Anterior suprapatellar fad pad impingement syndrome

A 24-year-old female was referred with recurrent anterior knee pain since 6 months. Clinical examination revealed restricted motion and cracking sounds with flexion and extension of the knee. Clinical history included radiographically documented leg length discrepancy with shortening of the right leg of 1.5 cm and bilateral thigh splints.

Sagittal T1-weighted MR image showed suprapatellar fat pad swelling with heterogeneous signal intensity compared to subcutaneous fat with a convex posterior border and mass effect on the suprapatellar joint recess (Fig 1a). Swelling is quantified by a maximum anteroposterior diameter of 12.2 mm (Fig 1b).

Fat-suppressed (FS) T2-weighted MR images demonstrated edematous enlargement of the suprapatellar fat pad with increased signal intensity compard to subcutaneous fat and slightly increased joint fluid (Fig 2a:sagittal, Fig 2b:axial, and Fig 2c: coronal). Protrusion of the  posterior border toward the suprapatellar pouch was also noticed (Fig 2a) with normal appearance of the quadriceps tendon (Fig 2b).

Anterior knee pain (AKP) is commonly encountered in daily clinical practice.  AKP may be caused by a variety of traumatic and non-traumatic disorders affecting various peripatellar structures, including the peripatellar fad pads [1]. Peripatellar fat pads play an important role in facilitating normal knee movements. They are subdivided according to their location: 1. suprapatellar fat pad,  posterior to the quadriceps tendon and anterior to the suprapatellar recess 2. prefemoral fat pad, posterior to the suprapatellar recess and anterior to the distal shaft of the femur 3. infrapatellar fat pad (Hoffa fat pad), posterior to the patellar tendon, and anterior to the intercondylar notch [2].

The suprapatellar fat pad prevents direct friction of the quadriceps tendon against the femoral condyle, allowing for normal movement of the knee [2]. It is hypothesized that repetitive microtrauma or overuse injury may cause mechanical impingement resulting in oedema and enlargement of the suprapatellar fat pad [2, 3].

Imaging plays a pivotal role in the diagnostic workup of AKP. In case of suspicion of  suprapatellar fat pad impingement syndrome (SPIS), MRI is the preferred imaging modality. On T1-weighted images, the oedematous and enlarged fat pad is of heterogeneous signal compared to subcutaneous fat with posterior protrusion of the inner margin of the fatpad into the suprapatellar recess [2]. Size is an important aspect to take into account when evaluating the suprapatellar fat pad, by measuring the anteroposterior, craniocaudal, and oblique diameters, with the anteroposterior diameter being the most significant  [3]. The normal anteroposterior diameter of the suprapatellar fat pad measures 6+/-2  mm in females and 7+/-2  mm in males [4]. Fat pad oedema is best demonstrated by high signal intensity compared to subcutaneous fat on T2-weighted images. Mass effect may result in slightly increased joint fluid [5].

MRI also plays a pivotal role in the differential diagnosis of AKP by specifying the precise localization of the abnormalities. Prefemoral fat pad impingement and patella tendon-lateral femoral condyle friction syndrome affects the prefemoral fat pad and superolateral Hoffa’s fat pad respectively.  Quadriceps tendinosis involves the quadriceps tendon. Trochlear dysplasia and lateralisation of the tibial tuberosity is seen in patella maltracking.

As these MR findings may be seen in patients without AKP, strict correlation of MRI findings with clinical presentation is mandatory to confirm the diagnosis [3,6,7].

Available literature on treatment and outcome is scarce. Complete resolution of pain has been reported by ultrasound-guided corticosteroid injection [3,8].

Written informed patient consent for publication has been obtained.