The most influential papers in unicompartmental knee arthroplasty

Purpose Unicompartmental knee arthroplasty (UKA) is a treatment option for anteromedial osteoarthritis of the knee. The number of UKA has been increasing constantly worldwide in recent decades. The aim of this study was to determine the most frequently cited scientific articles addressing this subject and to establish a ranking of the 50 most influential papers. Methods The 50 most cited articles related to UKA were searched in Web of Science® (Clarivate Analytics, Penn., USA) by the use of defined search terms. All types of scientific papers with reference to this topic were ranked according to the absolute number of citations and analyzed for the following characteristics: journal title, year of publication, number of citations, citation density, geographic origin, article type, and level of evidence. Results The 50 most cited articles had up to 453 citations. Most papers were published in the Journal of Bone and Joint Surgery (British volume). More than half of the articles were published in the 2000s and 2010s (n = 30). Ten countries contributed to the top 50 list, with most contributions from the UK (n = 17). Most articles could be attributed to the category of Clinical Science (n = 33), and most reported level IV studies. Conclusion Most of the frequently cited articles in UKA are clinical studies that have a low level of evidence. Few basic scientific studies could be identified, which suggests that most product development is done by commercial companies.


Introduction
Unicompartmental knee arthroplasty (UKA) dates back to the 1950s, following the introduction of the MacIntosh and McKeever hemiarthroplasty [1]. Since then, patient management, implant design, and surgical techniques, such as patient-specific instrumentation, have advanced [1,2]. Nowadays, UKA is considered a less-invasive treatment option for anteromedial osteoarthritis than total knee (TKA) arthroplasty [3][4][5]. As a result, the number of UKA has been increasing constantly worldwide in recent decades [6][7][8]. Due to the clinical relevance of UKA, numerous scientific papers related to this topic were published, that is, more than 2000 articles.
A citation is a reference or a quotation from previous scientific work that has been published in books or scientific journals [9]. The number of citations of published scientific articles is considered a parameter of its influence and impact in the scientific community. The impact factor of a journal is a widely accepted as a parameter of scientific quality and importance. It is calculated by the number of citations [9].
Analyses of citations in a specific scientific field allow us to give an overview of the most influential articles and offer physicians, researchers, and residents insight on the relevant current literature. Such studies have been done in different medical disciplines such as orthopedic surgery or general surgery; in a variety of orthopedic subspecialties including arthroplasty, arthroscopy, or hand surgery; and for conditions such as osteoporosis or anterior cruciate ligament rupture [10][11][12][13][14][15][16][17][18][19][20][21].
The aim of this study was to determine the most cited scientific articles related to UKA and to establish a

Search strategy
In March 2020, Web of Science® (Clarivate Analytics, Penn., USA) was searched for the following search terms "Unicompartmental Knee Arthroplasty," "Unicondylar Knee Arthroplasty," "Unicompartmental Knee Replacement," "Knee Arthroplasty," "Knee Replacement," "Implant," and "Prosthesis." The search output was documented after completion of the search. All scientific articles related to UKA that could be identified were included and ranked according to the absolute number of citations (times cited in the Web of Science core collection). If the absolute number of citations was the same in two or more papers, the publication that had the higher citation density (see below) was ranked higher. The fifty most cited articles were chosen and represent the list of the most influential papers in UKA. The literature search and subsequent data analysis were done by a board-certified orthopedic surgeon.

Data analysis
The fifty most cited articles were studied in detail for the following characteristics: article title, journal title, year of publication, and origin of the corresponding author. Each publication was assigned to a single country of origin. This decision was based on the corresponding author's address, as the corresponding author is in charge of the article and the guarantor of the data [22].
A citation density (the number of citations per year since publication) was calculated to show the relative impact of the included articles [17].
The included articles were allocated to different scientific categories as follows: "Clinical Science," "Basic Science," "Registry & Database," and "Review." The level of evidence was assessed in all articles that were attributed to the Clinical Science category. This assessment was done according to the guidelines for clinical articles by the Oxford Centre for Evidence-Based Medicine 2016 Levels of Evidence [23].

