The Skyline Patella Projection

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 <a class="external" href="http://www.google.com/" rel="nofollow" target="_blank">google</a> 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 above.

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

Conservative management of sports injuriesBy Thomas E. Hyde, Marianne S. Gengenbach, Jones & Bartlett Learning, 2007, p684

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

"The patella initially ossifies at between three and five years, commencing as multiple foci that rapidly coalesce" <a class="external" href="http://www.ncbi.nlm.nih.gov/pubmed/6729496" rel="nofollow" target="_blank">(Ogden JA. Skeletal Radiol. 1984;11(4):246-57. Radiology of postnatal skeletal development. X. Patella and tibial tuberosity.)</a> The skyline projection will not demonstrate a patella that has not ossified.

Resultant skyline patella image.

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 position of the patella may be evaluated on the basis of the ratio of the greatest diagonal length of the patella to the length of the patellar tendon on lateral radiographs (the Insall-Salvati ratio). This measurement is relatively independent of knee flexion, and a ratio of less than 0.80 indicates patella alta quoted from <a class="external" href="http://www.jbjs.org/Image_Quiz/2004/jun04/iqjun04_p2.shtml" rel="nofollow" target="_blank">The Journal of Bone and Joint Surgery Image Quiz</a>
adapted from <a class="external" href="http://www.jbjs.org/Image_Quiz/2004/jun04/iqjun04_p2.shtml" rel="nofollow" target="_blank">The Journal of Bone and Joint Surgery Image Quiz</a>	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 <a class="external" href="http://wiki.answers.com/Q/What_is_an_enthesophyte" rel="nofollow" target="_blank">enthesophyte</a>.
	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 tendon

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