Femoral tunnel positioning using an anteromedial technique for ACL reconstruction: A radiographic study with a cadaveric model

Abstract

BACKGROUND:

We studied the anatomic positioning of the femoral tunnel during simulated anterior cruciate ligament reconstruction using an anteromedial portal approach in cadaveric models.

METHODS:

In thirty cadaveric human knee specimens, simulation of an arthroscopic anterior cruciate ligament reconstruction was performed and the femoral tunnel was drilled using an anteromedial portal. A Kirschner wire was passed into the tunnel and radiographs were obtained. These radiographs were then evaluated in the coronal and sagittal planes. Angles between the axis of the femoral tunnel and the joint line in the coronal plane (alpha, $\alpha$ ) or the femoral long axis in the sagittal plane (beta, $\beta$ ) were calculated for each specimen. The external aperture of the femoral tunnel was defined as the point of exit of the Kirschner wire from the lateral femoral cortex. This was evaluated relative to a prescribed rectangle and coordinate axis, with the radiographic quadrant method of Bernard, to assess the accuracy of femoral tunnel placement.

RESULTS:

The mean $\alpha$ in the coronal plane was 48.53 ${}^{\circ}$ , the mean $\beta$ in the sagittal plane was 32.23 ${}^{\circ}$ . All of the femoral tunnel external apertures were located outside of the rectangle

CONCLUSION:

We evaluated the positioning of the femoral tunnel and the external aperture of the femoral tunnel with the anteromedial portal technique. This study provides a reference standard to assess accurately femoral tunnel positioning on postoperative radiographs.

Keywords

Femoral tunnel external aperture postoperative radiographs ACL reconstruction

1. Introduction

The anterior cruciate ligament (ACL) is important for maintaining knee stability. A tear of the ACL often warrants reconstruction, especially in a young and active patient. Essential to successful ACL reconstruction is an accurate anatomical placement of the femoral insertion site and tunnel [1, 2, 3, 4].

The importance of correct positioning in both the sagittal and coronal planes in ACL reconstruction was recognized many years ago, and it has been demonstrated in numerous studies that an incorrect position of the femoral tunnel leads to treatment failure [5, 6, 7]. A graft that is placed too vertically in the coronal plane will result in surgical failure, and it has been proved in various studies of reconstruction that there are significant advantages to an oblique ACL graft in the coronal plane [8, 9, 10].

The transtibial technique was conventionally preferred because it required a single incision and less surgical time, and consequently, low morbidity. However, studies have shown that the placement of the femoral tunnel via a transtibial approach results in non-anatomical placement of the femoral tunnel, which causes biomechanical instability and unacceptable clinical results. Specifically, in the transtibial technique the placement of the femoral tunnel is guided by the tibial tunnel, and it is only possible to reach a femoral entry position corresponding to the 11 o’clock or 12 o’clock position (i.e., for the right knee), rather than the recommended 10 o’clock position [11]. Consequently, the anteromedial portal technique has been suggested for positioning the femoral tunnel, without any increased complexity [12, 13]. Fu was the first to recommend anatomic ACL reconstructions for improved biomechanics and to decrease the failure rates [14]. However, cadaveric studies that have defined and evaluated parameters for an anatomical femoral tunnel placement are limited.

The primary purpose of this prospective study was to describe the anatomy of the femoral tunnel and the external aperture of the femoral tunnel in detail. An arthroscopic ACL reconstruction using the anteromedial technique was simulated on cadaver models and a radiographic assessment performed to determine the parameters for accurate femoral tunnel placement. The study should provide a reference for the postoperative evaluation of femoral tunnel placement, and also can be used clinically to identify the correct placement of tunnels intraoperatively.

2. Materials and methods

2.1 Materials

Thirty cadaveric human knee specimens were used for the purpose of this study. Cadavers with evidence of joint degeneration, deformity, or injury around the knee joint were excluded. There were 13 men and 17 women (11 left, 19 right) with a mean age of 37.3 years. This procedure was approved by the local ethics committee.

