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Dislocated or subluxated hips became more stable after a shelf operation, and the telescoping disappeared. However, the Trendelenburg sign remained positive, and consequently the limp persisted in all the patients with hip dislocation, treated by the shelf operation with the femoral head in a secondary acetabulum or in no acetabulum. When the shelf was built over the femoral head, after it had been placed in the primary acetabulum, the Trendelenburg sign became negative in only 18 per cent. of the patients. This sign remained positive in many patients, because, after the shelf operation, the femoral head usually became displaced under the shelf away from the line of gravity of the body. When the patient started to walk, the hip muscles were impotent to neutralize the increased rotatory effect produced by gravity upon this displaced center of motion. The Trendelenburg sign became negative only when the femoral head remained well centralized in the primary acetabulum. In these patients the shelf became smaller or even disappeared. On the other hand, in the patients whose Trendelenburg sign continued to be positive, the shelf became thicker and wider, due to the intermittent pressure of the displaced femoral head against the shelf.

From these observations, it was concluded that the aim of treatment for dislocation of the hip is not the construction of a big shelf, but the exact and permanent centralization of the femoral head in the acetabular cavity. The shelf alone is unable to accomplish this. The hip muscles must become strong and the capsule and ligaments of the hip joint must shrink to make secure the complete reduction of the femoral head. This may be accomplished in young patients by prolonged and well-directed functional after-treatment following reduction, with or without a concomitant shelf operation.

In the author's experience, the two main indications for the shelf operation are as follows: (1) In children over four years of age with congenital dislocation, reduced by the open method, a shelf may be created if the acetabulum is shallow; and (2) in patients with congenital or paralytic subluxation, or with subluxation of an old reduced congenital dislocation, the shelf operation is indicated when pain of static origin and a limp appear, usually after puberty. When reduction of the femoral head cannot be maintained in the primary acetabulum, the shelved hip will always be mechanically defective; a Schanz osteotomy is indicated in these cases.

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