Introduction

Randomized clinical trials (RCTs) and meta-analyses are considered the gold standard of evidence-based medicine nowadays [1]. The strength of RCTs rests on their excellent internal validity, which is based largely on the power of randomization to ensure that the only difference between two treatment arms is their exposure to the treatment of interest [2]. But the applicability of RCTs to the care of patients in routine practice is limited. In particular, patients, providers, and concurrent care in the general population are different from those in RCTs, and the generalizability or external validity of RCTs may be limited. Although observational research does not reach the same level of internal validity as RCTs, well-designed observational studies can offer high external validity and provide a unique opportunity to evaluate treatments and their outcomes in routine practice [2]. Many important clinical questions have not, cannot, and will not be addressed in the context of an RCT. In these situations, clinicians rely on information provided by observational research [2]. In a comparison of observational studies and RCTs, the estimates of the treatment effects from observational studies and RCTs were similar in most cases [3]. Registries are ongoing prospective observational data-collection repositories [4]. A registry is defined as a systematic collection of a clearly defined set of health and demographic data for patients with specific health characteristics, held in a central database for predefined purposes [5]. Medical registries can serve different purposes, for instance as a tool to monitor and improve the quality of care or as a resource for research [5]. To be useful, data in a medical registry must be of good quality [5]. To optimize the quality of medical registry data, the participating centers should follow certain procedures designed to minimize inaccurate and incomplete data [5]. The intended use of registry data determines the necessary properties of the data [5].

In 1992, surgeons from eight Swedish hospitals initiated a registry for inguinal and femoral hernia repair [6]. The aim of the registry was to report on the operative techniques used and to analyze outcome measures in order to stimulate quality improvement [6]. A number of national and international registries have since been added [6,7,8,9,10,11,12].

The aim of this manuscript is to explore the databases of these hernia registries and compare them in content and outcome variables.

Materials and methods

The CORE (Comparison of Hernia Registries in Europe) project was initiated with representatives from all established European hernia registries in March 2015 in Berlin. Initially perceived as a European project, the scope was broadened to also include the Americas Hernia Society Collaboration (AHSQC) Registry. Each registry representative was contacted to present and verify information regarding the registry (Table 1).

Table 1 Representatives of the participating registries
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The following information was obtained: Country(ies) of use, start date of registry, procedures included, compulsory or voluntary data entry, overseeing body, funding, user cost, access route, language, number of active users, whether data are validated and by what method, data analysis provided, and how the data are published. The following categories were used to compare the registries: initiation and funding, data collection and use for certification of hernia centers, patient data and data protection, operative data, registration of complications and follow-up data.

Results

The timeline for launch of registries included in the CORE project is shown in Fig. 1. Prospective hernia surgery registration was pioneered by Erik Nilsson in 1992 with the Swedish Groin Hernia Registry (SGHR) [6]. In 1998 the Danish Groin Hernia Database (DGHD) was established and was subsequently extended to ventral hernias (Danish Hernia Database) in 2007 [7]. The German Herniamed Registry included both inguinal and ventral hernias and was launched in 2009 [9]. In France the Club Hernie (CH) started their ventral hernia registry in 2011 across 30 specialized hernia surgeons [10]. Two registries were launched in 2012: EuraHS [8], and the Spanish Registro Español de Eventraciones (EVEREG) [11]. The Americas Hernia Society Collaboration (AHSQC) Registry followed in 2013 [12].

Fig. 1
figure 1

Timeline of hernia registries

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Compulsory or voluntary participation

The Danish Hernia Database is the only one to qualify as a genuine national registry where participation is compulsory for entry of all procedures by all surgeons performing a hernia operation. All other registries have to be considered as voluntary and completeness of data depends upon the participating hospitals and surgeons (Table 2).

Table 2 Initiation and funding of registries
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National vs international registries

Most hernia registries only record data on hernia operations conducted in their own country. The Herniamed Registry is used in the German-speaking countries Switzerland, Austria and Germany. EuraHS with a multilingual interface is intended for use at international level (Table 2).

Funding

Only the Danish Hernia Database and the Swedish Hernia Registry are publicly funded. All other registries are reliant on financial support from the medical technology industry (Table 2).

Case numbers

The case numbers in the various registries will of course greatly differ in accordance with how long a hernia registry has been in existence, the number of participating hospitals and surgeons as well as with the size of the respective country (Table 3).

Table 3 Data collection and certification
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Certification of participation

As an incentive for voluntary participation in a hernia registry, hospitals or surgeons are issued a certificate (EuraHS, AHSQC, Herniamed) confirming that they are taking part in a quality assurance study for hernia surgery. Since participation in the Herniamed Registry constitutes a basic prerequisite for obtaining certification as a hernia center from the German Hernia Society (DHG), the DHG has defined certain outcome criteria (Table 3).

Data protection

Due to data protection and privacy regulations, most registries are obliged or have chosen to enter their patient data anonymously or coded. Registries often use only the patient’s age or year of birth and mostly only a unique case identification number. The DHDB and SHR use a national personal patient code. In the Herniamed Registry, patient data are saved in a coded and anonymous format after obtaining the patient’s informed consent. The latter can be deleted at any time upon the patient’s request. All data classified as sensitive may be read and edited only by the treating institution for follow-up of the patients (Table 4).

