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: