X-ray Imaging for Patellar Malalignment

The patella is ideally suited for imaging, and when it comes to detecting malalignment the doctor will usually start with plain x-rays.

Page first published 2006, and reviewed August 2023 by Dr Sheila Strover (Clinical Editor)

Here the standard of care would be to obtain four X-rays. These would include the so-called AP (front-to-back), the side view (otherwise known as the lateral), the standing tunnel view and the so-called Merchant view – otherwise known sometimes as the axial view or sunrise view.

I would like to start by talking about the Merchant view which is the most well-known view for imaging patellar malalignment. This view is taken with the patient lying on his or her back with the knees resting on a special device called the merchant view board which is at the end of the imaging table. Conversely the patient can flex his or her knees 30 degrees and hold the X-ray cassette in his or her hands.

The image that is obtained reveals the kneecap and the groove into which the kneecap glides -or the groove with which the patella articulates. And the surgeon is looking for two things.

Firstly, the surgeon is looking for the position of the patella (kneecap) relative to this trochlear groove. Both the kneecap and the groove are roughly shaped like a U - the letter ‘U’ - and the U of the patella should roughly fit the U of the trochlea. If the kneecap is displaced right or left it will be clear that the two shapes do not match and this is a form of malalignment. 

The other thing that the doctor will be looking for is tilting of the kneecap. The normal patella sits like a hat on your head - it is horizontal - and yet in some patients with some forms of malalignment the kneecap is tilted like a beret and the patella will very clearly be seen to be tilted. 

The patella can be tilted but centred within the trochlea, it could be displaced right or left but not tilted, or it could be tilted and displaced. These are various forms of malalignment. Some patients with malalignment also have an anatomic variation in the shape of the trochlea – the groove – or a variation in the shape of the patella – or both – and on this merchant view the doctor can see whether there is an abnormality in the shape of this trochlea and/or of the patella.

The side view – the lateral x-ray – also gives the doctor information on the patellofemoral portion of the knee. If the lateral view is well done, the two condyles (these are the two knobs at the end of the femur or thigh bone) will be superimposed and it will look like there is really just one condyle - that is when you know that the lateral X-ray has been well performed. On that X-ray the surgeon can tell whether the depth of the groove is adequate or not. The difference between the outline of the groove and outline of the lateral femoral condyle represents the depth of the groove. So if these two lines are far apart that means that the groove is deep, if the two lines are close together the groove is shallow and if the two lines actually overlap that means that the groove is non-existent – there is no depth to this trochlear groove.

On the lateral X-ray the doctor can also assess the so-called height of the patella, that is the position of the patella relative to the groove when seen from the side. When the patella is too high (too close to the hip) this is called ‘patella alta’ (Latin for ‘elevated’) and when the kneecap is too close to the shinbone this is called ‘patella infera’ or ‘patella baja’, indicating that the kneecap is too low. Both patella alta and patella infera (baja) are forms of malalignment and these are readily appreciated on a lateral X-ray of the knee.

The other two views that I mentioned at the onset – the AP and the standing tunnel view – are extremely important for detecting problems in the other compartment of the knee but, relatively speaking, they do not provide much information with regards to the kneecap.