- Open Access
The capsular line reference, a new arthroscopic reference for posterior/anterior femoral tunnel positioning in anterior cruciate ligament reconstruction
© The Author(s). 2018
Received: 8 January 2018
Accepted: 8 March 2018
Published: 27 March 2018
Femoral malposition is the first cause for graft rupture during ACL reconstruction. Arthroscopic landmarks can be difficult to identify. So, landmark has to be found for reliable tunnel placement. A proximal-distal reference was described as “Apex reference” reported by Hart et al. but no posterior/anterior reference exists in the literature.
The purpose of this study was to do a 3D CT-scan assessment of the femoral tunnel positioning using the Capsular Line Reference (CLR) as a landmark for posterior/anterior placement in ACL reconstruction. We hypothesized the CLR could provide a precise and reliable antero/posterior femoral tunnel positioning less than 2 mm from the Bernard & Hertel posterior quarter.
Seven cadaveric knee specimens with a mean age of 79.2 ± 11 years were used. Using standard approaches, the CLR was identified corresponding to a white line (the capsule) appearing at the posterior border of the femoral condyle after bony debridement of the medial and posterior part of the lateral femoral condyle. The center of the tunnel was marked. An inside-out technique with anteromedial drilling technique was performed using an 8-mm diameter reamer. The distal femurs were sawed and a CT-scan was done for each specimen to obtain 3-dimensional image reconstructions. These 3D reconstructions were analyzed to measure the position tunnel center on the posterior/anterior axis and the distance from the posterior/anterior quadrant according to the Bernard & Hertel method.
The mean position for the posterior/anterior axis was 27.0 ± 1.8% (25–28.9) with a median of 26.9%. The position from the first quarter of the Bernard & Hertel method was 0.9 ± 0.8 mm (0–1.8) with a median of 0.8 mm.
The CLR is a reliable and reproducible arthroscopic landmark to place the femoral tunnel for ACL reconstruction in the anterior/posterior axis. Proximal/distal position depends on the choice of the surgeon to reproduce anteromedial or posterolateral fibers.
The most cited reason for residual instability and graft failure in arthroscopic anterior cruciate ligament (ACL) reconstruction remains technical errors including inappropriate graft fixation, graft impingement in the notch, incorrect femoral and tibial tunnel placement (Samitier et al., 2015; Wylie et al., 2017).
Appropriate tunnel placement is critical for the success of such surgeries, nevertheless, incorrect femoral tunnel placement still represents 80% of these technical errors (Samitier et al., 2015; Wylie et al., 2017; Whitehead, 2013; Trojani et al., 2011). Graft behavior is supported by anatomical, biomechanical and isometric conditions (Zavras et al., 2005). According to the most recent understanding of ACL anatomy (Smigielski et al., 2016; Smigielski et al., 2015; Sasaki et al., 2012), the graft must be placed anatomically on the lateral femoral condyle. Full anatomic ACL reconstruction remains actually a challenge, and the best we can achieve is a compromise between anatomy, biomechanics and isometry. In order to guide the surgeon in this choice, the concept of I.D.E.A.L femoral tunnel position has been proposed (Pearle et al., 2015). Arthroscopic ACL reconstruction is a technically demanding surgery and surgeons need easily recognizable bony landmarks in order to secure the tunnel placement. The tunnel center can be situated in two directions, Proximal and Distal and Posterior and Anterior positioning (Davis et al., 2016; Colombet et al., 2016). Many surgeons use the “classic clock face” centered in the notch, to place the tunnel center in proximal and distal position. This clock face technique is not precise enough and has been reported as lacking accuracy and precision for femoral tunnel placement (Azzam et al., 2011; Loriaut et al., 2017; Momaya et al., 2017). Some authors have shown that proper placement is related to the angle of knee flexion (Markolf et al., 2009), so the apex of posterior cartilage was recently reported as a better anatomical bony landmark for the proximal/distal position (Hart et al., 2015). Proximal/distal positioning depends on the ACL fibers the surgeon wishes to reconstruct: Antero-medial (AM) fibers are in the proximal part of the notch while Postero-lateral (PL) fibers are distal and both are anatomical.
