J. Biomechanics Vol. 21. NO. 9, pp.769-779, 1988 Printed in GreatBritain OOZI-9290/88 $3.00 + .OO zyxwvutsrqpo Pergamon Press plc zyxwvutsrqpon LOAD-DISPLACEMENT PROPERTIES OF LOWER CERVICAL SPINE MOTION SEGMENTS SEAN P. MORONEY,* ALBERT B. SCHULTZ,* JAMES A. A. MILLER* and GUNNAR B. J. ANDERSSON~ *Department of Mechanical Engineering, University of Michigan, Ann Arbor, MI 48109-2125, U.S.A. and t Department of Orthopaedic Surgery, Rush-Presbyterian Medical Center, Chicago, IL, U.S.A. Abstract-The load-displacement behavior of 35 fresh adult cervical spine motion segments was measured in compression, shear, flexion, extension, lateral bending and axial torsion tests. Motion segments were tested both intact and with posterior elements removed. Applied forces ranged to 73.6 N in compression and to 39 N in shear, while applied moments ranged to 2.16 Nm. For each mode of loading, principal and coupled motions were measured and stiffnesses were calculated. The effect of disc degeneration on motion segment stiffnesses and the moments required for motion segment failure were also measured. In compression, the stiffnesses of the cervical motion segments were similar to those of thoracic and lumbar motion segments. In other modes of loading, cervical stiffnesses were considerably smaller than thoracic or lumbar stiffnesses. Removal of the posterior elements decreased cervical motion segment stiffnesses by as much as 50”,‘,. Degenerated cervical discs were less stiff in compression and stiffer in shear than less degenerated discs, but in bending or axial torsion, no statistically significant differences were evident. Bending moments causing failure were an order of magnitude lower than those for lumbar segments. INTRODUCTION Knowledge of the load-displacement properties of the spine is useful in understanding its mechanical be- havior and for investigating a number of spine patho- logies. A basic mechanical unit of the spine is the motion segment, which consists of two adjacent ver- tebrae and their intervening soft tissues. Lysell (1969) studied the motion patterns of lower cervical motion segments without reference to the loads required to produce the motions. Earlier studies of human spine motion segments which have reported load-displace- ment properties have largely considered the motion segments of the thoracic and lumbar regions. For example, Virgin (1951), Hirsch and Nachemson (1954) and Brown et al. (1957) tested lumbar motion segments in compression. Markolf (1970, 1972) tested thoracic and lumbar motion segments in compression, shear, bending and axial torsion. Panjabi et al. (1976) tested thoracic motion segments in these modes as well as in tension. Lin et al. (1978) tested lumbar motion seg- ments in shear and in axial and eccentric compression. Berkson et al. (1979) and Schultz et zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA al. (1979) tested lumbar motion segments in compression, shear, bend- ing and axial torsion. Tencer and Ahmed (1981) investigated the effects of secondary experimental variables on the measurement of the mechanical pro- perties of lumbar motion segments. More recently, Panjabi et zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA al. (1983) and Panjabi et al. (1986) tested cervical motion segments in shear, compression and tension. Zidel et al. (1985) tested cervical motion segments in tension, compression, shear, bending and axial torsion. The cervical region of the spine differs from the lumbar and thoracic regions in that it bears less body Received 11 August 1986: in reoisedJorm 2 February 1988. weight and is in general more mobile. On these grounds, the mechanical properties of its motion segments might be expected to differ from those of the lumbar and thoracic regions. This paper reports the load-displacement behavior of 35 fresh human cadaver cervical spine motion segments tested in compression, shear, bending and axial torsion. Motions in response to these loads were measured and corresponding stiffnesses were com- puted for the principal motions. The influences of both the intervertebral disc level and the degree of degener- ation were examined. Moments required to produce motion segment failure were measured. The influence of motion segment gross morphology was examined to determine to what extent it might explain variations in measured properties. The effect of a bony union of the facet joints, observed in two segments, was also explored. MATERIALS AND METHODS Thirty-five adult motion segments excised from 16 cervical spine sections were tested. The spine sections had been previously removed at autopsy within 24 h after death and stored at - 20°C. Data regarding age, sex, medical history, or cause of death were not available for these specimens. In preparation for testing, muscular tissue was removed from each spine section before it was divided into individual motion segments. Each segment was visually examined and palpated for evidence of damage. In addition, the segments were radiographed in sagittal, transverse and coronal planes and the films were examined to insure that no fractures or evidence of other pathologies existed. Segments were tested as ‘intact segments’ or as ‘disc segments’. An intact segment was anatomically com- 169