force-lateral bending curves of whole cervical spine specimens. Critical load values were ob tained from these curves. As an alternative to this method, bending st iffness in the frontal plane was experimentally determined and the cri tical load was computed using Euler's theory of columns. Results. Base d upon the study of seven spine specimens (CO-T1), the critical load f or the human cervical spine was found to be 10.5 (3.8) N obtained by d irect experimentation. The average critical load calculated with the E uler theory using bending stiffness data, was 11.9 (2.0), but there we re large individual differences when compared with the experimental re sults. Conclusions. The critical load of the osteoligamentous human ce rvical spine is about one-fifth to one-quarter the weight of the avera ge head. Relevance Without muscles, the spin buckles under very low co mpressive loads. To ascertain the stabilizing role of musculature, the load carrying capacity of the isolated osteoligamentous spin must be known. (C) 1998 Published by Elsevier Science Ltd. All rights reserved .