Anatomical anterior cruciate ligament reconstruction: transtibial versus outside-in technique

Abstract

Purpose

the aim of this study was to compare clinical results and location of the femoral tunnel with transtibial (TT) and outside-in (OI) techniques in anterior cruciate ligament (ACL) reconstruction using in vivo 3D CT analysis.

Methods

we prospectively followed up 40 ACL reconstructions in which femoral tunnel placement was performed using two different techniques: TT [20] and OI [20]. Clinical evaluation was based on IKDC and KOOS scores and radiographic analysis with specific 3D CT scans. Tunnel coordinates were calculated using the Bernard-Hertel quadrant method to define the insertion point of the ACL.

Results

excellent clinical results were achieved in both groups, which showed comparable IKDC and KOOS scores. Two failures were recorded, both in the TT group. In the high-to-low direction, the position of the femoral tunnel, as measured using the quadrant method, was too high in the TT group, compared to what was observed in the OI group: 10.5 ± 6.9% (0–29%) and 30.2 ± 5.4% (19–42%), (p=0.043).

Conclusions

we found that with the TT technique, compared with the OI technique, the femoral tunnel was located higher in the high-to-low direction and was in a slightly shallower position in the deep-to-shallow direction. Using the OI technique the femoral tunnel was in a position closer to the anatomical ACL footprint than with the TT technique. A femoral tunnel position far from the anatomical footprint of the native ACL would result in a higher failure rate.

Level of evidence

level II, prospective comparative study.

Introduction

Restoration of the anatomical center of the native femoral footprint during anterior cruciate ligament (ACL) reconstruction is crucial in order to obtain normal ligament orientation and function. Biomechanical studies have emphasized the importance of anatomical tunnel placement during ACL reconstruction in order to restore normal knee kinematics and stability (1–4). In efforts to replicate the function of the two bundles of the ACL with a single-bundle reconstruction, the anatomical femoral footprint is considered to be located between the anatomical insertion of the anteromedial and posterolateral fibers (5, 6). However the surgical technique through which to reach this area remains controversial.

While creation of the femoral tunnel via the tibial tunnel (transtibial, TT) continues to be the most widely practiced technique, it has been shown to result in a higher rate of non-anatomical femoral tunnel placement, primarily because of its dependence on the tibial tunnel (7, 8). For these reasons independent femoral drilling techniques, such as the anteromedial (AM) portal and outside-in (OI) techniques, have raised interest in the possibility of restoring anatomical insertion of the ACL.

All three approaches have advantages and disadvantages in comparison with each other, but there is a paucity of evidence demonstrating the biomechanical and clinical benefits of any one of them in particular. Many studies have compared femoral tunnel aperture positions in ACL reconstructions performed using the three techniques (AM portal, TT and OI), and most of them are cadaveric studies (9, 10). Instead, the literature contains few clinical studies comparing femoral tunnel positions obtained using the different techniques (11, 12); in these studies comparison of the TT and the AM portal technique showed better restoration of the anatomical femoral footprint with AM drilling. The OI technique, however, has been poorly studied and compared with the classical TT technique. Furthermore, the main limitation of these studies is that the location of the femoral insertion is assessed using conventional X-ray or two-dimensional computed tomography (2D CT). Recently, three-dimensional computed tomography (3D CT) was proposed as a means of determining, with more accuracy, the position of the ACL femoral tunnel aperture (11).

The purpose of the current study was to quantitatively compare the femoral tunnel locations obtained with the TT and OI techniques in ACL reconstruction using in vivo 3D CT analysis. We hypothesized that the TT drilling technique, compared with the OI technique, would result in less anatomically placed tunnels and therefore in inferior clinical outcomes.

Methods

Participants and study design

We prospectively followed up a series of 40 consecutive single-bundle ACL reconstructions performed at our institution between October 2011 and April 2013. We included in the study patients with unilateral isolated ACL ruptures, who were candidates for arthroscopically assisted ligament reconstruction. Exclusion criteria were: additional knee ligament injuries (posterior cruciate ligament, posterolateral and posteromedial knee complex injuries, medial and lateral collateral ligament injuries, medial and lateral meniscus tears), previous knee ligament surgeries, associated fractures of the femur or tibia, and patients not willing to be followed up with conventional radiography and 3D CT scans postoperatively.

