MRI features of subcutaneous anterior knee mass associated with a focal defect of the patellar retinaculum.

OBJECTIVE: The purpose of this study was to assess the frequency and MRI features of a subcutaneous anterior knee mass herniated from the infrapatellar fat pad (IPFP) through a focal defect of the patellar retinaculum (PR). MATERIALS AND METHODS: This study included 94 patients (44 men; age range, 1-80 years; mean age, 52 years) with clinically palpable subcutaneous anterior knee masses who underwent MRI between January 2007 and July 2022. Two radiologists retrospectively reviewed MRI findings of subcutaneous masses associated with a focal PR defect (location and size of the defect and characteristics of the mass). RESULTS: Among 94 patients, 15 (16%; 5 men; age range, 49-80 years; mean age, 67 years) had subcutaneous masses herniated from the IPFP through a focal PR defect. The defect was single (13/15, 87%) and more frequently observed in the lateral than in the medial (11/15, 73% vs. 4/15, 27%) PR. The defect occurred in the anterior segment (15/15, 100%) and was more frequently observed in the lower (10/15, 67%) than in the middle (5/15, 33%) and upper portions (0/15, 0%). The mean maximum length of the defect in axial and oblique planes was 14 mm and 25 mm, respectively. The defect-associated subcutaneous masses included lipomatous lesion (6/15, 40%), osteochondromatous lesion (5/16, 33%), and synovial fluid or ganglion cyst (4/15, 27%). CONCLUSION: Subcutaneous anterior knee masses were associated with a focal PR defect in 16% cases. The location of a focal PR defect was characterized by the lateral, anterior, and lower segments.