The effects of glenoid baseplate peripheral screw number on micromotion in reverse total shoulder arthroplasty

Background: Glenoid baseplate loosening continues to be a challenge when assessing failure of reverse total shoulder arthroplasty (RSA). The baseplate is commonly attached to the glenoid bone surface using a series of screws. This study looks to examine the impact of the number of peripheral screws (0-, 2-, or 4-screws) on the initial fixation of an RSA glenoid baseplate. The possibility of retaining fewer screws could allow for the preservation of the already limited bone stock available in the glenoid. Methods: Three constructs, with 10 samples each, were prepared with differing numbers of locking peripheral screws: 4-screw (2 superior/inferior, 2 anterior/posterior), 2-screw (2 superior/inferior), or 0-screw. In addition, all samples were fixed with a central screw that is placed within the central boss of the implant. Then each baseplate was implanted within 15 pounds/cubic foot rigid polyurethane foam and embedded within polymethylmethacrylate. An initial and final displacement test were conducted by statically applying a shear load (350N) and compressive load (430N) and measuring baseplate displacements via linear variable differential transformers. In-between these two static measurements each sample was cycled through an arc of abduction (+30° to −15°) at ¼ Hz for 10,000 cycles while maintain a normal load of 750N. The displacement of the baseplate was also measured real-time during the cycling phase using three-dimensional digital image correlation. Results: The results from the initial and final static testing indicated varying statistical significance between the different constructs, but the measurements taken during active cycling were able to indicate that the 0-screw construct was significantly less stable than the 2- or 4-screw constructs in terms of both shear (initial: 68.86 ± 23.77μm, 29.40 ± 8.34μm, 22.71 ± 11.23μm); (final:116.72 ± 73.20μm, 64.54 ± 8.36μm, 53.11 ± 33.86μm) and compressive displacements (initial: 65.81 ± 17.81μm, 12.57 ± 5.25μm, 30.60 ± 28.85); (final: 204.22 ± 126.22μm, 37.24 ± 14.58μm, 70.11 ± 59.39μm). Discussion: Differences were found in baseplate stability when comparing a 4- and 2- screw construct to a 0-screw construct. In addition, the results indicate that taking measurements real-time during cyclic loading tends to allow for more accurate and consistent measurement of baseplate displacement in this type of biomechanical testing compared to a simple static initial and final displacement test. This result indicates a promising option for the preservation of glenoid bone stock following RSA.