2.2 Methods

Each thigh was transected 20 cm from the joint line and anchored to an operating table to ensure the reproducibility of the tunnel directions and a 0 ${}^{\circ}$ to 130 ${}^{\circ}$ range of knee flexion. Sequentially shaving and resecting the native ACL was done to determine arthroscopically the anatomic location of the ACL femoral insertion site. A k-wire was then drilled through the center of the femoral insertion site of the ACL using the anteromedial portal at 120 ${}^{\circ}$ of knee flexion. The wire was drilled through to exit the lateral femoral cortex. The anteromedial portal was placed at the level of the medial joint line, 2 cm away from the medial margin of the patellar tendon. The use of the anteromedial portal allowed clear visualization and drilling of the tunnel at the desired location [15]. The external aperture of the femoral tunnel was considered the point at which the k-wire exited the lateral cortex of the femur.

2.3 Surgical simulation

The knee was placed in 120 ${}^{\circ}$ of flexion. A standard anterolateral portal was used for diagnostic arthroscopy and an anteromedial portal was used as a working portal. To visualize the course and insertion points of the ACL clearly, a planer shaver was used to clean the joint cavity. Arthroscopic scissors were used to debride the ACL to the anatomical footprint.

A femoral guide with an appropriate offset was introduced into the joint through the anteromedial portal. Using the aiming device of the femoral guide, a Kirschner wire (k-wire) was then placed into the center of the anatomic insertion of the ACL, which typically exits from the external aperture of the femoral tunnel (EAFT; at approximately the 10 o’clock position for the right knee, or at approximately the 2 o’clock position for the left knee). The femur was drilled over the k-wire to create the external aperture of the femoral tunnel.

2.4 Radiographic measurement and analysis

The radiographic measurements were performed with a Computed Radiographic system (Direct View CR800 Systems, Kodak, USA). Radiographs included anteroposterior and lateral images, for coronal and sagittal plane visualization, respectively. Two images were obtained in the coronal and sagittal planes that showed the femoral tunnel for almost its entire length. We made sure that the most prominent points of internal and external femoral condyle cortical are referenced in same plane so as to ensure the neutral rotation in the anteroposterior image and that both posterior condyles precisely overlapped in the lateral image. The angle formed between the k-wire and the joint line was recorded as the angle alpha ( $\alpha$ , Fig. 2) in the coronal plane using Hantes et al.’s method [16].

Figure 1.

In the coronal plane, the angle $\alpha$ between the joint line and femoral tunnel centerline was 36 ${}^{\circ}$ .

Figure 2.

In the sagittal plane, the angle $\beta$ between the femoral long axis and the femoral tunnel was 42 ${}^{\circ}$ .

In the sagittal plane, using Segawa et al.’s method [17], the k-wire orientation was recorded as the angle beta ( $\beta$ ), where $\beta$ was the angle between the axis of the femoral tunnel and the femoral long axis (Fig. 2).

Using Bernard et al.’s [18] radiographic quadrant method in the sagittal plane, two pairs of parallel marker lines were selected to create a rectangle (Fig. 4), in which line A represented Blumensaat’s line; line B is parallel to A and tangent to the medial femoral condyle; line C is perpendicular to A and tangent to the posterior edge of the medial femoral condyle, and line D is perpendicular to A and tangent to the anterior edge of the medial femoral condyle. To describe the position of any point within the rectangle, we define a ratio form with line A as 0% and line B 100%, and similarly line C to line D as 0–100%.

Figure 3.

In the sagittal plane, the position of the external aperture of the femoral tunnel relative to the rectangle. In this image, the distance from the external aperture of the femoral tunnel to line A is 14.8 mm, and as a percentage of the distance to line B is 21.44%.

Figure 4.

.In the sagittal plane, the distance between the external aperture of the femoral tunnel and the lateral epicondyle of femur; the coordinate origin is the lateral epicondyle of the femur. In this image, the external aperture of the femoral location distance from lateral femoral epicondyle was 3.6 mm in Lateral view, and 4.5 mm in AP view.

