Inferior tilt of the glenoid leads to medialization and increases impingement on the scapular neck in reverse shoulder arthroplasty.

BACKGROUND

In reverse shoulder arthroplasty, Inferior tilt was originally promoted to decrease rates of baseplate failure. However, the literature is conflicting regarding the effect of tilt on scapular neck impingement, which has been associated with an increased risk of notching, increased risk of impingement-related instability, and decreased range of motion. We hypothesized that inferior tilt of -10° would lead to increased medialization and increased scapular neck impingement compared with 0° of tilt.

METHODS

Twenty patients without glenoid bone loss undergoing reverse shoulder arthroplasty (RSA) at a single institution underwent computed tomography scans of the entire scapula and proximal humerus for preoperative planning. For each patient, we digitally implanted a 25-mm glenoid baseplate flush with the inferior rim of the glenoid. We then simulated impingement-free range of motion with 16 different implant configurations: glenoid tilt (0° vs. -10°), baseplate lateralization (0 mm vs. +6 mm), glenosphere size (36 mm vs. 42 mm), and neck-shaft angle (135° vs. 145°). The primary endpoint was external rotation with the arm at the side (ERS), which is the primary mode of both notching and impingement-related instability, and the secondary endpoint was adduction (ADD). We recorded the RSA angle, preoperative scapular neck length (SNL), and postoperative SNL. Data were compared by paired t tests and a multivariable regression analysis.

RESULTS

In every simulation, inferior tilt led to more impingement on the scapular neck. Inferior tilt of the glenoid component was associated with a mean 27% decrease in impingement-free external rotation (P < .01 in all cases) and a mean 32% decrease in impingement-free ADD (P < .01 in all cases). Inferior tilt removed 3.2 mm of additional SNL (P < .001). Multivariable regression analysis showed that lateralization had the most impact on impingement-free external rotation and ADD (P < .001), followed by glenosphere size (P < .001), neck-shaft angle (P < .001), postoperative SNL (P < .001), glenoid tilt (P = .001), inclination (P < .001), and RSA angle (P = .023 for ERS and P = .025 for ADD).

CONCLUSION

Relative to 0° of tilt of the baseplate, inferior tilt of -10° was associated with increased scapular neck impingement in ERS and ADD, likely a result of the increased medialization necessary to seat an inferiorly tilted implant, which shortens the scapular neck and brings the humerus closer to the scapula. This scapular neck impingement increases the risk of notching and impingement-related instability.