The anterior cruciate ligament (ACL) is one of the crucial structures maintaining knee joint stability, primarily functioning to limit anterior tibial translation and internal rotation. ACL injury is a common sports-related injury, with 100,000 to 200,000 cases occurring annually in the United States, showing an increasing trend. If not treated promptly, ACL injury can lead to joint instability, secondary meniscus damage, cartilage damage, and ultimately knee osteoarthritis. Currently, the most effective treatment for ACL injury is arthroscopic ACL reconstruction. The reconstruction outcome is influenced by factors such as tunnel position, graft type, graft fixation method, and postoperative rehabilitation, with tunnel position playing a primary role. A systematic review analyzing 3,657 failed ACL reconstruction cases found that the main reason for failure was ACL graft re-rupture (about 38%), followed by surgeon t    echnical errors (about 22%), with other reasons accounting for 40%. Among surgeon technical errors, incorrect femoral tunnel positioning was the primary cause (63%), while tibial tunnel positioning errors accounted for only 7%. Therefore, in ACL reconstruction surgery, femoral tunnel positioning is the key factor determining surgical success and a necessary condition for ensuring good postoperative knee joint function.

1. Isometric ACL Reconstruction

    Isometric reconstruction refers to maintaining constant length and tension of the graft between the femoral and tibial tunnels during knee flexion and extension after reconstruction. In 1911, researchers pointed out that some ACL fiber bundles remain tense during knee flexion and extension. Based on this theory, Artmann et al. proposed that the ACL is isometric during movement. They believed that there is an isometric zone in the ACL footprint area of the lateral femoral condyle, where ACL reconstruction can achieve isometric reconstruction (maintaining equal length throughout the full range of knee flexion and extension), providing good knee function and significantly reducing the risk of graft re-rupture. Biomechanical studies have shown that the isometric zone of the ACL on the lateral femoral condyle is approximately located in a band-like area 20-30 mm from the intersection of the trochlear groove and the intercondylar notch, with a size of about 2 mm.

2. Anatomic ACL Reconstruction

    The purpose of anatomic ACL reconstruction is to rebuild the original anatomical structure of the ACL and restore its normal physiological function. Compared to isometric ACL reconstruction, anatomic reconstruction better restores knee joint function and rotational stability. Numerous cadaveric anatomical studies have shown that the ACL is mainly divided into anteromedial and posterolateral bundles, separated by the lateral intercondylar ridge. The lateral intercondylar ridge serves as an anatomical landmark for anatomic double-bundle ACL reconstruction. The ACL femoral footprint is located below the resident's ridge on the lateral femoral condyle, which primarily serves as an anatomical landmark for anatomic single-bundle ACL reconstruction. The positioning point for anatomic single-bundle ACL reconstruction is the midpoint between the anteromedial and posterolateral bundles, while the positioning points for anatomic double-bundle ACL reconstruction are the centers of the anteromedial and posterolateral bundle footprints.

3. ACL Reconstruction Based on the Ribbon-like Theory

    In recent years, with ongoing research into ACL anatomical structure, some researchers have proposed the ACL Ribbon-like theory, suggesting that the ACL is flat and ribbon-like from its femoral attachment to the middle, with no clear boundary between the anteromedial and posterolateral bundles.

The ACL femoral footprint is distributed between the resident's ridge and the posterior edge of the lateral femoral condyle in a crescent shape, with its footprint continuous with the femoral posterior cortex. Using the double-bundle theory (ACL anteromedial and posterolateral bundles) for anatomic double-bundle ACL reconstruction involves multiple tunnels and complex techniques, increasing the probability of improper femoral tunnel positioning. In 2013, based on the Ribbon-like theory and the complexity and technical issues of anatomic double-bundle ACL reconstruction, Petersen et al. proposed a new oval femoral tunnel positioning method. The femoral tunnel is located within the ACL footprint on the lateral femoral condyle, with an average tunnel size of 9.3 mm × 7.2 mm. Wen et al., through a 2-year follow-up study of 108 patients with anatomic single-bundle ACL reconstruction (39 cases with oval femoral tunnels and 69 cases with round femoral tunnels), found that patients with oval femoral tunnels had better knee joint function and laxity recovery, as well as higher ACL graft maturity. Zhang et al. also believed that the oval femoral tunnel ACL reconstruction technique is superior to the traditional round tunnel technique in terms of knee joint function recovery and early postoperative graft maturity. However, long-term clinical effects have not yet been reported.

