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Anteromedial Portal Double-Bundle Anterior Cruciate Ligament Reconstruction Yields Similar Outcomes to Non-AMP Femoral Drilling Double-Bundle Techniques: A Systematic Review of Comparative Studies

Background: Biomechanical studies have shown double-bundle (DB) anterior cruciate ligament reconstruction (ACLR) to have increased anterior and rotational stability as compared with single-bundle ACLR. Various techniques exist to drill the femoral tunnel, such as anteromedial portal (AMP), outside-in (OI), and transtibial (TT) drilling. However, it is unclear whether one drilling technique is superior to others when a DB graft is used. Purpose: To systematically assess the outcomes and complications in patients undergoing DB ACLR through an AMP technique as compared with other femoral drilling techniques. Study Design: Systematic review; Level of evidence, 3. Methods: PubMed, Medline, and EMBASE databases were searched in April 2018. Nonrandomized studies were assessed with the MINORS (Methodological Index for Nonrandomized Studies), whereas randomized studies were assessed with the GRADE (Grading of Recommendations Assessment, Development and Evaluation) system. Results: Ten studies comprising 722 patients satisfied the inclusion criteria. A total of 351 patients underwent DB ACLR with AMP drilling (mean ± SD age, 32.7 ± 4.7 years); 318 patients had DB ACLR with OI drilling (age, 31.9 ± 4.1 years); and 53 received a DB ACLR with TT drilling (age, 26.5 ± 2.0 years). Graft types used included hamstring autograft (74.1%; n = 247), tibialis anterior autograft (6.0%; n = 20) and unspecified grafts (19.8%; n = 66). No significant difference in postoperative Tegner and Lysholm scores was found between the AMP and OI groups postoperatively. The AMP group had a lower anterior and posterior graft bending angle as compared with the OI group. Four patients (1.1%) in the AMP group had graft reruptures, as compared with 9 reruptures (2.8%) in the OI group. There were no reports of rerupture in the TT group. Conclusion: DB AMP ACLR results in significantly improved functional outcome scores postoperatively. AMP techniques yield similar functional outcomes to OI ACLR. No direct comparison in functional outcomes scores were available between the AMP and TT techniques. Low overall complication and revision rates were observed for patients undergoing DB AMP ACLR and were found to be similar to those of other femoral drilling techniques. Owing to a steeper graft bending angle in patients undergoing OI or TT ACLR relative to AMP ACLR, patients treated with OI or TT femoral drilling may have increased strain placed on the graft. Based on the various limitations in the available literature, it is not currently possible to make a definite conclusion of whether AMP is superior to non-AMP techniques in the setting of DB ACLR.

devastating injury, as it can cause functional impairments 6 and increase the lifetime risk of knee osteoarthritis. 21 Recently, there has been a steady increase in popularity for anatomic ACL reconstructions (ACLRs) as an accurate way to restore native ACL positioning and biomechanical characteristics. 11 Single-bundle (SB) and double-bundle (DB) ACLR techniques have been associated with positive functional outcomes. 4,27 SB ACLRs aim to restore only the AM bundle of the ACLR, whereas DB ACLR uses 2 grafts to re-create the AM and PL bundles. 36 Many biomechanical studies have indicated that DB ACLR has increased anterior and rotational stability as compared with SB ACLR. 29 However, prognostic studies have shown that there are no significant differences in clinical outcomes between the reconstruction techniques. 10 There are various options when considering how to drill the femoral tunnel during surgery. ACLR initially evolved from a 2-incision technique with an outside-in (OI) femoral drilling technique to a transtibial (TT) technique wherein the ACL femoral tunnel is drilled through a tibial tunnel. 31 The OI technique is useful, especially in revision settings, as it may allow the creation of longer tunnels, oblique tunnels of a wider range of angles, more consistent femoral tunnel placement, and clearer fields of vision allowing for easier femoral drilling. However, the OI technique has been associated with more acute graft bending angles (GBAs) leading to potentially poorer graft healing and greater risk of rerupture. 20 TT techniques are associated with less surgical pain and morbidity and reduced operative time. 28 Some disadvantages of TT ACLR include increased interference screw bone divergence with damage to the graft during fixation. 28 Newer techniques allow for independent drilling of the femoral tunnel through the AM portal (AMP). 28 The AMP technique allows for a more accurate and anatomic placement of the ACL femoral tunnel, while theoretically improving anteroposterior and rotational stability. 28 However, the drilling technique can be technically demanding for those not familiar with it and has an increased risk of injury to the common peroneal nerve. 28 There has been extensive research comparing drilling techniques in SB ACLR. In a recent systematic review of 479 patients, the authors reported that SB ACLR with the AMP technique yields superior stability and improved postoperative functional outcomes as compared with TT techniques. 5 In another meta-analysis, the SB ACLR AMP technique had better Lysholm scores and a larger proportion of knees with normal International Knee Documentation Committee (IKDC) grade as compared with patients treated with TT drilling. 28 However, to date, there have been no systematic reviews that have critically compared various femoral drilling techniques in DB ACLR. Hence, the purpose of this study was to systematically assess the outcomes and complications in patients undergoing DB ACLR through the AMP as compared with other femoral drilling techniques. The hypothesis was that DP AMP ACLR would provide superior outcomes and lower rerupture rates to TT and OI in the same population.

