Effects of Glenosphere Positioning on Impingement-Free Internal and External Rotation Following Reverse Total Shoulder Arthroplasty

Introduction While shoulder elevation can be reliably restored following reverse total shoulder arthroplasty (RTSA), patients may experience a loss of internal and external rotation. Several recent studies have investigated scapular notching and have made suggestions regarding glenosphere placement in order to minimize its occurrence. However, very few studies have looked at how changes in glenosphere placement in RTSA affect internal and external rotation. The purpose of this study was to determine the effect of glenosphere position on internal and external rotation range of motion at various degrees of scaption following RTSA. We hypothesized that alteration in glenosphere position will affect the amount of impingement-free internal and external rotation. Methods CT scans of the scapula and humerus were obtained from seven cadaver specimens and 3-Dimensional (3D) reconstructions were created. A corresponding 3D RTSA model was created by laser scanning the baseplate, glenosphere, humeral stem and bearing. The RTSA models were then virtually implanted into each specimen. The glenosphere position was determined in relation to the neutral position in 6 different settings: Medialization (5 mm), lateralization (10 mm), superior translation (6mm), inferior translation (6 mm), superior tilt (20°), and inferior tilt (15° and 30°). The humerus in each virtual model was allowed to freely rotate at a fixed scaption angle until encountering bone-bone or bone-implant impingement (180 degrees of limitation). Each model was tested at 0, 20, 40, and 60 degrees of scaption and the impingement-free internal and external rotation range of motion for each scaption angle was recorded. Results At 0Es scaption, only inferior translation, lateralization, and inferior tilt allowed any impingement-free motion in IR and ER. At mid ranges of scaption (20Es and 40Es) a predictable pattern was seen in which increased lateralization and inferior translation resulted in improved rotation. Supraphysiologic motion (>90Es rotation) was seen consistently at 60Es of scaption in internal rotation. Both superior and inferior tilt positions resulted in increased ROM in the mid-range of scaption. Acromial impingement was seen when the glenosphere was medialized, superiorly translated and with a superior tilt. Superior translation (6 mm) resulted in no rotation at 0 and 20 degrees of scaption (both IR and ER). Figure 1 and 2: Represents the amount of internal (fig. 1) and external rotation (fig. 2) range of motion measured to bony impingement. 180 degrees was set as the physiologic limit for all measurements Conclusion Glenosphere position significantly affected humeral internal and external rotation after Reverse Total Shoulder Arthroplasty in our computer model. Inferior translation (6 mm) or lateralization (10 mm) appears to have the most beneficial effects to internal and external rotation of the shoulder. Inferior tilt (15° and 30°) of the glenosphere also improved overall arc of motion in IR and ER when compared to superior tilt and neutral positions. Superior translation (6 mm) and medialization (5 mm) of the glenosphere caused marked limitations in internal and external rotation due to scapular notching and acromial impingement.