Patellofemoral Arthritis Workup

Updated: Dec 11, 2019
  • Author: Dinesh Patel, MD, FACS; Chief Editor: Thomas M DeBerardino, MD  more...
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Approach Considerations

Radiography is the principal study used in the diagnosis of patellofemoral arthritis. Computed tomography can also be used to determine patellar tilt and articular damage. Arthroscopy can be used for both diagnosis and therapy.

No laboratory tests are usually required for the initial workup for knee pain. However, if clinical suspicion arises, a workup for systemic causes of polyarticular arthritis may be appropriate. This workup includes but is not limited to an evaluation for Lyme disease, rheumatoid arthritis, psoriatic arthritis, and gout.



Radiographic views of the knee include standing anteroposterior (AP), lateral, and axial (Merchant view, 45° flexion). [21] These radiographic views should be examined closely for narrowing of the joint space, osteophytes, articular degeneration, and patella alignment. Congruence of the patella and patella tilt can be assessed on the axial view. Specific measurements, which can be obtained from the radiographs, include the sulcus angle, the congruence angles, and the Insall-Salvati ratio (described below).

A sulcus angle is the angle between the medial and lateral condyles and the sulcus. It normally averages 138°. (See the image below.)

Patellofemoral arthritis. The sulcus angle is the Patellofemoral arthritis. The sulcus angle is the angle formed by the condyles and the sulcus. This is a measurement of trochlear depth. A finding greater than 138° indicates the presence of patellofemoral dysplasia.

A congruence angle is the angle between a line bisecting the sulcus angle and the articular ridge. It normally averages 6° and is a measurement of subluxation.

The Insall-Salvati ratio is the ratio of the length of the patella ligament to patella length as measured on the lateral view. The ratio should be 1:1; a ratio of more than 1:2 indicates the presence of patella alta. (See the image below.)

Patellofemoral arthritis. The Insall-Salvati ratio Patellofemoral arthritis. The Insall-Salvati ratio is used to assess the position of the patella and determine if the patella alta or patella baja is present. The ratio of the height of the patella to the length of the patella tendon should be 1:1.

The patellofemoral index is a distance ratio of the articular ridge and medial condyle to the articular surface and the lateral condyle. (See the image below.)

Patellofemoral arthritis. The patellofemoral index Patellofemoral arthritis. The patellofemoral index is the ratio of M, the closest distance between the articular ridge and the medial condyle, and L, the closest distance between the lateral facet and condyle.

With radiography, the presence of osteophytes at the lateral femoral trochlea combined with narrowing of the joint has 90% sensitivity in diagnosing arthritic changes, as compared with MRI. [22]


Diagnostic Arthroscopy

Arthroscopy is an excellent tool for diagnosing patellofemoral arthritis. [22] The surface of the patellofemoral and tibiofemoral joints can be evaluated under direct visualization to assess the size, severity, and location of articular degeneration. The motion of the patella with flexion and extension can be observed to determine whether subluxation is present. Arthroscopy can also be used for therapy.


Histologic Findings

Normal articular cartilage is smooth and glistening. Degenerative changes observed in the articular cartilage of the patella are similar to those observed in other joints. In one of the earliest descriptions of the pathology of the patellofemoral joint, Outerbridge described dulling of the color and softening of the cartilage as the earliest changes in chondromalacia patellae. [3] With progressive degeneration, osteoarthritis develops, and fissures and erosion of the cartilage occur. These changes are typically more pronounced on the medial patellar facet than elsewhere, but eventually extend throughout the patella.