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Tomas Smith1, Alexandra Bäunker1, Manuel Krämer2, Christof Hurschler2, Melena Kaufmann1, Marc Frederic Pastor1, Mathias Wellmann PhD 1
1 Department of Shoulder-, Knee- and Sports-Orthopaedics, Hannover Medical School, Anna von Borries Street 1-7, 30625 Hannover, Germany
2 Laboratory of Biomechanics and Biomaterials, Hannover Medical School, Anna von Borries Street 1-7, 30625 Hannover, Germany
Correspondence Address:
Mathias Wellmann
Department of Shoulder-, Knee- and Sports-Orthopaedics, Hannover Medical School, Anna von Borries Street 1-7, 30625 Hannover
Germany
 Source of Support: None, Conflict of Interest: None
DOI: 10.4103/0973-6042.167932
Purpose: 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.
Materials and 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.
Conclusion: 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.
Level of Evidence: Basic Science Study
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