Anatomy of the ACL

6.1.1 tibial side

The double-bundle structure of the ACL was introduced already almost two hundred years ago (Weber 1836). Since then various anatomical studies, cadaveric and clinical, have been presented, in which the anatomy of the ACL has been resolved thoroughly on the tibial side (Colombet et al. 2006; Takahashi et al. 2006; Edwards et al. 2007;

Luites et al. 2007; Steckel et al. 2007; Purnell et al. 2008; Siebold et al. 2008; Tallay et al. 2008; Zantop et al. 2008; Doi et al. 2009; Katouda et al. 2011; Kopf et al. 2011;

Pietrini et al. 2011; Ziegler et al. 2011; Ferretti et al. 2012; Otsubo et al. 2012) and femoral side (Mochizuki et al. 2006; Takahashi et al. 2006; Ferretti et al. 2007; Luites et al. 2007; Steckel et al. 2007; Edwards et al. 2008; Purnell et al. 2008; Siebold et al.

2008; Zantop et al. 2008; Iwahashi et al. 2010; Katouda et al. 2011; Kopf et al. 2011;

Pietrini et al. 2011; Ziegler et al. 2011; Ferretti et al. 2012; Otsubo et al. 2012).

When the ACL arises from the tibia, it has two bundles, which can already be discerned macroscopically in the foetus (Ferretti et al. 2007). It has a wide attachment area located in the eminentia of the proximal tibia between the lateral and medial joint surfaces. The length and width of the tibial attachment area are 7.4–14.0 mm and 10.7–25.0 mm, respectively (Colombet et al. 2006; Edwards et al. 2007; Steckel et al. 2007; Purnell et al. 2008; Siebold et al. 2008; Tallay et al. 2008; Kopf et al. 2011;

Ferretti et al. 2012) with a surface area of about 114–206 mm² (Siebold et al. 2008;

Ferretti et al. 2012; Otsubo et al. 2012). The attachment is usually oval or triangular in shape (Colombet et al. 2006; Tallay et al. 2008; Ferretti et al. 2012). The individual insertion site areas are 60.9–69.3 mm² for AM bundle and 52.0–55.7 mm² for PL bundle (Takahashi et al. 2006; Steckel et al. 2007; Siebold et al. 2008; Katouda et al.

2011). Otsubo et al (2012) divided the AM bundle further into AM and IM bundles, in which the attachment areas were 34.5 mm² and 31.0 mm² respectively. Figure 1.

The anteromedial bundle attaches 15.9–17.8 mm and PL bundle 8.4–13.9 mm anteriorly from over the back ridge (Colombet et al. 2006; Edwards et al. 2007; Doi et al. 2009;

Ziegler et al. 2011). The AM bundle is located more medially than the posterolateral bundle, hence the names of these two structures. There is quite a lot variation in the sizes of the insertion sites, since the centres of the bundles have been reported to be 4.5–10.1 mm apart (Colombet et al. 2006; Luites et al. 2007; Siebold et al. 2008; Tallay et al. 2008; Ziegler et al. 2011). The length and width of the AM bundle insertion sites are 9.1–12.0 mm and 5.0–11.1 mm and that of PL bundle 7.4–10.0 mm and 4.0–7.9 mm respectively (Siebold et al. 2008; Kopf et al. 2011; Ferretti et al. 2012). Some fibres may also be attached to the anterior or posterior horn of the lateral meniscus.

The cross-sectional shape of the ACL is not circular, since the two bundles of the ACL work individually in the various flexion-extension angles of the knee joint.

They intersect in the mid-substance area, in which the bundles have their narrowest diameters (AM 8.5 mm, PL 7.7 mm) and areas (AM 20.3 mm², PL 17.7 mm²) (Steckel et al. 2007). The total lengths of the bundles of the ACL have been reported to be 37.7–38.5 for AM mm and 19.7–20.7 mm for PL as measured from the tibia to the femur (Steckel et al. 2007; Zantop et al. 2008).

