title: The scapulo-humeral rhythm: effects of 2-D roentgen projection.
abstract: OBJECTIVE
  The objective of this study is to illustrate the low accuracy of two-dimensional (2-D) X-ray projection methods for the quantification of the three-dimensional (3-D) shoulder motions.
  
  
  BACKGROUND
  The traditional method for the quantification of the gleno-humeral motion is by means of 2-D X-ray recording. The motion was characterized by the scapulo-humeral rhythm: the ratio of the nett humeral elevation over nett scapular rotation. The method was based on the quantification of the planar projection of the spatial positions of X-ray dense structures of the scapula. The deformations introduced by the central projection method, a feature of X-ray projection, cannot be compensated for by calibration: the position of the scapula with respect to the camera setting is unknown, and skeletal landmarks of the scapula cannot uniquely be identified. The transformation from 3-D orientations to 2-D angles will, therefore, be inaccurate.
  
  
  METHODS
  A 2-D X-ray projection of the scapula during a typical arm abduction was simulated. The 3-D motion was obtained by means of palpation and subsequent digitization of skeletal landmarks of the scapula. The 3-D positions of the recorded landmarks were projected on a plane by a simulation based on the parameters of the X-ray equipment. The scapulo-humeral rhythm was calculated for the different scapular landmarks, and for the orientation of the subject with respect to the projection axis. The results were compared with previous published scapulo-humeral rhythms.
  
  
  RESULTS
  The scapulo-humeral rhythm depends both on the choice of the skeletal landmarks, used to quantify the scapular rotations, and on the orientation of the subject in the X-ray setting. The full range of results obtained from earlier published experiments could be obtained from a simulation based on a single 3-D arm abduction.
  
  
  CONCLUSIONS
  The 2-D scapulo-humeral rhythm, obtained from planar X-ray projection, is an inaccurate parameter to define the scapular motions.
  
  
  CONCLUSIONS
  The 2-D scapulo-humeral rhythm is an insensitive parameter to identify clinical disorders in the gleno-humeral motions, 3-D motion recording should be applied. Only when stringent precautions are taken with respect to repeatability of positioning of the subjects, can the method be used to study intra-individual effects, e.g., the follow-up of patients during treatment.
authors.affiliation: Laboratory of Measurement and Control, Faculty of Mechanical Engineering and Marine Technology, Delft University of Technology, The Netherlands.
authors.forename: J H
authors.initials: JH
authors.lastname: de Groot
mesh_terms: D001696:Biomechanical Phenomena; D006801:Humans; D006811:Humerus / Q000000981:diagnostic imaging / Q000502:physiology*; D011859:Radiography; D016059:Range of Motion, Articular / Q000502:physiology; D012399:Rotation; D012540:Scapula / Q000000981:diagnostic imaging / Q000502:physiology*; D012785:Shoulder Joint / Q000000981:diagnostic imaging / Q000502:physiology*