**1. Introduction**

Several measures of acromion morphology, both in the sagittal and coronal planes, have been described and associated with various shoulder disorders [1]. In recent years, much research has been directed towards the characterization of the lateral extension of the acromion. The acromion index followed by the critical shoulder angle, both described on antero-posterior shoulder radiographs, have been shown to be predictors of glenohumeral osteoarthritis (OA) and rotator cuff tendon tears [2,3]. These initial findings were supported by subsequent biomechanical studies revealing increased glenohumeral joint reaction forces with decreased lateral extension of the acromion [4,5]. However, these two anatomical parameters are unable to assess the antero-posterior imbalance of the glenohumeral joint typically found in Walch type B glenoids [6].

Over the past three years, Beeler et al. analyzed shoulder computed tomography (CT) images to improve characterization of acromion roof morphology. Compared with osteoarthritic shoulders, the acromion was more externally rotated (axial plane), more downward tilted (coronal plane), and had wider posterior coverage of the glenoid (sagittal plane) in shoulders with rotator cuff tears [7]. The same research group further found

**Citation:** Terrier, A.; Becce, F.; Vauclair, F.; Farron, A.; Goetti, P. Association of the Posterior Acromion Extension with Glenoid Retroversion: A CT Study in Normal and Osteoarthritic Shoulders. *J. Clin. Med.* **2022**, *11*, 351. https://doi.org/ 10.3390/jcm11020351

Academic Editors: Markus Scheibel, Alexandre Lädermann and LaurentAudigé

Received: 10 November 2021 Accepted: 8 January 2022 Published: 12 January 2022

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a significant difference between shoulders with concentric and eccentric primary glenohumeral OA, and concluded that a flatter acromion roof with less posterior glenoid coverage could contribute to static posterior subluxation of the humeral head and posterior glenoid wear [8]. Furthermore, Meyer et al. were able to link a decreased posterior acromion slope and increased glenoid retroversion with static posterior subluxation of the humeral head and posterior glenoid wear [9]. More recently, Beeler et al. reported that the scapula of a shoulder with dynamic and static posterior instability was characterized by an increased glenoid retroversion and an acromion that was shorter posterolaterally and higher and more horizontal in the sagittal plane [10].

Static posterior subluxation of the humeral head and posterior glenoid wear are of particular interest to shoulder surgeons, as they have been related to both early glenohumeral OA in young adults [11], and higher complication rates after shoulder arthroplasty [12]. Although Walch et al. stated that static posterior subluxation of the humeral head preceded posterior glenoid wear, with glenoid retroversion as a risk factor [11,13], there is currently no consensus regarding this chicken-or-egg debate [6,14,15]. To our knowledge and despite the recent study by Beeler et al. [10], there are no published data on the correlation between the detailed three-dimensional (3D) acromion shape and glenoid retroversion.

Therefore, our objective is to investigate the potential association between the 3D acromion shape and glenoid retroversion in both normal and osteoarthritic shoulders. This could prove to be clinically useful to better understand the etiology of eccentric glenoid wear and possibly define an anatomical parameter to predict its risk of occurrence. We first examined the presence or absence of correlation between the 3D acromion shape and glenoid retroversion in normal scapulae. Then, we tested whether the same results held true in shoulders with primary glenohumeral OA.

#### **2. Materials and Methods**