In the sagittal plane, the origin of the coordinate axis was the lateral epicondyle of the femur (Fig. 4). The distance between the external aperture of the femoral tunnel and the lateral epicondyle of the femur in the sagittal plane was calculated. To eliminate variation in size, the image of each specimen was scaled so that the distance from the femoral epicondyle to the lateral epicondyle was standardized to 80 mm [19].

2.5 Statistical analysis

SPSS 12.0 statistical software (SPSS, USA) was used to analyze the data. Measurement data are expressed as mean $\pm$ standard deviation (SD), and the calculated coefficient variation of the marked line.

Table 1
A description of the femoral tunnel and the EAFT ${}^{*}$

	EAFT	EAFT	Percentage	EAFT	C to D	Percentage	EAFT	EAFT in	$\alpha$ ( ${}^{\circ}$ )	$\beta$ ( ${}^{\circ}$ )
	to A	to B		to C			near lateral	front of lateral
							epicondyler	epicondyle
							of femur	of femur
1	10.4	40.5	0.26	18.2	41.1	0.44	3.6	4.5	43	41
2	11.3	45.6	0.23	22.4	45.3	0.49	1.6	7.1	33	44
3	9.5	39.6	0.26	17.5	39.5	0.44	6.3	2.2	39	16
4	12.3	43.2	0.24	16.1	36.2	0.44	1.8	2.2	40	29
5	14.4	47.3	0.22	20.9	43.2	0.48	2.6	4.6	46	30
6	10.5	39.7	0.26	18.5	41.8	0.44	2.3	4.8	56	28
7	9.8	38.6	0.27	18.9	42.3	0.45	3.5	5.6	53	42
8	10.5	40.5	0.26	17.1	42.6	0.40	2.9	2.9	62	40
9	11.2	41.2	0.25	18.1	40.8	0.44	5.3	2.5	45	39
10	9.8	36.6	0.28	18.3	46.8	0.39	4.6	3.6	57	26
11	8.6	37.8	0.28	20.5	45.5	0.45	2.6	3.4	59	24
12	11.9	45.5	0.23	14.5	35.8	0.41	5.3	4.5	46	26
13	9.8	39.3	0.26	17.8	39.5	0.45	3.5	4.6	38	35
14	8.1	37.5	0.28	18.3	38.7	0.47	2.6	6.2	42	36
15	6.9	34.9	0.30	23.7	45.2	0.52	5.1	5.6	50	29
16	8.6	38.9	0.27	19.4	41.1	0.47	3.2	4.6	48	30
17	10.5	40.5	0.26	25.8	46.7	0.55	1.9	4.7	47	32
18	8.9	37.4	0.28	18.2	38.4	0.47	1.2	6.1	56	39
19	9.1	41.1	0.25	17.9	37.6	0.48	3.5	5.6	59	26
20	14.8	48.5	0.21	15.8	36.9	0.43	4.3	5.1	40	30
21	13.6	46.8	0.22	17.9	40.5	0.44	4.1	2.9	39	36
22	10.6	42.1	0.25	18.4	41.5	0.44	2.6	5.4	59	34
23	8.6	35.5	0.29	19.5	42.7	0.46	3.2	4.5	63	36
24	8.9	36.8	0.28	24.8	46.3	0.54	3	4.3	56	30
25	11.2	43	0.24	18.8	39.4	0.48	3.4	3.1	49	34
26	10.8	43.6	0.24	17.8	37.3	0.48	5.5	4	42	32
27	11.9	43.5	0.24	18.6	38.5	0.48	1.9	2.9	46	36
28	15.1	48.8	0.21	17.9	39.1	0.46	2.6	3.5	43	21
29	10.8	39.4	0.26	19.3	41.1	0.47	2.5	2.6	48	26
30	14.1	47.2	0.22	23.1	44.5	0.52	6.8	6.5	52	40
Mean $\pm$ SD	10.7 $\pm$ 2.1	41.4 $\pm$ 3.9	0.25 $\pm$ 0.02	19.1 $\pm$ 2.6	41.2 $\pm$ 3.2	0.46 $\pm$ 0.04	3.4 $\pm$ 1.4	4.3 $\pm$ 1.3	48.5 $\pm$ 8.0	32.2 $\pm$ 6.6

${}^{*}$ In mm, unless indicated otherwise.

