>> The analyzes are essentially based on an evaluation of the data in accordance with Section 21 KHEntgG (Hospital Fees Act) by the Research Data Center (FDZ) of the Federal Statistical Office as well as on data from the hospitals' quality reports, which are made available as an xml file by the Federal Joint Committee.2 Additional data sources were used for individual questions.
The two years 2010 and 2018 were considered (the most recent data year available at the time the study was conducted). To describe hospital care, in addition to the number of locations, specialist departments, cases and services, the Herfindahl-Hirschman Index (HHI) was used as a concentration measure, in which the value 1 indicates a monopoly situation and 0 indicates equal market shares.
The concentration was viewed from two perspectives: On the one hand, it was examined how the frequency of carrying out certain treatment services is distributed across the hospital locations throughout Germany or regionally. On the other hand, it was examined how the use of the resident population of a district or an independent city3 for certain services is distributed across the hospital locations that provide the service - which can be located both inside and outside the district under consideration.
The analyzes are carried out both exploratively (e.g. for all specialist departments, ICD-3 digits and OPS-4 digits in Chapters 5 and 8 of the OPS catalog) and in an examination of specific services. For this purpose, services were selected for which, in the current specialist discussion, the concentration of service provision was too low and for which quality improvements are expected through greater concentration. The concentration of services was analyzed at the federal level, at the level of individual districts and at the level of regional hospital clusters. Due to the FDZ's confidentiality requirements, limitations in the significance of the results cannot be ruled out.
Results
A1 Comprehensive analyses
The number of hospital locations in Germany has been declining for a long time, despite the number of cases increasing over many years. However, very different developments can be seen at the level of the specialist departments. While the number of specialist departments in gynecology and obstetrics fell by 16% between 2010 and 2018, the number of specialist departments for geriatrics increased by 38% (Fig. 1).
Due to the significant declines in the frequent and high-case numbers of specialist departments for internal medicine, general surgery and gynecology and obstetrics, there is an overall decline in the number of specialist departments.
At the district level there are clear differences in the concentration of use, i.e. H. how the patients who live in a district are distributed among different hospitals. This applies especially to geriatrics: Despite the strong increase in the number of geriatric departments, the median HHI across all districts was comparatively high at 0.45 (Fig. 2); in the quarter of the districts with the highest HHI values it was at least 0.73.
In general surgery, on the other hand, there were still 1,033 specialist departments despite a significant decline in 2018; The HHI was therefore only 0.23 in the median across the districts, and the regional differences in the
centration were significantly less severe in general surgery than in geriatrics.

A2 Selected performance areas
For a detailed analysis, three services were selected for which, according to studies, positive quality effects can be expected from a concentration of treatment cases: minimally invasive aortic valve implantation (including primarily transcatheter aortic valve implantation - TAVI), anatomical lung resection and the implantation of total hip endoprostheses. A legal minimum number of cases has not yet been set for any of these three services, but this is being discussed for TAVI and lung resections (as of April 2021). For all areas, moderate minimum case numbers can also be derived from certification requirements or studies.
The results for 2010 and 2018 show that performance concentration has not increased in any of the three areas. For the two smaller service areas, TAVI and lung resection, the concentration of services has actually decreased significantly. This applies nationally, but predominantly also at regional level. Almost a third of the service cases with anatomical lung resection were treated at clinic locations that did not reach the minimum number of cases for certification (75 per year) (Table 1).
In the case of TAVI, the corresponding proportion - based on the study based on a minimum number of cases of 157 per year - is significantly lower at just under 10 percent.
The majority of clinics (87 percent) that implanted total hip replacement achieved the minimum number of cases for certification of 50 per year in 2018. Regardless of the changes, the concentration of performance during THA implantations is significantly lower compared to the other two performance areas.
Finally, there are regional differences for all three service areas: In addition to a low concentration of services in metropolitan areas, there are some rural regions in which the distribution of all cases treated in the hospitals of the regional cluster is highly concentrated among the clinic locations. These regional differences in the concentration of hospital care are also evident from the perspective of the regional population, i.e. the distribution of patients from a region across clinic locations (in and outside the region).
Below, selected results for two service areas are presented: TAVI and the implantation of THA.4

TAVI
In 2010, TAVI was still highly concentrated in the individual districts: in only 76 districts the use of the resident population was spread across at least three different hospitals;5 in 2018 the corresponding number was 352 districts with almost the same number of hospitals. In the remaining 61 districts, the resident population was cared for by a maximum of two hospitals.
In these 76 districts, the median concentration of utilization by the respective resident population (HHI) rose slightly from 0.38 in 2010 to 0.41 in 2018. In all 352 districts, the HHI was 0.47 in 2018; and in the quarter of counties with the highest HHI it was at least 0.64 (Fig. 3). In 34 districts, the hospital with the highest number of cases had a market share of at least 90%. If one assumes that the districts for which no data for 2010 is available for confidentiality reasons6 were higher than in the other districts, the average concentration at the district level is likely to have fallen from 2010 to 2018.
The strongest changes in the HHI when comparing the years 2010 and 2018 result from market share shifts between hospitals; This should be illustrated using two examples: In the two districts with the greatest increase in the HHI, the house with the highest number of cases gained significant market share: from 33% to 90% in the Biberach district and in the Bad Tölz-Wolfratshausen district from 43% to 90%.
In the two districts with the greatest decline in the HHI, however, the market share of the house with the highest number of cases has reduced significantly: from 87% to 67% in the Böblingen district and from 75% to 44% in the Vogelsberg district. In three of the four districts, the number of hospitals used remained constant at three; Only in the Biberach district has it increased from four to five.