Patients were randomized to one of the two treatment arms with an allocation ratio of 1:1 to undergo the conventional TT technique (TT group, 20 cases) or the anatomical OI technique (OI group, 20 cases) for femoral tunnel drilling.

Interventions

All surgeries were performed by the same surgical team. A pneumatic tourniquet was applied around the upper thigh and inflated to approximately 250 mmHg in all cases. The knee was positioned in 90° of flexion. The surgical procedure then began with arthroscopic examination through conventional anterolateral and anteromedial portals. After the initial assessment and joint debridement the hamstring tendon was harvested from the affected limb and a four-stranded graft was created for single-bundle ACL reconstruction. A tibial drill guide (Arthrex Inc., Naples, FL, USA) set at an angle of 45°–50° was introduced through the AM portal and the tip of this guide was placed at the center of the ACL tibial footprint. The extra-articular landmark of the tibial tunnel was 1 cm above the insertion of the pes anserinus and 1.5 cm medial to the tibial tubercle. After insertion of the guide pin, a tibial tunnel was created with a cannulated reamer which had the same diameter as the graft. The same surgical technique was always applied for tibial drilling, irrespective of the femoral tunnel drilling technique used (13).

For the TT technique, a 5- to 7-mm femoral offset guide (Arthrex) was introduced, through the tibial tunnel, into the joint and placed over the posterior aspect of the lateral femoral condyle. The offset guide was then laterally rotated so as to position the drill guide as close to the center of the ACL footprint as possible. An appropriately-sized cannulated reamer was then passed through the tibial tunnel and over the guide pin to create a femoral tunnel 30 mm in depth. The graft was passed through the tibia into the joint and through the femoral socket and was secured on the femoral side with Endobutton CL BTB (Smith & Nephew, Andover, MA, USA). Tibial fixation was performed with Bio-Intrafix (DePuy-Mitek, Raynham, MA, USA) with the knee in 30° of flexion and applying 40N tension to the graft using an ACL Tie Tensioner (DePuy-Mitek).

The OI technique was performed with the Flipcutter system (Arthrex) with the femoral guide set at 115° and introduced via the AL portal with the knee maintained in 90° of flexion. The tip of this guide was positioned at the center of the ACL femoral footprint with an extra-articular visual angulation of 45° relative to the frontal and sagittal axes of the femur (14). A small incision on the lateral thigh through the iliotibial band allowed insertion of the femoral drill guide into the bone. Extra-articularly, the guide sleeve was situated proximal and anterior to the lateral epicondyle to avoid injury to lateral soft tissue structures and to maximize tunnel length. The flipcutter was then advanced as anterograde drilling until it reached the tip of the femoral aimer. After blade flipping the retrograde reamer was pulled back to create a femoral socket with a depth of 25 mm. A graft was passed retrograde through the tibial tunnel and was placed in the femoral tunnel and fixed with Tight Rope RT (Arthrex). The tibial fixation was performed as previously described.

Outcome measures

At the preoperative assessment and at postoperative follow-ups at six months and one year, the patients were evaluated using the International Knee Documentation Committee (IKDC) objective and subjective scores and the Knee injury and Osteoarthritis Outcome Score (KOOS). Laxity testing was performed using the KT-2000 arthrometer (MEDmetric, San Diego, CA, USA). During the 2^nd postoperative week all the patients underwent a dedicated CT scan (Siemens Somatom Sensation, Siemens Aktiengesellschaft, Munich, Germany). The images were obtained at 120 kV and 450 mA, with a slice thickness of 1.25 mm and a pitch of 0.5 mm. The data from the CT scans were processed into three-dimensional surface models using the Syngo CT Volume Rendering Technique (Siemens Aktiengesellschaft, Munich, Germany). The models were co-registered with properly scaled male or female base models, which had been pre-aligned to an anatomical coordinate system based on the femoral head and tibial malleoli centers, as recommended by the International Society of Biomechanics (15, 16).

