# Establishment of a Periprosthetic Acetabular Bone Defect in an In Vivo Model
## 4 Discussion
To avoid disproportionately good results due to the higher osteogenic potential of young animals, we only included adult female sheep [21]. To avoid a spontaneous bone healing, we used the definitions and results for \"critical size defect\" of previous studies for sheep [31; 32; 33; 34]. With an average defect size of 3.375 cm\({}^{3}\), all defects fulfilled the given definitions and can be graded as \"critical size defects\". These sizes prevent self-healing and create a comparable situation to the one found in humans, with need for revision total hip arthroplasty. Here, the main problem is an extensive bone loss in the load-bearing area of the acetabulum with need for reconstruction. Due to ethical concerns, we were not able to establish a fourth group (\(n=8\); arthroplasty without any kind of defect reconstruction). Nevertheless, we have to admit that pelvic bone defects have a higher regenerative capacity caused by the mechanical stress in comparison to cranial bone defects [35]. Some authors recommend preoperative induction of an osteoarthritis for a more reality-based experimental setup [26; 27]. As our study was designed to create a setting in which defect reconstruction is possible with the use of different materials, we did not chemically induce an osteoarthritis. To date, the significance of possible cross-reactions caused by chemical drug-induced osteoarthritis, aseptic periarticular inflammatory reactions, necrotic bone, and materials for bone defect reconstruction are not clear. In addition, inflammatory periprosthetic bone and soft tissue reactions in the present animal model are clearly caused by the interaction of the substitute materials with the surrounding bone and soft tissue. In this context, we have to admit that our primary aim was not the detailed comparison of different materials but to create a setting, which allows evaluation in subsequent experiments. Our study has limitations, the most relevant being the limited number of animals at the time of evaluation and the duration/standing time until the evaluation of the bone defect. As previous studies already outlined, economical aspects always play a role in animal models. As this study was planned to evaluate a new model, only a limited number of animals were operated, and a limited period for postoperative evaluation was chosen. Further studies might be helpful to provide long-term results. Another point that must be discussed is the necessary surgical technique for the implantation of the femoral component with the need for cementation of the whole femur to avoid a periprosthetic femur fracture at the tip of the stem. Here, preoperatively, we found no data in the literature that urge the surgeon to perform the cementation of the whole femur. As the fractures occurred at the tip of the stem/end of the cement, we suspect a mechanical problem. Additional studies with the sole purpose of evaluating the different mechanical probabilities might be helpful. In this context, we would like to outline that a limitation of this study is the low number of animals per group so that the postoperative exclusion of three animal led to an overall reduction of 10%. We tried to evaluate our three excluded sheep, and we think that the reason for the femoral fracture was the result of a spontaneous aggressive behavior of the other sheep while being in a group of two. The most likely explanation for the cervical abscess with septic circulatory instability 22 days after surgery of another sheep was a bite. In contrast, we are not able to determine the genesis of pneumonia three weeks after surgery. In summary, with the presented setup and surgical technique, we were able to create a standardized acetabular bone defect model and perform augmentation of the defect and implantation of a revision cup. This in vivo model might thus be a possible foundation to evaluate different methods for biological or augmentation-based defect reconstruction and gain new insights for further treatment strategies. Conceptualization, S.G., D.C.W. and W.G.