THA
The distribution of the population's use of clinic locations that provided THA treatment (in and outside the region) varies significantly in the districts (Fig. 4). The average resident population of a district (median) used 33 different hospitals for THA surgeries in 2018 (min: 12, max: 100); in 2010 there were 29 hospitals. The median market share of the clinic location with the highest number of cases was 36% (Min: 12%, Max: 80%; 2010: 35%). The median concentration level (HHI) was 0.204 (min: 0.054, max: 0.643), slightly below the concentration level in 2010 (0.213). The HHI was above 0.5 in only six districts in 2018; There the market share of the house with the highest number of cases was at least 70%. In four cases these are regionally isolated districts in peripheral locations (Berchtesgadener Land, Wittmund, Passau and Garmisch-Partenkirchen).7 The regions with the lowest concentration of performance are often independent large cities and neighboring districts.
The comparison of the level of concentration in the districts for the years 2010 and 2018 shows a slight leveling overall: In regions with an above-average concentration in 2010, the concentration tended to decrease, while in regions with an originally below-average concentration, it tended to increase.
The clear increase on the left in Figure 5 is the Berchtesgadener Land district. Here the HHI increased from 0.323 to 0.643 and the market share of the clinic with the highest number of cases increased from 52% to 80% (Fig. 5). All three hospitals in the district belong to Kliniken Südostbayern AG, two of which carry out THA implantations. One of the two was certified as an arthroplasty center in 2013; The number of cases there increased significantly compared to 2010, but decreased at the other location. The second significant increase in the HHI from 0.256 to 0.544 is in the Celle district. The hospital most frequently used by patients from this group was able to increase its market share from 47% to 73%. THA implantations are only carried out at one location in the district: this has doubled its number of cases since 2010; It is therefore reasonable to assume that this location was able to make better use of the regional case volume and reduce the number of treatments in surrounding regions.
The significant decrease in concentration on the left edge of Figure 5 from 0.672 to 0.316 affects the independent city of Schwerin: Here the market share of the clinic with the highest number of cases was 81% in 2010 and only 53% in 2018. The only clinic in Schwerin itself has significantly lost cases, while clinics in the area (e.g. Hagenow, Crivitz, Grevesmühlen, Wismar and Parchow) were able to increase their case numbers.
Summary and discussion
The number of hospitals in Germany has decreased by around 10% since 2010; There was also an overall decline at the department level. However, there have already been different developments here: While the number of specialist departments for ophthalmology, ENT, gynecology and obstetrics has decreased significantly, the number of geriatric and neurological departments, for example, has increased significantly. In order to be able to answer the question of whether quality improvements can be achieved through a greater concentration of hospital care, an analysis at this level remains too rough, because the available empirical evidence of higher quality in facilities with larger case numbers relates to individual services.
For this reason, detailed analyzes were carried out in three service areas for which, according to studies, positive quality effects can be expected from a concentration of treatment cases:
• TAVI,
• (anatomical) lung resection and
• THA.

When comparing the years 2010 and 2018, there was a significant increase in the number of cases in all three areas, but the number of facilities was essentially constant. However, the performance concentration has decreased significantly in the case of TAVI and lung resections. Locations that had a particularly high number of cases in 2010 often suffered losses in market share, but in some cases also suffered absolute declines in the number of cases. Despite the significant increase in the average number of facility cases, there were still fewer cases of TAVI and total hip replacement below the minimum quantities required in certification procedures. In the case of anatomical lung resections, however, around a third of all resections were carried out in clinics that carried out the minimum number of 75 procedures per year. a.
not reached.
At the regional level, the situation is quite different both in terms of the level of concentration and in terms of development: While the use of THA care by the resident population in 17 regions (particularly the large cities and the Ruhr area) is strongly deconcentrated (HHI < 0.1), the HHI TAVI supply in 32 districts is over 0.81 and the market share of the hospital with the highest number of cases is 90% and more. The development of the case concentration was similarly diverse: in many regions there were only small changes in the degree of concentration (although there were sometimes significant shifts in the proportion of cases between hospitals). In a few regions, the concentration increased significantly: In the examples examined, this was often the case when locations were completely closed or services within a hospital network were concentrated at one location.
Overall, there is still potential for quality improvements through a greater concentration of care in all three service areas examined and in many regions. In view of the regional diversity of hospital care and the identified drivers for concentration processes, various measures are recommended:
Further impulses should come from the federal legal level to align the development of hospital structures more closely with quality goals. An evidence-based further development of the minimum quantity requirements can also contribute to this. In addition, further options for access to centers in rural regions with low case numbers should be created (e.g. telemedicine collaborations with larger centers).
The federal states' approaches to more active hospital planning, which have so far been manageable, should be continued and expanded. This should be done on the basis of continuous and systematic monitoring of regional care provision with regard to the availability, accessibility and quality of care. The inclusion of clinic capacities in the federal states' hospital plans should be linked to compliance with quality standards. This requires staffing of the planning authorities and targeted, sufficient investment financing as well as the early and systematic involvement of hospital and cost bearers.
Closures of locations and internal reorganizations have led to a significant increase in the concentration of hospital care in some regions. Hospitals' own efforts to cooperate across providers and to concentrate services often encounter not only political, but also antitrust reservations. The antitrust assessment of hospital collaborations and mergers should therefore be expanded. It is advisable to measure the concentration of hospital care at least additionally for specific service markets and to take greater account of quality effects when examining hospital mergers. And there needs to be a regulatory discussion about the role of competition in regional hospital markets. <<