Hoffa's fat pad abnormalities, knee pain and magnetic resonance imaging in daily practice

Abstract

Hoffa's (infrapatellar) fat pad (HFP) is one of the knee fat pads interposed between the joint capsule and the synovium. Located posterior to patellar tendon and anterior to the capsule, the HFP is richly innervated and, therefore, one of the sources of anterior knee pain. Repetitive local microtraumas, impingement, and surgery causing local bleeding and inflammation are the most frequent causes of HFP pain and can lead to a variety of arthrofibrotic lesions. In addition, the HFP may be secondarily involved to menisci and ligaments disorders, injuries of the patellar tendon and synovial disorders. Patients with oedema or abnormalities of the HFP on magnetic resonance imaging (MRI) are often symptomatic; however, these changes can also be seen in asymptomatic patients. Radiologists should be cautious in emphasising abnormalities of HFP since they do not always cause pain and/or difficulty in walking and, therefore, do not require therapy. Teaching Points • Hoffa's fat pad (HFP) is richly innervated and, therefore, a source of anterior knee pain. • HFP disorders are related to traumas, involvement from adjacent disorders and masses. • Patients with abnormalities of the HFP on MRI are often but not always symptomatic. • Radiologists should be cautious in emphasising abnormalities of HFP.

Keywords: Fat pad; Knee; Magnetic resonance imaging; Pathology; Regional anatomy.

Fig. 1

Anatomy of the HFP. The HFP (Hoffa fp) is limited anteriorly by the patellar tendon (Pat ten) and the joint capsule, superiorly by the inferior pole of the patella (Pat) (a), inferiorly by the proximal tibia (Tib) and the deep infrapatellar bursa (asterisk), and posteriorly by the synovium (arrows) and femur (Fem). It is attached directly to the anterior horns of the menisci (Med men, Lat men) (b). Normal vascular supply consists of two vertical arteries, posterior and parallel to the lateral edges of the patellar tendon (c)

Fig. 2

Suprahoffatic recess and infrahoffatic recess. Sagittal proton density with fat saturation MRI images show fluid in suprapatellar pouch, suprahoffatic recess (arrow, a) and infrahoffatic recess (arrow, b); a communication between the vertical and horizontal recesses may be present (arrow, c)

Fig. 3

Infrapatellar thickened plica. Sagittal T1w (a) and proton density with fat saturation (b) MR images show thickened infrapatellar plica (arrows) with associated oedema of HFP

Fig. 4

HFP acute trauma (pivot shift). MRI proton density with fat saturation image shows HFP apex oedema (thin arrows), oedema of the external femoral condyle and of the corresponding tibial plateau (thin arrows), with joint effusion (wide arrows)

Fig. 5

HFP acute trauma (direct contusion). MRI proton density with fat saturation images show HFP oedema, patellar oedema and joint effusion (thin arrows) (a) with fluid-fluid levels in axial image (wide arrows) related to hemarthrosis (b)

Fig. 6

HFP acute trauma. MRI proton density with fat saturation image, after direct trauma, shows HFP fragmentation (arrows)

Fig. 7

Superolateral HFP impingement. Axial proton density with fat saturation MR image shows oedema in superolateral portion of HFP in a young athlete with persistent knee pain in superolateral portion of the knee

Fig. 8

Hoffa’s disease. Repetitive microtraumas with hyperextension and rotational strain in soccer player. MRI proton density with fat saturation images demonstrates an enlarged, oedematous HFP

Fig. 9

Post-arthroscopic fibrosis. Sagittal T1w image (a) post-arthroscopy shows scarring in the HFP (arrows) that is oedematous and hypervascularised (arrows) on proton density with fat saturation image (b)

Fig. 10

Cyclops lesion. Sagittal proton density image, post ACL reconstruction, shows hypointense mass in the fat pad, which is oedematous

Fig. 11

Post-surgical fibrosis. Axial T2w (a) and sagittal proton density with fat saturation (b) images show hypointense post-surgical fibrosis in the fat pad (arrow), which is oedematous

Fig. 12

Lateral meniscal flap tear. Sagittal T1w image shows a fragment of the flap tear (arrows) displaced in the fat pad (a) that is oedematous on sagittal proton density with fat saturation image (b); joint effusion (wide arrows) is present

Fig. 13

ACL tear. Sagittal proton density with fat saturation image shows ACL tear with residual femoral stump (arrow), joint effusion (wide arrows) and oedematous HFP

Fig. 14

Patellar tendinopathy after autograft. Sagittal proton density with fat saturation image shows tendinopathy of patellar tendon (arrows) with increased signal intensity and increased anteroposterior diameter of the tendon, and oedema of the HFP

Fig. 15

Osgood–Schlatter disease. Sagittal proton density with fat saturation image shows enlarged and oedematous patellar tendon insertion (arrow), bone marrow oedema of the tibial tuberosity and HFP oedema

Fig. 16

Meniscal cyst. Sagittal proton density with fat saturation image (a) shows meniscal cyst (arrow) extending into the fat pad (oedematous), arising from lateral meniscus

Fig. 17

PVNS. Sagittal T1w (a) and T2w (b) images show synovial vegetation in the infrapatellar bursa (arrows); synovial vegetations are hypointense due to the paramagnetic effect of hemosiderin

Fig. 18

Synovial chondromatosis. Sagittal proton density with fat saturation image shows joint effusion with loose bodies in the suprapatellar pouch and in the infra-hoffatic recess (wide arrows). The HFP is oedematous

Fig. 19

Ganglion. Sagittal proton density with fat saturation demonstrates a multilocular ganglion cyst (arrows) within HFP in a patient with anterior pain and oedema of the HFP

Fig. 20

Extraskeletal chondroma, Sagittal proton density with fat saturation image shows a heterogeneous mass within the fat pad, with high signal of chondroid matrix and oedema and low signal of calcification or ossification (courtesy of Dr. L. Pietrobono, IRCCS Policlinico San Matteo, Pavia)