Periprosthetic acetabular fractures (PPAF) are a rare but complex complication of total hip arthroplasty (THA). Peterson and Lewallen found a low incidence of 0.07 % of periprosthetic traumatic fractures of the acetabulum in primary hip arthroplasty [1]. However, a higher number of occult PPAF cases could be detected. Occult PPAF were demonstrated to occur in up to 8.4 % of primary total hip arthroplasty [2].

In 2003, Della Valle, Momberger, and Paprosky presented a new classification system, which is widely used for PPAF, the modified Paprosky classification [3]. A distinction is made between fractures that occurred intraoperatively during implant insertion, those that occurred during implant removal, traumatic fractures, spontaneous fractures, and cases of complete pelvic discontinuity (Table 1). Furthermore, the fractures were divided into subtypes according to stability/bone quality. The classification according to Paprosky is frequently used clinically, as it considers important aspects such as the time of fracture, implant stability, and possible osteolysis, as well as pelvic discontinuity. Further PPAF classifications exist [1,[4], [5], [6], [7]], but these are currently not widely established.

The steadily increasing number of patients with a THA, the longevity of implants, and the desire to maintain mobility even in old age are leading to an increase in the number of patients with a periprosthetic fracture, including PPAF. The trend of increasing case numbers can be seen in other forms of periprosthetic fractures and was described by Park et al. as a major national socioeconomic burden [8]. In contrast to the periprosthetic femoral fracture, PPAF have only been described in small cohorts and are limited to a small number of clinical studies in the literature [9]. In addition, PPAF with an unstable cup have rarely been evaluated.

We present the results of surgical treatment of PPAF with an unstable acetabular cup using the modified Stoppa approach in combination with a direct anterior approach if revision of the acetabular component was performed (Fig. 1). Patient outcomes were measured based on subjective pre- and postoperative mobility, modified Harris Hip Score (mHHS), complications, revision rates and 1-year mortality. First, we hypothesized that combined surgical treatment of PPAF provides satisfactory patient outcomes in patients with and without cup instability. Second, we hypothesized that the complication rate associated with the combined surgical approach would be lower than the rates reported in the existing literature. Finally, we hypothesized that combined approaches would lead to a low 1-year mortality rate.