Biomechanical evaluation of inferior scapula notching of reverse shoulder arthroplasty depending on implant configuration and scapula neck anatomy

Abstract

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

Reverse Total Shoulder Arthroplasty: Implant Design Considerations

“…The stability of the RTSA is dependent on the contact area between the glenosphere and polyethylene inlay, which can range from minimal (high mobility) to maximal (constrained or retentive) [5,80,81]. Retentive liners can afford additional implant stability; however, they do so with the risk of increased wear and possible polyethylene wear-induced aseptic loosening [82,83]. In fact, Carpenter and colleagues [82] performed an in vitro wear simulation study and demonstrated that retentive liners undergo significantly greater volumetric loss and surface deviation compared with non-retentive liners.…”

A Critical Review on Prosthetic Features Available for Reversed Total Shoulder Arthroplasty

“…The latter can be used in the event of instability of the prosthesis. Additional stability afforded by retentive liners should be balanced against the potential for increased wear and the potential for subsequent polyethylene wear-induced aseptic loosening [57, 58]. …”

Validation of a Redundant Robotic Manipulator for Shoulder in Vitro Biomechanical Testing.

“…This is essential for the assessment of complex joints such as the shoulder 5 . Several clinical applications have thus been proposed to assess acromioclavicular joint stability 6 , to evaluate the in uence of a reverse shoulder prosthesis design on scapula notching 7 , or to characterise percutaneous osseointegrated implant systems 5 .…”

“…bony or soft tissue constraints that may limit further joint motion) 1 , in vivo human motions applied by the operator on the specimen have to be adapted to the specimen joint characteristics. This can be done by a scaling data procedure 15 or by the use of a 6-axis universal force-moment sensor to limit forces and moments applied by the manipulator on the joint 7 . However, to the best of our knowledge, manually-induced motions recorded on the investigated specimen have never been directly replicated by the use of a robotic manipulator.…”