The purpose of this cadaveric dissection study was to assess femoral tunnel positioning using the Capsular Line Reference (CLR) as a bony landmark for the posterior/anterior placement in arthroscopic ACL reconstruction. The CLR can be identified as a “white line” (corresponding to the capsule) appearing at the posterior border of the medial side of the lateral femoral condyle after bony debridement using a shaver. So it’s a posterior reference which can be always found and prevent to place the tunnel too anteriorly. In effect, the Posterior/Anterior position is much more crucial because a position which is too anterior constitutes a main reason for graft rupture (Samitier et al., 2015; Wylie et al., 2017). A femoral offset guide in different sizes (from 4 mm to 7 mm) has been proposed to control the Posterior/Anterior positioning (Tuca et al., 2016; Sekiya et al., 2016). It is hooked above the lateral femoral condyle but there is no visual arthroscopic landmark to ensure that the chosen target has been reached: the technique with offset guide has the disadvantage the position of the tunnel cannot be seen when the guide is in place whereas with the CLR technique, the position can be identified during all the procedure, regardless of the degree of flexion. Moreover, a simple synovial resection at the posterior part of the condyle is not enough to make appear the CLR and the posterior edge of the lateral notch wall is sometimes difficult to identify.
A 3D CT-scan of the specimen was used to precisely measure tunnel placement using the Bernard and Hertel (B&H) quadrant method (Bernard et al., 1997). Our hypothesis was that the CLR provides a precise and reliable antero/posterior femoral tunnel positioning less than 2 mm from the Bernard & Hertel posterior quarter (center of the ACL antero-medial fibers as described in the literature (Sullivan et al., 2015)).
Cadaveric dissection and CT scan
A scaled illustration of the femoral footprint was produced using Adobe Illustrator® C26 graphics software. The tunnel aperture was drawn and its center located (Fig. 5b). The Bernard & Hertel quadrant was drawn and its dimensions measured (length. width) in Adobe illustrator were related to the real distance reported in Radian DICOM viewer picture. We measured the distance in millimeters (mm) from the posterior part of the quadrant to the tunnel center and was recorded as a percentage with the quadrant’s Blumensaat line. Then, we measured the distance from the tunnel center to the first posterior quarter of the Bernard & Hertel quadrant which is the center of the AM direct fibers of the native ACL (Bernard et al., 1997; Johnson et al., 2015; Jenny et al., 2011; Kawakami et al., 2012; Lee et al., 2007; Li et al., 2012; Youm et al., 2013). The first posterior quarter was considered as the position to target and the aim was to bas as close as possible from the first quarter. Statistical Analysis: Descriptive statistics (mean, median and standard Deviation) were used to describe the anatomical relationships measured in this study.
Results for the seven specimens with two analyses: The distance of the tunnel center from the Bernard & Hertel (B&H) deep quarter line according to the quadrant method
Deep/Shallow measurement from the deep
Distance from the AM target
Native AM center
The main result of this study is that the Capsular Line Reference seems to be a reliable and reproducible reference for the positioning of the femoral tunnel in arthroscopic ACL reconstruction. Our hypothesis was validated with a mean distance from the posterior edge of B&H quadrant at 0.9 ± 0.8 mm (0–1.8).
Initially described as a radiological measuring method in 1996 in a German journal (Bernard & Hertel, 1996) and popularized since 1997, the Bernard & Hertel quadrant method is the most widely used in the literature for analyzing the position of the femoral tunnel on a lateral view (Bernard et al., 1997), (Hwang et al., 2012). This method was validated by Kim et al. for CT-scan analysis (Kim et al., 2016) and its reliability was confirmed by Lertwanich et al. (Lertwanich et al., 2011). We chose CT-scan analysis because it is more precise as it allows for accurate orientation of the medial part of the lateral condyle and offers the possibility of having measurements in millimeters without parallax or length approximation.
In our study, average mean position of the femoral tunnel center was at 27.0 ± 1.8% and the median was 26.9% for posterior/anterior position according to the Bernard & Hertel method. This result corresponds to the literature. Parkar et al. (Parkar et al., 2016) recently did a systematic review of imaging and cadaveric analysis of the position of tunnels in ACL reconstruction and reported a mean of 28.6% and a median of 26.3% for the posterior/anterior position for single bundle reconstruction. Results were comprised between 23.5 and 37.3%. For double-bundle reconstructions, the anteromedial bundle was comprised between 15 and 33.9%. We also found a small standard deviation (27.0 ± 1.8%) showing the good reproducibility of the CLR technique.
