The fat pad is a normal structure in the knee. It is literally composed of fat and is bright yellow in colour and exists below the patella (kneecap) just behind the patellar tendon. In this position it buffers any shocks to the front of the knee, but itself commonly suffers injury and problems related to the fat pad are not uncommon.
The fat pad can be seen on any routine MRI scan but is rarely discussed in the radiological (X-ray/MRI) literature. Abnormalities within it are most commonly a result of direct injury, of which arthroscopy is the commonest cause! Other causes of trauma include posterior and superior impingements - where the fat pad is too big it can get caught in the joint at the top (superior) or at the back of it (posterior). The fat pad can also be damaged in patellar dislocation. Trauma to the fat pad can also damage the infrapatellar plica that is a normal anatomical structure posterior to the fat pad. Within the fat pad one also may have inflammatory disorders and synovial and chondral tumours (growths) (arising from joint-lining cells and cartilage cells). These tumours can sometimes be nasty.One can get cysts in the fat pad (fluid filled pockets). No-one really knows the cause of this, but they can sometimes be confused with a cyst of the meniscus or cruciate ligament. The posterior border of the fat pad may also become inflamed and may cause increased joint fluid (effusion).
The characteristic symptom of Hoffa's disease is chronic ('going on for a long time') anterior knee pain mostly under the patella. It is more of a discomfort than a sharp pain. The knee may feel weak and sometimes swell. If the knee has been damaged in an injury with Hoffa's pad rupture-detachment then the pain may be acute ('coming on suddenly), the functional impairment may mimic a ligament injury, and the knee may be very swollen with blood in the joint fluid.
In testing for Hoffa's disease, one tries to elicit a 'Hoffa's sign' which is highly specific. The examiner applies pressure to the margins of the patellar tendon when the knee is bent, and maintains the pressure while the knee is straightened. In a positive result this causes a sudden severe pain and the patient refuses to keep on straightening the leg. MRI is good for showing the fat pad, unlike arthroscopy where it cannot be clearly seen because it is usually behind the light source and instruments.
An injection of local anesthetic and steroid into the fat pad itself helps to decide if the fat pad is implicated in the pain that the the patient is experiencing. If it is then it will cause a transient relief of symptoms. Fat pad pathology is usually secondary to other knee joint problems, and its primary involvement is rare.