Hoffa's Fat Pad Pain
Anatomy and Physiology
The infrapatellar fat pad (figure 1) also known as Hoffa's fat pad is the primary adipose structure within the knee joint. It is found between the joint capsule anteriorly and the synovial membrane posteriorly. It is composed principally of adipocytes but there are also abundant blood vessels and nerves.
There are two other adipose structures in the anterior knee, namely the prefemoral (or supratrochlear) and quadriceps suprapatellar fat pads. In figure one these fat pads are not shown but lie anterior to and posterior to the suprapatellar bursa respectively
The physiological role of the infrapatellar fat pad is uncertain. The following functions have been proposed:
- It fills the anterior knee compartment during flexion, and so may be related to shock absorption and protection of adjacent structures.
- It increases the surface area of the synovial membrane and so promotes the free circulation of synovial fluid.
- It may supply nutrients to the patellar ligament[1]
Pathophysiology
Hoffa's disease refers to infrapatellar hyperplasia and fibrosis accompanied with anterior knee pain as described by Professor Albert Hoffa. There is inflammation, hypertrophy, and fibrosis of the fat pad with repetitive injury. It can also be a source of pain in knee osteoarthritis. In some chronic forms the fat pad may undergo metaplasia to form an osteochondroma.
Other factors can also cause focal oedema of the infrapatellar fat pad
- Anterolateral impingement of the posterior-superior-lateral patellar tendon on the lateral femoral condyle, often found in association with patella alta or abnormal patellar tracking
- Synovial folds causing impingement resulting in inflammation: infrapatellar synovial fold, suprapatellar synovial fold, and middle synovial fold. The latter is the most clinically relevant and runs from the internal aspect of the joint capsule along to the medial edge of the patella and may be large in size.
- Anterior cruciate ligament injuries.
- Patellar fractures or tibial plateau fracture
- Patellar tendon injuries
- Patellar dislocation
- Synovitis from any cause
- Knee surgery
- Knee osteoarthritis: One theory explaining the osteoarthritis-obesity association seen in knee osteoarthritis but not hip osteoarthritis is the presence of the infrapatellar fat pad. Adipose tissue can release proinflammatory mediators resulting in low grade inflammation. There is conflicting evidence regarding the association between infrapatellar volume or inflammation on imaging with pain and functional impairment.[1]
Epidemiology
Hoffa's disease most commonly affects those between 30-40 years of age and appears more common in females.
Clinical Features
History: Mechanical anterior knee pain is the most common symptoms. In those where pain is triggered by walking up and down stairs one should consider patellofemoral pain syndrome. The patient may complain of a grinding feeling.
Examination: Sometimes hypertrophy of the infrapatellar fat pad is palpable and there may even be an effusion. There is pain on the anteromedial or anterolateral part of the knee. There may be a decreased range of flexion.
Hoffa's sign: with the knee flexed at 90ยฐ, the examiner presses laterally at the level of the patellar tendon and then passively extends the knee. The sign is positive if severe pain occurs during the final 10ยฐ of extension.
Imaging
Imaging findings are unreliable on ultrasound and MRI in the chronic phase. In the acute phase MRI is the most sensitive modality which can show oedema, bleeding. It may also show fibrosis in the more chronic phase.[1] Under ultrasound dynamic assessment can evaluate for impingement.[2]
Treatment
Analgesics, exercise therapy, corticosteroid injection in the acute phase.
Surgery is an option for some where partial or total excision of the fat pad is done and/or removal of osteochondroma. There is only low level evidence.[1]
