Supraacetabular fossa (pseudodefect of acetabular cartilage): frequency at MR arthrography and comparison of findings at MR arthrography and arthroscopy.

PURPOSE: To evaluate the frequency of the supraacetabular fossa (SAF) (pseudodefect of acetabular cartilage) at magnetic resonance (MR) arthrography of the hip and to compare the MR findings with those from arthroscopy.
MATERIALS AND METHODS: All patients gave written permission for anonymized use of their medical data for scientific purposes before the imaging examination. The study was submitted to the institutional review board, and the need to obtain additional approval was waived. A medical student, a radiology fellow, and two senior radiologists reviewed 1002 consecutive MR arthrograms for the presence of an accessory bony fossa in the roof of the acetabulum, or SAF. SAF was classified into two types: type 1, which was filled with contrast material on MR arthrograms, and type 2, which was filled with cartilage. The width of the SAF was measured on coronal and sagittal MR images. MR arthrograms showing SAF were evaluated for subchondral reactions. Findings at MR arthrography were compared with those from arthroscopy in four hip joints with SAF type 1 and 13 with SAF type 2.
RESULTS: Sixteen of the 1002 hip joints (1.6%; four female and 12 male patients; mean age, 20.1 years) had SAF type 1 (mean width, 5.2 × 4.5 mm). Eighty-nine hip joints (8.9%; 43 female and 46 male patients; mean age, 37.8 years) had SAF type 2 (mean width, 5.1 × 4.7 mm). No subchondral changes were found around the SAF. No cartilage defect was seen at the site of the SAF at arthroscopy.
CONCLUSION: The high frequency of SAF on MR arthrograms (10.5%), the absence of subchondral reaction, and the absence of cartilage defects at arthroscopy indicate that the SAF of the acetabulum likely represents a variant.