Stemmed TKA in a femur with a total hip arthroplasty: is there a safe distance between the stem tips?

When a stemmed TKA is needed in a femur in which a THA is already present, choosing an appropriate length for the TKA stem is crucial. Many surgeons intuitively fear that the distance between the stem tips correlates with the femur risk for fracture (RF). However, to date, no biomechanical data to support this intuition are available. Therefore, in this study, the RF in such a configuration was determined and compared for several activities, using a finite element modeling technique. During gait and sideways falling no difference in RF among different stem lengths was shown. However, a clear threshold appears during four-point bending. Stem tip distances shorter than 110 mm dramatically increased RF and, in osteoporotic bone, will certainly lead to fracture (RF>1) and thus should be avoided.