Introduction
There is nothing quite like nailing a perfect skyline knee projection.
This page looks at some issues and tips for achieving the goal of
perfect skyline knee radiography.
The Positioning Sponge
In pursuit of the perfect skyline knee projection, I investigated the
idea that the ideal tube angle might be more consistent if the knee
flexion was always the same. It would also be desirable in a trauma
situation if you could flex the knee slightly and rest it onto a
positioning sponge such that it was positioned perfectly for a
horizontal ray lateral using a 24 x 30cm (12 x 10 inch) cassette.
Moreover, it would be desirable to leave the sponge in situ for the
skyline view.
I had a large number of off-cut sponge pieces that I
cut down to different heights until I achieved what appeared to be the
perfect height. The final result is shown below.
Sponge can be cut with a hot wire or an electric carving knife. I made my own hot wire cutter. You can do a google
search for more information if you want to make your own cutter. If
you're not familiar with electrical safety issues, don't do it!
Alternatively, you can take your foam to a foam cutter for a professional looking result |
The "Research"
I
was attempting to find what the ideal X-ray tube angle was to produce a
perfect skyline knee image when using this positioning sponge. A
sample image is shown left.
I made these same measurements on
about 50 patients and found that, in adults, the positioning sponge
caused the knee to be flexed to about 30 degrees. Importantly, the
patella was angled at about 10-15 degrees to the horizontal. Using this
information, you could produce reliable skyline positioning.
Note that the sponge positions the knee in the middle of the cassette |
The Image
The skyline patellar image using positioning based on the measurements from the lateral knee is shown left.
Skyline knee image with 10 degree cephalic angle based on measurements from the horizontal ray lateral knee shown abov
|
Discussion
My understanding of the skyline patella position is that a minimum of
knee flexion is more likely to reveal abnormal patellar tracking. The
most frequent problem that I found with this technique was that there
was too little knee flexion. Too little flexion of the knee joint can
cause the tibial tuberosity or other anatomical structures to be
projected over the anatomy of interest.
These images were
produced with a CR system which allows for the easy measurement of
angles. It would be possible to measure the skyline angle from the
lateral view for all patients and you are likely to achieve very
consistent quality images. The practicality of this approach is
questionable.
Tips
The toes are in the way
It is good practice to remove the patient's footwear.
You have a number of choices if the patient's foot anatomy is superimposed over the patella.
- plantar flex the ankle joint like a ballerina "on point"
- increase knee joint flexion
- position the X-ray tube so that you are directing the X-ray beam along the medial or lateral side of the patient's foot
Underexposure
The skyline projection requires considerably more radiation exposure
than the AP/lateral. Also, the less flexion of the patient's knee, the
more exposure you will need. Furthermore, for reasons of practicality,
you could end up with a long FFD.
Comparison Views
As much as I dislike routine comparison views, there is justification
for imaging both patellae in the one exposure. This is particularly
useful when assessing subtle abnormal patellar tracking.
Supporting the cassette and patient
Sitting the patient up in a semi-sitting position is very uncomfortable. The
DARRIN
sponge is perfectly shaped to place behind the patient's back to
provide support. One disadvantage of this approach is that the patient
may receive a primary beam radiation dose to the orbits and thyroid.
My preferred technique is to keep the patient supine and use the
LEDDRA skyline cassette holder.
Radiographic Techniques
| There
are a variety of radiographic techniques for achieving a tangential
image of the patella. This table details some (but not all) of the
techniques. |
Is the Skyline Projection Necessary?
This
17 year old male presented to the Emergency Department following a fall
from his bike. He was examined and the requested radiography included
his left knee.
Is there any evidence of fracture or dislocation? |
 |
|
The lateral knee projection image demonstrates no fracture or dislocation.
Is further imaging required? |
|
|
| The
skyline projection image demonstrates a possible fracture of the
lateral aspect of the patella. Is this a fracture or a secondary
ossification centre? If you scroll up the page and review the AP image
you will notice that there is a faint impression of a lucency over the
supra-lateral aspect of the patella- this is the most common position to
find a secondary ossification centre. If this was a fracture, would it
have been missed without the skyline projection included in the routine
radiographic series? |
What Went Wrong?
Case 1
Fault
This is an example of incorrect centring. The central ray is directed
too inferiorly. The central ray should be directed to the
patello-femoral joint. Apart from aesthetic considerations and failure
to demonstrate the patello-femoral joint clearly, there has been
unnecessary irradiation of the femur and lower leg.
Correction
Centre to the patello-femoral joint
|
Case 2
Fault
One of the major objectives of the skyline view is to achieve an image
of the patella which is unobscured by the patient's toes. This image
demonstrates the importance of that objective!
Correction
Plantarflex the patient's foot or direct the beam medially or laterally to the patient's foot
|
Case 3
Fault
This image has a slightly vertically elongated look. This is likely to
be attributable to a cassette that is not at right angles to the
central ray. This can happen when the patient is holding the cassette
above his/her knees and lets it tilt forwards/backwards.
Correction
Position the cassette/IR such that it is a right-angles to the central ray
|
Case 4
Fault
This image is suffering form movement unsharpness.
