Glenoid baseplate screw fixation in reverse shoulder arthroplasty: does locking screw position and orientation matter?

Background: The longevity of a reverse total shoulder arthroplasty prosthesis can be compromised by glenoid baseplate loosening. Circular baseplate designs can be secured with superoinferior (SI) locking screws and anteroposterior (AP) compression screws or vice versa (AP-locking and SI-compression screws). This biomechanical cadaveric study investigated screw position (locking screws SI vs. AP and compression screws AP and SI) and screw orientation (parallel vs. divergent) to determine quantitative differences in baseplate micromotion. Methods: Ten paired fresh-frozen cadaveric scapulae (n = 20) were implanted with a standard circular baseplate (empty set = 29 mm). The specimens were randomized into SI-locking or AP-locking screw configurations with the screw orientation directed either parallel or angled divergently at 15 degrees. This yielded a total of 4 groups for statistical comparison: SI-locking(parallel), SI-locking(divergent), AP-locking(parallel), and divergent, AP-locking(divergent), which were subjected to axial eccentric loading on the implanted baseplates, similar to the American Standard of Testing of Materials standard for shoulder joint arthroplasty. Results: In both static and cyclic testing, there were no statistically significant differences (P = .6) in micromotion between SI-locking (2.9 +/- 0.8 mu m) and AP-locking (3.5 +/- 1.5 mu m) configurations. In addition, there were no statistically significant differences (P - .2) in the divergent screw orientation group (2.0 +/- 0.7 mu m) vs. the parallel group (4.0 +/- 1.5 mu m). Conclusion: All configurations of screw position and screw orientation tested in a circular reverse baseplate have similar time-zero fix- ation in an intact glenoid bone model. In addition, the resultant micromotions for all configurations tested fell far below the 150 mu m threshold for bone ongrowth. (C) 2020 Journal of Shoulder and Elbow Surgery Board of Trustees. All rights reserved.