Experimental Biomechanical study on the consequences of dashboard injury of the pelvis after total hip replacement

Fractures of the pelvic ring and acetabulum are caused by high-energy trauma, usually by traffic accidents. Acetabular fractures occur when a force drives the head of the femur against the acetabulum. As the prevalence of patients living with a total hip prosthesis (THR) increases, the chance of suffering periprosthetic acetabulum fractures also elevates. However, the injury threshold of forces resulting in a periprosthetic acetabular fracture is unknown. The study aimed to analyze the results of a dashboard injury on the acetabulum after total hip replacement through a head-on collision in an ex-vivo experiment. A cemented cup was implanted into hemipelves removed from cadavers, and a dashboard injury was simulated with an impact of a pendulum-like structure released from different heights. The impact energy increased until inflicting acetabular fracture. Eleven hemipelves were examined, of which five were male and six were female. The average force required to cause damage to the pelvis was 5852 N (3950-8386). Isolated acetabular component loosening was noticed with cement fracture in one case (at 5344 N force), acetabular cup loosening occurred combined with posterior column fracture in three cases (at 8386, 3950, 6295 N force), and acetabular cup loosening combined with acetabular floor fracture occurred in six cases (at 4305, 4573, 6531, 4707, 8174, 6117 N force). A combination of all three mechanisms occurred in one case: acetabular cup loosening, with posterior column and acetabular floor fracture at 5986 N force. The results of the ex-vivo experiment indicate that in a dashboard injury, at least around 4000 N force and 4 J impact energy is necessary to create a periacetabular fracture. The results suggest that a larger force is necessary for damage to occur in male pelvises: fractures occurred mostly below 5000 N force in female pelvises, while they occurred above 6000 N in most males.