4. ACL Reconstruction Based on the I.D.E.A.L. Theory

    Building on the Ribbon-like theory, Pearl et al. comprehensively considered ACL isometry, anatomy, histology, biomechanics, and related clinical factors to propose the I.D.E.A.L. theory for femoral tunnel positioning. They argued that the ACL graft should be placed in a position that recreates the Isometric properties of the native ACL, covers the Direct fiber insertion area histologically, is Eccentrically located in the anterior (high) and proximal (deep) regions of the footprint, is within the Anatomical footprint, and replicates the Low tension flexion pattern of the native ACL throughout the range of flexion and extension. These key points are abbreviated as I.D.E.A.L.

    Since its proposal, the I.D.E.A.L. theory has been increasingly accepted and put into practice by sports medicine researchers. Shi Weili et al. used the apex of the deep cartilage (ADC) of the lateral femoral condyle as an anatomical reference landmark, with the knee in a "figure-4" position and flexed to 120°, to effectively position the I.D.E.A.L. femoral tunnel, with good clinical follow-up results. Su et al. applied ACL remnant preservation technique combined with the I.D.E.A.L. theory for femoral tunnel positioning, and through a study of at least 12 months follow-up on 31 patients, found good postoperative knee stability and clinical outcomes. Wei Jinan et al. compared the effects of I.D.E.A.L. positioning method and anatomical footprint method on knee joint function recovery after ACL reconstruction, concluding that both methods can restore knee joint function and stability, with no significant impact on early postoperative rehabilitation. Based on these studies, the I.D.E.A.L. theory has achieved satisfactory clinical results, but the exact position of the femoral tunnel guided by the I.D.E.A.L. theory is still unclear. Zhou Tianping et al., through comparative analysis on knee joint models, believed that the femoral tunnel supported by the I.D.E.A.L. theory is located in the posterior 1/3 region of the lateral intercondylar ridge and close to the anatomical attachment point of the ACL anteromedial bundle. However, whether the femoral attachment point of the ACL anteromedial bundle is the I.D.E.A.L. point has not been confirmed by relevant studies. The I.D.E.A.L. theory is a summary of previous clinical experience and a theoretically existing point, but there is currently no consensus on the specific I.D.E.A.L. point.

5. Near-Isometric Point ACL Reconstruction

    Tunnel positioning is the most important factor affecting functional recovery after ACL reconstruction, with reports indicating that about 47.6% of ACL revision patients have improper femoral tunnel positioning. Improper tunnel position will place the graft in a non-isometric state, potentially leading to abnormal stretching and relative movement between the graft and tunnel during knee movement, a phenomenon known as graft-tunnel motion (GTM). GTM was described in the late 1990s, with some scholars believing that GTM can affect the interface between the graft and bone tunnel by altering the local mechanical environment and impairing graft-tunnel healing. Other studies suggest that GTM is a manifestation of a "windshield wiper effect" and "bungee effect," increasing bone resorption activity around the bone tunnel and leading to tunnel widening. Therefore, in 2019, Wan et al., based on in-depth research on ACL reconstruction, proposed the concept of "ACL near-isometric reconstruction," suggesting that reconstructing the ACL graft in a near-isometric position would minimize GTM. The near-isometric point for ACL reconstruction is located on the resident's ridge, 2 mm posterior to the medial surface of the lateral femoral condyle. They believe that by using the near-isometric point theory in ACL reconstruction, GTM can be maintained within 2 mm after reconstruction, promoting tendon-bone healing of the graft and avoiding graft loosening or re-rupture, as well as bone tunnel widening. Currently, there is limited research on the near-isometric reconstruction theory in China. As a new interpretation of isometric reconstruction, it is expected that more clinical reports will validate its effectiveness in the future.