Results
The absolute number of citations for the fifty most cited articles on UKA range from 110 to 453 times. The fifty articles were cited, in total, 8436 times. The mean number of citations of an included paper was 169 times (± 69). The top ten papers reached at least 219 citations since publication.
The absolute numbers of citations for the fifty most influential articles on UKA are shown in Table 1, and the top ten papers, according to their relative number of citations (highest citation density), are presented in Table 2.
Thirty-three articles were allocated to the "Clinical Science" category, which was the most frequently noted category. Figure 1 presents the distribution according to the different categories.Within the category "Clinical Science," the level of evidence was analyzed. Eighteen studies of this category were categorized as level IV studies. Table 2 The ten highest cited papers in unicompartmental knee arthroplasty in relative numbers The distribution of the level of evidence is shown in Fig. 2.
The Journal of Bone and Joint Surgery (British volume) has published most papers of the list (n = 16). The Journal of Bone and Joint Surgery (American volume), Clinical Orthopaedics and Related Research, Acta Orthopaedica (Scandinavica), Journal of Arthroplasty, Lancet, and Knee were other journals that contributed to the list (Fig. 3).
Research institutions from the UK had the highest number of contributions (n = 17). Nine further countries contributed to the top 50 list: the USA with 16; Sweden with six; France with four; Finland with two; and Australia, Austria, Canada, India, and Norway with one each. Thirty-one publications were from Europe and 17 from North America.
The publication years of these articles span 1981 to 2014. Figure 4 shows the decade in which the research was published. Most articles were published in the 2000s (n = 23), followed by the 1990s (n = 13), and then the 1980s and 2010s with seven publications per decade.

Discussion
UKA is considered an alternative treatment option for anteromedial osteoarthritis to TKA. Because of the growing scientific interest in the field of UKA, we tried to identify the most influential articles related to UKA as guidance for clinical practice and future research. Publications on UKA were cited up to 453 times since publication, with the top ten articles in this field being cited at least 219 times. The citation number of articles in UKA is lower than in other fields of orthopedics, such as total hip and knee arthroplasty, cartilage surgery, and anterior cruciate ligament injury [13,16,20,21]. However, compared to articles on hand surgery or pediatric orthopedics, articles on UKA are well cited [15,18]. This indicates that the field of UKA is a highly driven industry such as the field of arthroplasty on the whole [24,25].More than two-thirds of the articles were attributed to the category "Clinical Science." Most of these were level IV studies, which indicates a low level of evidence in the field of UKA. This points to a need for clinical studies in UKA with better study design so as to gain more evidence in the future. Only three articles were level I studies. Two of these were prospective randomized controlled trials comparing UKA and TKA in the treatment of osteoarthritis of the knee.  The second most frequent category was "Registry and Database," with ten articles. This indicates the great interest in implant performance and survival. Only six papers were in the category "Basic Science." Product developments in the field of UKA are mostly done by commercial companies. These products are then used in clinical practice and studied clinically. Interestingly, the most highly cited contributions in the field of UKA were from the UK (n = 17). This is in contrast to most bibliometric analyses done so far, in which the USA contributed most of the scientific work [10][11][12][13][14][15][16][17][18][19][20][21]. Furthermore, in the field of UKA, a high number of contributions are from European countries, especially Scandinavian ones. This is also confirmed by the publishing journals. A large proportion of the articles was published in the Journal of Bone and Joint Surgery (British volume). Another seven articles were published in European journals. This finding is also in contrast to other bibliometric analyses in orthopedic surgery so far [10][11][12][13][14][15][16][17][18][19][20][21]. Interestingly, most articles have been published since 2000. This indicates an increasing trend in UKA in the last recent decades.
In general, bibliometrics has its limitations. The identification of the fifty most cited papers still remains a selection, even when using predefined search criteria. In this study, the absolute number of citations was considered an objective parameter. Among the scientific community, the impact factor is an accepted parameter of an article's influence. The impact factor is directly influenced by the number of citations. However, the citation numbers of articles can be manipulated by various factors such as self-citations (in small numbers) and therefore might not necessarily represent the objective value of scientific work [9].
The literature search was performed in the Web of Science®. Textbooks, doctoral theses, presentations, and new media are not included in this database. Therefore,  Another limiting factor might be the cross-sectional study design based on total citation numbers. As a result, more recent influential papers might not have been identified and were not considered eligible for the list.

Conclusions
This study provides a list of the fifty most influential articles on UKA, which will help physicians and scientists to obtain an overview on past and current trends in the field of UKA. Most articles could be attributed to the category Clinical Science and provided a low level of evidence. This provides a basis for both further discussion and highlights the need for future research.