Transcript of video
Welcome to this third part in our series on ‘How to interpret knee X-rays’.
In Part 1 we discussed the knee X-ray from the front (that is, the ‘A-P’ view), and in Part 2 we discussed the knee X-ray from the side (that is, the ‘lateral’ view). In this particular part, we are going to talk about a special view, called the ‘skyline’ view.
It is called a skyline view because, when it is taken properly, you can see the kneecap or patella apparently hanging on the ‘skyline’ above the ‘horizon’ of the femur bone. I have outlined it for you here so that you can appreciate what I meant. This particular angle gives a great view of the patella, and that is what it is mainly ordered for.
To obtain a skyline view requires some special positioning - the X-ray beam passes down from above the patella at a particular angle so that the patella is clearly differentiated from the groove of the femur underneath it, and the patella is captured in profile from this position. Here is a drawing to show you exactly what I am meaning, and an X-ray to allow you to compare to the drawing.
The groove of the femur underneath the patella is called the ‘trochlear groove’, and it is important to look at the congruency of the contact surfaces here (shown in white). Nice congruent articular surfaces suggest that this patellofemoral joint is likely to be healthy.
From the skyline view one can see clearly the shape of the patella - whether it or its underlying trochlear groove of the femur are abnormal or not. Abnormalities in the shape of the patella or trochlea, if one is born this way, may be due to the conditions known as patellar or trochlear ‘dysplasia’.
From a skyline view one can see whether the patella tracks over too far to one or other side - which is called ‘mal-tracking’, or whether it is tilted abnormally on its edge - which is called ‘patellar tilt’. Of course you need to appreciate that these are static views as the patient is lying still and when the patient is actually moving things might not be absolutely the same, but it does give an indication of potential problems in these respects.
Take a look at the pale articular cartilage seen here. Articular cartilage is radio-lucent and is generally not very evident on X-rays, but it can sometimes be seen quite nicely in this view with a softer beam.
The skyline view may also reveal bone breaks or arthritis. A fracture (or bone break) of either the patella or trochlea will show up as a dark line. Arthritic changes may reveal themselves as small bubbles of cystic change or irregular spurs at the joint edges.
There is so very much useful information that you can glean from a skyline view that it is very strange that it is so often left off the order. Knee surgeons in general would order at least these three views and many would order as a routine the fourth view that we are going to discuss in Part 4.
Take a look again at the three ways we have seen so far to appreciate the patella - via the A-P view, the lateral view and the skyline view. It is very clear that the latter two give loads more information about the patella compared to the first view, and also clear that the skyline view gives information about the trochlea not at all evident in either A-P or lateral.
That’s the end of this part - the presentation is only short but nonetheless the view is really important. Thank you for listening, and please remember to subscribe to our YouTube channel.
