Fig. 18.1
Anisometry profiles of anteromedial (AM), posterolateral (PL), central, and conventional single-bundle fibers are shown at different flexion angles. Fiber lengths were normalized to zero at full extension for the flexion/extension cycles (Reprinted from Pearle et al. (2008) with kind permission of American Journal of Sports Medicine [49])
Fig. 18.2
This figure shows the terminology used for the navigation of the femoral footprint. The high position is also called anterior and low position is also called posterior, while the deep location is sometimes referred to as proximal and shallow as distal (Reprinted from Amis et al. (1998) with kind permission of Springer Science and Business Media [5])
The importance of isometry in vivo has been shown by Beynnon et al. [10]. The authors measured the isometry of the graft intraoperatively and divided the patients into a group that did not show graft elongation and a group that did show graft elongation. Immediately after reconstruction, they found no difference between the groups. However, at 5-year follow-up, the group with graft elongation showed more anterior-posterior laxity. These studies made it clear that an isometric position is important in achieving a stable ACL reconstruction without the risk of increased laxity and subsequent graft failure.
Several factors were responsible for the subsequent shift in attention away from isometry, as the primary goal of ACL reconstruction. Some studies assessed the role of isometry in the native ACL and showed that the ACL is not an isometric structure [6, 31, 37]. Markolf et al. showed with a trial wire that during the last 30° of extension, the length of the ACL increased by approximately 3 mm [37]. In addition, other studies showed that the ACL fibers do not insert at the most isometric point but at the anatomical footprint [4, 31]. It was believed that these anatomical fibers contributed to rotational stability and therefore were of significant importance. These findings resulted in the search for a compromise between a position within the anatomical footprint and a position with isometric characteristics, the so-called anatometry [44]. Although the attention partially shifted toward an anatomic reconstruction, isometry remains an important goal of ACL reconstruction. The more isometric position is located proximal (deep) in the femoral condyle and more anterior (high) [75, 76].
18.2.2 Anatomy
Ernest William Hey Groves (1872–1944) is believed to be the first surgeon who performed a complete ACL reconstruction with the use of a tibial and femoral tunnel [20, 57]. He used the fascia lata as a graft and threaded it through new canals in the femur and tibia. Hey Groves emphasized the role of anatomic reconstruction in proper restoration of knee joint kinematics. In the following decades, more attention was directed toward conservative treatment, primary ACL repair, and isometric tunnel position, and therefore anatomic reconstruction became less important [57].
Over the last decade, however, anatomical femoral tunnel positioning gained popularity, due to recently published data. First of all, the aforementioned studies identified that several ACL fibers inserted within the anatomical footprint and were considered to play an important role in kinematics [4, 31]. Furthermore, the nonanatomic but isometric vertical graft orientation seen with transtibial drilling techniques provided good anterior-posterior stability but suboptimal rotational stability [34, 55, 58]. Finally, in 2005 Musahl et al. compared knee kinematics between an anatomic femoral tunnel position and an isometric femoral tunnel position outside the anatomic footprint in a biomechanical study [45]. Although none of the tunnels fully restored the kinematics to those of a native ACL, they found that the anatomic tunnel position better restored knee kinematics compared to the isometric tunnel position in a simulated Lachman and simulated pivot shift.
In order to determine the anatomy of the footprint, bony landmarks of the femoral ACL insertion have been identified [51]. The lateral intercondylar ridge is an important bony landmark that is located just anterior to the ACL footprint. Clancy Jr. described this bony landmark as the resident’s ridge because it can be mistaken for the over-the-top position by inexperienced surgeons [22]. This could result in an anterior positioning of the graft and subsequently failure of the graft [28]. Another osseous landmark, which more recently has been described, is the lateral bifurcate ridge [15]. This ridge connects anteriorly with the lateral intercondylar ridge and posteriorly with the posterior aspect of the femoral cartilage and separates the anteromedial and posterolateral bundles of the ACL [72]. The lateral intercondylar ridge is arthroscopically identified in 88–100 % of the cases, while it is more difficult to identify lateral bifurcate ridge arthroscopically (48–82 %) [15, 70].
