An Analysis of Planned Inclination Correction by Reaming in Reverse Total Shoulder Arthroplasty

Background: The optimal inclination of the glenoid component in reverse total shoulder arthroplasty (RSA) is debated; however, an inferiorly tilted or neutral glenoid baseplate typically requires corrective reaming, structural grafting, or an augmented baseplate. Significant corrective reaming of the inferior glenoid has been theorized to result in medialization, a shortened scapular neck, and impingement. Few reports have described the relationship between the amount of reaming and factors such as degrees of inclination correction, indication for RSA, and patient demographics. Purpose: To characterize these effects in a large series of RSA cases that were 3D planned with pre-operative planning software. Methods: A retrospective review of RSA cases planned by a single surgeon using a commercially available 3D CT software planning program between 2018 and 2020 was conducted to evaluate the relationship between the 3D virtual reaming depth (mm) required to execute the planned 3D glenoid inclination change along with RSA indication and patient factors. Cases planned for reconstruction using structural graft (defined as planned backside seating <75%), those using an augmented glenoid baseplate, and cases with planned glenoid inclination greater than native glenoid inclination were excluded. Pearson’s correlation coefficient was calculated to evaluate the relationship between the planned reaming depth and the above factors. Results: Sixty-three cases met the inclusion criteria in 29 males (46%) and 34 females (54%). The indications for planning of RSA included 22 cases of rotator cuff arthropathy (35%), 26 cases of massive rotator cuff tears or fracture with no osteoarthritis (41%), and 15 cases of cuff-intact osteoarthritis (24%). The median native 3D inclination was 9 degrees (IQR=6-13). Seventy percent of cases were planned to have a final implant inclination of 0 degrees. The median amount of inferior glenoid reamed was 8 mm (IQR=6-10). Corrective reaming depth was strongly, positively correlated with planned degrees of inclination correction, r (61) =0.67, p<0.001. No significant correlation was found between reaming depth and RSA indication or any patient factor such as age or sex. Conclusions: Corrective reaming depth of the inferior glenoid in 3D planned RSA cases is highly correlated with correction of superior glenoid inclination. Each 1-degree correction in inclination requires approximately 0.9 mm of inferior reaming. An average reaming depth of 8 mm in the current series was necessary to achieve an average 9-degree inclination correction to neutral tilt.