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Imaging and X-rays for patellofemoral pain (part 6 of a course on Patellofemoral Pain)

Imaging is the term used to describe the pictures a doctor might look at to assist him or her in coming to a diagnosis and formulating a treatment plan. 'Assist' is a key term because imaging studies are meant to supplement the interview and physical examination, not supplant them.

A Radiologist, by the way, is a doctor who has specialized in medical imaging. A Radiographer is a technologist, not a doctor. It is generally the case that the technologist is the person who performs the imaging procedure, but most films are nonetheless generally reported upon by the Radiologist.

Imaging studies include x-rays, MRIs, CTs, and (nuclear) bone scans.


X-rays essentially reveal the bones. The cartilage ends of the bones are poorly seen, so the joint space between two bones looks larger than it really is. Doctors are used to looking at these spaces as well, and a diminution in the width of the space will be interpreted as a thinning of the 'invisible' joint cartilage.

Contrary to what you might have been led to believe, a vast amount of information can be obtained from x-rays - provided that the right views are obtained! You may be surprised to find that x-rays will occasionally reveal information not found on an MRI.

AP and Lateral X-rays

The two basic x-ray views are called the AP and Lateral. These are the front (anterior-posterior) and side views. These are the views obtained in an Emergency Room and they are useful for detecting major fractures, large tumors, and advanced arthritis.

Evaluation of the patella, though, usually requires special views.

x ray of knee

Take a look at this x-ray. The bones are pale, and the spaces are dark. If one bone overlies another you may see the outline of both, but they appear to be on the same plane (see arrow, showing the pale circle of the patella overlying the femur). With a single film one can thus see only the one dimension. This view is called an 'AP' view (antero-posterior).

In an AP view the patella shows as a vague circle over the femur bone. Little information can be gleaned other than its height and basic outline, and maybe fracture (break) lines (which appear dark).

An improvement on the basic AP view is the Standing AP view. Here you are standing as the x-ray is taken. If you suffer from knee arthritis, the narrowing of the space between the bones that is a sign of arthritis is best seen when you stand.

Another variation on the AP x-ray is the Standing Tunnel AP. This view also goes by the name Standing Flexion view, Rosenberg view, or even Schuss view because the patient stands with the knees bent. This view will occasionally detect arthritis that even the regular Standing AP will not pick up.

x ray of knee

One needs generally to take another picture from the side to see the other dimension. This is called a 'lateral' view. Now you can see that femur and patella are in different planes.

The Lateral (side) view can be technically difficult. In order to get an x-ray that is in the correct plane, the x-ray technician may have to rotate the leg this way or that way until he or she gets it right. By always seeing a standard view doctors can develop confidence in interpreting the films.

X-ray quality

poor x ray of knee

Here you have a poor lateral x-ray. This image is rotated and hard to interpret. The pale 'shadows' of the two condyles of the femur should lie on top of each other, so that they appear as one. In this film, however, they are both visible (red arrows). The shadow of the fibula (yellow arrow) should lie almost behind the tibia, but in this film it is clearly apparent.

The patella in this film just looks weird because of the rotated image.

Unless the doctor specifically asks for a good lateral, the x-ray technician sometimes won't bother. And indeed the doctor won't usually know enough to complain. This is unfortunate, because a good lateral x-ray will provide a great deal of information regarding the kneecap and the underlying trochlea. Specifically, a good lateral will indicate whether the trochlear groove has a normal depth from top to bottom.


patella baja

Compare the next film. Here you can see that the technician has managed an almost perfect lateral x-ray - the two condyles (red arrows) look almost on top of one another.

Now the doctor can confidently say that the patella IS weird not just optically distorted. This patella is too low - it is a patella baja (infera).

patella alta

This next film does have some rotation (not good) - but nonetheless it can be seen that there is a patella alta - that is, the patella in ths patient is higher up than it should be.

Blumensaat's Line

x ray of knee

How can one tell if a patella is too high or too low? Look at this lateral - the bottom of the patella should line up with the oblique white shadow on the femur - known as 'Blumensaat's line'.

blumensaat's line

I have drawn a red line in that position to show you what I mean.

Check the relation of the bottom of the patella to Blumensaat's line in the 'baja' ('infera') and 'alta' X-rays above.

Note also that Blumensaat's line works for patella alta/infera only when the knee is flexed 30 degrees -another good reason to be careful when specifying what films one wants taken.

Merchant's View

merchant view

The position and orientation of the kneecap is best determined on a so-called Merchant view, named after Alan Merchant, MD from California who first described it.

The essential requirement for this view is a simple device placed at the end of the x-ray table. This critical x-ray is often omitted or poorly done.

Without the special board at the end of the table, it is impossible for the x-ray technician to perform a Merchant view with the knee bent less than 60° or so!

The equivalent of a Merchant view is often included on knee MRI studies, but this is an expensive alternative to the standard Merchant view that can be obtained in your orthopedist's office.

The Merchant view gives the doctor the position of the patella in the medial-lateral (right-left) direction and indicates whether the patella is flat or tilted like a beret as in the x-rays seen here.

patellar tilt

In the first x-ray patellar tilt is demonstrated on the Merchant view (right knee).

patellar tilt with subluxation

In the second x-ray you can see both patellar tilt and subluxation (left knee).

On a number of patients all x-ray views are normal except for the Merchant view.

It is important that this view be obtained with as little bend in the knee as possible. A 30° bend is ideal. Indeed, as the knee bends, the position of the kneecap tends to improve. Thus, if the knee is bent 45° or more the doctor can be fooled into thinking that the kneecap is well positioned. An orthopedist who is familiar with the kneecap can immediately tell whether or not the knee was too flexed (bent) on the Merchant view.


Updated: 21 Jan, 2015
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