Patellofemoral Arthritis

For Medical Students

What It Shows

Patellofemoral arthritis is cartilage loss and bone change confined to the joint between the patella and the trochlea. 3.12.10 covers patellofemoral arthritis.

A skyline radiograph shows the space between the patellar facets and the trochlea in profile. 5.3.3 covers the skyline view.

It matters because the anteroposterior view projects the patella over the femur and cannot show this joint space at all. 5.3.1 covers the AP view.

How It Is Acquired

Request a skyline view when the pain is anterior, because the standard series otherwise answers a different question. 5.3.3 covers the skyline view.

Add a lateral view, which carries the patellar height and the anterior osteophytes. 5.3.2 covers the lateral view.

Assess the medial and lateral facets separately, since lateral loss is the usual pattern. 3.12.2 covers contact areas.

Assess the other two compartments on the same series, because isolated disease changes the options. 5.3.17 covers joint space narrowing.

Note the flexion angle the skyline was acquired at, since it shows the joint space at that angle only. 5.3.3 covers the skyline view.

What Normal Looks Like

An even space between each patellar facet and the matching trochlear surface. 5.3.17 covers joint space narrowing.

No osteophyte at the patellar margins or along the trochlear edges. 5.3.18 covers osteophytes.

Normal subchondral bone density on both sides of the joint. 5.3.19 covers subchondral sclerosis.

A patella seated in the groove without tilt or displacement. 5.12.3 covers patellar tilt.

What Abnormal Looks Like

Narrowing of the space, usually lateral before medial. 5.3.17 covers joint space narrowing.

Osteophytes at the patellar margins and along the trochlear edges. 5.3.18 covers osteophytes.

Subchondral sclerosis under the loaded facet. 5.3.19 covers subchondral sclerosis.

Lateral tilt and lateral displacement accompanying the lateral loss. 5.12.3 covers patellar tilt.

Bone contacting bone at the lateral facet with the medial space preserved. 3.12.2 covers contact areas.

MRCS Level

How It Is Acquired

In a viva the expected answer requests the view before describing any finding.

The expected points are the skyline view, the lateral view, the facets separately, the other compartments, and the flexion angle.

Say that a standard series without a skyline view does not exclude the diagnosis. 5.3.3 covers the skyline view.

Say that a grading system designed for the tibiofemoral joint is applied here out of its scope. 5.3.16 covers the Kellgren-Lawrence grading.

Say that the alignment and the patellar position are part of the picture rather than separate findings. 5.12.3 covers patellar tilt.

What Normal Looks Like

The reasoning expected is that this joint is loaded by flexion rather than by standing, so a weight bearing film of the other compartments says nothing about it. 3.12.3 covers contact pressures.

The skyline is acquired flexed because the patella has to be engaged in the groove for a joint space to exist on the image. 2.12.1 covers the trochlea.

The angle chosen changes which part of the facet is in contact, so a space seen at one angle is not the space at another. 3.12.2 covers contact areas.

Lateral loss follows the load distribution produced by the usual malalignment pattern. 3.12.11 covers the effects of malalignment.

How Good It Is

The skyline radiograph is the best available test of this joint space and tells you nothing about why the joint wore.

Its best contribution is the pattern and the severity of the space loss, under load, quickly and cheaply. 5.3.17 covers joint space narrowing.

It is limited by the single flexion angle, by positioning variation between departments, and by projecting a curved joint onto one plane. 5.3.3 covers the skyline view.

No figures appear for accuracy on this page, under decision 028, and no joint space measurements or grade boundaries appear either: those are published quantities measured at particular angles and this environment cannot check them.

FRCS Level

How Good It Is

At decision level the question is whether the disease is isolated, because isolated patellofemoral arthritis sits in a different conversation from arthritis of the whole knee. 3.12.10 covers patellofemoral arthritis.

A skyline view with a weight bearing series answers that, and a scan usually does not change the answer. 5.3.5 covers the Rosenberg view.

The bony measurements explain the pattern and belong with the diagnosis. 5.12.1 covers trochlear dysplasia.

Magnetic resonance imaging is requested when the clinical picture does not match the radiograph rather than to confirm it. 5.1.3 covers choosing the correct investigation.

The same radiographs document the alignment, which decides whether the wear has a correctable cause. 5.3.7 covers the mechanical axis.

Where It Misleads

The commonest error is excluding the diagnosis on a series with no skyline view. 5.3.3 covers the skyline view.

Magnetic resonance findings in this joint are near universal in the middle aged knee and are read as the explanation for the pain. 3.12.9 covers patellofemoral pain.

A tight lateral facet on a poorly positioned skyline is read as narrowing. 5.1.6 covers imaging pitfalls.

Disease in the other compartments is understated because the skyline drew the attention. 5.3.17 covers joint space narrowing.

Anterior osteophytes on the lateral view are read as a change in patellar height. 5.3.2 covers the lateral view.

Fellowship Level

Where It Misleads

At fellowship level the imaging decides which operation is on the table, and the decision rests on the compartment pattern rather than on the severity. 3.12.10 covers patellofemoral arthritis.

A skyline acquired at a different angle on either side of a referral makes a comparison that measures positioning. 5.3.3 covers the skyline view.

Secondary wear after instability looks identical to primary wear and arises from a cause that is sometimes correctable. 5.12.1 covers trochlear dysplasia.

Wear in a knee that has had a tubercle transfer or a trochleoplasty is read as progression rather than as the operated state. 5.12.8 covers the post-operative imaging.

Quantitative magnetic resonance measures of this joint are offered for trials and are not yet decision grade. 5.18.2 covers T2 mapping.

The useful report names the compartments involved, the facet pattern, the flexion angle, and the alignment. 5.20.6 covers reporting osteoarthritis.