title: Biomechanical evaluation of inferior scapula notching of reverse shoulder arthroplasty depending on implant configuration and scapula neck anatomy.
abstract: OBJECTIVE
  The presence of inferior scapula notching is significantly affected by the anatomy the scapula and can be influenced by the glenosphere design and position and the onlay type.
  
  
  METHODS
  A biomechanical study was undertaken with 13 human shoulder specimens in a robot-assisted shoulder simulator. Inferior scapula contact during adduction of the humerus was detected using a contact pressure film. Computed tomography scans with three-dimensional reconstructions of each specimen were performed.
  
  
  RESULTS
  The greatest improvement of the scapula notching angle (SNA) was achieved by simultaneous implantation of a shallow humeral onlay and an eccentric glenosphere design: 16.3-19.0° (P < 0.005). The SNA was significantly decreased by 5.8° when shifting from a 38 mm centric glenosphere to a 42 mm centric glenosphere (P < 0.005) and by 8.9° comparing the 38 mm centric glenosphere with 38 mm eccentric glenosphere (P < 0.005). The solitary implantation of a shallow onlay significantly decreased the SNA depending on the glenosphere size between 7.4° and 8.0° (P = 0.001). A more inferior position of the metaglene as well as a long scapula neck (P = 0.029) and a large lateral scapula pillar angle (P = 0.033) were correlated with a lower SNA.
  
  
  CONCLUSIONS
  This study demonstrates the importance of inferior glenosphere placement and the benefit of eccentric glenosphere and shallow humeral cup design to reduce the adduction deficit of the reverse shoulder. The presence of a short neck of the scapula can have a negative prognostic effect on inferior impingement during adduction of the arm.
  
  
  METHODS
  Basic Science Study.
authors.affiliation: Department of Shoulder-, Knee- and Sports-Orthopaedics, Hannover Medical School, Anna von Borries Street 1-7, Hannover, Germany.
authors.forename: Tomas
authors.initials: T
authors.lastname: Smith
authors.affiliation: Department of Shoulder-, Knee- and Sports-Orthopaedics, Hannover Medical School, Anna von Borries Street 1-7, Hannover, Germany.
authors.forename: Alexandra
authors.initials: A
authors.lastname: Bäunker
authors.affiliation: Laboratory of Biomechanics and Biomaterials, Hannover Medical School, Anna von Borries Street 1-7, 30625 Hannover, Germany.
authors.forename: Manuel
authors.initials: M
authors.lastname: Krämer
authors.affiliation: Laboratory of Biomechanics and Biomaterials, Hannover Medical School, Anna von Borries Street 1-7, 30625 Hannover, Germany.
authors.forename: Christof
authors.initials: C
authors.lastname: Hurschler
authors.affiliation: Department of Shoulder-, Knee- and Sports-Orthopaedics, Hannover Medical School, Anna von Borries Street 1-7, Hannover, Germany.
authors.forename: Melena
authors.initials: M
authors.lastname: Kaufmann
authors.affiliation: Department of Shoulder-, Knee- and Sports-Orthopaedics, Hannover Medical School, Anna von Borries Street 1-7, Hannover, Germany.
authors.forename: Marc Frederic
authors.initials: MF
authors.lastname: Pastor
authors.affiliation: Department of Shoulder-, Knee- and Sports-Orthopaedics, Hannover Medical School, Anna von Borries Street 1-7, Hannover, Germany.
authors.forename: Mathias
authors.initials: M
authors.lastname: Wellmann
keywords: Adduction angle; biomechanics; implant configuration; reverse shoulder arthroplasty; scapula anatomy; scapula notching