QUESTION | MRI REPORTS ARE OFTEN 2 PAGES LONG. WHAT ARE THE KEY FEATURES YOU LOOK FOR ON AN MRI SCAN OF A PATIENT YOU SUSPECT HAS AN ACL INJURY?
ANSWER | As with all injuries of the knee, a careful clinical examination
is required after taking a detailed history. The history and clinical
examination will allow you to make an accurate diagnosis over 90% of the
Plain Radiographs: Imaging is then performed to confirm your diagnosis and a standard
set of x-rays is obtained. These are:
- Weight bearing AP (looking at alignment and for arthritis),
- Lateral (looking at patella height and tibial translation)
- Notch view (looking at the flexion portion of the condyles)
- Skyline view (looking at the patella).
| Weight Bearing AP
|| Lateral View
|| Notch View
|| Skyline Patella View
In most instances the plain x-rays will be normal but there are
some changes worth looking for:
- A Segond fracture (described by Dr Paul Segond in about 18791) where a fleck of bone is seen adjacent to the tibia. This almost certainly indicates an avulsion near the area of insertion of the anterolateral ligament and in most studies is pathognomonic of an ACL rupture.
- The deep sulcus (terminalis) sign, which is seen as a depression on the lateral femoral condyle at the terminal sulcus. This is the junction between the weight bearing tibial articular surface and the patellar articular surface of the femoral condyle. If you have a bony injury in this area (rather than just a bone bruise) it tends to indicate a higher energy of injury to the knee and the longer-term outcomes tend to be worse when this is present.
- Anterior tibial subluxation relative to the femur (often best seen on MRI) but this is usually a late sign and may indicate failure of an ACL reconstruction or stretching out of the capsule (not usually seen with an acute injury).
|| Segond Fracture
|| Sulcus Sign
|If you suspect there may be varus alignment of the limb then 3ft standing xrays are needed because a varus knee will tend to stretch an ACL graft over time and cause it to fail. Some patients have their weight bearing axis medial to the tibial spines or have an alteration in the slope of the tibia (Decreased slope is helpful in the ACL deficient knee). In these situations, the ACL reconstruction might need to be combined with a high tibial osteotomy, with or without a slope correction.
The x-rays will also show arthritic changes or something unexpected like a tumour
x-ray of leg.
Black line shows
centre of knee.
MRI Scanning: Magnetic resonance imaging (MRI) is a scan which uses a magnetic
field to take pictures of the inside the body. It is especially helpful to
collect pictures of soft tissue such as organs and muscles that don’t show up
One way to think of an MRI scan is that of a water imaging device.
Normal x-rays image calcium, so they are good to see bones but not good for
imaging soft tissues.
At a simplistic level, the magnet lines up all the water molecules
and then lets them go. As they go back to their more natural state they release
energy which is read by a sensor and then a mathematical equation (Fourier
Transformation) is used to convert this to shades of grey to be seen as images.
An example of this is a bone bruise. There should be a very limited amount of water
within bone. When the ACL tears and the knee dislocates the 2 bones hit against
each other and create damage to the bone with some bleeding internally. This
‘micro-fracture’ of the bone indicates an increased severity of injury.
The investigation of choice is, of course an
MRI scan, with most of the pathology being seen on T2 weighted images. The primary role of the MRI
is to confirm the diagnosis or exclude a tear in patients with equivocal
physical examination findings. Be aware that not all MRI scans are the same
with quality and the reporting of the images varying significantly. That is why
it is useful to be able to interpret the images yourself.
The MRI cannot replace careful clinical examination looking for
tears of the lateral collateral ligament (LCL), posterior cruciate ligament
(PCL), and the menisci. Patients with combined LCL/ACL or PCL/ACL injuries
usually need surgery within 3 weeks because an unoperated LCL tear makes early
graft failure more likely.
The majority of ACL tears are mid-substance with only 10% being
femoral or tibial avulsions. On most MRI scans the tibial ACL insertion will be
better visualised than the femoral side so if the rest of the ligament looks
normal and the clinical examination does not suggest instability then the
ligament is probably not damaged even if the femoral insertion is not perfectly
visualised. While inspecting the ACL itself is important there are also
secondary signs which correlate well the presence of an ACL injury.
