Chapter 46 Double-Bundle Anterior Cruciate Ligament Reconstruction

Kenneth R. Morse, Verena M. Schreiber, Freddie H. Fu

Single-bundle anterior cruciate ligament (ACL) reconstruction has largely been a successful surgery over the last several decades, with satisfactory subjective patient outcomes in the short term.⁵ However, more recent long-term studies have found significant sequelae of ACL injury and its reconstruction with the development of early arthritic changes in a large percentage of patients. Pinczewski and colleagues¹⁹ have reported arthritic changes in 39% of patients who underwent patellar tendon autograft ACL reconstruction at 10 years postoperatively. Additionally, others have reported a subset of patients, between 30% and 40%, who have persistent instability or are unable to return to their previous level of activity.² Although these results compare favorably with the nonoperative treatment of ACL injuries, potential areas for improvement obviously remain.

In an attempt to improve these clinical results, surgeons have revisited a technique that once received little interest, the double-bundle ACL reconstruction. With the primary goals of recreating the two functional bundles of the ACL and individualizing each surgical procedure, the double-bundle reconstruction was first described in the 1980s. Over the last decade, the technique has gained in popularity as a better understanding of the normal anatomy and biomechanics of the ACL has been achieved.

This chapter will explore the native anatomy and biomechanics of the anterior cruciate ligament and the double-bundle reconstruction technique.

Anatomy and Biomechanics

The anatomy of the ACL has been studied for many years, with the assumption that a better understanding of the anatomy would lead to a better reconstructive procedure. In general terms, the ACL originates on the medial aspect of the lateral femoral condyle and runs obliquely to its insertion at the medial tibial eminence. It is narrowest in the midsubstance, with an average cross-sectional area of 36 mm² in females and 44 mm² in males.¹ It then fans out to a broad insertion at both ends.¹⁸ The length of the ACL is depends on which fibers are measured; it can range from 22 to 41 mm.¹⁷

It is widely accepted that the ACL is composed of two functional bundles. This was first described in the 1930s and has since been confirmed by others.²⁷ These two bundles, anteromedial (AM) and posterolateral (PL), are named for their tibial insertion and have distinct functional properties (Fig. 46-1). Recent literature has shown that in response to an anteriorly directed tibial load, the in situ force in the PL bundle is highest at full extension and decreases with flexion, whereas the AM bundle is lower in full extension and increases to maximum at 60 degrees of flexion.⁷ Additionally, the PL bundle also resists anterior translation when the knee is subjected to a combined rotatory load (simulated pivot shift) when near full extension.¹⁸

Figure 46-1 Cadaveric specimen showing the AM and PL bundles of the ACL.

Although researchers continue to investigate the anatomy and function of the ligament itself, others have revisited the insertional anatomy of the ACL. The last several years have provided a much greater detailed understanding of both the femoral and tibial insertions. On the femoral side, the overall shape is that of a segment of circle, with its anterior border straight and its posterior border convex. Ferretti and associates ⁴ have studied the area of the femoral insertion site and found it to average 196.8 mm². Individually, the AM bundle averaged 120 mm² and the PL bundle averaged 76.8 mm²; however, there may be some ethnic variation. To help identify each bundle, there are distinct bony landmarks that signify the anatomic extents of each bundle as well as the ACL as a whole. The lateral intercondylar ridge (resident’s ridge) is the anterior border of the ACL. This bony landmark runs from proximal to distal with the knee straight, and no cruciate fibers insert anterior to this point. The lateral bifurcate ridge, which is subtle but often present, separates the anterior portion of the AM bundle from the PL bundle. Of importance is the understanding that these two insertion sites are vertical with the knee extended and horizontal with the knee in flexion. Therefore, the two bundles are parallel with the knee in extension but crossed with flexion.¹⁰ This explains the biomechanical features of the two bundles and aides in reconstruction.

On the tibial side, the insertion varies in shape in size.^13,²⁴ Medially, the ACL inserts up to a bony ridge that is an anterior extension of the medial intercondylar tubercle.²² Posteriorly, no fibers extend beyond a ridge of bone at the anterior aspect of the tibial spine between the medial and lateral tibial intercondylar tubercles. The portion of the ACL, the PL bundle, lies just anterior to the insertion of the posterior horn of the lateral meniscus. The anterior and lateral borders are less well defined and consist of diffuse expansions. Similarly, there is no bony landmark that separates the two bundles on the tibial side.

Understanding the biomechanical function and insertional anatomy of the ACL highlights the importance of each bundle. This is critical to the correct application of the double-bundle technique.