These bony landmarks identify the anatomical footprint of the ACL fibers. The anatomical footprint is crescent shaped with the lateral intercondylar ridge as a straight anterior border and the lateral femoral condyle as a convex posterior border [63]. The surface area of the femoral ACL attachment site varies in different studies between 70 and 200 mm2 [23, 30] and is thought to cover approximately 18 % of the lateral wall of the intercondylar notch [23]. The length of the anatomical footprint varies in several studies between 14 and 18.5 mm and the width between 7 and 11 mm [30].
The shape of the native ACL is more tubular at the midsubstance and is flat, ribbon-like at the femoral end with a thickness of 2–4 mm and width of 10–16 mm (Fig. 18.3) [63]. It is not surprising that with single-bundle reconstruction, the ACL graft can only cover 30–54 % of the femoral footprint [54]. Therefore, some thought must be given to positioning of the ACL graft within the large area of the footprint. With time zero biomechanical studies showing superiority of an anatomic tunnel position [34, 45, 55, 58], it would appear beneficial to place the femoral tunnel within the confines of the footprint in the most isometric location, as opposed to an isometric position that is nonanatomic. Several studies have shown that the region that is both anatomic and isometric is located proximal (deep) and anterior (just posterior of the lateral intercondylar ridge) within the anatomic footprint [76].
Fig. 18.3
The ribbon shape of the ACL after careful removal of the synovial tissue is shown. The ACL fibers form a flat ribbon 2 mm from its femoral attachment to midsubstance (Reprinted from Śmigielski et al. (2014) with kind permission of Springer Science and Business Media [63])
18.2.3 Direct Fibers
Over the last few years, there has been an increasing interest in the histological characteristics of the ACL following a study of Iwahashi et al. [27]. The authors in this study identified two categories of fiber insertions at the femoral side of the ACL, with different histological and biomechanical characteristics. The authors described these fibers as the direct and indirect insertions (Fig. 18.4). The direct insertion has a transitional zone between the ligamentous tissue and the femoral insertion, while the indirect insertion lacks this zone. This transitional zone consists of ligamentous tissue, noncalcified cartilage, and calcified cartilage and enables the distribution of loads. Therefore, many authors considered the direct fibers to be biomechanically more important [8, 9, 27]. The indirect fibers are thought to play a role in resisting shear movements by functioning as a dynamic anchorage of soft tissue to the bone [56].
Fig. 18.4
The oval ACL insertion is shown. The direct insertion was located at the anterior of the ACL insertion (shaded portion). The width of the direct insertion was narrow. The posterior of the ACL insertion was the indirect insertion (dotted portion) (Reprinted from Sasaki et al. (2012) with kind permission of Elsevier [56])
It was noted that the location and orientation of the indirect or “fanlike” fibers did not change through the flexion arc, while the direct fibers did change in location [43]. Pathare et al. examined the biomechanical role of both fiber insertions [48]. They assessed the kinematics of knees with an intact ACL and compared these kinematics with knees in which the indirect fibers were removed. They found that knee kinematics between the intact ACL and the transected indirect fibers did not significantly differ in the simulated Lachman, anterior drawer, and pivot shift test. Upon transection of the direct fibers, a large increase in anterior tibial translation and internal tibial rotation was noted. Another study showed that the direct fibers carry approximately 82–90 % of the load when an anterior drawer force is applied, while the fanlike fibers only contributed to a minor part of the overall load [29]. These studies suggest that the indirect fibers have a much smaller load-bearing function compared to the direct fibers. Therefore, it may be beneficial to aim the femoral tunnel in the region of the direct fibers. Furthermore, it has recently been shown that graft impingement was not significant when placing the femoral tunnel in the direct insertion [69] although this has also been shown in a cadaveric study [26].
The direct fibers form a narrow, linear band zone that inserts just posteriorly to the lateral intercondylar ridge, which means they are located anteriorly within the anatomical femoral footprint [27, 48, 56]. The anterior-posterior thickness of the direct fibers is 5.3 mm (±1.1) [56] and covers approximately 36 % of the anatomical femoral footprint [43]. The indirect fibers insert between the direct fibers and the posterior femoral condylar cartilage, which means a posterior location inside the anatomical femoral footprint. The anterior-posterior thickness of the indirect fibers is approximately 4.4 mm (±0.5) [56], and they cover approximately 64 % of the surface of the femoral anatomical footprint [43].
