BACKGROUND: Acromial fractures after Reverse Total Shoulder Arthroplasty (RTSA) are a common complication. Nevertheless, only a few studies have identified risk factors for acromial fractures after RTSA. High delta angle (combination of inferiorization and medialization of the center of rotation) after RTSA was identified as a risk factor in recent studies. The aim of this study was the biomechanical exploration of different delta angles and implant configurations with regard to the acromial stress.
METHODS: In a rigid body model of the upper extremity muscle, forces of the deltoid muscle were calculated before and after implanting RTSA in different arm and implant positions. The deltoid muscle was divided into an anterior, middle, and posterior part. Implant positions of the glenoid components were changed in the medialization, lateralization and inferiorization of the center of rotation (COR) as well as lateralization of the humeral component. Further, in a finite element model of the upper extremity, the stresses of the acromion in the same implant design configurations were measured.
RESULTS: Differences in acromial stress between different delta angle model configurations were observed. Lateralization (5 mm, 10 mm) of the glenosphere reduced maximal acromial stress by 21% (1.5 MPa) and 31% (1.3 MPa), respectively. Inferiorization (5 mm, 10 mm) of the glenosphere increased maximal acromial stress by 5% (2.0 MPa) and 15% (2.2MPa), respectively. Changes in positioning the humeral component was found to have the highest impact in this model configuration. A 10 mm lateralized humeral component reduced acromial stress by 37% (1.2 MPa) while in the 6 mm medialized configuration, an increase in acromial stress by 83% (3.48 MPa) was observed. There was a high correlation between delta angle and acromial stress (R-squared = 0.967).
CONCLUSIONS: Implant design configuration has an impact on the acromial stress. High delta angles correlate with an increase in acromial stress. Both lateralization of the COR and the humerus decreased the acromial stress in our study. The lateralization of the humerus has the highest impact in influencing acromial stress. Due to contrary results in the current literature, further studies with focus on the acromial stress influenced by different anatomical variants of the shoulder and the acromion are needed before a clinical recommendation can be made.

BACKGROUND: Fractures of the acromion and spine can have a major impact on the outcome of reverse shoulder arthroplasty (RSA) with respect to pain, motion, and function. Reports on internal fixation for these fractures are isolated to small series or case reports with variable outcomes. The purpose of this study was to report on the outcome of open reduction and internal fixation (ORIF) of acromion or spine fractures encountered before or after RSA and describes our evolution of fixation techniques.
METHODS: Between 2011 and 2023, 22 fractures or nonunions of the acromion or spine of the scapula underwent ORIF at a single institution and were followed for a minimum of 1 year. In 16 shoulders, fractures occurred after RSA, whereas 5 shoulders underwent ORIF prior to RSA. One shoulder had undergone prior failed ORIF elsewhere and revision ORIF was performed at our institution. There were 10 males and 12 females with a mean age of 67 (SD = 15.1) years. Fixation strategies included single (n = 11) and double plate fixation (n = 11). Kruskal-Wallis one-way analyses of variance were used to analyze continuous variables and Chi-square tests employed for categorical variables.
RESULTS: Of the 5 fractures treated with ORIF pre-RSA, 1 shoulder suffered an additional fracture medial to the hardware and 1 required additional bone grafting for incomplete union at the time of RSA. These 5 shoulders all underwent RSA uneventfully, but 1 fracture experienced late displacement of the scapular spine nonunion, leading to plate removal. Of the 16 post-RSA ORIF shoulders, radiographic union was confirmed in 14 and substantial residual inferior angulation identified in 3. New fractures occurred after ORIF in 5 shoulders. For patients who underwent ORIF after RSA, pain scores improved from a mean of 8 to 1.9 points, with more modest elevation gains (58.2°-91.3° pre and postoperatively, respectively).
CONCLUSIONS: ORIF of acromion and scapular spine fractures or nonunions in the setting of RSA have the potential to lead to union. When these fractures and nonunions are encountered prior to RSA, ORIF allows for uneventful RSA implantation, but secondary displacement may occur. ORIF seems to lead to improvements in pain, but more modest improvements in motion and function. Our fixation strategy has evolved to (1) dual plating, (2) spanning the whole length of the spine with 1 of the plates, (3) use of hook features under the acromion or os trigonum if possible, and (4) liberal use of bone graft.

BACKGROUND: Acromial and scapular spine fractures are common complications after reverse total shoulder arthroplasty (RTSA). There is limited information on the treatment outcome of these fractures. Therefore, the purpose of this study was to compare the clinical outcome of operative and conservative treatment of patients with acromial or scapular spine fractures.
METHODS: A total of 1146 RTSAs were performed in our institution between 1999 and 2016. In 23 patients (2%), we identified an acromial fracture, and in 7 cases (0.6%), a scapular spine fracture in the postoperative course. Of those patients, 7 patients (23%) were treated with open reduction and internal fixation and 23 (77%) were treated conservatively. We compared the outcome of operative vs. conservative treatment assessing the Constant score (CS), range of motion, and subjective shoulder value (SSV). Fractures were classified by the system of Crosby. Radiographic assessment consisted of measuring the healing rate, time to heal, and the displacement of the acromion before and immediately after the fracture as well as after treatment.
RESULTS: There were no statistically significant differences between operative and conservative treatment. The mean preoperative CS in the operative group was 32 points and improved to 45 points after surgery, whereas it was 35 points in the conservative group and improved to 61 points at the final follow-up. The mean SSV improved from 20 to 50 points in the operative group and from 22 to 58 points in the conservative group. Mean active flexion changed from 59° to 75°, mean abduction from 68° to 67°, and external rotation from 25° to 13° in the operative group and from 75° to 91°, 67° to 92°, and 28° to 24° in the conservative group.
CONCLUSIONS: In our study, operative treatment was not superior to conservative treatment, neither for CS, SSV, or range of motion. Both treatment forms, however, resulted in inferior results to those previously reported for RTSA without postoperative acromion fractures. Before better surgical methods have been developed, conservative treatment of acromial fractures may be the better treatment option for acromial fractures after RTSA.