Answer: arthrokatadysis aka Protrusio acetabuli (acetabular protrusion)

While you can have minimal medial “bowing” or medial depression of the medial acetabular wall (up to 3mm in males and 6mm in females as measured from the ilioischial line), the femoral head should never have any part medial to the lateral pelvic line drawn from the medial ilium to the lateral obturator foramen see: http://www.learningradiology.com/archives2012/COW%20512-Protrusio%20acetabuli/protrusiocorrect.html )

Protrusio acetabuli can be both:

Primary Form (aka Otto Pelvis). No recognizable cause (2/3 of cases)

Secondary Form: Rheumatoid, Paget’s osteomalacia, ankylosing spopndylitis, septic joint, etc

Another good case write-up: http://www.mskcases.com/index.php?module=article&view=97

Ortho reference: http://www.wheelessonline.com/ortho/acetabular_protrusio

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