Fig. 4

Diagram for the classification system of patients with long-term unreduced hip joint dislocation, secondary osteoarthritis and pseudoarthrosis. a Type A, the original acetabulum remains nearly normal in size and has morphological features that are not affected by the inferior-anterior wall of the pseudoacetabulum. b Type B1, the original acetabulum remains nearly normal in size, but the superior-posterior wall is invaded by the inferior-anterior wall of the pseudoacetabulum, which leads to bone deficiency of the posterior wall of the original acetabulum. c Type B2, the original acetabulum is atrophied and is not affected by the inferior-anterior wall of the pseudoacetabulum. d Type C, almost the whole original acetabulum is invaded by the pseudoacetabulum, which leaves a severe bone deficiency, or the original acetabulum cannot be identified