title: Instantaneous centers of rotation for lumbar segmental extension in vivo.
abstract: The study aimed to map instantaneous centers of rotation (ICRs) of lumbar motion segments during a functional lifting task and examine differences across segments and variations caused by magnitude of weight lifted. Eleven healthy participants lifted loads of three different magnitudes (4.5, 9, and 13.5kg) from a trunk-flexed (~75°) to an upright position, while being imaged by a dynamic stereo X-ray (DSX) system. Tracked lumbar vertebral (L2-S1) motion data were processed into highly accurate 6DOF intervertebral (L2L3, L3L4, L4L5, L5S1) kinematics. ICRs were computed using the finite helical axis method. Effects of segment level and load magnitude on the anterior-posterior (AP) and superior-inferior (SI) ICR migration ranges were assessed with a mixed-effects model. Further, ICRs were averaged to a single center of rotation (COR) to assess segment-specific differences in COR AP- and SI-coordinates. The AP range was found to be significantly larger for L2L3 compared to L3L4 (p=0.02), L4L5 and L5S1 (p<0.001). Average ICR SI location was relatively higher - near the superior endplate of the inferior vertebra - for L4L5 and L5SI compared to L2L3 and L3L4 (p≤0.001) - located between the mid-transverse plane and superior endplate of the inferior vertebra - but differences were not significant amongst themselves (p>0.9). Load magnitude had a significant effect only on the SI component of ICR migration range (13.5kg>9kg and 4.5kg; p=0.049 and 0.017 respectively). The reported segment-specific ICR data exemplify improved input parameters for lumbar spine biomechanical models and design of disc replacements, and base-line references for potential diagnostic applications.
authors.affiliation: Empa, Swiss Federal Laboratories for Materials Science and Technology, Mechanical Systems Engineering, Ueberlandstrasse 129, 8600 Duebendorf, Switzerland.
authors.forename: Ameet
authors.initials: A
authors.lastname: Aiyangar
authors.affiliation: Department of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, PA 15203, USA; Health Effects Lab Division, National Institute for Occupational Safety and Health, Morgantown, West Virginia, 26505, USA.
authors.forename: Liying
authors.initials: L
authors.lastname: Zheng
authors.affiliation: Department of Orthopaedic Surgery, University of Pittsburgh, Pittsburgh, PA 15203, USA.
authors.forename: William
authors.initials: W
authors.lastname: Anderst
authors.affiliation: Department of Orthopaedic Surgery, University of Pittsburgh, Pittsburgh, PA 15203, USA; Department of Bioengineering, University of Pittsburgh, Pittsburgh, PA 15203, USA; Department of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, PA 15203, USA. Electronic address: xuz9@pitt.edu.
authors.forename: Xudong
authors.initials: X
authors.lastname: Zhang
mesh_terms: D000328:Adult; D001696:Biomechanical Phenomena; D005260:Female; D064368:Healthy Volunteers; D006801:Humans; D008159:Lumbar Vertebrae / Q000502:physiology*; D008297:Male; D016059:Range of Motion, Articular; D012399:Rotation; D060726:Torso / Q000502:physiology; D016474:Weight-Bearing; D055815:Young Adult
keywords: Dynamic stereo X-ray; Instantaneous center of rotation; Lifting task; Lumbar spine; Vertebral motion