A variety of disease entities can affect this structure and cause AKP, including impingement syndromes, postarthroscopy changes, plica syndromes, and mass lesions [27-30].
Hoffa's Fat Pad Syndrome
Acute or repetitive trauma to Hoffa's fat pad can result in edema and hemorrhage. The resultant changes of enlargement put the fat pad at risk for impingement between the femur and tibia. Fibrosis and anterior knee pain can result . This is called Hoffa's disease or syndrome. Acutely, there is high T2 signal and mass effect with the fat pad. Chronically, fibrosis appears dark on both T1- and T2-weighted images .
A possibly related entity to Hoffa's fat pad syndrome is an abnormal infra-patellar plica. The infrapatellar plica is a synovial fold that runs parallel to the anterior cruciate ligament (ACL) in the intercondylar notch. It travels over and through the superior aspect of the Hoffa's fat pad. Normally it shows low signal similar to that of ligaments. This structure can be injured, resulting in abnormal signal in both the plica and the superior aspect of Hoffa's fat pad. Injury to this plica is logically associated with injury to the fat pad, and thus the two entities may appear together and have some similar imaging findings. Clinical differentiation may also be difficult .
Other plica syndromes, including medial, lateral, and suprapatellar, are described. The mediopatellar plica is most often symptomatic. It extends from the medial joint wall to the synovium covering Hoffa's fat pad (Fig. 10). When it is prominent, it can be impinged upon by the medial condyle of the
femur and the patella. This can result in chronic irritation and injury, with an increase in thickening, edema, and further impingement. The plica can then become fibrotic and cause damage to the articular cartilage and synovitis. Symptoms range from crepitation and swelling to joint pain medial to the patella .
Patellar Tendon Lateral Femoral Condyle Friction Syndrome
Patellar tendon lateral femoral condyle friction syndrome, so named by Chung and colleagues , is related to the clinical disease known as fat pad impindge-ment syndrome. Patients present with anterior knee pain, more pronounced at the inferior aspect of the patella. Abnormal increased T2 signal is seen in the in-ferolateral aspect of the patellofemoral joint and with possible involvement of the lateral fat pad. Cystic changes in the fat pad and enhancement may occur.
Several symptomatic mass lesions can occur in Hoffa's fat pad that may cause symptoms. Localized nodular synovitis is the localized form of PVNS. It most commonly occurs outside the knee, but can occur in Hoffa's fat pad. MR shows a mass-like lesion with variable signal characteristics. Hemosiderin will often be present with its associated artifacts on gradient echo sequences . Para-articular chondroma arises from connective tissue due to cartilaginous metaplasia and most commonly occurs in or near Hoffa's fat pad. The MR appearance is that of a lobulated mass obliterating the normal high T1 signal fat pad inferior to the patella. The lesion shows increased T2 signal and will enhance after IV gadolinium administration. Primary intra-articular sarcoma has been reported but is extremely rare. Imaging appearances are often nonspecific and biopsy is often required for diagnosis .
The major portals used in arthroscopy are anterolateral, anteromedial, and medial and all of these can cause fibrosis within Hoffa's fat pad . The fibrosis appears as well-defined strands of low signal (on T1 and T2 sequences) tissue coursing through the high-signal fat. Artifact from metallic fragments can also be seen after arthroscopy . These changes on MR are not usually thought of as a cause of pain, but are important to recognize and not be misdiagnosed.
Patient's who have undergone ACL repair may present with a postoperative fibrotic complication known as localized anterior arthrofibrosis, or ''Cyclops'' lesion. This complication can result in pain and limited extension of the knee and is thought to occur from impingement of the anterior intercondylar notch on a graft that has been positioned too far anterior. The MR appearance is a low to intermediate signal intra-articular structure anterior to the graft. MRI has been shown to be sensitive for detection of the lesion .
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