<prompt> What is the Primary pathology in Congenital displacement of hip :Large head of femurShallow acetabulumExcessive retroversionEveed limbus  <response>: Pathological changes observed in CDH : 1.Bone- acetabulum shows primary acetabular dysplasia and acetabulum is shallow.In addition there could be a gap or groove at posterosuperior aspect,triangular outer surface of ilium and acetabulum are in same line ,above acetabulum there is a depression containing the head of femur. Head of femur: head of femur normal initially ,later head is flat on its posterior and medial aspect,femoral head in the ilium is buffer or conical shaped. Neck of femur:shoening and ante version pelvis: tilted forwards,small and atrophied ,lordosis present is more veical than normal. Capsule: show hourglass constriction, one containing head and other containing acetabulum,constriction is produced by iliopsoas,ligamentum trees passes through this constriction, it is hyperophied. 2.muscles : pelvifemoral group -adductors ,saorius,gracious,rectus femoris,hamstrings,tensor fascia lata muscles.these muscles are shoened and they prevent reduction of head. pelvitrochanteric group obturators,quadratus femoris,iliopsoas These are elongated psoas forms an obstacle to reduction. glutei muscles -show little organic change but power is diminished. REF:Textbook of ohopedics,John Ebenezer,4th edition pg no.494 