Although the fat pad has been implicated as a common cause of anterior knee pain, there is insufficient evidence for this to be a definite fact. Additionally, as there are no clear clinical features, the condition is unable to be diagnosed and treated with any certainty.
AnatomyA horizontal cleft is located in the posterior aspect of the infrapatellar fat pad. It is a common and normal MR imaging finding with a prevalence of 90%. The horizontal cleft is lined with synovium and its roof is formed by the ligamentum mucosum (infrapatellar plica). This cleft communicates with the knee joint. A distended cleft can form a prominent recess mimicking pathologic processes; conversely, disorders can arise in the cleft.
DefinitionThere is an ill-defined syndrome that involves pain and swelling of the infra-patella fat pad, sometimes referred to as Hoffa’s disease. At times the fat pad may be entrapped between the tibia and femoral joint surfaces resulting in a block to extension of the knee, verified at arthroscopy. The condition is considered to be related to impingement and inflammation of the infrapatellar fat pad.
AetiologyIt is considered that the infra-patellar fat pad can become symptomatic following a direct blow, or by impingement between the femoral condyles and the tibial plateaus in extension. It can be severely inflamed by hyperextension trauma or direct rubbing by the inferior pole of the patella. Thus, the fat pad may be the cause of acute pain. It has been proposed that fat pad pain is nearly always secondary to other knee joint pathology.
Clinical FeaturesThe clinical features include pain in the retro- and infra-patellar regions aggravated by movement of the knee, and tenderness to palpation. Physical signs are said to be tenderness and swelling over the anterior knee, deep to the patella tendon. There is usually some swelling and tenderness at the inferior aspect of the patella but with a good range of flexion. Extension may be painful at the end of range. Interestingly, the fat pad undergoes fibrosis after patellar and hamstring ACL reconstruction.
Differential DiagnosisExclusion of pathology to adjacent tissues such as the patellar tendon and the anterior horn of the meniscus can be achieved using ultrasonography and/or MRI studies.
ManagementFat pad syndrome is a condition that may exist acutely or chronically. Acute management will include anti-inflammatory medication, ice and rest. Any quadriceps exercises should be avoided early on and even partial weight bearing should be encouraged. In later stages, modality management such as ultrasound, laser, magnetic field therapy etc. can be used, as well as range of motion and quadriceps exercises. These exercises should avoid hyperextension and may be assisted by use of "unloading" tape techniques (Figure 1), which aim to limit the compression on the fat pad. Stretches of the quadriceps, tensor fascia lata and lateral retinacular structures should be undertaken if clinically relevant.