Anatomic Single- and Double-Bundle Anterior Cruciate Ligament (ACL)
Patient Information Hand-out & Post-operative Instructions
Freddie H. Fu, MD, DSc (Hon), DPs (Hon)
Robin West, MD
Volker Musahl, MD
Dharmesh Vyas, MD
University of Pittsburgh, Department
of Orthopaedic Surgery
is the anterior cruciate ligament (ACL)?
- The ACL is one of the major
ligaments in the knee that connects the thigh bone (femur) to the shin
- When athletes “blow out” their knees
– this is the ligament that is commonly torn.
- The injury is very common and its
treatment is a popular topic in medical literature.
- The ACL is important during daily
activities but is absolutely critical to the stability of the knee during
sports that require cutting and pivoting, such as soccer, football and
does the anatomy of the ACL look like?
the ACL is referred to as one
ligament, it consists of two functional bundles. These two bundles are
named for the place where they attach on the tibia. There is an
anteromedial (AM) bundle, which inserts more anterior (Towards the front)
and medial (towards the inside) of the tibia. The posterolateral (PL)
bundle inserts most posterior (towards the back) and lateral (towards the
outside) of the tibia.
- When the ACL is carefully dissected away, it is even
more clear to see where the AM and PL bundle attach to the femur and
tibia. Below you see the AM and PL bundle attachment on the tibia (left)
and femur (right).
- On the femur, there are two ridges that outline the insertion
of the ACL to the bone. There
is one ridge that borders the top of the ACL
(the lateral intercondylar ridge) and there is one ridge that forms the
border between the AM and PL bundles (the lateral bifurcate ridge). When
your ACL is torn off the
femur, these two ridges serve as a map to help us to find the location
where your ACL used to
does the ACL do?
- The ACL
provides stability to the knee, while also allowing for normal knee
movement. The AM bundle is tight when the knee is bent and provides
stability in the forward (anterior) direction. The PL bundle is loose when
the knee is bent, and allows for rotation of the knee.7,
the knee is straight the two bundles are parallel to each other, but when
the knee is bent the two bundles cross each other. Although the two
bundles have slightly different functions, the bundles do not work
independently, but rather they work together to keep the knee stable while
still allowing you to jump, run and play sports.
ACL tears common?
§ ACL tears
are very common. Over 200,000 ACL tears occur each year in the United States.
The highest occurrence is in individuals between 15 to 25 years of age who
participate in pivoting and cutting sports. However, ACL tears can occur at all
ages and in all sporting activities after either contact or non-contact
symptoms could I have when my ACL is torn?
Usually an ACL tear occurs during sporting
activities. They can be a sudden pivoting or cutting motion, or planting of the
foot while the rest of the body turns.
Patients frequently report “hearing a pop” and often have large swelling
and pain of located in the knee.
is an ACL tear diagnosed?
The physical examination in clinic is used to
make the diagnosis. This is done through a thorough history of your injury as
well as through a variety of physical exams which include anterior drawer test,
Lachman, pivot shift and KT-2000 measurements. Each of these tests aids in
determining the functional status of the ACL.
MRI scans are
used to image the ACL, confirm the
diagnosis and evaluate for other possible injuries, like meniscus tears. Below we
show an MRI of a normal ACL. We also take x-rays of the knee. You cannot
see the ACL on x-ray, but we do
this to make sure there is no problem with the bones, such as a fracture.
- Below, an MRI of a torn ACL is shown.
- We can
also use the MRI to measure the size of your ACL. Below is a picture of
someone with a 13 mm ACL on the left, person with 17 mm ACL in the center
and someone with a 21 mm ACL on the right.
- There is much variation in ACL insertion site lengths
(see graph below).
- An ACL insertion site greater than 18 mm allows for
double-bundle reconstruction. If the insertion site is less than 14 mm,
there is only space available for a single-bundle procedure. Between 14 –
18 mm, we can perform either double- or single-bundle reconstruction.
- We can also measure the angle between the ACL and the
femur and tibia (the inclination angle). On the left is a 43° angle,
center is a 47° angle and on the right is a 53° angle.
my ACL is torn, do I absolutely need surgery?
§ No. There
are some patients who are able to function without an intact ACL. These
patients modify their lifestyle by eliminating sporting activities that require
pivoting and cutting. However, sometimes
during everyday activities the ACL-deficient knee can buckle or “give way”
(subluxate) resulting in painful episodes with swelling.
there is a risk of damage to the menisci (the cartilage shock absorbers) and
articular cartilage (the slippery gliding surface on the ends of the bones)
with each subluxation event. This damage can lead to degenerative arthritis and
subsequent meniscus tears.4
§ Because of
these concerns a majority of active patients elect to undergo ACL surgery when
the ligament tears.
just tore my ACL—when will I be ready for surgery?
§ In general,
there are three criteria that must be met before the ACL can be surgically
Swelling in the knee must go down to
Range-of-motion (bending and straightening)
of the injured knee must be nearly equal to the uninjured knee
Good Quadriceps muscle strength must be
present. This means that while lying flat on your back you should be able to raise
your leg off the ground while holding it is straight. We call this a “straight
§ Usually it
takes a couple of weeks after injury before ACL reconstruction can be
§ The presence
of any associated injuries to the knee joint involving cartilage, meniscus, or
other ligaments may change the time-frame for surgery.
happens during surgery?
§ The goal of
reconstructing the ACL is to restore the native anatomy and function of the ACL.
anatomy is the primary principle in Orthopaedics, like in the treatment of
broken bones. Below is an example of a broken leg, along with a dislocated
ankle. After restoring the normal anatomy with a metal plate and screws, the
function of the leg and ankle joint are restored.
§ The second set
of pictures illustrates a normal ACL on the left and on the right a
reconstructed ACL. The anatomy and function are restored after the surgery.
does the ACL anatomy need to be restored?
historically standard technique for ACL reconstruction failed to restore
the native anatomy of the ACL5. It
involved some removal of portions of the bone from the femur and then the
new ACL was placed in a position different from the normal ACL