Introduction

The anterior cruciate ligament (ACL) is one of the most common injured knee ligaments and at the same time, one of the most frequent injuries seen in orthopaedic practice.

There are various predisposing factors for ACL injury, including neuromuscular and biomechanical abnormalities, mutations within collagen producing genes, female sex hormones, abnormal joint laxity, and primary structural influences of the knee [1,2,3,4].

ACL rupture is a serious knee injury that affects mainly physically active young people [5]. It is usually the result of a non-contact injury whereby a combination of movements such as femoral adduction and internal rotation, knee flexion, or tibial rotation with the foot and ankle in valgus result in partial or complete anterior cruciate ligament tears [6]. The injury is characterized by joint instability, which is associated with an increased risk of early secondary osteoarthritis of the knee [7]. Treatment options range from non-operative strategies including physiotherapy, bracing and the modification of physical activity, to surgical reconstruction of the ligament. Especially for sports-active patients and athletes, as well as for patients with complex knee (−ligament) injuries, the operative treatment seems to be the procedure of choice [8, 9].

Given the clinical relevance of this injury, numerous scientific papers focusing on this topic, including biomechanical-, basic science-, clinical- or animal studies, were published.

The purpose of this study was to determine the most frequently cited scientific articles which address the subject of anterior cruciate ligament injuries and to establish a ranking of the 50 highest cited papers by the use of the Thomson ISI Web of Science® Database.

Analyses of most cited papers have been performed in various medical disciplines such as orthopaedic surgery or plastic surgery, various diseases such as burns or osteoporosis and a variety of orthopaedic subspecialties including arthroplasty, shoulder-, foot- or hand surgery [10,11,12,13,14]. However, no such study has been done focusing on “ACL injury”.

Purpose

The purpose of this study was to determine the 50 highest cited articles in ACL injury research in order to provide clinicians, researchers, and trainees a list of “citation classics” on the topic of ACL injury, including an analysis of the papers’ characteristics (year of publication, number of citations, citation density, geographic origin, article type, and level of evidence).

Methods

Search strategy

In July 2016, Thomson ISI Web of Science® was searched for the following search terms “Cruciate”, “Cruciate Ligament”, “ACL”, “Gracilis”, “Gracilis Tendon” “Semitendinosus”, “Semitendinosus Tendon”, “Patella Tendon”, “Hamstring”, “Knee Ligament”, “Ligament” “Quadriceps Tendon”, “Instability”, “Joint Instability”, “PCL”, “Unhappy Triad”, “Arthroscopy”, “Reconstruction”, “Drawer Test”, “Drawer Sign”, “Lachmann”, “Pivot Shift”.

After completion of the search, the search output was recorded. Scientific papers with no direct reference to the topic were excluded. All types of scientific papers with reference to the ACL were included in the study and ranked according to the number of highest citations. In cases with identical absolute numbers of citations, papers with higher citation density (see below) were ranked higher. A list of the 50 highest cited articles was established. The initial search with the above described search terms was done by a medical student (J.D.). Subsequently the articles were analyzed by an Orthopaedic surgeon (L.A.H.) who decided which studies were included in the definitive list of the 50 highest cited articles in ACL injury.

Data analysis

Each of the 50 highest cited articles was reviewed for the following information in order to create a list: article title, journal title, year of publication, and origin of the corresponding author. Each paper was assigned to a single country according to the corresponding author’s address because of the corresponding author’s primary responsibility for the study [15].

In order to determine the relative impact of a published paper, the citation density, the number of citations per year since publication, was calculated as mentioned above [16].

Furthermore, the level of evidence of each clinical article irrespective of the journal, where it was published, was set using the Journal of Bone and Joint Surgery, American Volume authors’ guidelines that are published online [17].

All papers were sorted according to the scientific category including “Clinical Science & Outcome”, “Basic Science”, “Classification & Scoring System”, “Review & Meta-analysis”.

At last, all papers’ content was screened and each article was assigned to one of the following categories: “Anatomy & Biomechanics”, “Classification & Scoring System”, “Imaging”, “Injury Mechanism”, “Prevention & Rehabilitation”, “Surgical Technique & Outcome” and “Tissue Engineering”.

