Loosening of threaded acetabular components. Radiographic manifestations.

Radiographic signs of loosening were assessed, and their sequential appearance recorded, in a retrospective analysis of 20 loosened, threaded acetabular cups. There was radiographic evidence of loosening in 19 (95%) cups. In decreasing order of frequency, the radiographic manifestations included bone resorption between the threads in 19 (95%) cups, increased bone density above the cup in 15 (75%), continuous lucency around the cup in 14 (70%), axial migration in 9 (45%), and rotation in 6 (30%). The order of appearance of these radiographic manifestations was the same as the order of frequency of occurrence of the alterations. In the one patient with no radiographically detectable signs of loosening, the cup did not move freely but was easily detached during surgery. Early radiographic diagnosis of loosening requires accurate positioning of the patient in order for the radiographs to reveal bone resorption between the threads of the cup.