Posterior acetabular arc angle of unstable posterior hip fracture-dislocation

Purpose Posterior hip fracture-dislocation needs stability evaluation. A previous study in the normal acetabulum has shown that the coronal posterior acetabular arc angle (PAAA) could be used to assess an unstable posterior hip fracture. Our study was designed to assess PAAA of unstable posterior hip fracture-dislocation and whether posterior acetabular wall fracture involves the superior acetabular dome. Methods Using coronal computed tomography (CT) of the acetabulum and 3D reconstruction of the lateral pelvis, we measured coronal, vertical PAAA and posterior acetabular wall depth of 21 unstable posterior hip fracture-dislocations and of 50 % normal contralateral acetabula. Posterior acetabular wall fracture was assessed to determine whether the fracture involved the superior acetabular dome and then defined as a high or low wall fracture using vertical PAAA in reference to the centroacetabulo-greater sciatic notch line. Results The coronal PAAA of unstable posterior hip fracture-dislocations and of 50 % of the posterior acetabular wall of normal the contralateral acetabulum were 54.48 degrees (9.09 degrees) and 57.43 degrees (5.88 degrees) and corresponded to 15.06 (4.39) and 15.61 (2.01) mm of the posterior acetabular wall without significant difference (p > 0.05). The vertical PAAA of unstable posterior hip fracture-dislocation was 101.67 degrees (20.44 degrees). There were 16 high posterior acetabular wall fractures with 35.00 (16.18) vertical PAAA involving the acetabular dome and 5 low wall fractures. High posterior wall fractures resulted in four avascular necroses of the femoral head, three sciatic nerve injuries and one osteoarthritic hip. Conclusion Coronal and vertical PAAA of unstable posterior hip fracture-dislocations were 54.48 degrees and 101.67 degrees. Vertical PAAA assesses high or low posterior acetabular wall fracture by referring to the centroacetabulo-greater sciatic notch line. High posterior wall fracture seems to be the most frequent and is involved with many complications.