Bibliometric Evaluation and Hot Spot Analysis of Developmental Dysplasia of the Hip

Purpose: To analyze the research development process and research hotspots of DDH in order to guide the future research direction. Methods: Using the Web of Science database to search the relevant English literatures of DDH from 1994 to May 31,2020 and analyze them with Bibliometric analysis methods. The statistics and the analysis of the collected publications are for the sake of hierarchical renement and quality assessment of the publications of different countries, authors, institutions, funding agencies, and journals. Using the VOS viewer software for visual conversion and researching direction and important elds of DDH simultaneously. Results: With a total of 2156 articles, the numbers of DDH published articles is increasing steadily. In funding agencies, the National Natural Science Foundation of China has funded most of the researches. In journals, the published articles of JOURNAL OF BONE AND JOINT SURGERY AMERICAN VOLUME are of the highest quality. For authors, KIM YJ is the author who not only published most of the articles, but also published the ones of the highest quality. In countries, the USA is ahead in the quantity and quality of published articles. In institutions, the published articles of UNIV BERN are of the highest quality. Through co-occurrence analysis, it can be seen that the research direction in the DDH eld consists of four categories: Diagnosis, Complications, Hip replacement, and Staging treatment before hip replacement. Current research hotspots are mainly focused on treatment. Conclusions: At present, arthroplasty and osteotomy are research hotspots in recent years. It is recommended that researchers start the study of DDH from osteotomy, trying to optimize the adaptation age of various osteotomy to DDH and reduce its complications.


Introduction
Developmental dysplasia of the hip (DDH) refers to the decline or the loss of stability of the hip joint caused by abnormal structural development of the hip joint which includes the acetabulum and femoral head, resulting in subluxation or complete dislocation of the hip joint [1,2]. Some of these babies had total hip dislocation before their birth, so DDH had been diagnosed as congenital dislocation of the hip (CDH). Since DDH further clari ed the undesirable changes in the hip structure that continue to develop after the birth and progress during their growth and development, CDH is replaced [3][4][5]. According to reports, the disease has a high prevalence in Asia, the Mediterranean, and the United States, where for every 1,000 newborns, there are 1-1.5 children suffering from DDH [6]. The incidence of DDH in women is relatively high, about 4 to 10 times higher than men [7]. The early detection of DDH can change the progress and prognosis of the disease, which is the current international consensus [8,9]. The study found that ultrasound is the best choice for the baby who less than 4 months; After the age of 6 months, femoral skullization will be more obvious, at when X-ray is the preferred method for evaluating and monitoring DDH [10]. For DDH after diagnosis, there are different treatment methods at different ages, and it is required for the orthopaedic doctor to grasp the dynamic development process of DDH in detail in order to respond in a proper way. The DDH, which was not found early, is closely linked to adult osteoarthritis(OA) [11], at the same time one-third of hip replacements in adults under the age of 60 are caused by DDH [12]. Therefore, DDH has attracted much more scholars from all over the world.
However, there is no bibliometric report on the current status of DDH research and the global development trends. Bibliometrics apply statistical methods to quantitatively evaluate research trends based on literature databases and their characteristics, which can not only help scholars grasp the development trends and research directions of speci c research elds but also evaluate journals, institutions, countries, and others in speci c research in the eld and provide a basis for the development of clinical guidelines [13]. The purpose of this study is to analyze the status of global DDH research publications to carry out layered re nement and quality assessment of the distribution of publications in different countries, authors, institutions, funding agencies, and journals. In addition, we also performed a cooccurrence analysis of keywords from DDH-related publications in order to understand their research directions and hotspot areas.

Methods
Web of Science's Science Citation Index Expanded (SCI-E) database was used for Bibliometric analysis [14,15]. Through advanced search, the limited time is from 1994 to May 31, 2020. Then we entered " developmental dysplasia of the hip" and free words to search for the required English literature, and obtained the information of the documents including authors, countries, journals, institutions, annual publications and funding agencies. We then downloaded and saved the information to TXT format and imported it into the Microsoft Excel 2019 for analyzing, by which we ensure that the information is correct. Subsequently, two independent researchers (Yinuo Fan, Haitao Zhang) separately veri ed and evaluated the data and uni ed the opinions through discussion. Finally, the subsequent literature measurement parameters were determined: Total Publications, Sum of Times Cited, Average citations per item, H-index (Journals including IF).

Bibliometric Analysis
Web of Science can analyze the retrieved literature, and the retrieved results are analyzed and extracted separately for funding agencies, journals, literature related to the authors, the countries, and the institutions. The data for the measurement parameters are tabulated. The frequency of citations represents the total number of citations in the corresponding years. H-index is the number of publications which have been cited at least H time, which could re ect the quantity and quality of the published articles by researchers and institutions [16,17]. Average citations per item are the average number of cited per articles, and it can also represent the quality of the publication to some extent., while the IF of a journal represents its in uence.

