title: Simulation of a Bead Placement Protocol for Follow-up of Thoracic Spinal Fusion Using Radiostereometric Analysis.
abstract: METHODS
  Computer simulation to detect intervertebral motion enabling future follow-up of spinal fusions performed on patients with multilevel thoracic scoliosis.
  
  
  OBJECTIVE
  To propose a method using computer simulation to evaluate a radiostereometric analysis (RSA) marker placement protocol for visibility and redundancy and validate the performance of the developed RSA system in detecting intervertebral motion.
  
  
  BACKGROUND
  Radiostereometric analysis is a stereo x-ray technique in which clusters of tantalum markers are implanted to label well-defined landmarks and measure the relative motion between rigid bodies.
  
  
  METHODS
  A model of the spine with the instrumentation and the RSA markers was developed. The vertebrae were aligned to mimic multilevel thoracic scoliosis after correction. The researchers performed virtual segment motion to validate the performance of the developed system. X-ray images were simulated and RSA was used to evaluate the proposed marker placement protocol and detect virtual motion. The authors performed a physical phantom study to evaluate marker visibility.
  
  
  RESULTS
  All markers were located and matched between simulations and the condition numbers were well below the recommended value of 100. Based on computer simulation, average translational accuracy was 0.14, 0.01, and 0.24 mm along the x, y, and z axes, respectively, and average rotational accuracy was 0.23°, 0.12°, and 0.11° about the x, y, and z axes, respectively. The translational and rotational precision of the simulated RSA system was generally high. The physical phantom study agreed with the computer simulation and validated marker visibility.
  
  
  CONCLUSIONS
  Computer simulation is a powerful tool that can be used to facilitate the development and refinement of an RSA system before its application in patients, particularly when the anatomy involved is complex. The proposed marker placement protocol yielded translational and rotational accuracy below the limits of clinical significance, which enables future follow-up of multilevel thoracic scoliosis with Lenke classification 1AN.
authors.affiliation: School of Biomedical Engineering, Dalhousie University, Halifax, Canada; School of Mechanical Engineering, University of Leeds, Leeds LS2 9JT, United Kingdom. Electronic address: a.boufrancis@leeds.ac.uk.
authors.forename: Antony
authors.initials: A
authors.lastname: Bou-Francis
authors.affiliation: School of Biomedical Engineering, Dalhousie University, Halifax, Canada; Department of Applied Oral Sciences, Dalhousie University, Halifax, Canada.
authors.forename: J Michael
authors.initials: JM
authors.lastname: Lee
authors.affiliation: School of Biomedical Engineering, Dalhousie University, Halifax, Canada; Department of Surgery, Dalhousie University, Halifax, Canada; QEII Health Sciences Center, Halifax, Canada.
authors.forename: Michael
authors.initials: M
authors.lastname: Dunbar
authors.affiliation: School of Biomedical Engineering, Dalhousie University, Halifax, Canada; Department of Surgery, Dalhousie University, Halifax, Canada; IWK Health Center, Halifax, Canada.
authors.forename: Ron
authors.initials: R
authors.lastname: El-Hawary
keywords: Computer simulation; Intervertebral motion; Multilevel fusion; Radiostereometric analysis; Thoracic scoliosis