Obturator internus bursitis
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Right hip pain after exercise and difficulty laying on right side.
Fluid deep to the obturator internus muscle and tendon, adjacent to the ischial spine. The fluid signal is extending into the hamstring tendons origin.
Undisplaced antero-inferior labral tear, paralabral cyst and gluteal tendinopathy are also noted.
No hip joint effusion.
Obturator internus bursitis is a rare form of bursitis around the hip joint. The bursa may either be inflamed or infected.
MRI findings of an obturator internus muscle abscess is muscle swelling and a rim-enhancing fluid pocket within, or deep to, the obturator internus muscle. There may be infection elsewhere within the pelvis, or within the hip joint (septic arthritis).
In this case, the features are more in keeping with inflammation rather than infection.
Normally, the obturator bursa is in a collapsed state and not visualized on MRI unless it is distended, as we see here. Transverse plane provides the best visualization because the bursa is parallel to the curved portion of the obturator internus tendon posterior to the ischium i.e. "boomerang" shaped.
Whilst the fat-saturated proton density sequences visualize the lesion nicely in this case, a fat-suppressed gadolinium-enhanced T1-weighted sequence also demonstrates these lesions nicely (not seen in this case).