title: The parallax effect in the evaluation of range of motion in lumbar total disc replacement.
abstract: BACKGROUND
  Range of motion (ROM) has been shown to influence clinical outcomes of total disc replacement (TDR). While the parallax effect in image acquisition has been shown in the literature to influence the accuracy of a variety of measurements, this concept has not been investigated in the assessment of ROM analysis following TDR. We performed an evaluation of the influence of radiograph beam angle on "by hand" and on "gold standard" flexionextension ROM measurements in lumbar total disc replacement. The purpose of this study is to determine (1) the influence of X-ray beam angle on index level angle (ILA) measurements in lumbar TDR using the keel method, and (2) whether the out-of-plane radiographic beam effects cause a difference between true and calculated range of motion.
  
  
  METHODS
  Eight blinded orthopaedic surgeons used the keel method to calculate ROM measurements from radiographs of a flexible Sawbones model (Pacific Research Laboratories, Inc., Vashon, Washington) implanted with a ProDisc-L device (Synthes Spine, West Chester, Pennsylvania). Radiographs were obtained at beam angles of 0°, 5°, 10°, and 15° in the sagittal plane from the device center. Calculations were compared to measurements obtained by a validated digitized software method (Quantitative Motion Analysis, QMA, Medical Metrics, Inc., Houston, Texas). Inter- and intraobserver precision and accuracy were determined.
  
  
  RESULTS
  Compared with QMA, the radiographic keel method had an average error of 3.7°. No significant effect of variation in beam angle on interobserver precision (N = 16, P = .92) or accuracy (N = 16, P = 0.86) or intraobserver precision (N = 8, P = .09) or accuracy (N = 8, P = 0.07) of ROM measurements was identified. Repeat testing with QMA also revealed no effect of parallax and resulted in nearly identical ROM measurements.
  
  
  CONCLUSIONS
  Accuracy and precision of the keel method to determine ROM from index level angle measurements after TDR was not affected by increases in X-ray beam angles up to 15° from the device center.
  
  
  CONCLUSIONS
  Our study demonstrates that range of motion measurements are not influenced by parallax effect when using the keel method to determine index level angle measurements in lumbar total disc replacement.
authors: The Department of Orthopaedic Surgery, Washington University School of Medicine, St. Louis, Missouri.
authors: Joshua D
authors: JD
authors: Auerbach
authors: The Department of Orthopaedic Surgery, University of Pennsylvania, Philadelphia, Pennsylvania.
authors: Surena
authors: S
authors: Namdari
authors: The University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania.
authors: Andrew H
authors: AH
authors: Milby
authors: The Department of Orthopaedic Surgery, Beth Israel Deaconess Medical Center, Boston, Massachusetts.
authors: Andrew P
authors: AP
authors: White
authors: The Department of Orthopaedic Surgery, University of Pennsylvania, Philadelphia, Pennsylvania.
authors: Sudheer C
authors: SC
authors: Reddy
authors: Department of Orthopaedic Surgery, NYU-Hospital for Joint Diseases, New York, New York.
authors: Baron S
authors: BS
authors: Lonner
authors: 3B Orthopaedics, Pennsylvania Hospital, Philadelphia, Pennsylvania.
authors: Richard A
authors: RA
authors: Balderston
keywords: disc replacement; lumbar; parallax; range of motion