Visual Analysis
Vos viewer software (version 1.6.11) exploited by Netherlands Leiden University was utilized for visual analysis of the literature. The .txt le containing all the literature information obtained from the Web of Science database is imported into the software for "co-occurrence analysis", and the network and overlay visualization analysis structure diagrams are separately exported. Co-occurrence analysis can be employed to discern current research directions and hotspot areas of research.
Results 2491 documents were found on the web of science, adding to a total of 2156, details are shown in Fig1of the explicit inclusion and exclusion criteria chart.

Number of global publications
From 1994 to 2020, the quantity of DDH articles has been steadily growing. It can be seen from Fig2 that the volume of annual articles are divided into two periods: the rst period is from 1999 to 2005, and the second period is from 2006 to 2019. In the rst period, the rate of publication uctuated slightly with the annual growth in publication rising and falling, but the overall upward. In the subsequent period, except  Table 1, and these funding agencies supported nearly 12% of the researches. Interestingly enough, half of the funding agencies are from the US, the rest are from the UK, Japan, and China. The National Natural Science Foundation of China that ranked the rst funded 81 studies (3.757%), followed by National Institutes of Health (40 studies, 1.855%) and United States Department of Health Human Services (40 studies, 1.855%).

Journals
There are 372 journals published researches about DDH in total. Table 2  Co-occurrence analysis

Research direction analysis
All the published literatures included in the study have a total of 4319 keywords. Each keyword is de ned to appear at least 5 times through the Vos viewer software, and a total of 519 keywords are retained. As the Fig3 shows, the research direction of the eld is divided into 4 categories:Diagnosis (cluster1, in blue), Complications (cluster2, in green and yellow), Hip replacement (cluster3, in red) and Staging treatment before hip replacement (cluster4, in purple).The high-frequency keywords in cluster1 are ultrasound, ultrasonography and diagnosis; in cluster2, the most frequently occurred keywords are DDH, acetabular dysplasis and Osteoarthritis; In cluster3, the most frequently used keywords are replacement, follow-up and dislocation; In cluster4, the most commonly used keywords are congenital dislocation, closed reduction and open reduction.

Hotspot analysis
The Vos viewer software can be used to classify the keywords in the literature according to the chronological order of their appearance, so that their research hot spots are expressed with different colors. As the Fig4 shows, it can be seen from the time scale under the gure that the color purple appears early and belongs to the previous research hotspot, followed by blue, green and yellow, and the latest is red, which is the current research hotspot. The color changes gradually from purple to red. By analyzing the emergence of keywords, we can tell that the current research hotspots mainly focus on treatment, such as arthroplasty, different osteotomy (for example, Bernese periacetabular osteotomy, salter innominate osteotomy and pelvic osteotomy, etc.) and postoperative follow-up.

Discussion
Although DDH has a long history and many scholars have studied it, little is known about its exact cause. What we can do is to diagnose, treat in advance, and minimize the use of total hip replacement (THR) at the end stage [18]. At present, there is already a mature program for the diagnosis and treatment of DDH. Therefore, it is time to review the development of DDH to better understand the past, the present and the future. Bibliometrics is a research method that can provide comprehensive, objective, and macro information, and can also nd the research directions and hot spots in a certain eld [19,20]. This study aims to summarize the development of DDH through bibliometrics, and evaluate publications in DDHrelated elds, and visually analyze the research directions and hot spots.

Assessment of global publications
Between 1994 and 2020, global publications on DDH have gradually grown. Interestingly, we found that China was catching up in this eld, the National Natural Science Foundation of China has funded most for the research, and SHANGHAI JIAO TONG UNIV has published the largest number of articles (simply in terms of institutions), But the United States is still a giant overall in the eld of DDH research. The United States not only has more funding agencies to fund research in this eld, but also has high-quality journals in this eld. In addition, it is also in a leading position in terms of the quantity and quality of national documents. In terms of the author's contribution to the DDH research, KIM YJ's has the largest contribution. He is the author with the largest number of articles published and the highest quality of published articles which inspires us that If we want to grasp the latest development in this eld, we can start with the journals with publications of the highest quality; if we consider cooperation issues, the USA and UNIV BERN are considered rst; if we want to fund the research of an author to overcome DDH Relevant problems, there is no doubt that KIM YJ is the most worthy author.

Co-occurrence analysis
The analysis of research direction and hotspot Visualization of statistical keyword clusters by Vos viewer software and global research directions in the eld of DDH. It was split into 4 major categories: Diagnoses, Complications, Hip replacement, and Staging treatment before hip replacement. By analyzing the occurrence of keywords, we determined that the current research focus is primarily on the treatment of Aspects such as arthroplasty, different osteotomy, and postoperative follow-up. We analyze each of these directions and research hotspots intending to nd breakthroughs regarding the diagnosis and treatment of DDH.