Correction
This fault can be associated with technique. If you are using the
"semi-sitting" patient position, it is best to put something behind the
patient's back so they are not left "rocking" while they are supporting
the cassette. I found that having the patient supine and using a
cassette holder is a safer bet. See the page on the Leddra Skyline Cassette Holder.
Shorter exposure time will also help reduce movement unsharpness
|
Case 5
Fault
The soft tissue density (arrowed) that is arcing through the middle of
the image is likely to be the soft tissues of the lower leg.
Correction
Increase knee flexion
|
Case 6
Fault
If the aim of this view was to produce a skyline projection of the
patient's patella, you would have to declare this imaging endeavour a
failure
Correction
If the
patient appears not appear to have a patella... ask the obvious
question! Always check previous imaging when possible- you might
discover that the patient has had a previous patellectomy.
|
Case 7
 |  |
A skyline knee projection taken in this position, with the central ray
as shown above, will project the tibial tuberosity over the
patello-femoral joint.
Note- patient has Osgood-Schlatter disease . | Resultant skyline patella image. The tibial tuberosity (arrowed) is
projected over the patello-femoral joint. To correct this error further
flex the patient's knee. |
Case 8
Case 9
 |  |
| This 13 year old boy fell of his bike and was referred for right knee radiography. He was unwilling/unable to flex his knee | |
| Resultant skyline patella image. This has no diagnostic value and should not have been attempted. |
Case Studies
Case 1
This
23 year old male presented to the Emergency Department with a
dislocated left patella. This is somewhat unusual in that patients who
experience a dislocated patella will often relocate the patella
themselves prior to presentation to the Emergency Department.
The
AP knee projection image demonstrates the patella to be dislocated
laterally. There appears to be a lucent defect in the inferomedial
aspect of the patella. This may represent an avulsion fracture. |
 |
|
| The cross-table lateral knee image demonstrates the dislocated patella. |
 |
|
| The
patella was relocated in the ED. The post-relocation AP knee image
demonstrates the relocated patella. The patella is not centrally located
due to external rotation positioning error. |
 |
|
| The
post-relocation lateral knee projection demonstrates the relocated
patella and a lipohaemarthrosis (arrowed). The lipohaemarthrosis
supports the possibility that the patient suffered an avulsion fracture
of the patella with the donor site demonstrated on the pre-reduction AP
knee image. |
 |
|
| The
post-reduction skyline patellar image demonstrates the patella to be
relocated. The possible site of the avulsion fracture is shown (white
arrow). A bony fragment (black arrow) demonstrated on the lateral aspect
of the lateral femoral condyle is of unknown significance. |
Case 2
This
13 year old boy presented to the Emergency Department with a sore left
knee following a sports injury. He was examined and found to be tender
in the region of this left patellar tendon. He was referred for left
knee radiography.
The AP knee image demonstrates no displaced fracture. |
 |
|
The
tibial tuberosity is fragmented (white arrow). Whilst this is not
diagnostic for Osgood Schlatter's disease (OSD), the appearance is
consistent with OSD.
Note the thickening of the patellar tendon
with a possible fluid collection between the patellar tendon and the
proximal tibial epiphysis (black arrow). Hoffer's fatpad demonstrates
mixed fat and fluid density consistent with an acute injury.
Patella alta noted |
 |
|
The Insall-Salvati ratio in a normal knee (top) and in one with patella alta (bottom).
LP = length of the patella
LT = length of the patellar tendon. |
 |
|
| The
skyline knee projection demonstrates good patient positioning but the
radiographer has had difficulty including all of the anatomy on the IR
because of the patella alta. |
Case 3
This
34 year old male presented to the Emergency Department following a fall
from his pushbike. On examination, he was found to have a painful and
swollen left knee. He was referred for knee radiography.
The AP
projection image demonstrates patella alta. Patella alta refers to a
condition in which the patella is located in an abnormally proximal
position. |
 |
|
The cross-table lateral knee image also demonstrates patella alta.
Skyline knee radiography will be difficult given the high-riding
position of the patella. It may be necessary to flex the patient's knee
more than would be normally required in order to project the tibial
tuberosity clear of the patello-femoral joint. |
 |
|
| The skyline projection demonstrates tibial tuberosity overlying the patello-femoral joint space. |
Case 4
| This
68 year old male presented to the Emergency Department following a
fall onto a concrete floor. |
His right knee was painful and deformed
with a very prominent patella. He described a hyperflexion injury to
his right knee. He was referred for right knee radiography.
His
right knee was seen to have a depression in the skin immediately
superior to the upper pole of the patella. This was not evident on the
contralateral side. |
 |
|
| The depression superior to the right patella(arrowed) was not evident on the contra-lateral side. |
|
|
| The AP knee image is unremarkable. |
 |
|
The lateral knee image demonstrates abnormal orientation of the patella. There is a some mixed density
within Hoffa's triangle. There is also loss of definition of soft tissue structures and mixed fat/fluid density in
the region of the suprapatellar pouch.
There is a well corticated bony density in the region of the suprapatellar pouch and a quadriceps tendon enthesophyte. |
 |
|
The patella is pulled and tilted inferiorly by the action of patellar tendon in the absence of a countering
traction from the quadriceps muscle associated with a quadriceps tendon rupture. |
 |
|
| The skyline projection was destined for failure given the rupture of the quadriceps ten |
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