According to Harner et al. the femoral insertion of the ACL was 113 ± 27 mm². The femoral attachment area of the AM bundle was 47 ± 13 mm², and the correspondent area for the PL bundle was 49 ± 13 mm² [15].

Mochizuki et al. reported that the femoral attachment of the ACL was composed of two different shapes of fibers: one is the main attachment of the midsubstance of ACL fibers, and the other is the attachment of the thin fibrous tissue which extends from the midsubstance fibers and broadly spreads out like a fan on the posterior condyle [19, 32]. In their recent study [33], they found that during knee flexion, a fold in the ACL femoral attachment was observed at the border between the midsubstance and the fanlike extension fibers (Fig. 16.1), because the fanlike extension fibers were adhered to the bone surface, and the fiber location and orientation in relation to the femoral surface were constant, regardless of the knee flexion angle, while the orientation of the midsubstance fibers in relation to the femur changed during knee motion [33]. The attachment of the midsubstance fibers was significantly smaller than the attachment of the fanlike extension fibers [33]. The insertion of the midsubstance fibers involved cartilaginous zone, which is regarded as the direct insertion. On the other hand, the fanlike extension fibers directly attached onto the bone without forming transitional cartilaginous zone, which is regarded as the indirect insertion. Recently, Sasaki et al. [34] reported similar observations concerning the femoral attachment of the ACL.

Mochizuki et al. [19] could divide the ACL into AM and PL bundles. The length of the major axis of the AM and PL bundles, parallel to the posterior femoral cortex, averaged 9.2 ± 0.7 mm and 6.0 ± 0.8 mm, respectively. The length of the minor axis of both ACL bundles was 4.7 ± 0.6 mm. The distances from the attachment center of AM and PL bundles of the ACL to the posterior border of the lateral femoral condyle averaged 6.3 ± 0.6 mm and 8.6 ± 0.6 mm, respectively. The distances from the center of the AM and PL bundles to the anterior border of the lateral femoral condyle averaged 16.0 ± 1.5 mm and 5.8 ± 0.9 mm, respectively. When they used the “lateral wall clock” technique to describe the center of the AM and PL bundles of the ACL, the found the attachments at 01:40 and 03:10 position, respectively, for the left knee [19].

Ferretti et al. observed the femoral footprint length and width was 17.2 ± 1.2 mm and 9.9 ± 0.8 mm, respectively. The footprint area averaged 196.8 ± 23.1 mm². The areas of the AM and PL bundle attachments were 120 ± 19.8 mm² and 76.8 ± 8.9 mm², respectively [21].

Colombet et al. found that the proximodistal diameter of the ACL femoral attachment area was 18.3 ± 2.3 mm and its anteroposterior diameter was 10.3 ± 2.7 mm. The distance between the center of the AM bundle and the center of the PL bundle was 8.2 ± 1.2 mm. The distance between the posterior border of the ACL femoral attachment and the adjacent articular surface was 2.5 ± 1.1 mm [18].

According to Takahashi et al., the distance from the center of the AM and PL bundle femoral insertions to the posterior margin of the articular surface of the lateral condyle was 7.6 ± 1.5 mm and 7.0 ± 1.4 mm, respectively. The long axis of insertion of the AM and PL bundles was 11.3 ± 1.6 mm and 11.0 ± 1.7 mm, respectively. The short axis of insertion was 7.5 ± 1.3 mm for the AM bundle and 7.6 ± 1.0 mm for the PL. The footprint area of the AM and PL bundles was 66.9 ± 2.3 mm² and 66.4 ± 2.3 mm², respectively [31].

They observed the lateral radiographs of the femoral condyles and found the center of the AM bundle to be, on average, 31.9 % from the posterior margin in the anteroposterior direction and 26.9 % from the roof in the proximal to distal direction, whereas that of the PL bundle was located, on average, at 39.8 % from the posterior margin and 53.2 % from the roof [31].

In the study of Edwards et al., the ACL attachment was 14.0 ± 2.0 mm long by 7.0 ± 1.0 mm wide. In all dissected pieces, the AM bundle extended to the posterior-proximal limit of the femoral notch, blending with the periosteum of the femoral shaft. The width of the AM bundle attachment was 7.6 ± 1.5 mm, and the center of the AM bundle was 4.3 ± 1.1 mm from the posterior edge of the notch, both measurements parallel to the femoral axis. The diameter of the AM bundle femoral insertion parallel to the femoral roof (Blumensaat’s line) was 7.0 ± 1.6 mm, and the distance between the center of the AM bundle and the posterior outlet was 4.6 ± 1.2 mm [29].