Sasaki et al. observed that the direct and indirect fibers are both microscopically and macroscopically identifiable with the positions as described above [56]. Indirect fibers do not contribute much to knee kinematics and load carrying and lack a transitional zone. Therefore, we recommend targeting the region of the direct fibers that consists of a 5 mm thick linear zone bordering the lateral intercondylar ridge and thus anteriorly within anatomical footprint.
18.2.4 Tension Pattern
Another possible explanation for higher failure rates of ACL reconstruction is the tension on the ACL graft. It has been suggested that a higher force on the ACL graft can cause graft failure, loss of fixation, or limited motion [21, 37, 38]. Several studies have shown that the largest tension on the ACL takes place during extension and hyperextension [21, 36]. As the knee moves through the flexion arc, the tension decreases. Markolf et al. compared the forces on the native ACL with forces on the ACL graft and found higher forces on the ACL graft (Fig. 18.5) [36]. They specifically found that in the native ACL, the tension decreased as the knee moves through the flexion arc, but this decline was less pronounced in the ACL graft. The authors found similar results in a second study using different ACL reconstruction techniques [40].
Fig. 18.5
This graph shows the graft tension of the different femoral tunnel position grafts. The pp position indicates the proximal position within the anatomical footprint, while the pd position indicates the distal position within the anatomical footprint. The two anterior positions (ap and ad) were positioned outside the anatomical footprint and show a higher graft tension pattern. The tunnels positioned within the anatomical footprint (pp and pd) and the isokinetic position showed the least graft tension (Reprinted from Zavras et al. (2005) with kind permission of Springer Science and Business Media [75])
The position of the femoral tunnel is known to play a role on the forces on the ACL graft [21, 32, 38]. Increased graft force can cause overconstraint; posterior, lateral, and external subluxation; and eventually slackening and failure of the graft [42]. Zavras et al. assessed the tension on the ACL graft and the anterior-posterior laxity in different femoral tunnel positions [75]. They compared different tunnel positions in the proximal isometric zone and found that the femoral tunnel location that imparted the lowest tension on ACL graft was located at the anterior-proximal corner inside the anatomical footprint. This position correlates with the previously stated position that is both isometric and anatomic. They found that a more anterior position of the femoral tunnel caused an increased tension pattern in knee flexion and subsequently can cause overconstraint. A more posterior position of the femoral tunnel can cause high tension in extension and slackening of the graft during flexion and thus an increased anterior-posterior laxity. Other studies have confirmed the correlation between several femoral tunnel positions and the tension in the graft [36, 37, 62]. In addition, as stated in the isometry discussion, placing the femoral tunnel more distal would cause an increased length change compared to proximal positioning within the anatomical footprint [76].
Markolf et al. assessed the tensioning of the ACL graft in different ACL reconstruction techniques [40]. They compared the intact ACL with single-bundle ACL reconstruction and with the “fill-the-footprint” ACL reconstruction. Their results showed that the single-bundle technique better restored the graft tension than the fill-the-footprint technique. These biomechanical studies show that the graft tension most optimally approximates the intact ACL graft tension when the femoral tunnel is placed in the anterior-proximal zone of the anatomical footprint.
18.3 Guidelines for Arthroscopic Surgery
During arthroscopic surgery, it can be challenging to identify the anatomical landmarks and subsequently place the graft in an anatomic and isometric position within the anatomical footprint. To locate the position within the anatomical footprint, we provide some guidelines that can be used during arthroscopic surgery.
18.3.1 Eccentric Position
Several studies have advocated a central position for the ACL within the femoral footprint [19, 34, 58, 74]. Wilson et al. recently advocated a central position of the femoral tunnel [74] in order to capture the function of the anteromedial and posterolateral bundles. However, as previously discussed, there are different loading characteristics for different regions of the femoral footprint. Because of these considerations, we advocate that the femoral tunnel insertion should be placed eccentrically within the anterior-proximal region of the footprint rather than in a more central position. We discussed that a graft in a central position showed more elongation (up to 8 mm) compared with a more anterior position within the footprint (up to 4 mm) and has lower tension in the graft than a central location. Furthermore, this position also occupies the region of the direct ACL fibers. Thus, a central position for the graft would not be optimal with respect to isometry, tension patterns, and biomechanics. Therefore, after identification of the native ACL footprint during arthroscopic surgery, an eccentric anterior-proximal location is recommended to prevent these risk factors of graft failure (Fig. 18.6).
