Double-bundle ACL reconstruction is based on anatomy

Correct tunnel placement is imperative for optimal outcomes.

Issue: March 2008

ByFreddie H. Fu, MD, DSc(Hon), DPs(Hon)

Freddie H. Fu, MD, DSc (Hon), DPs (Hon)
Freddie H. Fu

The principles of anatomic ACL reconstruction are to restore the native anatomy, insertion sites, and tension patterns. Utilizing these principles allows an individualized approach to ACL reconstruction. Anatomic ACL reconstruction is a concept, rather than just a technique. It can be applied to single-bundle reconstructions, double-bundle reconstructions, anteromedial (AM) or posterolateral (PL) bundle augmentations for partial ACL injuries, and revision ACL surgery in order to restore the native ACL anatomy.

Dr. Jack Hughston taught us that “Whenever you are having your anatomy sessions, pay particular attention because orthopedics is all about anatomy ... plus a little bit of common sense.” The ACL is composed of two functional bundles; the AM (anteromedial) bundle primarily controls anterior-posterior stability and the PL (posterolateral) bundle primarily controls rotation. These need to be restored to reproduce the native knee anatomy and function.

Studies have shown that ACL tibial and femoral insertion sites can vary from 9 mm to 21 mm in length. To always perform the same size and type of ACL reconstruction does not make sense, and a more individualized approach is needed.

There are distinct tensioning patterns and a dynamic motion unique to each ACL bundle. The PL bundle is taut in extension and loose in flexion, while the AM bundle is relatively taut throughout the knee motion, reaching a maximum between 45° and 60°. These tensioning patterns are necessary for normal ACL function.

By applying the principles of double-bundle surgery, the surgeon is better equipped to perform an anatomical ACL reconstruction regardless of technique. If single-bundle surgery is desired, an anatomical approach to restoring the correct anatomy and insertion sites can be performed based on double-bundle concepts.

Anatomy and basic science

Anatomic analysis of the ACL in fetal, arthroscopic, and cadaveric studies demonstrates that there are two distinct functional bundles with unique sizes and insertion sites. Recently, bony ridges have been identified as guides to the anatomic insertion sites for the ACL and its bundles. Dr. William Clancy referred to one of them as the “Resident’s ridge,” also known as the lateral intercondylar ridge. There is also a lateral bifurcate ridge that lies between the AM and PL femoral insertion sites.

It is important to note that the orientation of the femoral insertion sites change with knee flexion. In full extension, the femoral AM and PL insertion sites are oriented vertically but become more horizontal as the knee approaches 90° of flexion, with the PL insertion site lying more distal relative to the AM insertion. The “Figure 4” position is best to visualize the PL bundle, as this position stresses the PL bundle and moves its insertion site forward.

Robotic studies have shown that the two bundles of the ACL have different functions. The AM bundle is the major restrictor of anterior-posterior knee displacement, while the PL bundle primarily controls the rotational stability. In vivo kinematic studies using stereoradiographic techniques have shown that traditional single-bundle ACL reconstructions may restore anterior-posterior knee stability but do not successfully restore rotational stability. Furthermore, studies have shown that an inability to sufficiently restore rotational stability leads to increased loads/stresses on the cartilage of the medial femoral condyle that result in osteoathritic changes in the long term.

An analogy to ACL surgery is the hinge on a door. A door with one hinge will last some time, but will wear out quicker than if mounted with two or three hinges. The same idea can be applied to ACL surgery. Although, unlike a door that cannot modify its activity, a patient can change his or her level of activity to potentially live with a suboptimal ACL reconstruction.

Anatomic tunnel placement

There are often remnants of each ACL bundle after acute injury that should be evaluated to guide the surgeon to the anatomic location of each bundle. Therefore, it is important to pay attention to the rupture pattern of the bundles and identify the native insertion sites.

Also, determining the “ideal position” of the tunnels it is very important to understand the dynamic motion of the femoral insertion site and each bundle as the knee is carried through a range of motion, regardless of whether a single-bundle or double-bundle reconstruction is performed. Different surgeons find and drill their tunnels at different knee flexion angles, which change the bundle orientation and insertional-site anatomy. It is also important to look at the lateral femoral condylar notch through the medial portal to better visualize the soft tissue and bony ridge anatomy that define the insertion sites of the AM and PL bundles on the femur. A notchplasty is not advocated because it removes valuable landmarks and prevents an anatomic approach to ACL reconstruction.