3. Results

In the coronal plane, the angle $\alpha$ ranged from 33 ${}^{\circ}$ to 63 ${}^{\circ}$ (48.53 ${}^{\circ}$ $\pm$ 7.99 ${}^{\circ}$ ). In the sagittal plane, the angle $\beta$ was varied from 16 ${}^{\circ}$ to 44 ${}^{\circ}$ (32.23 ${}^{\circ}$ $\pm$ 6.61 ${}^{\circ}$ ). In the sagittal plane, the external apertures of the femoral tunnels were all located outside the rectangle, and the distance from the external aperture of the femoral tunnel to line A was 10.75 $\pm$ 2.06 mm and as a percentage of the distance to line B was 25 $\pm$ 2%; the coefficient of variation was 9.33%. The distance from the external aperture of the femoral tunnel to line C was 19.13 $\pm$ 2.57 mm and as a percentage of the total distance to lines C and D was 46 $\pm$ 4%, with coefficient variation 7.75%. The external aperture of the femoral tunnel (EAFT) was located at a mean distance of 3.44 $\pm$ 1.42 mm in lateral radiographs and 4.34 $\pm$ 1.33 mm in AP radiographs.

4. Discussion

To provide a reference for femoral tunnel assessment, in this study we used the anteromedial approach in a knee cadaver model. Recent studies have clearly shown the benefits of anatomical placement of the femoral tunnel during ACL reconstructive procedures, that is, placement based on the native insertion site [20, 21, 22]. It has also been recognized that the closer the femoral tunnel is to the center of the ACL insertion site, the better the knee joint kinematics after reconstruction. Some researchers have found that the angle of the grafts in the sagittal and coronal planes also influences the joint kinematics, and correlate with the ability of the femoral tunnel walls to withstand stress [23, 24]. Therefore, it is necessary to identify the angle of the femoral tunnel in anatomical ACL reconstruction. In our study, the femoral tunnel was drilled through the ACL insertion site with the knee at 120 ${}^{\circ}$ flexion, ensuring the precision of the femoral tunnel placement.

According to biomechanical studies, in comparison with a more vertical position, an oblique femoral tunnel placement in the coronal plane improves rotatory knee stability [10, 22]. Probably the biomechanical and clinical advantages of the oblique graft in the coronal plane are due to its near anatomic placement as the native ACL. Yamamoto et al. [25] also found that the horizontal femoral tunnel showed less tension of the graft, better movement of the knee, and less incidence of graft impingements. Markolf et al. [25] in his study demonstrated that moving the femoral tunnel from the standard location, using the transtibial technique, to a more oblique position would result in improved knee kinematics. The study also showed that a vertically oriented graft in the coronal plane had poor clinical results and a persistent pivot shift during clinical examination was observed [8]. In the present study, we used the anteromedial portal technique approach to create an oblique femoral tunnel placement in the coronal plane. Results showed that doing so resulted in an average $\alpha$ angle of 48.6 ${}^{\circ}$ , and average $\beta$ angle of 31.3 ${}^{\circ}$ .The findings suggest that optimal tunnel positioning can be obtained by drilling the tunnel through the anteromedial portal with the knee at 120 degrees of flexion. The angle calculated postoperatively may thus prove a surrogate indicator of the long-term prognosis of the ACL reconstruction.

