Incidence of and risk factors for femoral fractures in the gap between hip and knee implants

The purpose of this study was to analyse the incidence of interprosthetic femoral fractures and describe risk factors for them. Between 2009 and 2015, we selected patients who were carrying two implants (hip and knee) in the same femur. We collected demographic and clinical data and performed a radiological evaluation to analyse the gap between implants—the femoral canal area and total femoral area—in the axial plane. We defined interprosthetic fracture as that corresponding to a Vancouver type C fracture and types 1 and 2 according to the Su classification. We studied 68 patients who had total knee arthroplasty (TKA), and 44 patients who had total hip arthroplasty (THA); 24 patients an intramedullary nail. We found six interprosthetic fractures (8.8 %), all in patients with a non-cemented THA. There was a tendency towards statistical difference (p = 0.08). Patients with an additional implant at the proximal femur were statistically less likely to have an interprosthetic fracture (p = 0.04). In radiological results, we found more interprosthetic fractures in patients who had an increased femoral canal area in the axial plane just distal to the tip of the hip implant. Identifying risk factors for this specific type of fracture may facilitate their prevention. Better implant stability and the presence of a gap between stems in a lower canal zone appear to hinder the occurrence of interprosthetic fractures.