Rates of subacromial notching are low following reverse shoulder arthroplasty with a 135° inlay humeral component and a lateralized glenoid

Background Lateralization in reverse shoulder arthroplasty (RSA) decreases bony impingement and improves rotational range of motion, but has been theorized to increase the risk of subacromial notching (SaN). The purpose of this study was to evaluate the presence of SaN following RSA and its relationship with lateralization with a 135° inlay humeral component. The secondary purpose was to assess the association of SaN with functional outcomes. Methods A retrospective review was performed from a multicenter prospectively collected database on patients who underwent primary RSA from 2015 to 2021. All RSAs were performed with a 135° inlay humeral component. SaN was defined as bony erosion with sclerotic margins on the undersurface of the acromion on final follow-up radiographs not present preoperatively. Postoperative implant positioning (inclination, distalization, and lateralization) were evaluated on minimum 1-year postoperative radiographs. Regression analyses were performed on implant and clinical variables to assess for risk factors. A separate analysis was performed to determine the association of SaN with clinical outcomes. Results SaN was identified in 13 out of 442 shoulders (2.9%). Age, sex, body mass index, smoking status, diabetes mellitus, arm dominance had no relationship with SaN. Neither glenoid sided lateralization nor humeral offset were associated with SaN risk. Other implant characteristics such as distalization, glenosphere size, and postoperative inclination did not influence SaN risk. The presence of SaN did not affect patient-reported outcomes (American Shoulder and Elbow Surgeons: P = .357, Visual Analog Scale: P = .210) or range of motion. Conclusion The rate of SaN is low and not associated with glenoid or humeral prosthetic lateralization when using a 135° inlay humeral component. When SaN occurs, it is not associated with functional outcomes or range of motion at short-term follow-up.

Lateralizing the center of rotation in reverse shoulder arthroplasty (RSA) has been reported to increase impingement-free rotational range of motion (ROM), improve rotator cuff tensioning, and enhance stability. 2,5,7,12Additionally, adduction impingement is a well-known consequence of a medialized RSA design 6,8,14,24 and glenoid lateralization decreases scapular notching/adduction impingement. 15,21iomechanical studies have raised concerns with lateralization and abduction impingement. 17,18,23The consequent subacromial notching (SaN) is an incompletely described phenomenon with limited clinical studies to date.One study with a 145 component and a medialized glenoid reported a 12.8% rate of SaN. 13 However, this cohort was limited to an Asian population and a 36-mm glenosphere.
Therefore, the purpose of this study was to evaluate the presence of SaN following RSA and its relationship with glenoidsided lateralization with a 135 inlay humeral component.The secondary purpose was to assess the presence of SaN on functional outcomes.We hypothesized that the SaN incidence would be low and not related to lateralization when utilizing a 135 inlay stem.

Database and study patients
A retrospective review was performed on a prospectively maintained multicenter database on patients who underwent primary RSA from 2015 to 2021.Inclusion criteria were as follows: minimum 1-year follow-up and primary RSA performed with a 135 inlay humeral component.Exclusion criteria were as follows: revision procedures, primary RSA for proximal humerus fractures, use of custom implants, and the presence of preoperative or postoperative acromial fracture.Institutional review board approval and patient consent was obtained before study inception as part of the prospective database enrollment.

Surgical technique
RSAs were performed at 12 sites.In all cases, a deltopectoral approach was used with a 135 inlay humeral component (Univers Revers; Arthrex, Inc., Naples, FL, USA).For the glenoid, an anatomically shaped baseplate was used before 2018 (Universal Baseplate; Arthrex, Inc., Naples FL) and a modular circular baseplate (Modular Glenoid System; Arthrex, Inc., Naples, FL, USA) was used from 2018 to 2021.Glenoid-sided lateralization occurred through the baseplate and/or glenosphere and varied from 0 to 8 mm in 2 mm increments based on surgeon preference, patient anatomy, and soft-tissue tension.Humeral offset included the polyethylene liner and metallic spacer if used.Glenospheres with diameters ranging from 33 mm to 42 mm were implanted based on surgeon preference with a goal of matching to patient size and avoiding excessive anterior or posterior overhang.Subscapularis repair and postoperative rehabilitation were not standardized.

