title: Defining the impaction frequency and threshold force required for femoral impaction grafting in revision hip arthroplasty. A human cadaveric mechanical study.
abstract: OBJECTIVE
  The two most common complications of femoral impaction bone grafting are femoral fracture and massive implant subsidence. We investigated fracture forces and implant subsidence rates in embalmed human femurs undergoing impaction grafting. The study consisted of two arms, the first examining the force at which femoral fracture occurs in the embalmed human femur, and the second examining whether significant graft implant/subsidence occurs following impaction at a set force at two different impaction frequencies.
  
  
  METHODS
  Using a standardized impaction grafting technique with modifications, an initial group of 17 femurs underwent complete destructive impaction testing, allowing sequentially increased, controlled impaction forces to be applied until femoral fracture occurred. A second group of 8 femurs underwent impaction bone grafting at constant force, at an impaction frequency of 1 Hz or 10 Hz. An Exeter stem was cemented into the neomedullary canals. These constructs underwent subsidence testing simulating the first 2 months of postoperative weight bearing.
  
  
  RESULTS
  No femurs fractured below an impaction force of 0.5 kN. 15/17 of the femurs fractured at or above 1.6 kN of applied force. In the second group of 8 femurs, all of which underwent femoral impaction grafting at 1.6 kN, there was no correlation between implant subsidence and frequency of impaction. Average subsidence was 3.2 (1-9) mm.
  
  
  CONCLUSIONS
  It is possible to calculate a force below which no fracture occurs in the embalmed human femur undergoing impaction grafting. Higher impaction frequency at constant force did not reduce rates of implant subsidence in this experiment.
authors.affiliation: Trinity Centre for Bioengineering, School of Engineering, Trinity College Dublin, Ireland. fionnanc@gmail.com
authors.forename: Fionnan
authors.initials: F
authors.lastname: Cummins
authors.forename: Peter O'
authors.initials: PO
authors.lastname: Reilly
authors.forename: Olivia
authors.initials: O
authors.lastname: Flannery
authors.forename: Danny
authors.initials: D
authors.lastname: Kelly
authors.forename: Paddy
authors.initials: P
authors.lastname: Kenny
mesh_terms: D000328:Adult; D019644:Arthroplasty, Replacement, Hip / Q000009:adverse effects / Q000379:methods; D001696:Biomechanical Phenomena; D016025:Bone Transplantation; D002102:Cadaver; D005264:Femoral Fractures / Q000209:etiology; D006622:Hip Prosthesis / Q000009:adverse effects; D006801:Humans; D011475:Prosthesis Failure; D012086:Reoperation