Results

The 50 highest cited articles on ACL injury were cited from 315 to 1624 times. The top ten papers according to absolute numbers were cited at least 600 times. The 50 highest cited papers according to the absolute number of citations can be seen in Table 1.

Table 1 The 50 highest cited papers in anterior cruciate ligament injury
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The majority of articles could be attributed to the category Clinical Science & Outcome (n = 20). Within this category, level of evidence could be analyzed in 14 clinical papers, primarily distributed in level II (n = 10) and level III (n = 3) papers. One paper was categorized as level of evidence V (expert opinion). The distribution of the other categories can be seen in Fig. 1.

Fig. 1
figure 1

Distribution of highest cited papers according to categories

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Most papers (n = 22) were published in the American Journal of Sports Medicine. Other journals include the Journal of Bone and Joint Surgery; American and British Volume, Clinical Orthopaedics and Related Research, Biomaterials, Medicine and Science in Sports and Exercise, Orthopaedics, Radiology, Clinical Biomechanics, Clinical Journal of Sports Medicine, Arthroscopy, Journal of Athletic Training and Arthritis and Rheumatism. The distribution of the highest cited papers in the various journals can be seen in Fig. 2.

Fig. 2
figure 2

Distribution of the highest cited papers according to the published journal

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Seven countries contributed to the top 50 list, with the USA having the most contributions (n = 40), followed by Sweden with three, the UK and Norway with two each, and Canada, Italy and South Korea with one each.

The number of highest cited papers according to the decade of publication can be found in Fig. 3. The majority of papers were published since 1990 (n = 32), the remaining publications are related to the 1970s and 1980s. Within the list of the 50 most cited articles with regard to our topic, none was published during the last six years.

The distribution of the various scientific topics covered in the list of the 50 highest cited articles can be seen in Fig. 4. Papers in in the basic scientific fields of “Anatomy & Biomechanics” were most frequent.

Fig. 3
figure 3

Number of highest cited papers according to decade of publication

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Discussion

The purpose of this study was to determine the 50 highest cited articles in ACL injury research in order to provide clinicians, researchers, and trainees a list of “citation classics” on the topic of ACL injury, including an analysis of the papers’ characteristics, content, and clinical significance.

The ACL is one of the most common injured ligaments of the knee. Therefore, ACL injuries have a high impact on the individual, but also on healthcare systems worldwide. Given the strong scientific interest raised by this topic, both in the older as well as in current literature, we tried to identify the most significant articles on the subject of ACL injury. Therefore, we did a literature search based on Thomson ISI Web of Science®, in order to create a list of highest cited papers on that topic, regardless of the age and category of these articles.

Although a high number of total citations may not be the only parameter to determine the influence or clinical importance of an article, it can be considered as a parameter for the scientific relevance on a specialized subject [18,19,20, 23]. Furthermore, such an analysis allows identifying the research activity in various fields.

Fig. 4
figure 4

Distribution of various topics covered in the list of the 50 highest cited articles

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Articles on ACL injury were cited up to 1624 times, with the top ten articles on this topic being cited at least 600 times. These numbers are lower than the highest cited papers in the industry driven field of arthroplasty [15]. However, the number of citations seems to be quite high when compared to the most cited articles in the subspecialty “Orthopaedic Cartilage Surgery”, for example, which may be a comparable “hot topic” in the recent orthopaedic literature. The highest cited paper on this topic has reached 989 citations [19]. This is even more clear, when compared to highest cited papers in hand and shoulder surgery or paediatric orthopaedics [10,11,12,13,14].

The majority of articles could be attributed to the category Clinical Science & Outcome. Most of the articles in this group, on which a level of evidence could be assigned to, were based on level II and level III studies. Despite of one article, which was referred to as a level V article (expert opinion), no clinical studies with lower levels of evidence could reach a sufficient number of citations to join the list of the 50 highest cited papers. This will unlikely change or will even provide papers with a higher level of evidence in future publications due to the trend toward prospective-randomized trials and the focus on evidence-based medicine. [24].

These point to the fact that there are numerous high quality papers in the field of ACL injury research, especially when compared to other fields such as arthroplasty, where most papers were level of evidence IV [15].