Table 4 Patient data
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Patient variables

In addition to the patient’s age and gender, most registries also record details of previous operations, risk factors and comorbidities (Tables 4, 5). Only a few registries record the patient’s occupation or information on sporting or exercise activities.

Table 5 Operative data
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Operative data

Most registries record details of the operation such as urgency of the operation, hernia classification, hernia localization, operating time, operative technique, anesthesia type, mesh type, fixation technique, defect closure, drain utilization and antibiotic prophylaxis (Table 5).

Intra- and postoperative complications

Intra- and postoperative surgical and general complications are recorded and vary among registries (Table 6).

Table 6 Registration of complications
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Follow-up data

Further variations are observed in the follow-up parameters and protocols as well as the follow-up achievements of the registries (Tables 7, 8). This can be explained by a huge variation in the structure of healthcare systems in different European countries. The quality and frequency of routine clinical follow-up varies due to clinical and financial limitations. Patients who experience postsurgical complications often do not present to the initial operating surgeons or institution.

Table 7 Follow-up data part 1
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Table 8 Follow-up data part 2
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Outcome measurement tools

All registries deliver feedback to their participating hospitals, surgeons and research groups via annual reports and Excel exported files (Table 9). Since registries have no proven system for checking the validy of entered data, they can suffer from selection and input bias. This is always a limitation of all data analyses from registries.

Table 9 Provision of data and validation
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Discussion

Within the scope of the CORE project, representatives from seven hernia registers gathered to compare different aspects of their hernia registers. The CORE project examined aspects such as financing, data collection, certification, patient data, operative data, complications and follow-up of the patients. As registries were developed during various time periods where hernia surgery techniques and focus on outcomes have differed over time, differences between registries can be found. Financial resources have also had an impact on the quality of registries as have the ideas of individual surgeons.

It would be desirable to directly compare and combine data from the various hernia registries; therefore, the present analysis suggests potential adjustments to the way data are collected to improve data comparability in the future. The recommendations for reporting outcomes should be given particular attention [13].

Despite the differences in the way data are collected for each of the listed hernia registries, the data are indispensable in clinical research. As a consequence of the numerous innovations in hernia surgery (surgical procedures, meshes, fixation devices), hardly any other area of surgical study has such a high need for clinical trials and data collection, comparison and analysis. Registries play a vital role in this innovation process [14]. In addition, there is insufficient public funding available to perform RCTs [15, 16]. Furthermore, the costs for conducting RCTs have increased dramatically over the last decades [17]. Therefore, RCTs should be more feasible embedded within registries [18].

It has been shown that the introduction of the Danish Hernia Database improved the quality of inguinal hernia surgery from a national perspective [19]. A review based on three European hernia registries demonstrated the range of insightful findings that can be gleaned from hernia registries [20]. Registries can also play an important role in monitoring new devices by the industry (post marketing surveillance) [21]. This is of paramount importance as registries are called upon to provide more data for this specific purpose, because in the context of the current regulation environment at least in the European Union countries, the need of post marketing surveillance of medical devices has increased. As the main aim of the new European Union Medical Device Regulation is better patient safety industry, insurance companies and governments should ultimately contribute to fund hernia registries.

Currently, over 170 analyses from various hernia registries (Danish Hernia Database—http://www.herniedatabasen.dk 84; Swedish Hernia Registry—http://www.svensktbrackregister.se 55; Herniamed—http://www.herniamed.de 22; EuraHS—http://www.eurahs.eu 5; AHSQC—http://www.ahsqc.org 5; Club Hernie—http://www.club-hernie.com 1; EVEREG—http://www.evereg.es 1) have been published. The number of published articles clearly indicates that RCTs and registry-based observational studies have become partners in the evolution of medical evidence in hernia surgery [20]. As there is a discrepancy between the actually published data from hernia registries and the number listed in PubMed the use of the registry name as key word for the publication should be obligatory.

Many important questions in the field of hernia surgery have only been studied in registry studies [20]. Thus, the registers in hernia surgery are of great importance for clinical research. One clear advantage of the registry concept is having the ability to detect and analyze low rate potentially clinically relevant or even catastrophic events. Due to the increasing complexity in hernia surgery, hernia centers are increasingly being established worldwide [22].

Public media are increasingly aware of the fact that surgery can only be improved if its results are known [23]; the registry data are increasingly used for quality control [24], for example, in the certification of hernia centers [25]. A hernia center should be required to participate in a registry and submit as complete as possible data on all hernia patients [25].

Limitation of all data analysis from registries is always selection and input bias. The American College of Surgeons National Surgical Quality Improvement Program (NSQIP) mandates that participating hospitals assigns a NSQIP trained clinical reviewer to collect data on a stratified sampling of patients. Ongoing education for the reviewers as well as auditing is designed to ensure data reliability. This can be a model for the future, but calls for adequate financial support. This model can also prevent misuse of a registry by participating hospitals for marketing purposes.

In summary, while the seven existing hernia registries worldwide may differ in structure, together they contribute to raising the quality of hernia surgery. Assurance of data quality is critical to registries. This aspect should be taken into account in the evaluation of registry data. It would be desirable to harmonize outcome variables. The registries are of great importance for clinical research and are complimentary to RCTs for quality assurance, monitoring innovations, and potential certification of hernia expert centers. Combining all registry data in a common database would be desirable to allow additional knowledge to be gained.