Search Strategy
Three online databases (PubMed, Embase, and MEDLINE) were searched for studies comparing AMP femoral tunnel drilling with other non-AMP femoral drilling techniques for skeletally mature patients from data inception to April 24, 2018. The search terms included "anterior cruciate ligament," "anterior cruciate ligament reconstruction," "double-bundle," "anteromedial portal," and similar phrases (Appendix Table A1). The search terms were entered onto Google Scholar to ensure that articles were not missed. The research question and inclusion and exclusion criteria were established a priori. Inclusion criteria were (1) comparative studies (ie, AMP vs non-AMP drilling techniques; OI, TT, etc), (2) skeletally mature patients (ie, closed femoral and tibial physis), (3) DB reconstruction, (4) outcomes reported and stratified for population of interest, (5) human studies, and (6) English language. Exclusion criteria included (1) multiligament knee reconstruction, (2) partial tears treated with ACL augmentation, (3) review articles, (4) nonsurgical treatment studies (eg, conservative treatment, technique articles without outcomes), (5) case reports, (6) SB reconstruction, and (7) cadaveric/nonhuman studies.

Study Screening
A systematic screening approach in accordance with PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) 22 and R-AMSTAR (Revised Assessment of Multiple Systematic Reviews) 15 was employed from the title to full-text screening stages in duplicate by 2 independent reviewers (T.T., A.G.). Discrepancies were discussed and resolved with input by a third reviewer. The references of included studies were also screened through the same systematic approach to capture any additional relevant articles.
Quality Assessment evidence (1-4) for each study was determined by the 2 reviewers independently and in duplicate. 35 The methodological quality of nonrandomized comparative studies was evaluated with the MINORS (Methodological Index for Nonrandomized Studies) checklist. 30 A score of 0, 1, or 2 is given for each of the 12 items on the MINORS checklist, with a maximum score of 16 for noncomparative studies and 24 for comparative studies. Methodological quality was categorized a priori as follows for noncomparative and comparative studies, respectively: a score of 0-8 or 0-12 was considered poor quality; 9-12 or 13-18, fair quality; and 13-16 or 19-24, excellent quality.
Risk of bias in randomized controlled trials was assessed with the GRADE (Grading of Recommendations Assessment, Development and Evaluation) system. 8 The GRADE tool begins its assessment by rating randomized controlled trials as high quality of evidence and observational studies as low quality. These studies are then evaluated via 5 factors associated with the quality of evidence: risk of bias, inconsistency, indirectness, imprecision, and publication bias. 1,9

Data Abstraction
The 2 reviewers then independently abstracted relevant data from included articles and recorded the data onto a Google spreadsheet designed a priori. Demographic data included author, year of publication, sample size, study design and location, level of evidence, and patient demographics (eg, sex, age). Information was documented regarding rehabilitation protocols and postoperative outcomes (surgical and radiographic), including complications.

Statistical Analysis
Given the inconsistent reporting of outcomes, a metaanalysis could not be performed, and the results are summarized descriptively. Descriptive statistics, such as mean, range, and measures of variance (eg, standard deviations, 95% CIs), are presented where applicable. The intraclass correlation coefficient (ICC) was used to evaluate interreviewer agreement for the MINORS score. A kappa (k) statistic was used to evaluate interreviewer agreement at all screening stages. Agreement was categorized a priori as follows: ICC/k of 0.81-0.99 was considered almost perfect agreement; ICC/k of 0.61-0.80, substantial agreement; ICC/k of 0.41-0.60, moderate agreement; ICC/k of 0.21-0.40, fair agreement; and a ICC/k of 0.20 was considered slight agreement. 17

Study Characteristics
The initial search yielded a total of 3494 articles. After 1182 duplicates were excluded, a systematic screening process yielded 10 articles that met inclusion ( Figure 1). There were no additional studies found upon reviewing references of included studies or a manual search through Google Scholar. Of the included studies, there was 6 randomized controlled trials, 2 prospective cohorts, and 2 retrospective cohorts. The included studies were conducted in South Korea (60%; n ¼ 6) and Japan (40%; n ¼ 4) ( Table 1).

