Background Acromial fractures are a substantial complication following reverse shoulder arthroplasty, reported to affect up to 7% of patients. Previous studies have shown that implant placement affects acromial stress during elevation of the arm in the scaption plane. The purpose of this study was to investigate the results of arm loading and variation in plane of elevation on acromial stresses. Methods Nine elevation angles (0°–120°), in three planes of elevation (abduction (0°), scaption (30°), and forward elevation (60°)), and three hand loads (0, 2.5, 5 kg) were investigated. Finite element models were generated using computed tomography data from 10 cadaveric shoulders (age 68 ± 19 yrs) to determine acromial stress distributions. Models were created for a lateralized glenosphere (0, 5, 10 mm), inferiorized glenosphere (0, 2.5, 5 mm), and humeral offset (−5, 0, 5 mm). Results For all planes of elevation (0°, 30°, 60°) and hand loads (0, 2.5, 5 kg) investigated, glenoid lateralization consistently increased acromial stress, glenoid inferiorization consistently decreased acromial stress, and humeral offset proved to be insignificant in altering acromial stress. Abduction resulted in significantly higher peak acromial stresses (p = 0.002) as compared to scaption and forward elevation. Conclusions In addition to implant position and design, patient activity, such as plane of elevation and hand loads, has substantial effects on acromial stresses. Level of evidence Basic science study

Background The humeral head osteotomy during shoulder arthroplasty influences humeral component height, version and possibly neck-shaft angle. These parameters all potentially influence outcomes of anatomic and reverse shoulder replacement to a variable degree. Patient-specific guides and navigation have been studied and utilized clinically for glenoid component placement. Little, however, has been done to evaluate these techniques for humeral head osteotomies. The purpose of this study, therefore, was to evaluate the use of patient-specific guides and surgical navigation for executing a planned humeral head osteotomy. Methods The DICOM images of 10 shoulder computed tomography scans (5 normal and 5 osteoarthritic) were used to print 3D polylactic models of the humerus. Each model was duplicated, such that there were 2 identical groups of 10 models. After preoperative planning of a humeral head osteotomy, Group 1 underwent osteotomy via a patient-specific guide, while group 2 underwent a real time navigated osteotomy with an optically tracked sagittal saw. The cut height (millimeters), version (degrees) and neck-shaft angle (degrees) were recorded and statistically compared between groups. Results There were no statistically significant differences between patient-specific guides and navigation for osteotomy cut height ( P = .45) and humeral version ( P = .059). Navigation, however, resulted in significantly less neck-shaft angle error than the patient specific guides ( P = .023). Subgroup analysis of the osteoarthritic cases showed statistical significance for navigation resulting in less version error than the patient specific guides ( P = .048). Conclusion No significant differences were found between patient specific guides and navigation for recreation of the preoperatively planned humeral head cut height and version. Neck-shaft angle, however, had significantly less deviation from the preoperative plan when conducted with navigation.

Introduction: There is limited research on the surgical management techniques for scapular spine fractures after reverse shoulder arthroplasty (RTSA). As such, the purpose of this in vitro biomechanical study was to compare 4 fixation constructs to stabilize scapular spine insufficiency fractures. Methods: Twelve paired fresh-frozen cadaveric scapulae (N ¼ 24) were randomized into 4 fixation groups: subcutaneous border plating (AE hook) and supraspinatus fossa plating (AE hook). A Levy type II fracture was simulated. Each specimen was cyclically loaded incrementally up to 700 N in 50 N steps or until failure. Between 50 and 200 N construct stiffness was measured, and stability failure was defined as displacement greater than 2.5 mm. Results: Seventy-nine percent (19 of 24) of the specimens failed before the maximum load of 700 N. The average survival force with subcutaneous border plating was 480 AE 80 N compared with 380 AE 30 N for supraspinatus fossa plating (P ¼ .3). Fixation construct failure was significantly more likely with fossa plating over subcutaneous plating (P ¼ .012). The presence of the lateral plate hook was beneficial in preventing failure of the lateral acromion (P ¼ .016). Conclusion: When appropriately surgically indicated, a dorsally applied plate to the subcutaneous border of the scapular spine with a lateral inferior supporting hook may be advantageous for internal fixation of type II scapular spine insufficiency fractures after RTSA.

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