Introduction

The goals of anterior cruciate ligament (ACL) reconstruction are the restoration of normal knee anatomy and function, reestablishment of biological and biomechanical homeostasis and prevention of osteoarthritis (OA). The traditional transtibial technique has come under recent scrutiny as it has been shown that it results in a non-anatomic reconstruction with regards to the native ACL footprints and therefore may cause suboptimal knee kinematics [45, 46, 74, 92]. Focus is now on anatomic reconstruction, with greater emphasis on tunnel position in the native ACL footprint, functional restoration of the ACL to its native dimensions, collagen orientation and insertion sites. Both biomechanical and clinical trials have shown superior results in support of this technique [11, 30, 57]. It must be stressed that double-bundle (DB) ACL reconstruction is not synonymous with anatomic ACL reconstruction [39]. It is merely a step closer to replicating the native ACL anatomy; it can still be performed non-anatomically. The theoretical advantage is that DB ACL reconstruction allows for individualizing the surgical procedure to each patient, of more complete footprint restoration, and that the two bundles can be tensioned separately, mimicking the native tension patterns of the ACL bundles.

With the emergence of the term anatomic ACL reconstruction, many authors have claimed their adoption of anatomic ACL reconstruction techniques. However, two previous systematic reviews indicated that often only limited information is provided by the authors when outlining the specifics of their anatomic ACL reconstruction [84, 85]. Furthermore, variations in anatomic techniques were found. To further evaluate the possible benefit of performing anatomic reconstruction and to be better equipped when comparing studies, it is important to use standardized and thoroughly evaluated criteria for what constitutes anatomic ACL reconstruction. This has led to the development and validation of the anatomic ACL reconstruction scoring checklist (AARSC) by van Eck et al. [82]. An international expert panel of orthopaedic surgeons listed potential criteria for what constitutes an anatomic ACL reconstruction. These criteria were evaluated and expanded by a global panel of orthopaedic surgeons. A modified list containing 17 items with the maximum score of 19 was created (Table 1). The new list was verified by a large number of peer reviewers from four high-impact orthopaedic sports medicine journals. The score was then tested for validity and reliability, and all items were evaluated for internal consistency. This scoring system allows for the grading of ACL reconstruction procedures in individual patients, for review of the documentation of surgical methods in published studies on anatomic ACL reconstructions and for peer review of scientific manuscripts.

Table 1 Items from the anatomic ACL scoring system and the frequency of the reported data
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The goal of this systematic review was to apply the AARSC on current clinical studies comparing SB and DB ACL reconstructions. It was hypothesized that reported data in studies comparing SB and DB ACL reconstructions would fall below a proposed minimum required score of 10 for anatomic ACL reconstruction, illustrating the heterogeneity of current literature on the subject.

Materials and methods

This systematic review was conducted following the preferred reporting items for systematic reviews and meta-analyses (PRISMA) guidelines [53].

Eligibility criteria

Inclusion criteria were clinical therapeutic studies that compare SB and DB primary ACL reconstructions. Prognostic and diagnostic studies were generally excluded unless there was a clear relation between the outcome measures and the surgical technique. Economical and decision analysis studies were excluded. Studies with skeletally mature, living human subjects with isolated total ACL rupture were eligible for inclusion. Concomitant meniscus and minor cartilage injuries were not criteria for exclusion. Only papers written in English were included [65].

Information sources and search

Electronic search

A systematic electronic search was performed using PubMed (MEDLINE), EMBASE and Cochrane Library. Studies that were published from January 1995 to August 2011 were included. An additional updated search was performed in January of 2014 only from the PubMed (MEDLINE) database, and relevant studies published between August 2011 and January 2014 were included. Execution and validation of the search was attained from two experts in electronic search methods at the **MASKED** Library. The following search terms were used in the fields title, Abstract and Keywords: ‘anterior cruciate ligament’ OR ‘ACL’ AND ‘reconstruction’ OR ‘surgery’ AND ‘1995:3000’ (see “Appendix” for complete search string) [65].

Data collection and analysis

Study selection

Three researchers sorted the studies based on the abstracts from the electronic search. Each reviewer sorted one database, which in turn was validated twice by the other reviewers. The included studies were categorized into study types proposed by the Oxford Centre for Evidence-Based Medicine and into the category SB, DB or SB versus DB reconstruction. Only studies comparing SB to DB ACL reconstruction were included in this systematic review regardless of graft type or fixation method. The study was processed in full text and analysed if the abstract did not provide enough data to make a decision. The analysis was not performed in a blinded fashion, i.e. the researchers were not blinded to author, year and journal of publication. Disagreement between the reviewers was resolved by consensus or by discussion with the senior author when consensus was not reached.