Study Quality
There was substantial agreement among reviewers at the title (k ¼ 0.865; 95% CI, 0.841-0.889) and abstract (k ¼ 0.650; 95% CI, 0.593-0.706) screening stages and almost perfect agreement after full-text screening (k ¼ 0.761; 95% CI, 0.685-0.837). The mean (± SD) MINORS score across all nonrandomized studies was 17.5 ± 1, indicating fair quality of evidence for nonrandomized studies. The risk of bias across all randomized studies was low to moderate (Appendix Table A2). There was almost perfect agreement among the reviewers for the quality assessment based on the MINORS criteria (weighted ICC ¼ 0.953; 95% CI, 0.924-0.971) ( Table 1).

Patient Characteristics
There were 722 patients in the included studies. Among the patients, 48.6% (n ¼ 351) were enrolled in an AMP group, whereas 51.3% (n ¼ 371) of patients were assigned to a non-AMP group. Of the 371 patients who underwent non-AMP

Surgical Technicalities
In this systematic review, all studies performed a DB-ACLR comparing AMP techniques with either OI ACLR or TT ACLR.
Graft Sizes. The mean overall graft length was 16.24 cm (n ¼ 335).  32 Prospective cohort (2)   Rehabilitation Protocol. Two studies reported rehabilitation protocols. 11,19 One study reported partial weightbearing at 4 weeks with crutches and an increase in knee flexion range of motion to 120 at 4 weeks. 11 At postoperative 6 weeks, patients were allowed full weightbearing with a brace. 11 The other study reported use of an ACL limited-motion brace applied 4 to 5 days after surgery, and joint motion exercise was carried out at 15 increments per week. 19 A pair of crutches was used to allow partial weightbearing from 3 days to 6 weeks after surgery. Patients were educated on performing proprioceptive balancing exercises at 3 months after surgery. Return to competitive sports involving jumping, pivoting, or sidestepping was prohibited until 6 months after the reconstruction. 19

Functional Outcome Scores
The Tegner activity score was measured in 53 patients in the AMP group. 11 A significant within-group increase in Tegner activity score was observed (P < .05). The Tegner activity score was measured in 58 patients in the OI group. 11 A significant within-group increase in Tegner activity score was observed (P < .05). No significant differences in outcomes were noted between techniques. The Lysholm score was measured in 85 patients in the AMP group. 11,19 A significant within-group increase in Lysholm score was observed in 1 study (P < .05). The Lysholm score was measured in 107 patients in the OI group. 11,19 A significant within-group increase in Lysholm score was observed in 1 study (P < .05). No significant differences in outcomes were noted between techniques. No studies compared functional outcomes scores between AMP and TT.

Rotational Stability
Pivot shift was measured in 107 patients in the AMP group. 11,23,24 The percentage of patients with pivot-shift results of glide or greater was 85.8% (n ¼ 54) preoperatively and 12.1% (n ¼ 13) postoperatively.
Pivot shift was measured in 3 studies (n ¼ 90 patients) in the OI group. 11,23,24 The percentage of patients with pivotshift results of glide or greater was 91.4% (n ¼ 53) 4 preoperatively and 12.2% (n ¼ 11) postoperatively. 11,24 No studies reported postoperative pivot shift as an outcome in TT groups.

Radiographic Outcomes
The radiographic outcomes reported in the included studies are summarized in Appendix Table A3. In all cases, the GBA was measured by computed tomography. The AM GBA was found to decrease from 110.8 (95% CI, 110.2 -111.4 ; n ¼ 52) at 3 days postoperatively 13,18 to 73.8 (SD, 11.2 ; n ¼ 32) 2 years postoperatively in patients who underwent ACLR with the AMP technique. 24 This contrasts with patients who underwent ACLR with the OI technique, where it decreased from 101.3 (95% CI, 100.5 -102.1 ; n ¼ 51) 13,18 at 3 days postoperatively to 90.7 (SD, 8.8 ; n ¼ 32) at 2 years postoperatively. 24 No studies reported GBA measured by computed tomography as an outcome in TT.