The position of the tibial tunnel was determined using a true proximal-to-distal view on the tibial plateau according to the anatomical coordinate axes method described by Tsukada et al. (17). The anterior-to-posterior position was calculated as a percentage of the distance from the line (T1) running through the anterior border of the tibial plateau (where the plateau edge drops down to the shaft) to the line (T2) running through the most posterior border of the tibial plateau [normal value = 43.85% (17)]. The medial-to-lateral position was calculated as a percentage of the distance from the line (T3) running through the medial border of the tibial plateau to the line (T4) running through the lateral border of the tibial plateau [normal value = 48.85% (17)] (Fig. 1).

Fig. 1.

Identification of the tibial tunnel position - anatomical coordinate axes method [Tsukada et al. (17)].

The center of the femoral tunnel was measured using the quadrant method as described by Bernard and Hertel (18). The parallel and the perpendicular to the Blumensaat’s line positions were determined using a true medial view of the lateral femoral condyle. The parallel to the Blumensaat’s line position was calculated as a percentage of the total sagittal diameter of the lateral condyle measured along Blumensaat’s line (F1: deep-to-shallow distance) [normal value = 28.5% (19)]. The perpendicular to the Blumensaat’s line position was calculated as a percentage of the maximum intercondylar notch height (F2: high-to-low distance) [normal value = 35.2% (20)] (Fig. 2).

Fig. 2.

Identification of the femoral tunnel position - quadrant method [Bernard and Hertel (18)].

Data analysis

Statistical analysis was performed using SPSS Statistics version 20 software (IBM, Armonk, NY, USA). A p value < 0.05 was considered statistically significant. All data were presented as mean values with standard deviations (SDs).

Results

The mean age of the patients (32 males and 8 females) at the time of surgery was 31 years (range, 18 to 48 years). They had a mean body mass index (BMI) of 25.1 (range, 21 to 30.4). The mean follow-up period was 28 months (range, 13 to 32 months). The two groups were comparable for age, gender, height, weight and BMI (Tab. 1).

Table 1.

Summary of patients’ preoperative characteristics ^a.

Variables	Trans-tibial (TT) N = 20	Outside-in (OI) N = 20	p-value
Gender (Male/Female ratio)	16/4	16/4
Age (years)	29 (20–44)	33 (18–48)	0.927
Height (cm)	180 (173–192)	176 (162–190)	0.229
Weight (Kg)	82 (65–95)	78 (55–92)	0.389
Body Mass Index (BMI)	25.5 (21.7–29.4)	24.7 (21–30.4)	0.372

^avalues are expressed as median (range) unless otherwise specified.

The clinical results are shown in Table 2. The IKDC scores and total KOOS showed significant improvements in all cases (p 0.032) from the preoperative assessment to the 12-month follow-up. Two failures were recorded for the TT technique, at 13 and 7 months of follow-up. No failures were recorded for the OI technique. No clinical differences, in IKDC and KOOS scores, were recorded between the TT and OI groups. The side-to-side difference on the manual maximum displacement test performed with a KT-2000™ arthrometer revealed a significant difference between the pre-operative and 12-month postoperative follow-up status in both groups [from 6.14 ± 1.22 mm to 2.07 ± 0.67 mm for TT (p = 0.001), and from 6.23 ± 1.96 mm to 1.54 ± 0.94 mm for OI (p = 0.001)]. No differences were recorded between the TT and OI techniques in terms of postoperative anteroposterior translation.

Table 2.

Clinical outcomes ^a.

Variables	Trans-tibial (TT) N = 20	Outside-in (OI) N = 20	p-value
KT-2000 side-to-side difference
Baseline	6.14 + 1.22	6.23 + 1.96	0.901
6-month follow-up	2.7 + 0.97	2.35 + 1.01	0.352
12-month follow-up	2.07 + 0.67	1.54 + 0.94	0.256
Objective IKDC grade
Baseline	D	D
6-month follow-up	B	B
12-month follow-up	B	B
Subjective IKDC score
Baseline	44.3 + 14.86	45.75 + 15.09	0.793
6-month follow-up	60.34 + 5.84	65.23 + 8.92	0.275
12-month follow-up	65.98 + 6.42	71.55 + 1.96	0.115
KOOS score
Baseline	61.79	60.13 + 19.62	0.843
6-month follow-up	74.06	83.51 + 6.95	0.196
12-month follow-up	78.83	86.75 + 4.22	0.106

^avalues are expressed as median ± standard deviation.

