Surgical Methods

1. Over-The-Top Positioning

        Over-The-Top is a classic ACL positioning method, first proposed by Macintosh et al. in 1974. Initially, the Over-The-Top technique was mainly used for primary ACL reconstruction, where the reconstructed graft passed through the medial border of the lateral femoral condyle and was fixed to the distal femoral shaft. In recent years, this positioning method has been primarily used for patients with immature skeletal development or revision patients with failed femoral tunnels. The Over-The-Top point is located at the intersection of the lateral wall of the femoral intercondylar notch and the posterior aspect of the intercondylar notch roof.

Indications for ACL reconstruction using the Over-The-Top positioning method include:
① When femoral bone loss during revision surgery prevents further use of femoral tunnel techniques for graft positioning or fixation;
② When the posterior wall of the femoral tunnel has burst, preventing graft fixation;
③ When other fixation techniques are needed to secure soft tissue grafts to the lateral femoral diaphyseal-epiphyseal junction;
④ For patients with open growth plates;
⑤ When extra-articular repair is required during ACL reconstruction.

    Nagai et al. performed primary Over-The-Top ACL reconstruction on 14 patients with immature skeletons and Over-The-Top ACL revision surgery on 21 patients with mature skeletons. After an average follow-up of 2.2 years, both groups achieved good clinical outcomes. However, the ACL graft in the Over-The-Top position is non-anatomical, which can lead to increased graft stress and ultimately graft failure. Therefore, the value of the Over-The-Top positioning method in ACL reconstruction surgery still requires further research.

2. Clock Face Positioning Method

    Since the introduction of arthroscopic ACL reconstruction in the 1980s, the clock face positioning method has been widely used for femoral tunnel positioning. In 2003, Loh et al. conducted biomechanical experiments on 10 human cadaver specimens and concluded that positioning the femoral tunnel at the 1 o'clock position for the left knee and the 10 o'clock position for the right knee could provide better rotational stability for the knee joint. This technique requires a clear view of the knee's intercondylar notch and uses a "simulated clock face" to measure the ACL femoral tunnel position. In the clock face positioning method, with the knee flexed at 90°, the top of the femoral intercondylar notch corresponds to the 12 o'clock position, with the tunnel position for the left knee at 1-2 o'clock and for the right knee at 10-11 o'clock.

    Although the clock face positioning method is simple to operate, it is no longer suitable. Firstly, the femoral tunnel established using this method is non-anatomical because the knee's intercondylar notch is a three-dimensional structure, while the clock face is two-dimensional, considering only coronal plane positioning while neglecting sagittal plane positioning. During knee flexion, both the apex and depth of the intercondylar notch change, leading to changes in the clock position. Secondly, due to differences in individual habits and experience among surgeons, using the clock face positioning method may result in significant deviations in femoral tunnel placement.

3.Lateral Femoral Condyle Bony Landmark Positioning Method

    Resident's Ridge and Bifurcate Ridge: The femoral attachment of the ACL is located in the posterior 1/3 of the lateral wall of the femoral intercondylar notch, forming an elongated oval shape. The resident's ridge is located at the junction of the middle and posterior 1/3 of the line connecting the top of the intercondylar notch and the posteroinferior edge of the lateral wall of the intercondylar notch, adjacent to the ACL femoral footprint area. Studies report that the resident's ridge is present in almost 100% of cases, making it a stable bony reference landmark for intraoperative anatomic ACL reconstruction femoral tunnel positioning. The lateral femoral condyle bifurcate ridge is a bony elevation between the anteromedial and posterolateral bundles of the ACL, separating the two bundles. The midpoint between the centers of the two bundle remnants can serve as the femoral tunnel position for anatomic single-bundle ACL reconstruction, while the centers of the two bundle remnants can serve as femoral tunnel positions for anatomic double-bundle ACL reconstruction.

    ADC (Apex of Deep Cartilage): From an anatomical perspective, the ADC represents the proximal and anterior corner of the articular cartilage edge on the medial aspect of the lateral femoral condyle. The ADC corresponds to the deep and high angle of the cartilage under arthroscopy. Hart et al. conducted an anatomical study on 8 fresh knee joint specimens and found that the center of the ACL femoral footprint was consistently higher and shallower than the ADC, with median high and shallow values of 3 mm (1-4 mm) and 12 mm (11-17 mm), respectively. Shi et al. analyzed postoperative 3D CT scans of 134 patients who underwent ACL femoral tunnel positioning using the ADC as an anatomical reference landmark and concluded that the ADC is a good marker for femoral tunnel positioning in ACL reconstruction. They also described a method for positioning the femoral tunnel using the ADC: first identify and mark the center of the anteromedial bundle remnant, then with the knee flexed at 120°, position the auxiliary locator on the ADC to accurately locate the femoral tunnel.

    Both of these methods for femoral tunnel positioning based on bony landmarks can accurately achieve anatomic ACL reconstruction. However, unstable bony landmarks will lead to poor tunnel positioning, ultimately resulting in poor postoperative knee joint function. Previous studies considered the lateral femoral intercondylar ridge to be a relatively constant bony positioning landmark, with an occurrence rate of about 100%. However, Tsukada et al.'s anatomical study of 318 femoral specimens found that the lateral femoral intercondylar ridge was absent in 6% of specimens, and the lateral femoral condyle bifurcate ridge was not found in about 50% of specimens. The ADC, being the apex of the boundary between bone and cartilage, always exists and is easily identifiable under arthroscopy, making it currently considered the best bony positioning landmark.

    However, the bony landmark positioning method also has some disadvantages. Firstly, in the process of surgically exposing the bony landmarks, the ACL femoral remnant needs to be cleaned thoroughly to facilitate observation and subsequent tunnel positioning. This, however, eliminates the benefits of preserving the ACL remnant, which plays a role in sealing the tunnel, avoiding a certain degree of tunnel widening, preserving proprioception, and promoting graft revascularization. Secondly, for patients with chronic ACL injuries, long-term knee instability can lead to severe lateral femoral condyle hyperplasia, making it difficult to observe the bony landmarks clearly.

4. Computer-Assisted Navigation Positioning Technology

    In 1995, Dessenne et al. first used computer-assisted navigation for arthroscopic ACL reconstruction, improving the accuracy of intraoperative tunnel positioning. A systematic review by Cheng et al. concluded that compared to traditional surgery, the computer-navigated ACL group had a lower postoperative graft impingement rate and better accuracy and consistency in femoral tunnel positioning, although there was no significant difference in tibial tunnel positioning. Computer-assisted navigation for tunnel positioning eliminates the need to identify bony positioning landmarks intraoperatively and avoids human factor interference, thus achieving highly accurate tunnel positioning, especially for femoral tunnel positioning. However, computer-assisted navigation also has its drawbacks, including increased surgical costs and the invasive nature of the procedure, which can increase periarticular injury and prolong surgery time.

5. 3D Printing Technology-Assisted Positioning

    Currently, the application of 3D printing technology in ACL reconstruction is still in the exploratory stage, mainly including 3D-printed personalized navigation templates and 3D-printed anatomical models. The former allows for more anatomically accurate tunnel placement, shortens surgery time, and makes ACL reconstruction more personalized and precise. The latter enables thorough preoperative planning and simulation, resulting in accurate and reproducible tunnel positioning, while reducing subjective errors by surgeons, shortening surgery time, and flattening the learning curve for surgeons.