Diagnosis
Ultrasound (US) is the preferred imaging modality for infants younger than 4 months of age. In the United States, ultrasound is recommended for selective screening of newborns, rather than universal screening, because the latter has not been turned out to help. Decrease the diagnosis of DDH [21][22][23]. Femoral ossi cation becomes more pronounced after 6 months of age when x-rays are used to diagnose DDH [10]. Since the diagnostic imaging of DDH after birth is improving, it is challenging for us to achieve a breakthrough in this area [9]. Nowadays, some scholars are gradually focusing on the direction of genetics, and it is found that genetic factors play an important role in the development of DDH and the role of CX3CR1 has been more recently studied as demonstrated in Fig4 [24]. CX3CR1 was found to be involved not only in in ammatory processes but also in the differentiation of mesenchymal stem cells into chondrocytes, and the error of this gene expression may contribute to the abnormal formation of acetabular cartilage [7]. We reviewed the literature and found that not only CX3CR1, WISP3, PAPPA2, HOXB9, and HOXD9, among others, also play a role in the genetics of DDH [24]. Although every patient with DDH needs to be evaluated individually by precise radiological and clinical examination, the discovery of the causative gene will greatly assist in diagnosis and treatment [25]. Therefore, in the future, gene-based prediction of infants and children with a positive family history of DDH will have a signi cant impact on the etiology and pathogenesis of DDH.

Complications
The most common and serious complication of DDH in the population is hip OA, and when the disease makes progress towards end-stage, the THR is required [11,12]. Therefore, the most essential thing is the early diagnosis and early intervention to delay the progression of OA and thus reduce the likelihood of surgery.
Hip replacement THR is a last resort for when DDH develops into terminal OA. However, concerning that this complication does not only occur in the elderly but also the young. What's more, THR is performed at a young age [25]. While THR is exposed to the possibility of infection and revision, this procedure is not suitable for young and middle-aged patients. Although studies regarding THR and its related aspects are still commonplace as saw in Fig3, we have to acknowledge the signi cance of early diagnosis and treatment of DDH.
Staging treatment before hip replacement Pavlik harness is a common treatment for children with DDH less than 6 months of age. We can see from Fig 4, that this is a hotly researched issue. It has been found that after successful Pavlik stenting if the acetabulum is developing normally by age 2 years, at an average of 10 years from the acetabulum continues to develop normally at all follow-up visits [26]. However, failure of Pavlik harness treatment can also lead to a series of complications [10], such as avascular necrosis (AVN) of the femoral head and femoral nerve palsy. Therefore, future research could be aiming at improving the success rate of Pavlik harness and reducing its complications.
We usually use closed reduction if the patient is diagnosed at 6-18 months of age. Although there is a better understanding of surgical outcomes of closed repositioning and ways to reduce its failure rate [27], it still has a Complications, such as AVN, a current concern and problem for orthopedic surgeons to address [28].
In general, open reduction is usually considered if patients are older than 12 ~ 18 months or ithey have failed to closed reduction, but AVN is still an issue to consider. The upper age limit at the time of open reduction is currently unclear. It has been found that younger children (<8 years old) may be the best time to perform the procedure and that the results of surgery are poor for patients with DDH diagnosed after 8 years of age at open reduction, so it is still debatable whether to operate on these patients [29,30].
Pelvic osteotomy is an excellent option for children with DDH who have failed initial treatment and are older (3-5 years). We can perform osteotomies to restore normal development of the acetabulum, which can be reconstructed up to the age of 5 [31]. The most commonly used pelvic osteotomies are the Salter, Pemberton, and Dega osteotomies, which are performed at the age of 5 years. Fig4 hotspots are also seen in the graph. However, the osteotomy is at the discretion of the orthopedic surgeon, as this procedure is not superior to other procedures [10].
The recent rapid development of hip-sparing surgery aimed to prevent OA associated with DDH. Bernese Periacetabular osteotomy (PAO) is a common hip-sparing surgery performed in the mid-to late-1980s. PAO aims to improve coverage of the femoral head in the acetabulum, thereby delaying hip osteoarthritis and hip arthroplasty. The study found that a signi cant improvement in the quality of life is brought to patients after PAO in the short to medium term and THR is not required within 10 years [32][33][34]. In a study of a long-term follow-up that lasts more than 30 years shows that 71% of patients had continued progression of pain symptoms, the presence of osteoarthritis on imaging, and the presence of chronic disease, evidence, or require THR [35]. In addition, although the age limit for this PAO has not been clari ed, studies have found a poorer postoperative prognosis in patients older than 35 years of age [36]. Therefore, future research should focus on how to delay the progression of OA and THR for a longer period, and expand the age scope to that ts PAO.

Limitations
Firstly, this research is restricted to English literature. DDH is not only found in Europe and the United States, but other languages were not included in the literature. Secondly, studies are limited to the period up to May 31, 2020, after which studies have not yet been included. Finally, due to the still brief period of recently published literature, which may have resulted in a lower number of citations and affected the overall citation frequency and H-index, thus the statistical results could be affected.

Conclusion
With the United States currently leading the way in DDH-related research and China having a fairly active presence in the eld, we consider that scholars could pay more attention to the progress of U.S. research in this area, and collaborate with leading authors or research institutions to major insights from DDH. The key point of the research related to DDH is gradually moving from diagnosis to treatment. Hip replacement and staging treatment before hip replacement are the main research directions, while Arthroplasty and Osteotomy are the hotspots of recent research. It is recommended that researchers start the study of DDH from genetic early diagnosis and osteotomy, and try to optimize the adaptation age of various osteotomy to DDH and reduce its complications.   Table 3. The top 10 active authors with most publications from 1994 to 2020.