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Immature Sheep Spines Are More Flexible Than Mature Spines: An In Vitro Biomechanical Study

Clarke, Elizabeth C., BE, BSc*†; Appleyard, Richard C., PhD; Bilston, Lynne E., PhD

doi: 10.1097/BRS.0b013e31815cde16

Study Design. Dynamic triaxial biomechanical testing of immature and mature ovine spine motion segments.

Objective. To compare torque-deflection parameters of mature and immature spine motion segments and to investigate whether scaling relationships apply between mature and immature motion segment torque-deflection responses.

Summary of Background Data. While previous studies have examined the cervical region in a limited number of loading directions, a comprehensive multiaxial study of the response of the pediatric spine at all 3 spinal levels (cervical, thoracic, and lumbar) has not been performed.

Methods. Motion segments from cervical, thoracic, and lumbar levels were tested under moment application about 3 axes for newborn and 2-year-old sheep. Range of motion, neutral zone, and stiffness were calculated for each motion segment and compared for immature and mature spine.

Results. Immature spine motion segments exhibited a significantly larger range of motion (P < 0.001) and neutral zone (P < 0.001) and significantly lower stiffness (P < 0.001) in comparison to mature spine segments about the 3 moment axes, at the 3 spinal levels tested. There were statistically significant interactions between specimen age and the moment axis and/or spinal level for some torque-deflection parameters.

Conclusion. The significantly greater neutral zone of immature spine suggests greater ligament laxity. Significantly higher range of motion and lower stiffness of the immature spine may be implicated in spinal cord injury mechanisms and implies a change in relative tolerance of the spine to damage with spinal maturity. Significant statistical interactions between spinal maturity and moment axis or motion segment level suggest that scaling torque-deflection parameters from mature to immature spine may not be appropriate.

Immature spinal joints are significantly more flexible than mature joints under dynamic torque application. This biomechanical difference suggests a change in relative tolerance of the spine to damage with spinal maturity and may explain some clinical differences between common mechanisms of spinal cord injury in adults and young children.

From the *Department of Aerospace, Mechanical and Mechatronic Engineering, University of Sydney, Sydney, Australia; †Prince of Wales Medical Research Institute, University of New South Wales, Sydney, Australia; and ‡Murray Maxwell Biomechanics Laboratory, Department of Orthopaedics and Traumatic Surgery, Kolling Institute of Medical Research, Royal North Shore Hospital, University of Sydney, Sydney, Australia.

Acknowledgment date: January 17, 2007. Revision date: March 21, 2007. Acceptance date: May 30, 2007.

Supported by an Australian Postgraduate Award and the University of Sydney Engineering Postgraduate Scholarship (to E.C.), an NHMRC Senior Research Fellowship (to L.E.B.), and the Lincoln Foundation (Royal North Shore Hospital) (to R.C.A.).

The manuscript submitted does not contain information about medical device(s)/drug(s).

No funds were received in support of this work. No benefits in any form have been or will be received from a commercial party related directly or indirectly to the subject of this manuscript.

Address correspondence and reprint requests to Lynne E. Bilston, PhD, Prince of Wales Medical Research Institute, Barker St, Randwick, NSW 2031, Australia; E-mail:

© 2007 Lippincott Williams & Wilkins, Inc.