Fig. 18.6
This figure summarizes the ideal tunnel position (black circle) since this is in the anatomical footprint and captures both the direct fibers and the most isometric position (Reprinted from Pearle et al. (2015) with kind permission of The American Journal of Orthopedics [48])
18.3.2 Equidistant
The other guideline that can be used to check whether the proposed femoral tunnel is correctly positioned is the equidistance between anatomical landmarks. The anterior-proximal femoral tunnel position within the anatomical footprint is roughly equidistant between the top of the femoral notch and the bottom of the notch or the most posterior aspect of the femoral cartilage. This point can be easily identified and functions as a last check before the femoral tunnel is drilled. Moreover, the bony anatomy of the femoral footprint is not always clear [15, 70]. This equidistant position halfway between the top of the notch and the most posterior aspect of the femoral cartilage can be identified when the osseous landmarks of the anatomical footprint are not entirely clear since these landmarks are outside the footprint (Fig. 18.7).
Fig. 18.7
An arthroscopic view of the femoral footprint is shown with the intercondylar ridge (upper black line), bifurcate ridge (lower line between two colored dots), and the centers of the anteromedial (red) and posterolateral (blue) bundle
18.3.3 Transtibial and Anteromedial Technique
There has been much debate about whether the transtibial technique (TT) or the anteromedial (AM) technique should be used for the drilling of the femoral tunnel. With the use of the transtibial technique (TT), the tibial tunnel dictates the femoral tunnel position and can result in a more vertical graft [1, 64, 65]. A more vertical graft is correlated with an increased graft tension [62] and rotational instability [34, 55, 58] although this latter finding has been questioned [39]. With the AM portal drilling technique, also referred to as independent drilling technique, a good identification of the anatomy of femoral footprint is necessary in order to identify the position of the femoral tunnel [68].
Many systematic reviews and meta-analyses have lately been published to determine which technique is superior [2, 12, 33, 52, 53]. The general conclusion is a good femoral tunnel position is possible with both techniques although some studies showed a small preference for the AM or independent drilling technique [2, 12, 33]. However, a Danish registry-based study showed that the revision rate of the AM technique (5.2 %) is higher than the revision rate of the TT technique (3.2 %) [52]. Their explanation was that the introduction of this new and more complex AM technique causes more technical failures, while it is also possible that the femoral tunnel position plays a role. A retrospective in vivo MRI comparison performed by Bowers et al. found no differences between the positions of the femoral tunnel at the femoral condyle [11]. However, a more posterior position of the tibial tunnel in the tibial footprint was necessary to ensure femoral insertion at the anatomical footprint, and therefore the graft obliquity in the sagittal plane was more vertical with the TT technique. This finding is similar to earlier reports [1, 64, 65].
Taking the systematic reviews and meta-analyses into consideration, it seems that an acceptable femoral tunnel position can be achieved with both techniques. Some studies suggest that an AM technique will result in better functional outcomes and a better tunnel positions although the Danish registry showed that the revision rate could be slightly higher with the AM technique.
Conclusion
Due to the large surface area of the anatomic femoral ACL footprint and the inability of a tubular ACL graft to fill the footprint, some thought must be given to positioning of an ACL graft within the confines of the native footprint. Based on the critical review of the literature that we have presented in this chapter, we recommend an anterior (high) and proximal (deep) position within the anatomical femoral footprint. With the femoral tunnel in this location, the graft (I) remains anatomic, (II) is relatively isometric, (III) has low tension, and (IV) is located in the biomechanically advantageous direct insertion.
This position can be identified reliably at arthroscopy and is located just posterior to the lateral intercondylar ridge and proximal (deep) to the lateral bifurcate ridge. Further studies are needed to clarify whether the position we propose in this chapter results in clinically superior outcomes.
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