DISCUSSION
The most important finding of this systematic review is that DB AMP ACLR has similar subjective and objective outcomes as well as a similar complication profile in comparison with non-AMP techniques. Furthermore, radiographic studies revealed a greater decrease in the GBA with time in the AMP group versus the OI group.
It has recently been proposed that a steep GBA may be a biomechanical factor that contributes to graft damage. In ACLR, sharp angulation of the graft at the tunnel aperture increases the local strain and abrasive forces, and repetitive bending may cause excessive stress on the bone-graft interface, slowing down the rate of graft healing. 33 The generally more horizontal tunnels created in the OI ACLR technique create a steep angle, which may increase graft wear. 28 In this systematic review, the GBA was reported only for patients who underwent OI drilling and AMP drilling. At 2 years postoperatively, the GBA for OI and AMP was 90.7 and 73.8 , respectively. In another study, the GBA in the TT technique was significantly larger than that of the AMP technique at low flexion angles when the graft was fully stretched. 25 Anatomic ACLR leads to a more oblique femoral tunnel orientation, resulting in an ACL graft that bends at a less acute angle at the femoral tunnel aperture. 33 However, the GBA is just one of many factors, such as tunnel positioning, the shape of the intercondylar fossa, graft selection, and graft tension, that likely plays a role in graft rupture. Unfortunately, the majority of the studies did not include information on these other possible confounders to allow us to comment on their impact on graft rupture rates with different drilling techniques.
AMP ACLR allows for a more anatomic graft position as compared with TT ACLR, which may play a role in decreasing rerupture rates. Femoral tunnels performed through the AMP are more precise and closer to the anatomic femoral ACL insertion as compared with the TT drilling, allowing for a more horizontal placement. 7 It has been identified that in TT ACLR techniques, a vertical and anterior graft placement is required owing to drilling through the tibial tunnel, increasing biomechanical demand during rehabilitation. 28 There were very few complications reported by the studies included in this systematic review, likely in large part because of the small sample sizes. For this reason, accurate comparison in rerupture rates and complication rates between the femoral tunnel drilling techniques was difficult. In general when using an AMP drilling technique, surgeons must be cognizant of the possibility of a short tunnel length. 2 AMP ACLR techniques provide for a more oblique femoral tunnel position to improve the rotational stability. However, this more oblique tunnel position significantly decreases the femoral tunnel length and can cause difficulties such as the risk of unintentionally penetrating the outer cortex. Additionally, a number of studies have shown that the DB AMP technique can result in posterior-wall blowout and potential damage to the posterior articular cartilage. 14 Complications involved in the TT drilling technique include interference screw-bone plug divergence in the femoral tunnel, damage to the graft during fixation, and graft-tunnel length mismatch. 14 OI techniques have a greater risk of abrasion of the graft at the intra-articular edges of the tunnels. 14

Strengths
The strengths of this systematic review stem from the rigorous methodology used. The use of multiple databases, a broad search strategy, and a duplicate systematic approach to reviewing the literature ensured that relevant articles were not overlooked. Excellent agreement at all screening stages and quality assessment were obtained.

Limitations
Despite the included studies being comparative in nature, their statistical and methodological heterogeneity (eg, follow-up period, outcome measures reported) precluded a meta-analysis. There is also a need for more consistent reporting in outcomes across studies of all drilling techniques to make more definitive analyses. The studies that were included in this review were also limited in their generalizability, as all of the patients were from Asia.
Future studies should use large prospective cohorts and randomized controlled trials with long-term follow-up to further assess the results presented in this review. Ideally, future studies would have more consistent reporting of important information, such as functional outcome scores, objective anterior and rotational stability testing, and tunnel positioning, which was found to be often lacking in the currently available literature.

CONCLUSION
DB AMP ACLR results in significantly improved functional outcome scores postoperatively. AMP techniques yield similar functional outcomes to OI ACLR. No direct comparison in functional outcomes scores was available between AMP and TT techniques. Low overall complication and revision rates were observed for patients undergoing DB AMP ACLR and were found to be similar to those of other femoral drilling techniques. Given a steeper GBA in patients undergoing OI or TT ACLR relative to AMP ACLR, patients treated with OI or TT femoral drilling may have increased strain placed on the graft. Based on the various limitations in the currently available literature, it was not possible to make a definitive conclusion on whether AMP is superior to non-AMP techniques in the setting of DB ACLR.