**3. Results**

#### *3.1. Historical Data: Baseline Years 2010–2019*

The procedural volume of primary shoulder arthroplasties increased by approximately 14% each year from 13,678 to 25,294 patients, amounting to an incidence rate of 30.4 per 100,000 in 2019 (Figure 1). This development has mainly been attributed to increased utilization of rTSA, for which the number of procedures has almost quadrupled since 2010. In addition, the use of HA decreased by over 70%, while the number and incidence of anatomic total shoulder arthroplasty procedures (aTSA) showed almost steady progress over time. Regarding the demand in different age groups, in 2010, 5.2% of all primary shoulder arthroplasties and only 1.8% of all rTSA procedures were performed in younger patients (55 years or younger). By the end of 2019, the relative size of this population had decreased to 4.1% and 1.4%, respectively. Overall, an enormous increase in fracturerelated arthroplasty could be identified, with the number of fracture-related rTSA showing an almost ten-fold increase during that time span (Figure 2). Additionally, the overall number of osteoarthritis (OA)-related procedures also almost tripled, with rTSA becoming increasingly important compared to aTSA or HA.

**Figure 1.** German population, shoulder arthroplasty incidence ( **A**) and type of procedure (**B**) from 2010 to 2017.

**Figure 2.** Historical and projected main indications for reverse shoulder arthroplasty from 2010 to 2040.

During the study period, 19,190 revision procedures (replacements or explanations) were undertaken, which included 5748 revisions for HA and 13,241 for TSA, with an overall RB of 12.9% for HA and 7.0% for TSA. However, we identified a slight RB decrease for TSA, while the RB for HA significantly increased (from 7.1% to 18.4%). For both procedure types, a 1.2-fold higher relative risk for revision was calculated for males compared with females.

#### *3.2. Projection of Primary Shoulder Arthroplasty: Years 2020–2040*

Based on our quasi-Poisson projection model, the annual number of shoulder replacements is estimated to grow to 95 × 10<sup>3</sup> (95% CI 79–112 × 103) by 2040 (Figure 3). The forecasted incidence rate was projected to be 112 (95% CI 91–134) per 100,000 German residents, resulting in a more than seven-fold growth from 2010 to 2040. Additionally, we used a negative binomial approach with monotone B-splines to achieve a different projective view on the future demand for shoulder arthroplasty. This model projected the annual number of shoulder replacements to rise to 37 × 10<sup>3</sup> (95% CI 32–44 × 103) by 2040, leading to an incidence rate of 47 (95% CI 41–55) per 100,000 inhabitants, which still represents a rise of approximately 175% since 2010.

**Figure 3.** *Cont*.

**Figure 3.** Projections of the incidence (**A**) and total number (**B**) of shoulder arthroplasties by the year 2040 based on a Poisson (red) and a negative binomial regression model using a monotone B-spline approach (blue). The black points represent the historical numbers.

#### *3.3. Projections of Shoulder Arthroplasty as a Function of Age*

Considering age, the incidence rates almost doubled in each age group, while the total amount of shoulder replacements showed the highest increase in older patients (Figure 4). This increase can mainly be attributed to the rising utilization of rTSA, which will be responsible for over 90% of all shoulder replacements in 2040 if our projections hold true (Figure 5). These arthroplasties will be performed due to a massive increase in fracture-related scenarios, which could reach an eight-fold increase by 2040 (Figure 2).

**Figure 4.** Reported and predicted case numbers per age group from 2010 to 2040.

**Figure 5.** Projected share of reverse total shoulder arthroplasties (rTSA) in total shoulder arthroplasties from 2010 to 2040.

Despite a simultaneously rising number of TSA in younger patients, the demand for primary shoulder arthroplasties among patients aged 55 y or younger was projected to decrease from 5.2% in 2010 to 3.2% (95% CI, 2.7–3.6) of all recipients by 2040.

#### *3.4. Projections of Revision Shoulder Arthroplasty*

Along with the rising number of primary replacements, the number of revision procedures is projected to increase as well. Based on our models, this number will reach its maximum of approximately 4000 (upper 95% CI limit: 9000) procedures in 2040, which represents an increase of approximately 300%. However, because of the higher future number of rTSA, which was associated with a lower RB, the overall estimated RB will decline. For better comparison, we also calculated the number of revision procedures using a constant-rate approach. Here, we calculated the highest number of revision procedures of 4100 (95% CI 3568–4852, monotone B-spline approach) and 10,500 (95% CI 8817–12,558, Poisson modeling), respectively, which represents a 2.5 to 8.4-fold increase in revision procedures (Figure 6).

**Figure 6.** Projections for revision shoulder arthroplasty procedures using a "constant-rate" approach based on a Poisson (red) and a negative binomial regression model using a monotone B-spline approach (blue).
