The Aetiology of Primary Protrusio Acetabuli

Abstract

Theories so far advanced to explain primary protrusio are reviewed and an alternative hypothesis is proposed. An investigation to test the hypothesis is described in three sections. (1) The effects of stress on an artificial Y-cartilage in an adult pelvis were examined, and a distinctive alteration in pelvic shape recognised as resulting from this stress. (2) The pelvic shape in nine cases of primary protrusio and in eight cases of congenital dislocation of the hip was compared with the shape found in normal subjects. It was shown that the pattern of pelvic deformity detected in Part I is present in primary protrusio whereas the dislocated hips show a converse pelvic deformity. (3) The phenomenon of juvenile beaking of the Y-cartilage was examined. It was shown that the beaking was associated with a bulge of the acetabulum into the pelvis, involving sometimes the whole acetabulum, but more commonly its posterior half. The age incidence of this juvenile protrusion was determined radiologically and found to increase from birth to the age of eight and subsequently diminish. Cases are illustrated in which the juvenile bulge has failed to correct, leaving primary protrusio. It is suggested in the light of these findings that juvenile beaking is a true protrusion of the acetabulum, resulting from normal shear and rotation stresses without any associated bone disease; that this protrusion is normally reversible; and that failure of correction of it is the cause of primary protrusio acetabuli.