Patient characteristics and outcome measures
Patient characteristics and patient-reported outcomes (PROs) were prospectively collected in a secure database.Baseline data collected included age, sex, body mass index (BMI), smoking status, history of diabetes mellitus, surgical side dominance.PROs and ROM were assessed at baseline and at the final follow-up.PROs obtained included American Shoulder and Elbow Surgeons (ASES) and Visual Analog Scale (VAS) scores.ROM was measured by the treating surgeon with a goniometer for active forward flexion, active external rotation (ER) in adduction (ER0), active ER with arm at 90 (ER90), and internal rotation (IR) with the arm at 90 (IR90).Internal rotation was also estimated to nearest spinal level (IR spine).Implant characteristics such as glenosphere size, glenoid-sided lateralization, and humeral offset were also recorded.

Radiographic measurements
Preoperative, immediate postoperative and final follow-up radiographs were obtained including Grashey, axillary, and scapular Y views.Radiographs were reviewed by a fellowship-trained shoulder surgeon not involved in the surgeries (T.P.) in DICOM (digital imaging and communications in medicine) using Horos (Pixmeo, Bernex, Switzerland).SaN was defined as bony erosion with sclerotic margins on the undersurface of the acromion 13 on final follow-up radiographs not observed on preoperative or immediate postoperative radiographs (Fig. 1).An independent examiner (T.P.) evaluated radiographs and any discrepancies were resolved by consensus with two senior authors (B.W., P.J.D.).
Measurements for postoperative inclination, distalization, and lateralization were made on postoperative radiographs.Glenoid inclination was determined by the ß angle as described by Maurer et al. 20 Acromial humeral distance (AHD) was one metric used to determine distalization.The AHD was measured as described by Haidamous et al 11 on Grashey views.To account for positioning of the arm, an initial line down the axis of the humerus was drawn.From there, intersecting lines from the undersurface of the acromion to the most superior aspect of the greater tuberosity were made.
Additional measurements for distalization and lateralization were also made according to the distalization shoulder angle and lateralization shoulder angle, respectively 3 (Figs. 2 and 3).The lateralization shoulder angle was further divided into glenoid (gLSA) and humeral (hLSA) contributions at the most lateral point of the glenosphere as described by described by Schippers and Boileau 22 (Fig. 3).

Statistical analysis
All statistical analyses were performed in Statistical Package for the Social Sciences version 29 (IBM, Armonk, NY, USA).Evaluation of patient and implant risk factors for SaN was performed using a binary logistic regression analysis, with the presence of SaN as the dependent variable.For each independent variable, odds ratios, 95% confidence intervals and P values were generated.To assess any relationship between the presence of SaN on PROs and ROM, linear regression analyses were performed with the outcome of interest as the dependent variable, and SaN as well as numerous other potential confounders as independent variables.For each of these analyses, unstandardized beta coefficients, 95% confidence intervals and P values were generated for SaN.

Results
A total of 442 RSAs met the study criteria.Baseline characteristics are summarized in Table 1.Glenoid metallic lateralization was 4 mm or larger in 88.9% of cases.Humeral lateralization was largely 6 mm or less (Table 2).SaN was observed in 13 patients, for an overall rate of 2.9%.Age, sex, BMI, history of diabetes mellitus, arm dominance had no relationship with SaN (Table 3).
Neither glenoid-sided lateralization as determined by metallic glenoid-sided lateralization and gLSA, nor humeral lateralization as determined by implant humeral offset and hLSA increased SaN risk.Distalization as determined by AHD and distalization shoulder angle was neither predictive nor protective of notching.Other implant characteristics such as glenosphere size or postoperative inclination did not have an effect on SaN risk (Table 3).From there, a second line is made from 2) the glenoid tubercle to 3) the superior most aspect of the greater tuberosity.A measurement between those 2 lines forms the DSA.DSA, distalization shoulder angle.
Figure 3 The LSA as described by Boutsiadis et al. 3 A line connecting the 1) superior glenoid tubercle and 2) the lateral most aspect of the acromion is made.From there, a second line connects 2) the lateral most aspect of the acromion to 3) the lateral most aspect of the greater tuberosity.A measurement between those 2 lines forms the LSA.The LSA was further divided from the lateral most aspect of the glenophere to determine glenoid (gLSA) and humeral (hLSA) contributions.LSA, lateralization shoulder angle; gLSA, glenoid contribution to lateralization shoulder angle; hLSA, humeral contribution to lateralization shoulder angle.

