Imaging findings of Hoffa's fat pad herniation

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.

Anterolateral knee pain related to thrombosed lateral patellar retinaculum veins: Unusual anterolateral pain of the knee

We describe the ultrasound (US) features of venous thrombosis involving the lateral patellar retinaculum (LPR) veins presenting as anterolateral knee pain. Four male patients, aged 16 to 35 years, were referred for lateropatellar pain with focal tenderness. In two cases, physical examination also demonstrated soft tissue swelling at the anterolateral aspect of the knee. In all four cases, US showed subcutaneous fat edema surrounding a thrombosis involving the LPR veins. Magnetic resonance imaging performed before US in one patient revealed only nonspecific subcutaneous fat edema and was therefore unable to make the correct diagnosis of LPR venous thrombosis.

The lateral patellar retinaculum defect: anatomical study using ultrasound

OBJECTIVE

The purpose of this study was to evaluate whether ultrasound allows precise assessment of a focal defect at the lateral patellar retinaculum (LPR) and to determine its anatomical significance.

MATERIALS AND METHODS

This work was initially undertaken in four cadaveric specimens and followed by high-resolution ultrasound study in 48 healthy adult volunteers (96 knees) by two musculoskeletal radiologists. Dimension and location of the LPR defect and its relations to adjacent anatomical structures were analyzed.

RESULTS

A focal defect of the LPR through which vessels penetrated was constantly identified by ultrasound in our population. The mean transverse diameter of the defect was 2.4 mm (range, 1-6 mm). The defect was located a mean of 23.7 mm (range, 18-30 mm) proximal to the lateral tibial plateau and 6.6 mm (range, 4-9 mm) lateral to the patellar tendon. Anatomical dissection demonstrated that the LPR defect is related to perforating vessels that originate from the peripatellar anastomotic ring.

CONCLUSIONS

The focal defect of the LPR can be depicted by ultrasonography. Reproducible anatomical landmarks for its detection could be defined. Our data suggest the LPR defect may be considered a normal variant rather than a pathologic change.

Soft Tissue Masses of the Knee Related to a Focal Defect of the Lateral Patellar Retinaculum

We report on the imaging features of 6 soft tissue masses in the anterolateral aspect of the knee related to a focal defect of the lateral patellar retinaculum. In 4 patients (3 female and 1 male; 6-65 years) presenting with nontender palpable soft tissue masses in the anterolateral aspect of their knees, ultrasonography showed a focal defect of the lateral patellar retinaculum with herniation of the Hoffa fat pad, which was only visible in flexion. A magnetic resonance imaging examination performed in 1 case confirmed the defect in the lateral patellar retinaculum but did not show a mass, as the knee was examined only at 10° of flexion. The 2 remaining patients (one male and 1 female; 62 and 55 years) presented with soft tissue masses in their anterolateral knees in both flexion and extension. These masses were related respectively to a lipoma of the Hoffa fat pad and to a ganglion cyst, both herniated through a defect of the lateral patellar retinaculum. Our cases suggest that a focal defect of the lateral patellar retinaculum may be a route for Hoffa fat pad herniation as well as a route for superficial extension of infrapatellar fat lesions such as lipomas and ganglion cysts.

A Focal Defect at the Lateral Patellar Retinaculum on Clinical Knee MRI and Cadaveric Study: A Normal Variant or Pathologic Lesion?

OBJECTIVE

The purpose of this study is to retrospectively investigate the frequency of a focal defect at the lateral patellar retinaculum on knee MRI and to determine the variables that are significantly associated with the defect.

MATERIALS AND METHODS

Two radiologists independently reviewed 99 knee MR images for the presence of a focal defect at the lateral patellar retinaculum and categorized patients with concordant results into two groups: a group with the defect (≥ 3 mm; group 1) and a group without the defect (< 3 mm; group 2). Clinical and radiologic variables, including the Kellgren-Lawrence radiographic grade, subcutaneous fat thickness, infrapatellar fat pad area, and the amount of joint effusion, were evaluated. The size and location of the defect were measured in group 1. To correlate MRI and histologic findings, MRI was performed for 11 cadavers, and three histologic specimens were obtained.

RESULTS

Of the 99 knee MR images, concordant results between two reviewers were obtained for a total of 96 knees (97%): 25 knees (26%) in group 1 and 71 knees (74%) in group 2. A statistically significant difference between groups (p = 0.033) was noted for the infrapatellar fat pad area only. In all three cadaveric specimens, the lateral patellar retinaculum was more loose and sparse than the medial patellar retinaculum, and this site corresponded to the location of the defect on MRI.

CONCLUSION

A focal defect of the lateral patellar retinaculum was not found to be a rare finding on knee MRI (frequency, 26%), and it may be a normal variant rather than a pathologic change. In addition, we suspect that a focal defect can be a route of infrapatellar fat herniation and a route of superficial extension of the infrapatellar fat pad lesion without a lateral patellar retinaculum tear or invasion.