Data collection process

The data were extracted from the included papers by the first two authors through utilization of a computerized database created in Microsoft Access (version 2010, Microsoft Corporation, Redmond, WA, USA). Each study was validated twice by the first author.

Data items

The data that were obtained from the included papers were the following: author, year, title, journal, volume, issue, pages, ISSN, DOI, abstract, author address, database provider, category, study type, level of evidence and country. In addition, the items from the AARSC were extracted for both SB and DB groups (Table 1). In some studies, there were comparisons for more than two groups. In these cases, the scoring system was applied to all groups, and thereafter, only the highest scoring group from the SB and/or DB group was included. The study needed to state clearly the item from the scoring system, e.g. the authors needed to state that they visualized the ACL insertion sites, otherwise no points were given. For a point to be awarded for a specific criterion in the score, the criterion in question had to be applied to all patients in the SB or DB group. Furthermore, outside-in drilling of the femoral tunnel was classified as transportal drilling.

Synthesis of results

Statistical analysis of the data was performed using the IBM SPSS Statistics (version 21, IBM Corporation, USA). Descriptive statistics were used to summarize all data recorded. The comparison of AARSC score depending on the level of evidence and year of publication for the SB and DB groups was performed using one-way ANOVA. Alpha was set to 0.05.

Results

Study selection

An initial electronic search yielded 5,608 studies in PubMed (MEDLINE), 5,421 studies in EMBASE and 700 studies in the Cochrane Library. Duplicates were removed. There were 7,154 studies left in which 3,757 were excluded based on the abstracts and 1,887 based on full-text assessment. A total of 1,510 studies were included in the database and categorized into SB, DB and SB versus DB ACL reconstruction. Fifty-one studies were categorized as studies comparing SB and DB ACL reconstructions. An updated search was performed in January 2014 only from the PubMed database. Of these, a total of 26 studies were included. This amounted to 77 studies in total included in this systematic review (Fig. 1) [1–10, 12–17, 20, 22–24, 26–31, 33–38, 40–44, 47–52, 54–56, 58, 59, 61, 62, 64, 66–68, 71–73, 75–81, 86–91, 93–99].

Fig. 1
figure 1

Flow diagram of the selection of studies for the systematic review

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Characteristics of studies

The time span of the included studies stretches from 2004 to 2014. Most studies were published in 2011 (19; 25 %) and thereafter 2012 (12; 16 %). The top three journals were Arthroscopy (19; 25 %), Knee Surgery Sports Traumatology Arthroscopy (KSSTA) (18; 23 %) and Am J Sports Med (AJSM) (16; 21 %). Together they represented 69 % of all included studies. Subgroup analysis revealed that the most randomized clinical trials (RCT) were published in KSSTA (n = 7) followed by AJSM (n = 6) and Arthroscopy (n = 5). Of these RCTs, AJSM had six studies classified as category 1 in level of evidence, KSSTA five studies, and Arthroscopy four studies. The top five countries in terms of number of studies were Japan (17; 22 %), Italy (12; 16 %), South Korea (9; 12 %), China (7; 9 %) and Germany and Finland both (5; 6 %). Most studies were RCTs (29; 38 %), and the least common were case series (4; 5 %) (Table 2). Level of evidence category 2 was most common (30; 39 %) (Table 3). The most common graft types were hamstrings tendon graft for both groups (Table 4).

Table 2 Types of studies and frequencies for the included studies
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Table 3 Distribution of level of evidence for the included studies
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Table 4 Frequency of graft types utilized categorized into single- and double-bundle reconstructions
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Synthesis of results

Femur-related items

Thirty (39 %) in the SB group and 41 (53 %) in the DB group described visualizing the femoral ACL insertion site. Twenty-eight (36 %) in the SB group and 42 (55 %) in the DB group reported placement of the femoral tunnels in the ACL insertion site. Subgroup analysis revealed that 6 (8 %) in the SB group and 8 (10 %) in the DB group stated placement in the femoral ACL insertion site but did not report visualization of the insertion site (Table 5). Moreover, 22 (29 %) in the SB group and 34 (44 %) in the DB group reported both placement and visualization of the femoral ACL insertion site. Also, 17 (22 %) in the SB group and 24 (31 %) in the DB group stated transportal drilling of the femur tunnel(s) and placement of the tunnel(s) in the femoral ACL insertion site. Eleven (14 %) in the SB group and 18 (23 %) in the DB group reported placement of the tunnel(s) in the ACL insertion site but did not report transportal drilling. The use of an accessory medial portal was reported more often in the DB group (25; 32 %) than in the SB group (9; 12 %). Six studies (8 %) in the SB group and 16 (21 %) in the DB group documented the use of an accessory medial portal and placement of the tunnel(s) at the insertion site of the femur. Measurements of the femoral ACL insertion site dimensions were only reported in one study for SB (1; 1 %) and DB (1; 1 %) groups. Femoral fixation methods were documented in all but three studies (74; 96 %) for SB groups and in all but two studies (75; 97 %) for DB groups.

