Keywords:
Musculoskeletal soft tissue, Anatomy, MR, Education, Pathology
Authors:
A. C. Vieira, A. Vieira, R. Cunha; Porto/PT
DOI:
10.1594/essr2016/P-0097
Background
The knee is the largest joint in the body,
and one of the most easily injured.
It is made up of four main structures: bones,
cartilage,
ligaments,
and tendons with the fat pads occasionally being forgotten as anatomical structures.
There are three anterior knee fat pads:
- Infrapatellar fat pad or Hoffa fat pad – limited by the inferior pole of the patella superiorly,
the joint capsule and patellar tendon anteriorly,
the proximal tibia and deep infrapatellar bursa inferiorly and the synovium-lined joint cavity posteriorly
- Anterior suprapatellar fat pad - located behind quadriceps tendon that lies anteriorly,
the suprapatellar recess posteriorly and the superior aspect of the patella inferiorly
- Posterior suprapatellar fat pad – located anteriorly to the femur but posteriorly to suprapatellar bursa with the quadriceps tendon and vastus intermedius muscle superiorly and femoral trochlea inferiorly
Fig. 1: Anterior fat pads of the knee; Anterior suprapatellar (red), Posterior suprapatellar (blue), Infrapatellar (green).
Therefore is easy to comprehend that the infrapatellar fat pad is an intracapsular but extrasynovial structure and it plays an important role in stabilizing the patella in extremes of extension and flexion.
The infrapatellar fat pad may have two clefts (horizontal and vertical) that are easier to identify on MR images in the presence of an effusion.
They represent normal synovial recesses separated by fat projections or alar folds.
The horizontal cleft is more common than the vertical cleft and both are synovial-lined and may communicate with one another and with the synovial cavity of the knee joint.
Fig. 2: Fig. 2- Sagittal T2 fat sat showing horizontal (horizontal arrow) and vertical (vertical arrow) clefts.
Fat pads are sometimes the only anatomic structure where we can find alterations related to injury of the other surrounding structures but they can be also the source of the pathology.