Discussion
The main findings of this study were that the rate of SaN following RSA with a 135 inlay humeral component was low at 2.9%, and glenoid-sided lateralization even up to 8 mm did not increase SaN risk.Furthermore, the presence of SaN did not affect PROs or ROM.
18]23 Neither glenoid nor humeral sided lateralization conferred SaN risk (gLSA: P ¼ .459,glenoid lateralization: P ¼ .884,hLSA: P ¼ .275,humeral offset: P ¼ .561).However, in the one other clinical study on patients with a 12.8% rate of SaN, humeral lateralization was found to be a risk factor. 13In their series, a 36-mm glenosphere was implanted in all cases without additional lateralization, indicating that if any lateralization was introduced, it was originating from humeral offset.In contrast, humeral offset in the current study was primarily 6 mm or less with lateralization primarily being introduced from the glenoid.Given the comparably low SaN incidence of 2.9% in the present study, preferentially lateralizing through the glenoid may have a role in decreasing SaN.
The low SaN incidence in the present investigation may further be explained by racial morphologic differences and implant size mismatch.Prior studies have demonstrated morphologic differences around the shoulder girdle between East Asian and Western populations. 1,4Current implants may not accommodate the smaller morphometrics of the East Asian population. 4As mentioned, in their series of an Eastern Asian study population, a 36-mm glenosphere was implanted in all cases. 13In comparison, the current study utilized glenospheres with diameters from 33 mm to 42 mm with a goal of matching to patient size.The availability of smaller glenospheres may indicate an enhanced ability to decrease size mismatch, which has been demonstrated to affect outcomes. 19Further study in Asian populations with the use of smaller glenospheres may thus be warranted.Although precise guidelines on glenosphere size matching are currently lacking.
When SaN did occur, it did not affect 1 year ASES or VAS (P ¼ .329,P ¼ .191).ROM (FF, ER0, ER90, IR spine, IR90 -P > .05)was also not affected.With the exception of ASES score, this is largely consistent with what has been reported previously. 13As there are limited clinical data on SaN, further studies are needed to investigate the effect on functional outcomes.In the short term, however, SaN does not appear to impair function.
This study is not without its limitations including its retrospective nature.Secondly, given the very low incidence, patients with 1-year follow-up were included.However, as SaN has been up to this point a largely undescribed phenomenon, the inclusion of 1year follow-up is justified.Third, although patient demographics such as age, sex, and BMI were included in the analysis, preoperative indication for RSA was not included.Additionally, it is possible the implant characteristics and patient demographics investigated  may have an association with SaN; however, the low number of SaN cases introduces the potential for type II error in the findings.Finally, defining SaN remains unvalidated and this study was limited to use of plain radiographs.As such, preoperative films were scrutinized and compared to final follow-up films.All cases of SaN were reached by consensus.

Conclusion
The rate of SaN is low and not associated with glenoid or humeral prosthetic lateralization when using a 135 inlay humeral component.When SaN occurs, it is not associated with functional outcomes or ROM at short-term follow-up. Disclaimers:

Figure 1
Figure 1 Grashey view of a left shoulder with subacromial notching as evidenced by bony erosion with sclerotic margins on the undersurface of the acromion (circle) at (A) final follow-up radiographs not present (B) preoperatively.

Figure 2
Figure2The DSA as described by Boutsiadis et al 3 is made by drawing a line connecting 1) the lateral most aspect of the acromion and 2) the superior glenoid tubercle.From there, a second line is made from 2) the glenoid tubercle to 3) the superior most aspect of the greater tuberosity.A measurement between those 2 lines forms the DSA.DSA, distalization shoulder angle.

Table I
Baseline patient characteristics.DM, diabetes mellitus; PRO, patient-reported outcome; ASES, American Shoulder and Elbow Surgeons; VAS, visual analog scale; ROM, range of motion; FF, forward flexion; ER0, external rotation; ER90, external rotation with the arm at 90 ; IR, internal rotation; IR90, internal rotation with the arm at 90 ; s.d., standard deviation.

Table IV
Regression results e presence of surbacromial notching.ASES, American Shoulder and Elbow Surgeons; VAS, visual analog scale; ROM, range of motion; FF, forward flexion; ER0, external rotation; ER90, external rotation with the arm at 90 ; IR, internal rotation; IR90, internal rotation with the arm at 90 ; CI, confidence interval.