In two cases of posterior fracture-dislocation of the hip, we found an impaction and plastic deformation of the retroacetabular surface, in addition to other typically associated lesions. This deformation creates a mismatch between the stable and the free fragments of the posterior wall, and if unrecognized makes an anatomic reconstruction of the posterior wall impossible. On the computed tomography scan, the most characteristic sign is a concave deformation of the retroacetabular region as compared to the contralateral side. To our knowledge, this specific fracture-dislocation pattern has not been previously described in the literature. If orthopedic surgeons know about the existence of this deformation, then they will easily recognize the impaction on preoperative computed tomography scan and will be able to achieve an anatomic reduction of the acetabulum. Diagnostic aspects and the operative technique to approach this problem are described and illustrated in two cases.