*2.1. Study Population*

All subjects gave their informed consent for inclusion before participating in the study. The study was conducted in accordance with the Declaration of Helsinki, and the protocol was approved by the local Ethics Committee of the state of Upper Austria (Study number 1167/2020). The case number consisted of 25 patients (15 women) who underwent shoulder arthroplasty in the period from 01/2009 to 07/2020. Included in the study were all patients who received an aTSA with a flat MBG in the specified period. The indication for implantation was an intact RTC without severe fatty infiltration (Fuchs grade ≤ 2) as well as radiographically determined omarthrosis, which was accompanied by severe pain in the shoulder joint and restricted movement of the affected arm and glenoid morphology according to Walch A1, 2 and B1 [9,10]. The exclusion criteria for performing aTSA included a full-thickness RTC tear and/or fatty infiltration of the RTC (Fuchs grade > 2), glenoid morphology according to Walch B2, B3, C, and D.

The minimum follow-up time from prosthesis implantation to the last reevaluation was 24 months, with a mean follow-up time of 68.6 months (range: 25.9–100.7). All patients were required to have pre- and postoperative radiographic images of the operated shoulder. Exclusion criteria for participating in the study were neurologic abnormalities or inability to fulfill the study requirements.

#### *2.2. Data Collection and Assessment*

Clinically, the postoperative Constant Murley Score (CMS), UCLA-Score, and the Simple Shoulder Test (SST) were assessed at the final follow-up. Radiologically, every patient had preoperative X-rays in two planes (anterior-posterior (AP) and axillary or y-view) and MRI or CT. CT and MRI were used for the classification of the preoperative glenoid morphology, according to Walch et al. [10], and RTC degeneration, according to Fuchs et al. [9]. Immediately postoperatively and at final follow-up, all patients received at least an X-ray in two planes (AP and axillary or y-view).

The postoperative X-ray images were calibrated over the known head size of the implanted humeral head. Radiolucent lines (RLs) around the humeral and glenoid components were assessed from the postoperative X-rays. Postoperative X-rays were also used to evaluate the center of rotation (COR). Postoperative radiographs were needed to measure the humeral head migration (HHM) and lateral glenohumeral offset (LGHO). HHM was measured via the smallest distance between the COR and the dense cortical bone

marking the underside of the acromion. The difference between immediately postoperative AP X-rays and AP X-rays at final follow-up was calculated. The COR was determined as described by Alolabi et al. [11]. The debridement of the PE was measured using the LGHO as a difference (millimeter) of LGHO from immediately postoperative AP X-rays and LGHO from AP X-rays at final follow-up, which was determined by the distance from the medial edge of the baseplate to the most lateral point of the greater tuberosity (Figure 1).

**Figure 1.** Postoperative radiographs of a left shoulder demonstrate the measurement method of the lateral glenohumeral offset (above X-ray image) and the humeral head migration (below X-ray image).

To evaluate RLs around the glenoid, the Lazarus scoring system, originally described for pegged glenoid components, was applied [12]. To evaluate the RLs around the humeral component in our study, the AP radiographs were assessed, and the axillary view was taken by dividing the implant-bone interface into three different sections.

Similarly, the radiolucent lines around the humeral component were assessed using eight distinct zones. For the humeral components, the analysis was based on the classification by Molé et al. [13] (Figure 2).

**Figure 2.** Postoperative radiographs of the left shoulder. The above X-ray image shows the anatomical circle (blue) and the implant-matched circle (orange), according to Alolabi et al. [11]. The distance between the two centers was measured (ΔCOR). In the X-ray image below, an assessment of radiolucent lines (RLs) for the glenoid and humeral components is shown. Glenoid RLs were quantified in 5 zones (1–5) considering their thickness, while the humeral RLs were quantified in 3 different zones (a, b, and c). The Radiograph shows radiolucency in zone a.