Table 5 Cross table with frequencies calculated for placement of tunnels in insertion sites and certain surgical techniques
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Tibia-related items

Twenty-nine studies (38 %) in the SB group and 39 (51 %) in the DB group described visualizing the tibial ACL insertion site; however, 44 (57 %) in the SB group and 57 (74 %) in the DB group reported that the tibial tunnel was placed in the insertion site of the ACL (Table 1). Subgroup analysis revealed that 22 (29 %) in the SB group and 23 (30 %) in the DB group did not document visualization of the insertion site but claimed placement in the insertion site (Table 5). Furthermore, 7 (9 %) in the SB group and 5 (6 %) in the DB group documented visualization of the insertion site but did not claim placement in the insertion site. Twenty-two (29 %) in the SB group and 34 (44 %) in the DB group reported both visualization and placement of the ACL insertion site. Measurements of the tibial ACL insertion site dimensions were only reported in one study for SB (1; 1 %) and DB (1; 1 %) groups. Tibial fixation methods were documented in all but three studies (74; 96 %) for SB groups and in all but two studies (75; 97 %) for DB groups.

Documentation of tunnel position

The highest level of documentation used for ACL tunnel position for both groups was most often the scoring item with zero point allocation (84; 55 %), i.e. drawing, diagram, operative note, dictation or o’clock reference (Table 1). The second most common was two-dimensional images (44; 29 %) and thereafter three-dimensional images (26; 17 %).

Single-bundle versus double-bundle

The DB reconstruction was reported more thoroughly in several items, especially the usage of accessory medial portal, visualization of the insertion sites, drilling technique and placement and fixation of the graft(s) (Table 1).

Anatomic ACL score

Calculation of the AARSC score reveals a mean score of 6.9 ± 2.8 for the SB group and 8.3 ± 2.8 for the DB group (Table 6). There were no significant differences in the AARSC score for either the SB or DB group depending on the level of evidence of the study with p = 0.126 (n.s) and p = 0.226 (n.s) respectively, or the year of publication with p = 0.745 (n.s) and p = 0.717 (n.s)  for SB and DB groups respectively.

Table 6 Calculated anatomic ACL score for single- and double-bundle ACL reconstruction
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Discussion

The most important finding from this systematic review is that there is a substantial underreporting of surgical data with means in AARSC well below the proposed required minimum score of 10. This makes any pooling and comparing of outcomes measures across studies difficult, if not impossible.

Most studies are published in recent years and in the journals Arthroscopy (25 %), KSSTA (23 %) and AJSM (21 %). The trend is an annual increase in the number of studies. This is not surprising as the surgical technique is relatively new. A positive finding is that most studies are RCTs and that there are fewer case series. This is promising and a trend that hopefully will continue in the future. Japan and Italy are the countries with most studies, contributing 22 and 12 % of all studies in this review, respectively. However, it must be stressed that despite the trend of increasing Level of evidence with time, we see that studies are still lacking in documentation [65]. It is at the level of peer review that we can stress the importance of adequate documentation and improvements are expected in the future.

A majority in both the SB group and the DB group report placement of the tibial tunnel(s) at the insertion site for the native ACL. However, only approximately half of those claim visualization of the insertion site. Only 29 % in the SB group and 44 % in the DB group report both visualization and anatomical placement on the tibial side. Absent reporting may not necessarily be synonymous with absent visualization; however, this is related to how the scoring system is implemented.

The bony landmarks of the femoral notch, the lateral inter-condylar ridge and the lateral bifurcate ridge constitute an important aid in the placement of the tunnels, and this is a significant finding in recent years [100]. In total for both groups, only approximately 10 % report visualization of the lateral inter-condylar ridge and 6 % the lateral bifurcate ridge. The lateral inter-condylar ridge is of special interest in both groups as it defines the superior border for the footprint [19, 60, 70]. The lateral bifurcate ridge marks the border between the AM and PL bundles [18]. Therefore, the latter is a vital component in the placement of the tunnels in DB reconstruction. Also, the landmarks are present in both sub-acute and chronic cases without a significant difference in presence [83]. Thirty-nine per cent in the SB group and 53 % in the DB group claim visualization of the femoral insertion sites, which is comparable with the number of studies that state placement on the femoral insertion sites. Overall this illustrates a substantial underreporting that creates an uncertainty when assessing the outcomes of the studies. The utilization of an accessory medial portal facilitates both the visualization of the ACL footprint on the femur and also provides a portal for drilling [8]. Only 35 % in the SB group and 42 % in the DB group report transportal drilling. Some studies did, however, report the use of transportal drilling of the PL bundle and transtibial drilling of the AM bundle. These studies were not awarded a point for transportal drilling, as it was necessary that all tunnel drillings can be performed independently. This is not optimal as the femoral insertion site for the AM bundle can in some cases be reached through the tibial tunnel; however, a consensus was reached in order to create homogenous reporting [46]. The implementation of transportal drilling can to a certain extent reduce the inherent restrictions when drilling through a tibial tunnel [21, 25, 32, 63]. Furthermore, there are several studies that report difficulty in reaching the anatomic femoral insertion sites through transtibial drilling [11, 45, 46]. Caution is therefore warranted when studies claim anatomical placement solely facilitated though transtibial drilling, especially in DB ACL reconstruction.

