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.