Smaller glenosphere size and increased baseplate retroversion improve postoperative internal rotation after reverse total shoulder arthroplasty performed with a 135° humeral implant and lateralized glenoid

Abstract

Background: Optimal placement of the glenosphere in reverse shoulder arthroplasty (rTSA) is a key component affecting postoperative range of motion (ROM) but remains a subject of ongoing research. The purpose of this study was to evaluate the relationship between three-dimensional (3D) glenosphere position and orientation relative to anatomic scapular landmarks and postoperative patient-reported outcomes and ROM following rTSA.

Methods: A retrospective multicenter cohort study was conducted on primary rTSAs performed with a 135° humeral inlay component and a lateralized glenoid component between November 2016 and March 2022. Surgeries performed with a 3D plan and patient-specific transfer instrumentation with minimum 2-year clinical follow-up were included. Implant position was extracted from preoperative planning software, focusing on pin position (center of the glenosphere) and glenosphere diameter, version, and overhang relative to scapular anatomic landmarks. ROM and American Shoulder and Elbow Surgeons (ASES) scores were assessed at 2-year follow-up, with linear regression models utilized to analyze the relationships between preoperative and intraoperative variables and postoperative outcomes while adjusting for confounding variables.

Results: A total of 75 rTSAs met the study criteria. For every 1 millimeter increase in glenosphere diameter, there was a 0.5 spinal level decrease in internal rotation (IR) spine (P ≤ .005) and a 2.5° decrease in forward flexion (P ≤ .005). For every 4° increase in baseplate retroversion, there was a 1 spinal level improvement in IR spine (P = .009). Superior tilt of the baseplate was associated with a decrease in internal rotation at 90° of abduction (3° decrease per 1° of increased superior tilt, P ≤ .001). ASES scores were also significantly affected, with a 3.5 point decrease per millimeter increase in glenosphere diameter (P ≤ .001), but improved by a 1 point per millimeter increase in pin-to-coracoid distance (P = .015).

Conclusion: In patients with 3D planning and patient-specific instrumentation, smaller glenosphere diameter, increased baseplate retroversion, and avoidance of superior tilt improve IR after rTSA performed with a 135° humeral component and lateralized glenoid. A smaller glenosphere diameter and increased distance from the coracoid also improved ASES scores. This data suggests that with the use of a lateralized glenoid in rTSA, efforts should be made to increase the glenosphere distance from the coracoid, avoid a superior tilted positioning of the baseplate, and consider a smaller glenosphere when in between sizes.

Keywords: 3D software; Baseplate retroversion; Glenosphere positioning; Humeral inlay component; Patient reported outcomes; Postoperative outcomes; Preoperative planning software; Reverse shoulder arthroplasty.

Figure 1

Center of rotation to acromial distances. The center of rotation was measured on scapular Y radiographs. A best-fit circle was made on the glenosphere and the center was marked. From there, three measurements were made: Center of rotation to coracoid (CC), Center of rotation to anterior acromion (CAA), and Center of rotation to posterior acromion (CPA).

Figure 2

Glenoid height and width. The center of rotation was measured on scapular Y radiographs. A best-fit circle was made on the glenosphere and the center was marked. The superior and inferior aspects of the glenoid were marked and a line was drawn through the Center of rotation (COR) to determine the glenoid height (GH). A second line was drawn perpendicular to the glenoid height (GH) through the COR to determine the glenoid width (GW).

Figure 3

Glenosphere overhang. The center of rotation was measured on scapular Y radiographs. A perfect-circle was made on the glenosphere and the center was marked. From there, three measurements were made: anterior glenoid to anterior glenosphere (A), posterior glenoid to posterior glenosphere (P), and inferior glenoid to inferior glenospehere (I).