The measurement of the tibial and femoral ACL footprint aids the orthopaedic surgeon in both placement of the tunnel and individualization of the reconstruction to the patient and is seldom technically challenging for the surgeon. These measurements were only provided in one instance (1 %) in the SB group and one instance (1 %) in the DB group, and only two studies claimed individualization. Today, experienced orthopaedic surgeons often describe attempts to individualize the surgical procedure to most patients. In this individualization, several factors are taken into account, e.g. native ACL insertion site size, graft thickness, acceptance of harvest-site morbidity and notch size. The quadriceps tendon and the patellar tendon offer possibilities of preoperative measurement and therefore preoperative individualization; this is not the case with hamstrings tendon autografts. Of course, individualization is not possible if patients are randomized, and this should be taken into account as most included studies in this review are RCTs. Also, in comparative studies in general there is an inherent difficulty to individualize in this way as most studies are conducted following strict surgical methods. Regardless of these obvious restrictions, we propose increased attention to intraoperative measurements and reporting detail, to achieve more individualization of ACL reconstruction in the future.

The majority of the studies document tunnel position with diagrams or operative notes, which does not yield any points in the AARSC. Documentation of the tunnel position is especially important in studies comparing two different surgical techniques. Two-dimensional images especially clear arthroscopic pictures are easy to use and store, and present valuable information to the reader. In the future, image documentation of tibial and femoral tunnel placement should be a prerequisite with all major journals, especially in scientific papers claiming anatomic ACL reconstruction. Several studies utilize the o’clock reference which has several limitations making it difficult to reproduce [69]. Hence, when radiographic figures are not presented, one should thoroughly and meticulously describe the tunnel position, preferably with reference to the bony landmarks [83].

The AARSC has only recently been developed and not yet implemented in a systematic review or clinical study. The reliability and the validity of the scoring system have been assessed [82]. The maximum score is 19 points. In this systematic review, the calculated mean for the SB group was 6.9 ± 2.8 and 8.3 ± 2.8 for the DB group. It is difficult to evaluate these values, as it has not previously been reported. A classification and grouping with cut-offs for the score is planned. Based on their clinical experience, the senior authors of this systematic review propose a minimum score of 10 for anatomic ACL reconstruction. The mean score for SB and DB reconstructions in this study fell well below this minimum required score. As previously stated, unreported items are not synonymous with not performed. In many cases, these procedures are performed by experienced surgeons who may not report their technique in the papers, regarding this as redundant or obvious information. With today’s high level of medical research and in the light of new fields of focus in ACL reconstruction, researchers and surgeons, experienced and less so alike, should be required to provide sufficient information on their surgical technique allowing readers a fair comparison and interpretation of the outcomes. Without this information, it may be difficult to pool the outcomes of studies and understand what we actually are comparing.

A limitation of this study was that the data extraction was not performed in a blinded fashion. However, two independent reviewers selected all the papers and extracted all the data. Furthermore, the first authors validated the extracted data by processing the included studies once again after data extraction. Outcomes were not extracted, as it was not the aim of the systematic review. Hence, several topics regarding clinical outcomes were removed from the PRISMA guidelines, as they were not applicable to this systematic review. The utilized scoring system is a newly developed one and has not previously been implemented; however, it has been tested for reliability and validity. Test–retest reliability still needs to be confirmed over a longer period of time. As a result of this, many of the papers included in the systematic review are older than the score itself.

In the future, comparison between clinical trials could be facilitated by implementing tools such as standardized clinical tests or standardized surgical procedures. The AARSC could be a valuable tool to assist with clinical outcomes research. Future research should focus on improving surgical techniques and their documentation.

Conclusions

The most important findings in this systematic review were that there was a substantial underreporting of surgical data in both SB (6.9 points) and DB (8.3 points) groups. The calculated means of AARSC were below the proposed required minimum score of 10/19 for anatomic ACL reconstruction. This underreporting creates difficulty when comparing and pooling the results of these studies and warrants improvement in future studies on ACL reconstruction.