Analysis of micromovements and stress state of an acetabular cup in a Paprosky type IV fracture

Periprosthetic acetabular fractures can lead to very serious consequences and therefore have an impact on people's quality of life. Their incidence has increased significantly in recent years, due to epidemiological factors such as increased life expectancy and changes in age distribution. Periprosthetic acetabular defects are classified using the Paprosky classification, which identifies type IV defects as the most severe, with pelvic discontinuity and significant bone loss. In the case of such fractures, typical acetabular prostheses fail to stabilise the acetabulum and thus to lead to fracture healing. It is therefore necessary to develop custom-made prostheses. The aim of this study is to provide an analysis of the post-operative status of a patient to whom a custom-made revision prosthesis was implanted following a periprosthetic type IV fracture of the acetabulum according to Paprosky. The revision prosthesis consists of a cup-hook construct connected by cables. Three models were analysed. With the first model, the preoperative situation is analysed, whereby a simulated femoral head is inserted into the hip where the acetabular fracture is present. In the second model, connection forces are applied between the two fracture edges, in order to ideally study the effect of the presence of the connecting cables. The third model represents the final postoperative situation, in which the prosthesis is present. The three models were analysed in a single leg stance condition. The results obtained showed that in the absence of a prosthesis or connection force, the joint resultant force causes the two fracture edges to move apart with displacements on the order of millimeters. Prosthesis insertion stabilizes the situation, leading to much smaller and micro-metric displacements. The stress in the acetabulum is low overall and no excessive stresses are evident. From a clinical point of view, it has been seen that the patient's acetabular fracture has healed at the three-year check-up. In conclusion, the cup-hook construct connected by pre-tensioned cables seems worthy of further analysis, as it has proven to be a promising option in the treatment of periprosthetic acetabular fractures.