Numerous techniques describe landmarks and references to correctly place the femoral tunnel center, including: clock face method, bony landmarks, ACL remnant, Computer-Assisted Surgery (CAS) and radiographic method (Davis et al., 2016). The clock face method is not precise because it depends on the position of the knee and cannot be proposed as a reliable method (Davis et al., 2016). In 2012, Piefer et al. (Piefer et al., 2012) summarized the different bony landmarks (lateral intercondylar ridge, “back” of femur, notch roof, “over-the-top” position, posterior cortical border of femoral diaphysis). They specified that ACL insertion is located in an area below the notch roof and posteriorly to the lateral intercondylar ridge but this corresponds to a large area. Pansard et al. also proposed to only use bony landmarks to place the tunnel because they found no difference between two types of drilling techniques (Pansard et al., 2015). But as written by Steckel et al., visualization of the footprint varies with degrees of flexion, and so, a slight variation could modify the position of the tunnel even though bone is seen (Steckel et al., 2010). In addition, bony landmarks such as the lateral intercondylar ridge can be difficult to identify. Another landmark described is the stump of the ACL. It can be used in addition to bony landmarks but the size of the footprint is an oval with a length ranging from 12.9 to 18.4 mm and a width between 4.7 and 9.3 mm (Piefer et al., 2012) so it cannot be fully covered and a choice has to be done to place the tunnel inside the stump. Radiographic and CAS are time consuming or expensive and difficult to use in a daily practice everywhere. Davis et al. (Davis et al., 2016) published a technique to place the femoral tunnel by measuring the height and the depth of the lateral notch but this implies parallax approximation and does not take into account variations in patient anatomy. Hart et al. (Hart et al., 2015) proposed the Apex of Posterior Cartilage (ADC) as a reference. This method aims to place the tunnel at the center of the footprint by measuring a fixed length from the ADC. Once again, this technique ignores variations in anatomy and using only fixed measured distance does not seem to be optimal. Furthermore, the center of the footprint is not the position we target with the CLR technique, we aim to place the center of the tunnel in the middle of the anteromedial fibers of the native ACL. The position of these anteromedial fibers corresponds to the first posterior/anterior quarter line of the Bernard & Hertel quadrant (Bernard et al., 1997; Johnson et al., 2015; Jenny et al., 2011; Kawakami et al., 2012; Lee et al., 2007; Li et al., 2012; Youm et al., 2013). We found a distance of 0.9 ± 0.8 mm from this line showing the CLR as a precise posterior/anterior landmark to place the tunnel at insertion sites of anteromedial fibers. Ahn et al. measured the distance from a reference point using the quadrant method, but the results were given as a percentage and not in millimeters (Ahn et al., 2013).
We chose the insertion site of anteromedial fibers because Pearle et al. advocated that the aim of ACL reconstruction could not be just to try to fill the footprint and centralize the center of the tunnel and so that a compromise should be an option (Pearle et al., 2015). Although the anatomy of the footprint varies from patient to patient, there is some consistent data in the anatomy of the ACL which structure was recently described as ribbon like (Smigielski et al., 2016). The anterior margin of the direct fibers is the intercondylar bony ridge and there is a fan-like extension posteriorly close to the cartilage, and in continuity with the capsule attachment on the lateral condyle (Mochizuki et al., 2014). Iwahashi reported an average width of the insertion of 8 mm. Given its direction, the graft does not fill up the whole tunnel aperture but occupies the anterior part of it (Segawa et al., 2003). When the tunnel is placed behind the intercondylar bony ridge and anteriorly to the capsule, the graft is situated in the native anterior direct fiber location. With the CLR technique, this criterion is fulfilled because the reference is the capsule; the tunnel is placed behind the interconyldar ridge. Moreover, the proximal and anterior part of the native ACL insertion (called anteromedial fibers) is the most isometric location (Simmons et al., 2003), these fibers are subjected to small elongations during flexion extension of the knee and thus correspond to a placement with low tension applied on the graft. So, with the CLR technique, the tunnel is placed at the proximal and anterior part of the femoral footprint, which corresponds to the I.D.E.A.L. position for the center of the tunnel, as recommended by Pearle et al. (Pearle et al., 2015). So the CLR is a posterior reference that can be found in all patients and allows to be as posterior as possible. It’s necessary to do the bony debridement to make it appear at the posterior edge of the condyle.
There are, however, some limitations. We studied seven specimens but this number corresponds to the literature (Hart et al., 2015), (Johnson et al., 2015), (Musahl et al., 2003) and we found a small standard deviation showing the reliability of the technique. One surgeon performed the technique for all the specimens but analysis was done by an independent observer. Our study is a cadaveric analysis but we applied the exact same operative conditions with the same instruments so we could expect similar results in human surgery. A prospective clinical study with the CLR technique is on the way.
This study shows that the CLR reference appears to be a reliable and precise technique for femoral tunnel placement during an ACL reconstruction. This arthroscopic reference could substantially help in posterior/anterior femoral tunnel positioning.
Availability of data and materials
Data will be available upon request.
PC performed the experiment, analyzed the acquired data, and drafted the manuscript. NB analysed the data and helped to draft the manuscript. AS helped for CT-scan analysis. All authors read and approved the final manuscript.
Consent for publication
A part of funding in this study was from SBM France. The funding body was not relation to the design of the study and collection, analysis, and interpretation of the data and in writing the manuscript.
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