Patellofemoral friction syndrome: MRI findings of an underdiagnosed disease.

Clinical History

A 30-year-old woman who presents pain with hyperextension of the knee and by pressing on the lower pole of the patella.

Imaging Findings

Inflammatory changes in Hoffa fat located between the lateral femoral condyle and the patellar tendon (figures 1, 2 and 3). The patellar tendon shows discrete lateralization. High patella is also observed (Insall-Salvati Index 1.4;figure 4). The described findings are compatible with lateral femoral condyle-patellar tendon friction syndrome.

Discussion

Frequently underdiagnosed, the patellofemoral friction syndrome, also called Hoffa's fat cushion compression syndrome, is mainly represented by 2 characteristic symptoms, which are mainly found in women (especially if they present obesity or rapid weight gain, as in pregnancy): patellofemoral pain and instability in the patellar region [1].

Edema of the superolateral Hoffa fat pad is associated with pathology of the patellofemoral region, such as malalignment and friction syndrome (object of this clinical case).

This pathology that must be diagnosed with MRI, since MRI is the imaging technique of choice for the diagnosis of this entity. Radiographs and CT are usually normal. [1, 2]
The upper patella, the increased distance TT-TG (distance between the anterior tuberosity of the tibia and the deepest part of the femoral trochlea), and a short distance between the lateral trochlear (external) facet and the patellar ligament, this is, the distance PL-T [1, 4].
The upper patella and the distances described are also independent predictors of the appearance of a hypertensive signal that translates oedema in the Hoffa fat region.
In addition, once the hyper signal in Hoffa's fat is detected, the lateral aspect of the patellar ligament should also be assessed in an active search for pathology indicative of focal tendinopathy [3]

The most typical image findings of this entity is a hyperintense focal area on STIR and T2-weighted sequences (representing the oedema on the inferolateral aspect of the femorotulin joint), in the region included in the most superolateral portion of the infrapatellar fat of Hoffa. It is not uncommon to find a lesion of a cystic nature located between the lateral retinaculum and the external femoral condyle, but this finding is not too frequent [1, 4].
Associated findings: high patella;lateral patellar subluxation. These superimposed findings can appear in >90% of patients.
The edema in the superolateral region of the fat of Hoffa is should make us suspect that there is a femorotullar misalignment, with the distances described above altered. In addition, when there is oedema, there is a tendency to present greater external deviation of the patella and the existence of a trochlear groove that is less deep than normal (to assess signs of trochlear dysplasia). The existence of less deep trochlear grooves or external subluxations will cause a greater friction (excessive if the syndrome appears) in the region of the external femoral condyle and the superolateral zone of the infrapatellar fat [4].

Written informed patient consent for publication has been obtained.

Differential Diagnosis List

Final Diagnosis

Patellofemoral friction syndrome.

URL:	https://www.eurorad.org/case/15628
DOI:	10.1594/EURORAD/CASE.15628
ISSN:	1563-4086