Basic Science Characterization and prediction of rate-dependent flexibility in lumbar spine biomechanics at room and body temperature Dean K. Stolworthy, MS, Shannon A. Zirbel, MS, Larry L. Howell, PhD, Marina Samuels, BS, Anton E. Bowden, PhD* 435 CTB, Department of Mechanical Engineering, Brigham Young University, Provo, UT 84602, USA Received 3 May 2012; revised 8 June 2013; accepted 23 August 2013 Abstract BACKGROUND CONTEXT: The soft tissues of the spine exhibit sensitivity to strain-rate and temperature, yet current knowledge of spine biomechanics is derived from cadaveric testing con- ducted at room temperature at very slow, quasi-static rates. PURPOSE: The primary objective of this study was to characterize the change in segmental flex- ibility of cadaveric lumbar spine segments with respect to multiple loading rates within the range of physiologic motion by using specimens at body or room temperature. The secondary objective was to develop a predictive model of spine flexibility across the voluntary range of loading rates. STUDY DESIGN: This in vitro study examines rate- and temperature-dependent viscoelasticity of the human lumbar cadaveric spine. METHODS: Repeated flexibility tests were performed on 21 lumbar function spinal units (FSUs) in flexion-extension with the use of 11 distinct voluntary loading rates at body or room temperature. Furthermore, six lumbar FSUs were loaded in axial rotation, flexion-extension, and lateral bending at both body and room temperature via a stepwise, quasi-static loading protocol. All FSUs were also loaded using a control loading test with a continuous-speed loading-rate of 1-deg/sec. The visco- elastic torque-rotation response for each spinal segment was recorded. A predictive model was de- veloped to accurately estimate spine segment flexibility at any voluntary loading rate based on measured flexibility at a single loading rate. RESULTS: Stepwise loading exhibited the greatest segmental range of motion (ROM) in all load- ing directions. As loading rate increased, segmental ROM decreased, whereas segmental stiffness and hysteresis both increased; however, the neutral zone remained constant. Continuous-speed tests showed that segmental stiffness and hysteresis are dependent variables to ROM at voluntary loading rates in flexion-extension. To predict the torque-rotation response at different loading rates, the model requires knowledge of the segmental flexibility at a single rate and specified temperature, and a scaling parameter. A Bland-Altman analysis showed high coefficients of determination for the predictive model. CONCLUSIONS: The present work demonstrates significant changes in spine segment flexibility as a result of loading rate and testing temperature. Loading rate effects can be accounted for using FDA device/drug status: Not applicable. Author disclosures: DKS: Nothing to disclose. SAZ: Nothing to dis- close. LLH: Grant: Crocker Spinal Technologies, Inc. (E, Paid directly to institution); Stock Ownership: Crocker Spinal Technologies, Inc. (1.25% ownership (personal), 5.5% ownership (Paid to Institution)). MS: Nothing to disclose. AEB: Grant: Crocker Spinal Technologies, Inc. (E, Paid directly to institution), National Science Foundation (F, Paid directly to institution); Stock Ownership: Crocker Spinal Technologies, Inc. (1.25% ownership (personal), 5.5% ownership (Paid to Institution)); Con- sulting: Bridging Medical (B), ZSX Medical (B). The disclosure key can be found on the Table of Contents and at www. TheSpineJournalOnline.com. Research funding for this work was provided by Crocker Spinal Tech- nologies, Inc., the BYU Technology Transfer Office, and the National Sci- ence Foundation (NSF Grant CMMI-0952758). IRB approval: All human cadaveric specimens were obtained and tested following an institutional research board–approved acquisition and testing protocol. * Corresponding author. Department of Mechanical Engineering, Brig- ham Young University, Provo, UT 84602, USA. Tel.: (801) 422-4760;fax: (801) 422-0516. E-mail address: abowden@byu.edu (A.E. Bowden) 1529-9430/$ - see front matter Ó 2014 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.spinee.2013.08.043 The Spine Journal 14 (2014) 789–798