title: Coaptation/elevation role of the middle deltoid muscle fibers: a static biomechanical pilot study using shoulder MRI.
abstract: OBJECTIVE
  It is generally recognized that the middle deltoid muscle fibers (MDMF) have an elevating effect on the humeral head at small abduction angles. These forces are normally counterbalanced by the coaptation effect of the rotator cuff (RC) muscles to maintain shoulder stability. With RC tears, this balance may be jeopardized leading to shoulder dysfunction. Conversely, not all patients with RC tears develop shoulder dysfunction. The purpose of this study was to investigate the coaptation/elevation forces (CEF) ratio of the MDMF in RC tears subjects and in volunteers, using magnetic resonance imaging (MRI).
  
  
  METHODS
  Magnetic resonance imaging images in 10 subjects with RC tears and in five volunteers were used to build a three-dimensional model of the shoulder. The CEF ratio of the MDMF was estimated by representing the glenohumeral joint as a pulley.
  
  
  RESULTS
  The CEF ratio increased from the posterior to the anterior fibers of the MDMF.The median and interquartile range (IQR) of the CEF ratios of the posterior-third, middle-third, anterior-third and anterior-surface segments of the deltoid were: 0.16 (IQR = 0.23), 0.61 (IQR = 0.4), 1.42 (IQR = 0.41), 1.94 (IQR = 0.56) in subjects, and 0.06 (IQR = 0.24), 0.45 (IQR = 0.28), 1.32 (IQR = 1.01), 1.49 (IQR = 0.39) in volunteers. In the subjects, the CEF ratio of the anterior-surface segment was greater than all other segments (P ≤ 0.03).
  
  
  CONCLUSIONS
  The CEF ratio of the MDMF increased from its posterior segment to its anterior segment, indicating a greater stabilizing effect of the anterior segment both in RC tears subjects and in volunteers. Strengthening of the anterior fibers of the MDMF could potentially improve shoulder function in subjects with RC tears.
authors.affiliation: Department of GPA, Laboratoire de recherche en Imagerie et Orthopédie (LIO), École de Technologie Supérieure (ÉTS), Montréal, QC, H2L2W5, Canada, jhereter@gmail.com.
authors.forename: Joan
authors.initials: J
authors.lastname: Hereter Gregori
authors.forename: Nathalie J
authors.initials: NJ
authors.lastname: Bureau
authors.forename: Fabien
authors.initials: F
authors.lastname: Billuart
authors.forename: Nicola
authors.initials: N
authors.lastname: Hagemeister
mesh_terms: D000328:Adult; D000368:Aged; D001696:Biomechanical Phenomena; D057645:Deltoid Muscle / Q000033:anatomy & histology / Q000473:pathology*; D005260:Female; D006801:Humans; D007592:Joint Diseases / Q000473:pathology; D007593:Joint Instability / Q000473:pathology*; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008297:Male; D008875:Middle Aged; D010865:Pilot Projects; D016059:Range of Motion, Articular; D017006:Rotator Cuff / Q000473:pathology; D000070636:Rotator Cuff Injuries; D012785:Shoulder Joint / Q000033:anatomy & histology / Q000473:pathology*