title: A patient-specific measurement technique to model shoulder joint kinematics.
abstract: BACKGROUND
  Measuring dynamic in vivo shoulder kinematics is crucial to better understanding numerous pathologies. Motion capture systems using skin-mounted markers offer good solutions for non-invasive assessment of shoulder kinematics during dynamic movement. However, none of the current motion capture techniques have been used to study translation values at the joint, which is crucial to assess shoulder instability. The aim of the present study was to develop a dedicated patient-specific measurement technique based on motion capture and magnetic resonance imaging (MRI) to determine shoulder kinematics accurately.
  
  
  OBJECTIVE
  Estimation of both rotations and translations at the shoulder joint using motion capture is feasible thanks to a patient-specific kinematic chain of the shoulder complex reconstructed from MRI data.
  
  
  METHODS
  We implemented a patient-specific kinematic chain model of the shoulder complex with loose constraints on joint translation. To assess the effectiveness of the technique, six subjects underwent data acquisition simultaneously with fluoroscopy and motion capture during flexion and empty-can abduction. The reference 3D shoulder kinematics was reconstructed from fluoroscopy and compared to that obtained from the new technique using skin markers.
  
  
  RESULTS
  Root mean square errors (RMSE) for shoulder orientation were within 4° (mean range: 2.0°-3.4°) for each anatomical axis and each motion. For glenohumeral translations, maximum RMSE for flexion was 3.7mm and 3.5mm for empty-can abduction (mean range: 1.9-3.3mm). Although the translation errors were significant, the computed patterns of humeral translation showed good agreement with published data.
  
  
  CONCLUSIONS
  To our knowledge, this study is the first attempt to calculate both rotations and translations at the shoulder joint based on skin-mounted markers. Results were encouraging and can serve as reference for future developments. The proposed technique could provide valuable kinematic data for the study of shoulder pathologies.
  
  
  METHODS
  Basic Science Study.
authors.affiliation: Artanim Foundation, Medical Research Department, Geneva, Switzerland. Electronic address: caecilia.charbonnier@artanim.ch.
authors.forename: C
authors.initials: C
authors.lastname: Charbonnier
authors.affiliation: Artanim Foundation, Medical Research Department, Geneva, Switzerland.
authors.forename: S
authors.initials: S
authors.lastname: Chagué
authors.affiliation: Rive Droite Radiology Center, Geneva, Switzerland.
authors.forename: F C
authors.initials: FC
authors.lastname: Kolo
authors.affiliation: Department of Geomatics Engineering, University of Calgary, Calgary, Canada.
authors.forename: J C K
authors.initials: JC
authors.lastname: Chow
authors.affiliation: Division of Orthopaedics and Trauma Surgery, La Tour Hospital, Geneva, Switzerland; Faculty of Medicine, University of Geneva, Geneva, Switzerland.
authors.forename: A
authors.initials: A
authors.lastname: Lädermann
mesh_terms: D000328:Adult; D001696:Biomechanical Phenomena; D005240:Feasibility Studies; D005471:Fluoroscopy; D006801:Humans; D007091:Image Processing, Computer-Assisted; D008279:Magnetic Resonance Imaging; D008297:Male; D016059:Range of Motion, Articular / Q000502:physiology*; D012016:Reference Values; D012785:Shoulder Joint / Q000033:anatomy & histology* / Q000502:physiology*
keywords: Fluoroscopy; Glenohumeral translation; Global optimization; Motion capture; Shoulder kinematics