The circular fibers, the zona orbicularis, are most abundant at the lower and back part of the capsule where they form a sling or collar around the femoral neck.

[3] The zona orbicularis and proximal hip joint capsule are poorly understood.

Recent studies seem to confirm that the proximal to middle part of the articular capsule, including the zona orbicularis, acts biomechanically as a locking ring wrapped around the femoral neck and thus is a key structure for hip stability in distraction.

[4][5] It tightens the joint capsule of the hip when iliopsoas muscle contracts.

This article incorporates text in the public domain from page 334 of the 20th edition of Gray's Anatomy (1918)