It is ideal to have a sequence that you follow each time when
looking at a knee MRI scan.
Start laterally in the knee and move medially on the T2 weighted
images. The first thing we usually see is bone bruising on the femoral condyle
(although this may not be present in all injuries and on recurrent instability
episodes - about 70% of people with an ACL injury have a bone bruise). This
bruising is due to the joint subluxation that happens and is seen (1) in the
lateral femoral condyle overlying the anterior horn of the lateral meniscus and
(2) on the posterior aspect of the tibial plateau. The femoral condyle bone
bruise is seen at the anterior aspect of the lateral femoral condyle between
the femoral condyle and the trochlear (where an impaction fracture can take
A lateral meniscal tear is seen in up to 85% of patients with an
ACL injury but few of these need treatment clinically.
As you move through the notch you will see the ACL first and then
the PCL but if the MRI slices are too thick or the axis is not altered to 10-20
degrees of internal rotation the ACL might not be seen on an MRI scan. These
days this is rarely the case but this is another reason that clinical
correlation with the imaging findings is essential.
Normally, the ACL is a dark structure in the center of the knee.
It can be seen starting on the tibia and heading upwards and backwards towards
the tibia. It is usually a straight line with no sag (the ‘hammock sign’
showing sagging of the ACL usually means the ligament is functionally deficient
even if it seems to be attached to both the tibia and femur).
As we move more medial we start to see fluid in the joint, which
is consistent with bleeding from an ACL tear. It is also common for there to be
a tear to the posterior horn of the medial meniscus with an ACL injury.
Sagittal view - bone bruise ACL
| Sagittal View
|Coronal View: We then move to the coronal images. The normal ACL will be traced from the postero-superior and lateral origin to it’s antero-inferior and medial insertion. As we start to move more posterior we look for bone bruising and we start to see a stump of a torn ACL. Instead of seeing normal contour of the dark ACL substance, we start to see more a balled-up appearance. In this view we can also see the injury to the posterior horn of the medial meniscus.
Coronal View missing ACL
|Axial Views: Finally, we look at the axial views, although these are not as useful when looking at ACL tears. The ACL is an oval low intensity structure which becomes elliptical in the midsubstance. This is much harder to appreciate if the cuts are not in exactly the right plane. You can see if there is fluid present within the joint and you may be able to see the Segond fracture at the tibia on these images (depending on the slice thickness used).
|| Axial View - ACL
In situations where there has been a prior ACL reconstruction then
the tibial and femoral tunnels should initially be evaluated on plain xrays.
Ideally these are taken with the knee in full extension to accurately
The Tibial Tunnel is best seen on lateral view knee with the knee
in full extension. The anterior margin of the tibial tunnel should be behind a
line drawn along the roof of the femoral notch (Blumensatt’s line). This
ensures that the graft is not impinged on by the bone with the knee in full
extension. The center of the graft tunnel should be one-quarter to one-half the
distance from the anterior to the posterior tibial cortex but this is more
variable that the Blumensaat's line measurement. In general if the tibial
tunnel is too far forward it leads to graft impingement and if it is too far
back it leads to ongoing instability.
The Femoral tunnel origin should be posterior to a vertical line
drawn along the posterior cortex of the femur. In general, anterior femoral
tunnel placement results in excessive tension on the graft in flexion, which
restricts ROM, causing tension on the graft fixation site and eventual
stretching of the graft.
Posterolateral instability is the most commonly (unrecognized)
combined injury and can be seen in 10% to 15% of chronically ACL-deficient
knees. The MCL, posterior horn of the medial meniscus, and posterior capsule
provide secondary stability in the ACL-deficient knee and must be checked both
clinically and on MRI scans because there are no reliable xray signs of this
| Tibial Tunnel marked
|| Tibial Tunnel with plateau markings
| Femoral Tunnel origin
| Femoral and Tibial Tunnel
This is clearly not an overview to read all parts of the MRI but
ACL injuries are common and it is worth knowing some of the common and
important imaging signs to look for. In a young active person with these
imaging signs early referral for a surgical opinion is appropriate. Not all ACL
injuries require surgery but unfortunately many do to return the person to
their desired level of sport.
 Segond P. Recherches
cliniques et expérimentales sur les épanchements sanguins du genou par entorse.
Progres Med 1879
Dr Doron Sher