Tibial tunnel position, as evaluated using the anatomical coordinate axes method, was similar with both surgical techniques (Fig. 3) as shown in Table 3. The femoral tunnel positions are reported in Table 4.

Fig. 3.

Identification of the location of the tibial tunnel aperture using anatomical coordinate axes method.

Table 3.

Mean tibial tunnel positions in the anterior-to-posterior and medial-to-lateral directions ^a.

Direction	Anatomic Point	Trans-tibial (TT) N = 20	Outside-in (OI) N = 20	p-value
Anterior-to-Posterior	43.85	42.72 + 7.7 (28–58)	44.08 + 7.9 (29–58)	0.644
Medial-to-Lateral	48.85	47 + 5.2 (39–59)	46.67 + 3.1 (42–51)	0.829

^avalues are expressed in percentages as mean ± standard deviation (range).

Table 4.

Mean femoral tunnel positions in the deep-to-shallow and high-to-low directions ^a.

Direction	Anatomic Point	Trans-tibial (TT) N = 20	Outside-in (OI) N = 20	p-value
Deep-to-Shallow	28.5	39.4 + 7 (24–64)	31.1 + 6.5 (19–48)	0.082
High-to-Low	35.2	10.5 + 6.9 (0–29)	30.2 + 5.4 (19–42)	0.043

^avalues are expressed in percentages as mean ± standard deviation (range).

In the high-to-low direction, the position of the femoral tunnel, as measured using the quadrant method, was too high in the TT group, compared to what was observed in the OI group (p = 0.043). In the deep-to-shallow direction no significant differences were recorded between the two techniques (Fig. 4).

Fig. 4.

Identification of the location of femoral tunnel aperture using the quadrant method.

Discussion

The present study was conducted to analyze quantitatively the location of the femoral tunnel aperture, comparing the results obtained with the TT and OI techniques. The analysis, based on postoperative in vivo 3D CT scans, was performed using the quadrant method. We also analyzed clinical outcome and revision rates, again comparing the two different techniques. Our hypothesis was that the TT technique would result in vertical non-anatomical femoral tunnel placement and would therefore be associated with inferior clinical results. We found that the femoral position obtained with the TT technique was too high in the roof of the intercondylar notch, and the TT technique showed a higher failure rate than the OI technique.

Certainly, anatomical femoral footprint restoration is one of the most important prerequisites for successful ACL reconstruction. Biomechanical studies have demonstrated that a femoral tunnel that is placed inside the anatomical footprint results in knee kinematics closer to those of the intact knee than does a tunnel position located for best graft isometry (3, 4, 20). Restoration of an anatomical femoral insertion and thus native ligament orientation also has an important impact on the clinical results of ACL reconstruction. Significantly superior IKDC and Tegner scores, higher anterior stability and less pivot shift have been described for ACL reconstructions in which the femoral tunnel is located inside the anatomical footprint compared to ones in which the femoral tunnel is located outside this area (1, 21).

However, the surgical technique by which to achieve the best restoration of anatomical ACL insertion remains controversial. The TT technique, because of the constraints of the tibial tunnel, is associated with a tendency to place the femoral tunnel in a shallow position and high in the notch. Many studies have demonstrated the limitations of the TT technique in placing the femoral tunnel within the native ACL insertion site (7–9). Moreover, it should be considered that the percentage of the femoral footprint covered by the tunnel in the TT technique is dependent on the entry point and orientation of the drill. A cadaveric study demonstrated that it was possible to achieve anatomical placement with the TT technique, but only with a proximal and medial tibial drilling starting point that is located prohibitively close to the joint line (22). To avoid this complication and better restore anatomical ACL insertion, the independent AM portal and OI femoral drilling techniques have become a focus of interest.