The second frequent article type fell within the category of Basic Science. These articles are predominantly based on anatomic- and/or biomechanical cadaver studies. This shows that there is ongoing research incorporating novel techniques and technologies. Therefore, further “citation classics” are to be expected in this field. Interestingly, the three highest cited papers fall within the category “Classification & Scoring System”. These articles are simultaneously the ones with the highest citation density. These include the frequently applied and worldwide known Tegner-Lysholm Knee Scoring Scale and the Knee Injury and Osteoarthritis Outcome Score, initially developed for the evaluation of knee ligament injuries [21,22,23]. These manuscripts are the real “classics”, as they continue to be highly cited in research and are used in clinical practice frequently. A comparable high number of citations of classification or scoring systems have also been seen in other disciplines or subspecialities [25,26,27,28].

Particularly remarkable, when considering the article categories and contents, is the poor representation of papers focussing on surgical ACL repair techniques. The evolution of surgical treatment has undergone a paradigm shift over the decades, starting with the technique of open, primary repair of the ACL with all its biases as described in the corresponding literature. The subsequent developments, such as MRI and arthroscopy resulted in minimally-invasive techniques for ACL reconstruction- or augmentation. Solely, semitendinosus versus patellar tendon as an autograft used for ACL reconstruction were discussed in three of the listed papers. Anatomic and biomechanical studies, focussing mainly on anatomical, functional, and experimental analysis of the tissue-texture of both the ACL as well as ACL grafts, followed by clinical follow-up studies following ACL reconstruction and/or conservative treatment are the most contributing articles within the list of the “top 50” [29].

In total, seven countries contributed to the top 50 list, with the USA having by far the most contributions (n = 40). A fact that may reflect the high level of orthopaedic research in the United States. This is also confirmed by the publishing journals. A large proportion of the articles were published in the American Journal of Sports Medicine, followed by the Journal of Bone and Joint Surgery, American Volume. Similar results have been seen in other fields were the USA led most rankings [11,12,13, 27].

The oldest article in this category, focussing on the mechanics of the ligaments and menisci of the knee joint, was published in 1941 and is still cited 4.3 times per year in average [20].

A last interesting point is that the majority of the articles have been published between 1990 and 2005. Within the last ten years, no article on this topic has reached a comparably high number of citations. This might be due to cross-sectional design of the present study.

Bibliometric studies like ours reflect a “snapshot” of the most frequently cited articles in their specific field. Furthermore, the listing of articles according to the number of absolute citations remains a selection [30]. Important and influential articles on the topic might have been excluded. The citation numbers may be improperly increased by several practices including self-citations, subpublications, and timing of publication [31]. These limitations point out the necessity for other metrics to identify prestigious articles [30]. Although more objective journal metrics, for example the “SJR indicator” or the “Eigenfactor-score” with all their benefits, including the evaluation of the importance of citations from various sources, the eliminated influence of self-citations, and the greater number of journals and languages included in the database, it was not possible to establish a ranking of the most influential and important articles on a specific topic by using these scores since they were developed to measure the total importance of a scientific journal but not single articles [31]. A recent development is web platforms, which provide alternative metric scores for articles, for example article views, downloads or mentions in social media and news media [30,31,32]. Given the rapid development and growing importance of these platforms, altmetrics, as here provided, may have a huge impact on the assessment of the importance of articles and journals in the near future. Therefore, we decided to search for the most cited articles on our topic as has been done in various other fields. Since we have performed the search in the Thomson ISI Web of Science®, another specific limitation of our search strategy arises from the fact that articles published in non-indexed journals as well as articles cited in textbooks, lectures or digital media could not be taken into account [32].

Despite these limitations, we think that citation analysis remains one of the best currently available tools to assess the impact of scientific articles on their field [30].

We believe that studies like ours represent a valuable summary of scientific milestones on the topic of ACL injury. The diversity of the listed articles regarding the article types and study design for clinical papers as well as the heterogeneity of the content, ranging from biomechanical studies on native ligament and graft analyses to tissue engineering and reconstruction techniques, prevention measures, and outcome analyses following surgical, non-surgical or combined treatment strategies may reflect the comprehensive, active research conducted on this subject both current as well as in the past few decades. Furthermore, the results may reflect the quality of prior scientific research [30].

In summary, this article provides a selection of the most read and cited articles on the specific topic of ACL injury. This “reading list” may be helpful and interesting for both “unexperienced” readers as well as for senior readers or clinicians in order to provide a basis for both further discussion as well as future research.