Anatomy of the zona orbicularis of the hip: a magnetic resonance study.
OBJECTIVE
This study aimed at describing the anatomy of the zona orbicularis (ZO), based on magnetic resonance arthrography (MRA) and magnetic resonance imaging (MRI) scans and to assess the presence of synovial folds in relation to the ZO.


METHODS
A retrospective review was performed using consecutive hip and pelvic MRA and MRI examinations from our institution. We identified 25 normal scans of each variety. Patients were scanned in a neutral hip position and 3D FIESTA sequence images were included in a number of cases. Using electronic callipers, measurements were obtained of the ZO thickness and of the location of the ZO with respect of the femoral head and neck.


RESULTS
On MRA, the ZO appeared as a horseshoe in 18/25 patients, being absent anteriorly. On MRI the ZO was less consistent and absent in 12/25 posteriorly, in 8/25 inferiorly and in 2/25 anteriorly. Where present, the ZO usually coincided with the boundary of femoral head sphericity and the narrowest point of the isthmus of the femoral neck. The medial synovial fold was identified in all MRA studies (25/25).


CONCLUSIONS
The ZO of the hip is most consistently identified when the joint is distended and in approximately 75% of cases appears as a horseshoe-shaped structure. Superiorly, the ZO is aligned perpendicular to the long axis of the femoral neck. The ZO twists from postero-lateral to antero-medial as it moves inferiorly. Our findings are consistent with the hypothesis that the ZO functions as a ring that resists femoral head distraction and contributes to dynamic circulation of synovial fluid.
South West London Elective Orthopaedic Centre, Dorking Road, Epsom, Surrey, KT18 7EG, UK, fmalagelada@gmail.com.
Francesc
F
Malagelada
Rene
R
Tayar
Samuel
S
Barke
Giles
G
Stafford
Richard E
RE
Field
D000293:Adolescent; D000328:Adult; D000368:Aged; D005260:Female; D006621:Hip Joint / Q000033:anatomy & histology*; D006801:Humans; D008279:Magnetic Resonance Imaging; D008297:Male; D008875:Middle Aged; D012189:Retrospective Studies; D055815:Young Adult