The Effect of the Central Post and Screw Constructs on the Univers Revers Total Shoulder System.

Introduction: The use of lateralized glenoid components in reverse total shoulder arthroplasty (rTSA) is increasing to avoid scapular notching and improve strength and impingement-free range of motion. However, maximizing glenoid lateralization increases stress at the bone-baseplate interface. The ideal type and length of central fixation remains a subject of debate. The purpose of this study was to compare baseplate micromotion and load to failure in a biomechanical model with either a central or post for fixation of an rTSA baseplate. The primary hypothesis was that bicortical post or screw placement would improve baseplate stability compared to fixation contained within the vault. Secondary hypotheses were that larger amounts of construct lateralization would increase micromotion and decrease baseplate stability, regardless of central fixation method.

Methods: Based on previously published work with similar methods and reported measures, a power analysis determined an adequate sample size of n=6 for statistical comparisons between 6 groups, with an alpha of 0.05 and power of 0.8. The 6 study groups are shown in figure 1. Thirty-six shoulder scapulae (12 pcf Sawbones; Pacific Research Laboratories) were implanted with a non-augmented baseplate, glenosphere, and four peripheral screws as well as either a central screw or post. The post groups were either contained within the glenoid or penetrated the vault (bicortical). All groups with a central screw were bicortical. Lateralization was tested at both 4 mm and 8 mm. All implants (Univers Revers Total Shoulder System; Arthrex Inc., Naples, FL, USA) were placed using a glenoid specific guide for optimal and consistent positioning of the central guide pin and confirmed by both X-Ray and CT analysis. Cyclic testing was performed with increasing load until baseplate micromotion exceeded 150 μm. Load to failure testing was performed with failure defined as baseplate displacement of 1000 μm or scapula fracture. ANOVA testing was performed to evaluate for statistical significance between groups (p<.05).

Results: There was no difference in micromotion testing in all six groups. (p=0.390). Lateralization at 4 mm or 8 mm did not significantly affect micromotion testing. Test groups with 4 mm of lateralization and a central post contained within the vault (PC4, p=0.01) and 4 mm of lateralization with a central bicortical screw (SB4, p=0.005) had statistically significantly greater load to failure compared to the other groups.

Conclusion: With a non-augmented glenoid baseplate and 4 to 8 mm of lateralization, central posts within the vault, central posts exiting the vault, and bicortical screw fixation were equivocal in terms of micromotion. Load to failure was highest with 4 mm of lateralization and a central post contained within the glenoid vault or a bicortical screw. With 8 mm of lateralization and the use of 4 peripheral screws there is no difference between a central post exiting the vault, post within the vault and bicortical screw fixation.