The width of the PL bundle attachment was 6.2 ± 2.3 mm, and the center of the PL bundle was 8.9 ± 2.1 mm from the posterior edge of the notch, both measurements parallel to the femoral axis. The diameter of the PL bundle femoral insertion parallel to the femoral roof (Blumensaat’s line) was 5.5 ± 3.1 mm, and the distance between the center of the AM bundle and the posterior outlet was 7.3 ± 1.8 mm [29].

According to Siebold et al., the femoral ACL insertion area was 83 ± 19 mm², with a mean width of 8 ± 2 mm and a mean length of 15 ± 3 mm. They found the mean insertion area in men was significantly larger (98 ± 22 mm²) than in women (76 ± 13 mm²). The mean femoral insertion area of the AM bundle was 44 ± 13 mm² (52 % of the femoral insertion), with a mean width and length of 7.2 ± 1.5 mm and 7.1 ± 1.5 mm, respectively. The mean femoral insertion area of the PL bundle was 40 ± 11 mm² (48 % of the femoral insertion), with a mean width and length of 7.0 ± 1.0 mm and 7.0 ± 2.0 mm, respectively. They found the mean femoral ACL insertion area of right knees was significantly smaller compared with left knees [23].

Kai et al. [35] pointed out that the AM bundle was not attached on a flat aspect of the femur but on a cylindrical surface of the femoral intercondylar notch around the proximal outlet. Therefore, they suggested that three-dimensional clock system is needed to measure the center of the femoral attachment of the AM midsubstance fibers. They showed that the averaged center of the direct attachment of the AM bundle midsubstance fibers was located on the cylindrical surface of the femoral intercondylar notch at 10:37 (or 01:23) o’clock orientation in the distal view and at 5.0 mm from the proximal outlet of the intercondylar notch in the lateral view [35].

Kawaguchi et al. found the length of the femoral ACL attachment was 17.9 ± 2.0 mm. The width of the femoral attachment of the anterior fanlike extension was 4.3 ± 0.9 mm, the width of the central direct attachment was 8.5 ± 1.1 mm, and the width of the posterior fanlike extension was 5.7 ± 1.6 mm [30].

Iwahashi et al. evaluated the position and area of direct insertion of the ACL and found that the footprint length was 17.4 ± 0.9 mm and its width was 8.0 ± 0.5 mm. The ACL insertion area was 128.3 ± 10.5 mm².

Recently Śmigielski et al. found that the midsubstance portion of the ACL has a ribbonlike structure from its femoral insertion, and the ligament fibers are in continuity with the posterior femoral cortex. They could find no clear separation into two bundles [10].

Hutchinson and Ash described a distinctive change in the slope of the femoral notch roof that occurs just anterior to the femoral attachment of the ACL. They named it as the “resident’s ridge” [36]. A different osseous landmark also called resident’s ridge was described by William Clancy Jr. (direct communication), and it is a thick ridge in the medial wall of the lateral femoral condyle that runs through the entire ACL footprint, reaching the articular cartilage, with no ACL attached anterior to this ridge.

Ferretti et al. performed a 3D assessment and arthroscopic study of the ACL femoral attachment. They studied the “resident’s ridge” first described by Clancy. However, to avoid discordance, they called it the lateral intercondylar ridge [21] (Fig. 16.3).

Farrow D. et al. studied the morphology of the intercondylar femoral notch and found the lateral intercondylar ridge in 194 of the 200 specimens [37]. Tsukada et al. [38] observed a great degree of positional and dimensional variation in the lateral intercondylar ridge, specifically concerning the distal part and slightly differences between men and women. They found that although the proximal part of the lateral intercondylar ridge (LIR) almost corresponded to the anterior margin of the ACL attachment, the anterior margin of the ACL attachment was commonly located anterior to the middle and distal parts of the LIR, having the greatest margin-ridge distance averaged 4.2 mm [38].

There is another ridge or a change of slope of the lateral femoral condyle that separates the AM and PL bundle insertions. This ridge, described as lateral bifurcate ridge, supports the concept that the ACL has two bundles, each one with distinct attachments. This is an important landmark to guide knee surgeons during the anatomic reconstruction of the ACL [21].

To facilitate the understanding, the authors prefer to name lateral intercondylar ridge to describe the anterior limit of the ACL femoral insertion and the lateral bifurcate ridge referring to the slope modification between the AM and PL bundles.