Comparison between sheep and human cervical spines - An anatomic, radiographic, bone mineral density, and biomechanical study

Study Design. The quantitative anatomic, radiographic, computerized tomogra phic, and biomechanical data of sheep and human cervical spines were evalua ted. Objectives. To compare the anatomic, radiographic, computerized tomographic , and biomechanical data of human and sheep cervical spines to determine wh ether the sheep spine is a suitable model for human spine research. Summary of Background Data. Sheep spines have been used in several in vivo and in vitro experiments. Quantitative data of the normal sheep cervical sp ine are racking, yet these data are crucial to discussion a bout the result s of such animal studies. Methods. In this study, 20 fresh adult female Merino sheep cervical spines and 20 fresh human cadaver cervical spines were evaluated anatomically, rad iographically, computerized tomographically, and biomechanically. Three lin ear and two angular parameters were evaluated on four digital radiographic views: anteroposterior, right lateral in neutral position, flexion, and ext ension. Quantitative computed tomography scans at the center of each verteb ral body and 3 mm below both endplates were analyzed for bone mineral densi ty measurements. Biomechanical testing was performed in flexion, extension, axial rotation, and lateral bending by a nondestructive stiffness method u sing a nonconstrained testing apparatus. Range of motion and stiffness of e ach motion segment were calculated. Additionally, 10 linear anatomic parame ters of each vertebra were measured using a digital ruler. Results. Anterior and mean disc space height in the sheep cervical spine in creased constantly from C2-C3 to C6-C7, whereas middle disc space height de creased and posterior disc space height remained unchanged. Anterior and me an disc space height were significantly higher in sheep. In both sheep and human cervical spines, intervertebral angles were not significantly differe nt. Standard deviations of bone mineral density in the human cervical spine were fourfold higher than in the sheep cervical spine, yet no significant differences were found in bone mineral density values between the two speci es. Range of motion differed significantly between the two species except i n flexion-extension of C3-C4, C5-C6, axial rotation of C2-C3, and lateral b ending of C2-C3, C3-C4, and C4-C5. Stiffness also was significantly differe nt except in flexion-extension of C2-C3, C4-C5, C5-C6, and lateral bending of C2-C3, C3-C4, and C4-C5. Anatomic evaluation showed no difference in upp er endplate parameters for C4 and C5. Conclusions. Although several differences were found between human and shee p cervical spines; the small intergroup standard deviations and the good co mparability: with the human spine encourage the use of the sheep cervical s pine as a model for cervical spine research. On the basis of the quantitati ve data obtained in this study, the sheep motion segment C3-C4 seemed to be the most reliable model for the corresponding human motion segment.