Anatomic Double-Bundle ACL Reconstruction

Patient Information Handout / Frequently Asked Questions

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

Kenneth R. Morse, MD

Verena M. Schreiber, MD

Kenneth D. Illingworth, MD

University of Pittsburgh, Department of Orthopaedic Surgery

Updated: 07/27/2009

What is the ACL and what does it do?

§ The anterior cruciate ligament (ACL) connects the femur bone to the tibia bone in the center of the knee joint. (Ligaments connect bones to bones)

§ When athletes “blow out” their knees – this is the ligament that is commonly torn.

§ The ACL is important during daily activities but absolutely critical to the stability of the knee during sports.

What is the native anatomy of the ACL?

§ The ACL is made up of two functional bundles of tissue, the anteromedial (AM) and posterolateral (PL) bundles. These bundles are first seen during fetal development and persist throughout life. [1]

§ The AM bundle of the ACL primarily controls anterior (forward) movement of the tibia underneath the femur, and the PL bundle controls rotational stability of the knee, such as in pivoting, twisting, running, and jumping. [9,10]

§ In other words, each bundle has a different function, and this is reflected in the anatomy. When the knee is straight the AM and PL bundles are parallel. As the knee is flexed, the two bundles cross each other:

§ Here is a closer look at the femoral attachment sites – we can see the upper margin of the ACL attachment site (intercondylar ridge) and the bifurcate ridge (probe on ridge) which separates the insertion sites of both the AM and PL bundles.

§ Interestingly – different animals have different numbers of ACL bundles – likely a reflection of Darwinian selection.

§ This is seen in both the bone and soft tissues. Above is a comparison of the human bony ridge (left) for the ACL insertion, and on the right is the bony ridge of the ACL in monkeys – reflecting different ACL bundles.

§ Below is a goat with 3 bundles on the left, and a rhesus monkey also with 3 bundles on the right.

Goat Monkey

· Here are some normal human knees showing both the AM and PL bundles of the ACL - on MRI scan and during arthroscopy:

Are ACL tears common?

§ ACL tears are very common. Over 200,000 ACL tears occur each year in the United States. The highest occurrence (incidence) is in individuals between 15 to 25 years of age, who participate in pivoting sports (like soccer and football). However, ACL tears can occur at all ages and in all sporting activities. [18]

How is an ACL tear diagnosed?

§ Tear of the ACL can be diagnosed by a history of trauma to the knee (contact or non-contact) and physical examination. MRI scan can confirm the diagnosis: