Rationale for Small Glenoid Baseplate: Position of Central Cage within Glenoid Vault (Exactech® Equinoxe® Reverse System)

Background: Glenoid baseplate location is important to good clinical outcomes of reverse total shoulder arthroplasty (RTSA). The glenoid vault is the determining factor for glenoid baseplate location, but, to date, there are no reports on the effect of central cage location within the glenoid vault on RTSA outcomes when using the $Exactech^{(R)}$ $Equinoxe^{(R)}$ Reverse System. The purpose of this study was to determine the appropriate cage location in relation to the glenoid vault and monitor for vault and/or cortex penetration by the cage. Methods: Data were retrospectively collected from the Samsung Medical Center (SMC) and Seoul National University Bundang Hospital (SNUBH). Patients who underwent RTSA between November 2016 and February 2018 were enrolled. Glenoid vault depth, central cage location within the vault were examined. Inferior glenoid rim-center distance, inferior glenoid rim-cage distance, and center-cage center distances were collected. Results: Twenty-two patients were enrolled. Three SNUBH patients had inappropriate central cage fixation (33.3%) versus 4 SMC patients (30.8%). All cage exposures were superior and posterior to the glenoid vault. Mean center-cage distance was 5.0 mm in the SNUBH group and 5.21 mm in the SMC group. Center-prosthesis distance was significantly longer in the inappropriate fixation group than in the appropriate fixation group (p<0.024). Conclusions: To ensure appropriate glenoid baseplate fixation within the glenoid vault, especially in a small glenoid, the surgeon should place the cage lower than usually targeted, and it should overhang the inferior glenoid rim.