Andrew D. Pearle, MD, David McAllister, MD, and Stephen M. Howell, MD
Authors’ Disclosure Statement: Dr. Pearle reports that he consults for and receives royalties from Biomet. Dr. McAllister reports that he consults for and receives royalties from Biomet. Dr. Howell reports that he consults for Biomet Sports Medicine.
Histologic analysis has further clarified the relationship between the femoral footprint and functional aspects of the native ACL. The femoral origin of the ACL has distinct direct and indirect insertions, as demonstrated by histology and 3-dimensional volume-rendered computed tomography.21 The direct insertion consists of dense collagen fibers anterior in the footprint that is attached to a bony depression immediately posterior to the lateral intercondylar ridge.19 Sasaki and colleagues22 found that these direct fibers extended a mean (SD) of 5.3 (1.1) mm posteriorly but did not continue to the posterior femoral articular cartilage. The indirect insertion consists of more posterior collagen fibers that extend to and blend into the articular cartilage of the posterior aspect of the lateral femoral condyle. Mean (SD) width of this membrane-like tissue, located between the direct insertion and the posterior femoral articular cartilage, was found by Sasaki and colleagues22 to be 4.4 (0.5) mm anteroposteriorly(Figure 2). This anterior band of ACL tissue with the direct insertion histologically corresponds to the fibers in the anterior, more isometric region of the femoral footprint. Conversely, the more posterior band of fibers with its indirect insertion histologically corresponds to the more anisometric region and is seen macroscopically as a fanlike projection extending to the posterior articular cartilage.7
The dense collagen fibers of the direct insertion and the more membrane-like indirect insertion regions of the femoral footprint of the native ACL suggest that these regions have different load-sharing characteristics. The direct fibers of the insertion form a firm, fixed attachment that allows for gradual load distribution into the subchondral bone. From a biomechanical point of view, this attachment is extremely important, a key ligament–bone link transmitting mechanical load to the joint.23 A recent kinematic analysis revealed that the indirect fibers in the posterior region of the footprint, adjacent to the posterior articular cartilage, contribute minimally to restraint of tibial translation and rotations during stability examination.24 This suggests it may be strategically wise to place a tunnel in the direct insertion region of the footprint—eccentrically anterior (high) in the footprint rather than in the centrum.
3. Isometric Considerations
Forty years ago, Artmann and Wirth25 reported that a nearly isometric region existed in the femur such that there is minimal elongation of the native ACL during knee motion. The biomechanical rationale for choosing an isometric region of an ACL graft is that it will maintain function throughout the range of flexion and extension. A nonisometric graft would be expected to slacken during a large portion of the flexion cycle and not restrain anterior translation of the tibia, or, if fixed at the wrong flexion angle, it could capture the knee and cause graft failure by excessive tension. These 2 theoretical undesirable effects from nonisometric graft placement are supported by many experimental and clinical studies demonstrating that nonisometric femoral tunnel placement at time of surgery can cause recurrent anterior laxity of the knee.26-28 Multiple studies have further clarified that the isometric characteristics of an ACL graft are largely determined by femoral positioning. The most isometric region of the femoral footprint is consistently shown to be localized eccentrically within the footprint, in a relatively narrow bandlike region that is proximal (deep) and anterior (along the lateral intercondylar ridge within the footprint)19,29,30 (Figure 3).
A large body of literature has demonstrated that a tunnel placed in the center of the femoral footprint is less isometric than a tunnel in the more anterior region.25,29,31,32 Indeed, the anterior position (high in the footprint) identified by Hefzy and colleagues29 demonstrated minimal anisometry with 1 to 4 mm of length change through the range of motion. In contrast, a central tunnel would be expected to demonstrate 5 to 7 mm of length change, whereas a lower graft (in the PL region of the footprint) would demonstrate about 1 cm of length change through the range of motion.31,32 As such, central grafts, or grafts placed in the PL portion of the femoral footprint, would be expected to see high tension or graft forces as the knee is flexed, or to lose tension completely if the graft is fixed at full extension.32
Importantly, Markolf and colleagues33 reported that the native ACL does not behave exactly in a so-called isometric fashion during the last 30° of extension. They showed that about 3 mm of retraction of a trial wire into the joint during the last 30° of extension (as measured with an isometer) is reasonable to achieve graft length changes approximating those of the intact ACL. Given this important caveat, a primary goal for ACLR is placement of the femoral tunnel within this isometric region so that the length change in the ACL graft is minimized to 3 mm from 30° to full flexion. In addition, results of a time-zero biomechanical study suggested better rotational control with anatomical femoral tunnel position than with an isometric femoral tunnel34 placed outside the femoral footprint. Therefore, maximizing isometry alone is not the goal; placing the graft in the most isometric region within the anatomical femoral footprint is desired. This isometric region in the footprint is in the histologic region that corresponds to the direct fibers. Again, this region is eccentrically located in the anterior (high) and proximal (deep) portion of the footprint.