Previous cadaveric studies to evaluate the femoral insertion site were performed by manually resecting the femoral attachment site of the ACL and using the long axis and short axis meeting points as a rough guide to the insertion site. This technique was crude and may not accurately localize the true femoral tunnel placement. In contrast, in the present study the femoral tunnel was drilled and orientated arthroscopically and the cadaveric knee specimens were intraoperatively intact, thus permitting a realistic surgical simulation. We believe that this improved the scientific and practical assessment compared to former studies. The procedure and the results closely replicated the clinical setting. The technique described appears highly reliable and reproducible.

There has been an increase in the number of multi-ligamentous injuries in the recent past and surgeons often tend to reconstruct multiple ligaments such as the ACL and posterior cruciate ligament together in a single procedure. Occasionally when ACL and posterior cruciate ligament reconstructions are performed in a single sitting, the drilled tunnels may interfere with each other. It is therefore crucial to know the exact anatomy and accuracy of tunnel placement before the surgery.

There are few reports available regarding radiological analyses of the external aperture of the femoral tunnel. In this study, we referred to the method of Bernard et al. [18] concerning the radiological position of the ACL femoral tunnel. Our study found that the ratio as expressed in percentage terms of the distance between the external aperture of the femoral tunnel and line A and the external aperture of the femoral tunnel and line B is 20.17 $\pm$ 4.35%. Similarly the ratio as expressed in percentage terms of the distance between the external aperture of the femoral tunnel and line C and the external aperture of the femoral tunnel and line D is 41.13 $\pm$ 3.87%. The external aperture of the femoral tunnel located proximal lateral epicondyle of femur 3.4 $\pm$ 1.4 mm, and front 4.3 $\pm$ 1.3 mm. Our former study of the posterior cruciate ligament found that the ratio of the distance from the insertion site center of the lateral collateral ligament to line A and the distances from line A to line B is 14. 80 $\pm$ 5. 76%; the ratio of the distance from the center to line C and the distances from line C to line D is 38.06 $\pm$ 4. 60%.

The distance between the insertion site center of the popliteus tendon and line A as a percentage of both the distances to lines A and B is 39.85 $\pm$ 6.86%; the distance between the external aperture of the femoral tunnel and line C as a percentage of both distances to lines C and D is 48.27 $\pm$ 8.01%. The distance of the EAFT from the insertion site center of the lateral collateral ligament located in the proximal epicondyle of the femur was 1.16 $\pm$ 2.81 mm, and posteriorly in the sagittal plane was 3.09 $\pm$ 1.45 mm; the popliteus tendon insertion site center located anatomically at the distal epicondyle of the femur 8.69 $\pm$ 2.83 mm and posteriorly in the sagittal plane was back 4.07 $\pm$ 1.76 mm. Our study shows that the external aperture of the femoral tunnel in anatomical ACL reconstruction does not affect the femoral insertion sites of the lateral collateral ligament and popliteus tendon, but may interfere with femoral tunnel placement of the popliteus tendon [26].

Our study is based on the anatomy of the femoral ACL attachment, radiological measurement, and statistical analysis of the femoral tunnel in anatomical ACL reconstruction. The results are clinically relevant for improving outcomes of ACL reconstruction. The study has its limitations, primarily because it is a cadaver study, and therefore may not accurately reflection the population typically undergoing an ACL reconstruction, especially in terms of age, gender, and other morphometric characteristics. However, we believe that since all the cadavers were devoid of any joint deformity or disease, our results are an important reference for follow-up studies. In addition, since the parameters were calculated as ratios or percentages, the individual variations may not be that important. Another issue could be the difficulty in getting standardized radiographs that conform to the standards set in this study. Although difficult, trained radiographers that conformed to a standardized protocol ensured reproducibility of the standard operating protocol.

In conclusion, this cadaveric study reports various radiographic reference parameters that were observed when an anteromedial portal approach was used for an anatomical femoral placement. The data from this study is likely to be valuable in evaluating, planning, and executing anatomical ACL reconstructions.

Conflict of interest

The authors have no conflict of interest to report.

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