title: Continuous cervical spine kinematics during in vivo dynamic flexion-extension.
abstract: BACKGROUND
  A precise and comprehensive definition of "normal" in vivo cervical kinematics does not exist due to high intersubject variability and the absence of midrange kinematic data. In vitro test protocols and finite element models that are validated using only end range of motion data may not accurately reproduce continuous in vivo motion.
  
  
  OBJECTIVE
  The primary objective of this study was to precisely quantify cervical spine intervertebral kinematics during continuous, functional flexion-extension in asymptomatic subjects. The advantages of assessing continuous intervertebral kinematics were demonstrated by comparing asymptomatic controls with patients with single-level anterior arthrodesis.
  
  
  METHODS
  Cervical spine kinematics were determined during continuous in vivo flexion-extension in a clinically relevant age group of asymptomatic controls and a group of patients with C5-C6 arthrodesis.
  
  
  METHODS
  The patient sample consisted of 6 patients with single-level (C5-C6) anterior arthrodesis (average age: 48.8±6.9 years; 1 male, 5 female; 7.6±1.2 months postsurgery) and 18 asymptomatic control subjects of similar age (average age: 45.6±5.8 years; 5 male, 13 female).
  
  
  METHODS
  Outcome measures included the physiologic measure of continuous kinematic motion paths at each cervical motion segment (C2-C7) during flexion-extension.
  
  
  METHODS
  Participants performed flexion-extension while biplane radiographs were collected at 30 images per second. A previously validated tracking process determined three-dimensional vertebral positions with submillimeter accuracy. Continuous flexion-extension rotation and anterior-posterior translation motion paths were adjusted for disc height and static orientation of each corresponding motion segment.
  
  
  RESULTS
  Intersubject variability in flexion-extension angle was decreased 15% to 46% and intersubject variability in anterior-posterior translation was reduced 14% to 33% after adjusting for disc height and static orientation angle. Average intersubject variability in continuous motion paths was 1.9° in flexion-extension and 0.6 mm in translation. Third-order polynomial equations were determined to precisely describe the continuous flexion-extension and anterior-posterior translation motion path at each motion segment (all R2>0.99).
  
  
  CONCLUSIONS
  A significant portion of the intersubject variability in cervical kinematics can be explained by the disc height and the static orientation of each motion segment. Clinically relevant information may be gained by assessing intervertebral kinematics during continuous functional movement rather than at static, end range of motion positions. The fidelity of in vitro cervical spine mechanical testing protocols may be evaluated by comparing in vitro kinematics to the continuous motion paths presented.
authors.affiliation: Department of Orthopaedic Surgery, University of Pittsburgh, 3820 South Water St, Pittsburgh, PA 15203, USA. Electronic address: anderst@pitt.edu.
authors.forename: William J
authors.initials: WJ
authors.lastname: Anderst
authors.affiliation: Department of Orthopaedic Surgery, University of Pittsburgh, 3820 South Water St, Pittsburgh, PA 15203, USA.
authors.forename: William F
authors.initials: WF
authors.lastname: Donaldson
authors.affiliation: Department of Orthopaedic Surgery, University of Pittsburgh, 3820 South Water St, Pittsburgh, PA 15203, USA.
authors.forename: Joon Y
authors.initials: JY
authors.lastname: Lee
authors.affiliation: Department of Orthopaedic Surgery, University of Pittsburgh, 3820 South Water St, Pittsburgh, PA 15203, USA.
authors.forename: James D
authors.initials: JD
authors.lastname: Kang
mesh_terms: D000328:Adult; D001696:Biomechanical Phenomena; D002574:Cervical Vertebrae / Q000033:anatomy & histology / Q000502:physiology* / Q000601:surgery; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D016059:Range of Motion, Articular / Q000502:physiology; D012016:Reference Values; D012399:Rotation; D013123:Spinal Fusion
keywords: Adjacent segment disease; Anterior cervical fusion; Percent contribution