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.