The last thing that dictates tunnel location is the femoral and tibial insertion site size. If both are larger than 12 mm, a double-bundle reconstruction can be performed, with each bundle’s tunnel being placed in its anatomic location. If either insertion-site is less than 12 mm, an anatomic single-bundle reconstruction should be performed by placing the tunnel in the center of the AM and PL native insertion sites.

The native ACL does not impinge. Nonanatomic reconstruction is result of the fear of graft impingement and the trend to utilize a transtibial tunnel drilling technique. To avoid impingement surgeons have had to perform a notchplasty and move the tibial tunnel site more posteriorly in the PL bundle location. Studies have shown that transtibial techniques do not allow an anatomic reconstruction of the femoral insertion site, usually leading to a higher AM tunnel. Therefore, instead of connecting the tibial AM bundle to the femoral AM bundle and the tibial PL bundle to femoral PL bundle, the tunnels have been mismatched by connecting the tibial PL to a high femoral AM position. To avoid these problems, the soft tissue, bony landmarks, and insertional site anatomy of the ACL has been better defined, and it should be applied by surgeons to place the femoral tunnel in a more anatomic position regardless of which technique they choose.

Outcome of ACL surgery

Traditional single-bundle ACL reconstructions have been considered the gold standard. However, long-term follow-up data have shown problems with arthritic changes and residual instability with conventional single-bundle reconstruction techniques. Recently, there have been three level I and three level II prospective randomized studies regarding the outcomes of double-bundle ACL reconstructions reporting favorable clinical outcomes compared to traditional single-bundle ACL reconstructions at mid-term follow-up. Current research endeavors are awaiting future longitudinal studies to report the long-term results of double-bundle reconstructions and fully define the benefits of the double-bundle technique.

Learning curve

As with any surgery there will be a learning curve. The curve associated with double-bundle techniques is fairly steep, but a great way to lessen the risks associated with that curve is to start with an anatomic reconstruction. The concepts behind ACL surgery are evolving and just because someone does 10 to 15 ACL reconstructions a year, it does not mean they should not do it anatomically. Orthopedics is a field based on anatomy and restoring it. Whether fracture fixation or ACL reconstruction, the same rule applies. Although technically demanding, orthopedic surgeons should be able to learn the concepts of anatomical double-bundle reconstruction and benefit from it. After three decades of arthroscopic ACL surgery, it is time for us to revisit the anatomy of the ACL and try to restore it.

Whether someone switches to doing double-bundle ACL reconstructions depends on the individual surgeon and their dedication to learning new techniques and principles. With the correct training and application of the double-bundle concept, all orthopedic surgeons should be able to stay current and provide the best care for their patients, whether it be with anatomic single-bundle or double-bundle ACL surgery.

Tips for beginners

First: Utilize the three-portal technique for all arthroscopic cases to view the intercondylar notch and femoral ACL insertion site. The lateral parapatellar portal alone does not allow adequate visualization of the ACL to anatomically reconstruct the two bundles. By viewing through the medial portal, a notchplasty is not needed because of the ability to see the entire ACL femoral insertion site from the anteromedial portal.

Second: Pay attention to the normal ACL anatomy whenever possible. With a Figure 4 position, the PL bundle is visualized in its entirety.

Third: Evaluating the acute injury of the ACL is a great way to see the distinction between the AM and PL bundles during arthroscopy. In chronic ACL deficient cases, it can be difficult to distinguish the two different bundles as they have been scarred over time.

Finally, the operating surgeon should practice marking and measuring the ACL and bundle insertion sites to begin understanding the double-bundle anatomy. With time it will become clear that insertion-site sizes and locations vary, adding validity to the concept of individualizing each patient’s ACL reconstruction. Again, the double-bundle concept is based on anatomic reconstruction principles and should be applied regardless of whether a double- or single-bundle reconstruction is being performed.

For more information:

Freddie H. Fu, MD, DSc(Hon), DPs(Hon), can be reached at the University of Pittsburgh, Department of Orthopedic Surgery, 3471 Fifth Ave., Suite 1011, Pittsburgh, PA 15213; 412-687-3900; e-mail: ffu@msx.upmc.edu. He has no direct financial interest in any companies or products mentioned in this article.

Reference:

Fu FH. Why double-bundle ACL reconstruction. Presented at Orthopedics Today Hawaii 2008. January 13-16. Lahaina, Maui, Hawaii.