6.1.2 Femoral side

The attachment sites in the femoral side are more complex, but the anatomy can be seen more clearly because of bony landmarks, which obviously remain in the chronic phase of a rupture (van Eck et al. 2010). The lateral intercondylar ridge, alias resident’s ridge, is found on the lateral wall of the intercondylar notch (Ferretti et al. 2007; Purnell

Figure 1. Anatomical picture of the tibial insertion sites.

AM PL

et al. 2008; Iwahashi et al. 2010; Shino et al. 2010; van Eck et al. 2010; Ziegler et al.

2011; Otsubo et al. 2012). It is divided by the lateral bifurcate ridge, which separates the insertion sites of the AM and PL bundles of the ACL (Ferretti et al. 2007; van Eck et al. 2010; Ziegler et al. 2011). The whole femoral insertion site of the ACL is located posterior to resident’s ridge when the knee is fully extended. Figure 2.

The length and width of the ACL femoral insertion are 13.9–17.4 mm and 6.0–13.0 mm respectively (Colombet et al. 2006; Ferretti et al. 2007; Steckel et al. 2007; Edwards et al. 2008; Purnell et al. 2008; Siebold et al. 2008; Iwahashi et al. 2010; Kopf et al. 2011) and that of individual bundles AM 7.1–11.3 mm and PL 4.7–9.8 mm (Mochizuki et al. 2006; Takahashi et al. 2006; Ferretti et al. 2007; Edwards et al. 2008; Siebold et al. 2008; Kopf et al. 2011). The distance between the AM and PL bundle centres is 6.2–10.0 mm (Colombet et al. 2006; Luites et al. 2007; Siebold et al. 2008; Zantop et al. 2008; Ziegler et al. 2011). The area of the whole ACL insertion in the femur varies considerably (83.0–196.8 mm²) (Ferretti et al. 2007; Luites et al. 2007; Siebold et al. 2008; Iwahashi et al. 2010). The area of the AM bundle has been reported to be 36.1–120.0 mm² and the area of the PL bundle 32.1–103.0 mm² (Takahashi et al.

Figure 2. Anatomical picture of the femoral side attachment points.

AM PL

2006; Ferretti et al. 2007; Luites et al. 2007; Siebold et al. 2008; Katouda et al. 2011).

Otsubo et al. (2012) also divided the AM bundle on the femoral side into AM and IM bundles, giving the areas of the separate bundles as AM 36.1 mm², PL 53.6 mm² and IM 34.9 mm².

The terminology frequently used in literature on femoral insertion sites differs from the traditional medical language. The reason for this is that it is easier to comprehend the femoral anatomy in the arthroscopic view when the terms used are shallow, deep, low and high instead of distal, proximal, posterior and anterior respectively. The anatomical placements of the individual bundles remain the same, obviously, regardless of the flexion angle of the knee, but the dynamic anatomy changes on arthroscopy.

Hara et al. (2009) in their human cadaver study found that the bundles originating from the anteromedial portion of the tibial attachment were inserted into the high and deep portion of the femoral attachment in flexion, whereas those from the anterolateral portion were inserted into the high and shallow portion. Bundles originating from the posteromedial portion were inserted into the low and deep portion and posterolateral into the low and shallow portion of the femoral footprint. In another study by Steckel et al. (2010) the finding was that at full extension of the knee the PL bundle attached to the posterior-distal aspect of the femoral insertion site.

Wolters et al. (2011), in their clinical study of 82 patients, found a correlation between the femoral notch width and the insertion site size. Another finding was that women had smaller notch width than men. This was also the conclusion of the cadaver study by Siebold et al. (2008). A similar finding was reported by van Eck et al. (2011) in their clinical and MRI study of 100 patients. They concluded additionally that notch volume correlated with increased height and weight, but not with the BMI of the subject and that their ACL injury group had larger notch volume than the healthy control group.

6.2 Biomechanics of the ACL and other