The comparative accuracy of these techniques in restoring the ACL femoral footprint has been largely investigated in cadaveric models, although most of the studies compared only the TT and AM portal techniques (9, 10, 23). Few in vivo studies have analyzed femoral tunnel positioning using the OI technique in comparison to the standard TT technique (11, 12, 24). The traditional methods used to measure tunnel position, such as plain radiographs, are projected in 2D, and this may affect the accuracy of the measurements, with the result that significant errors may be introduced in the measurement of the tunnel position. In our study in vivo 3D CT was employed to assess quantitatively femoral tunnel location, comparing the TT and OI techniques. We found that the TT technique resulted in a significant higher placement of femoral tunnel in the high-to-low direction. Similarly to our findings, a recent investigation using in vivo CT 3D showed that the TT technique resulted in more highly positioned femoral tunnels than did the AM portal and OI techniques (11). In our study, the femoral tunnel measured in the deep-to-shallow direction was found to be in a slightly shallower position in the TT group than in the OI group, although the difference was not significant. In line with our findings, Shin et al., comparing the TT, OI and AM techniques, reported no differences in the deep-to-shallow location (11).

Giron et al. (23) compared the location of the femoral tunnel aperture after single-bundle ACL reconstruction using the TT, AM, and OI techniques found that aperture placement did not differ significantly between the three techniques either in the high-to-low or the deep-to-shallow directions (23). The authors, however, used cadaveric knees and measured the location of the femoral tunnel entrance on lateral radiographs.

In accordance with our findings, a recent investigation using in vivo 3D CT demonstrated that the traditional TT technique for arthroscopic ACL reconstruction results in bone tunnel apertures that are located in a position that is too high and too shallow in the roof of the intercondylar notch (25).

The exact location of the anatomical femoral insertion of ACL is still debated. It is commonly considered to be located between the anatomical insertion of anteromedial and posterolateral fibers, although other authors support the thesis that the functional fibers of the ACL originate mostly from the “resident’s ridge” and that tunnels positioned close to the insertion of anteromedial fibers would be desirable (5, 19). Many parameters have been described to quantitatively define the exact location of ACL femoral footprint and different quantification methods have been used. Nevertheless a recent review reported a mean value of 28.5% in the deep-to-shallow direction and a value of 35.2% in the high-to-low direction (19). These values were taken into consideration as reference points in our study. Comparing our results with these standards, we found showed that with the OI technique the femoral tunnel aperture location (31.1 % in the deep-to-shallow direction, 30.2 % in the high-to-low direction) was in a more anatomical position than with the TT technique (39.4% in the deep-to-shallow direction, 10.5% in the high-to-low direction). Our data are in line with those reported in a recent meta-analysis in which the independent drilling techniques resulted in better restoration of the center of the anatomical femoral footprint than drilling through the tibial tunnel, which instead resulted in femoral tunnels that were too anterior and proximal to the “ideal” location (12).

The clinical impact of restoration of the anatomical femoral footprint is still controversial. Some authors have reported better clinical outcomes for anatomical single-bundle reconstructions, with less anteroposterior and rotational laxity than is observed with non-anatomical single-bundle reconstructions (1, 21).

Conversely Riboh et al., in a recent meta-analysis, showed not significantly different clinical outcomes, in terms of IKDC score, Tegner score and Lysholm score, between TT and independent drilling of the femoral tunnel (12). This analysis however had an important limitation, namely the inclusion of both the AM and the OI technique in the same independent drilling group, on the basis of the assumption that each of these techniques is equivalent to the other. From our data no significant difference in the overall outcome was recorded between these two techniques, but the TT reconstruction was found to be associated with a higher failure rate.

There are limitations to the present study that need to be considered. First, we had a small number of patients for each group and the sample size was under-powered to detect any statistically differences in clinical scores. That said, the primary purpose of this study was to analyze the possible presence of differences between the two techniques in the location of the femoral tunnel on CT scan. Second, we had a relatively short-term follow-up. However, when analyzing failure in ACL reconstruction, it must be considered that most re-ruptures occur within two years of surgery. Third, the study was underpowered to detect any statistical difference in revision rate, even though our data showed a tendency toward a higher failure rate for TT technique.

In conclusion, we found that with the TT technique the femoral tunnel was located higher in the high-to-low direction and, considering its deep-to-shallow position, was slightly shallow compared to what was observed with the OI technique. The femoral tunnel obtained using the OI technique was found to be in a position closer to the anatomical ACL footprint than the femoral tunnel obtained using the TT technique. A femoral tunnel position distant from the anatomical footprint of native ACL may result in higher failure rate.