Although the exact cause of congenital dysplasia of the hip is unknown, genetic and biomechanical factors are probably important in producing the malformation, which may be hereditary and frequently involves both hips in varying degrees. Determination of the CE angle of Wiberg is helpful in classifying the various degrees of subluxation of the hips, as revealed by roentgenogram.

The purpose of the shelf operation is to increase the weight-bearing surface of the malformed acetabulum and thus to afford stability for the subluxated or dislocated femoral head. The operation consists in construction of an adequate bone ledge or shelf on the side wall of the ilium over the head of the femur, after the femur has been displaced downward by traction (Figs. 2-A and 2-B).

In a series of eighty operations for congenital subluxation or dislocation of one or both hips, fifty-eight (72.5 pcr-cent.) were successful in producing partial or entire relief of limp, pain, and hip fatigue, while a good range of motion was retained in the hip operated upon. Twenty-two of the operations (27.5 per cent.) were unsuccessful, in that the patients' symptoms were unimproved. Poor results were attributed to construction of shelves which were inaccurately placed. were too small, or were absorbed. Arthritic symptoms persisted or developed after operation in a few instances.