Postoperative periprosthetic acetabular fractures can be divided into acute traumatic fractures and pelvic discontinuity, Berry reported that the incidence of pelvic discontinuity was 0.9%. Pelvic discontinuity indicates the need for complex surgical treatment, and the prognosis is poor in the case of segmental or cavitary bone defect. There are few reports about traumatic periprosthetic acetabular fractures. Peterson reported that 50% of the periprosthetic fractures of the acetabulum were caused by trauma in the follow-up study of 23850 patients after total hip arthroplasty in 1996. Miller reported that all periprosthetic fractures of the acetabulum were caused by trauma in 1972. Emily Cha reported a case of traumatic periprosthetic acetabular fracture, both-column fracture combined with pelvic fracture, GAURAV  reported two cases of periprosthetic fracture caused by slight trauma in the early postoperative period, and underwent revision surgery.
The first step in the treatment of periprosthetic acetabular fractures is to evaluate and classify the fractures. The assessment of acetabular component stability is critical for further treatment. Radiographic evaluation includes antero-posterior view and Judet views. CT scan is usually required for evaluation, which can obtain the information between the bone and the component. Pascarella considered that the stability of the component and the time of fracture were important factors to determine the treatment strategy and predict the prognosis, and proposed the classification of periprosthetic acetabular fractures (Table 2).
Acetabulum periprosthetic fractures classification by Pascarella
1. Intraoperative fractures
2. 2. Postoperative/Traumatic fractures
a. Prosthesis stable
b. Prosthesis unstable
a. Prosthesis stable
b. Prosthesis unstable, mobilized simultaneously trauma
c. c. Prosthesis unstable, mobilized before trauma (osteolysis/bone loss)
The typical radiographic view of cemented acetabular component loosening is the translucent zone around the component and the position of the component changed. The performance of cementless component loosening is gradual bone loss, cup migration and cup position change. If osteolysis occurs around the acetabular component, the component is considered unstable. It is challenge to determine the stability of the component by preoperative radiographic examination, because the typical radiographic findings are not common, sometimes orthopedics doctors can’t determine whether the component is stable pre-operation.
Acute traumatic periprosthetic acetabular fracture and pelvic discontinuity are sometimes difficult to differentiate. Even with a history of trauma and pain symptoms, fractures may be caused by migration after progressive osteolysis. In the 5 cases reported in this paper, the history of trauma was clear. One case had typical radiographic findings of periprosthetic translucent line, and the component was considered loosening before trauma. The other 4 cases did not show typical signs of component loosening, and there was no history of hip pain before trauma, so it was impossible to determine whether the component was loosening before surgery. Therefore, the acetabular component was treated by direct vision examination to determine the stability. The preparation for revision was also made.
In addition to the stability of the component, acetabular fractures should also be evaluated pre-operatively. The periprosthetic acetabular fracture is no displacement and can be considered stable. In this situation, it should be noted that the risk of secondary displacement. Displaced fracture is unstable and usually requires surgical treatment. Laflamme believed that posterior column fracture plays a great role in the stability of component, because the failure rate of posterior column instability was 67%. At the same time, we should also pay attention to the exclusion of vascular injury after trauma. Harvie reported a case of internal iliac artery injury caused by acetabular fracture in a patient with periprosthetic acetabular fracture.
Periprosthetic acetabular fractures can be divided into two types: component stable or unstable. For unstable acetabular component, revision surgery is needed. If the acetabular component is stable, conservative treatment or surgical treatment is an option. The fracture type, component selection and patient related risk factors should be considered preoperatively. For conservative treatment, the component stability and fracture displacement should be accurately evaluated, and the risk of fracture displacement or component loosening should be carefully considered. The choice of the final treatment depends on whether the component is loose and the displacement of the fracture. If there is no displacement of fracture and the acetabular component is stable, conservative treatment could be considered. In Petersen report, 8 patients were treated conservatively, 6 patients healed, 2 patients required revision surgery, and of the 6 cases healed, 4 needed revision due to follow-up complications. When conservative therapy is underwent, surgeons should inform patients of the possibility of fracture displacement and need revision surgery, and recommend that radiographic examination be carried out frequently to evaluate the fracture healing.
When acetabular fracture is serious displacement or the component is unstable, revision surgery is needed. The goal of revision surgery is to firmly fix the fracture, with acetabular bone healing as the prerequisite, and also reconstruct the acetabular bone mass to stabilize the component. The acetabular rotation center and biomechanical condition of the hip joint is restored. There are a variety of options for surgical treatment, such as impaction bone grafting, posterior column plate fixation, antegrade or retrograde anterior column screws, bi-column fixation ,acetabular cup augmentation, highly-porous metal cups, anti-protrusion cages, and cup/cage constructs.
Anatomical reduction is not necessary as a reason of traumatic arthritis no longer occurred. Inappropriate reduction can be corrected by acetabular reaming. Evaluate the condition of the fracture bed to decide whether to allogeneic bone graft or using augment. Allogeneic bone graft makes the fracture gap to achieve secondary stability. Finally, the fracture bed needs to be reamed again, and fragment can be fixed by adding multiple angle screws on the acetabular cup. If the stability of the fracture is still uncertain, it can be additional bridged by protrusion cages. In cases with obvious bone defects, fractures should be augmented with bone grafts and could be stabilized by so-called jumbo cups, which bridge the fracture with screws into the superior and inferior part of the fracture. For the treatment of pelvic discontinuity, it is recommended to provide support for the component by particle bone graft or structural bone graft, and use anti-protrusion cages to increase the stability.
When the component is stable, the acetabular fracture can be fixed. From the biomechanics, achieve both column fixation, which could be done through both column plates or anterior column screw and posterior column plate, is the most stable way of fixing. Posterior column fixation is very important because the posterior column can provide the stability of bone -component interface. Plate is usually used for posterior column fracture fixation to obtain the stability of component.
The reverse press fit technology proposed in this paper, is to compress the gap on acetabular fracture while the acetabular component is not loose. This technology could obtain the maximum range of contact between the acetabular bone and the component. The plate is used to firmly fix the fracture, so as to wrap the acetabular component and achieve the press fit effect from the opposite direction, and provide a stable biomechanical environment for secondary bone ingrowth between component and bone. This technique also has some limitations. In the case of severe osteoporosis, it may not be able to provide enough pressure when using screw reduction forceps to compress the fracture, and the fixation strength of pelvic plate can’t be guaranteed. If the fracture around acetabular component is comminuted, the fracture could not be compressed. The long-term prognosis of reverse press fit technology need further follow up.
The literature reported a variety of surgical methods for the treatment of periprosthetic acetabular fractures. Emily Cha reported a case of pelvic fracture combined with periprosthetic acetabular fracture after total hip arthroplasty. The ilioinguinal approach was used to determine the stability of the component and posterior column was fixed with plates during the operation. The patient could walk on 18 months follow up. Falzarano  reported 24 cases of periprosthetic acetabular fractures treated with tantalum component. The average HHS score was 89.3 on 24 months post-operation. Gras reported a case of periprosthetic fractures of the acetabulum treated with navigation assisted screw placement.
Periprosthetic acetabular fracture is a severe challenge to surgeons, which is required to have not only surgical techniques to deal with acetabular fractures, but also rich experience in hip revision surgery. The treatment of periprosthetic acetabular fractures mainly depends on the stability of the component, which can be evaluated both preo-peratively and intra-operatively. More research on periprosthetic acetabular fractures is needed to provide the best treatment strategy for patients to achieve better prognosis.