title: Techniques to improve the shear strength of impacted bone graft: the effect of particle size and washing of the graft.
abstract: BACKGROUND
  When fresh morselized graft is compacted, as in impaction bone-grafting for revision hip surgery, fat and marrow fluid is either exuded or trapped in the voids between particles. We hypothesized that the presence of incompressible fluid damps and resists compressive forces during impaction and prevents the graft particles from moving into a closer formation, thus reducing the graft strength. In addition, viscous fluid such as fat may act as an interparticle lubricant, thus reducing the interlocking of the particles.
  
  
  METHODS
  We performed mechanical shear testing in the laboratory with use of fresh-frozen human femoral-head allografts that had been passed through different orthopaedic bone mills to produce graft of differing particle-size distributions (grading).
  
  
  RESULTS
  After compaction of fresh graft, fat and marrow fluid continued to escape on application of normal loads. Washed graft, however, had little lubricating fluid and better contact between the particles, increasing the shear resistance. On mechanical testing, washed graft was significantly (p < 0.001) more resistant to shearing forces than fresh graft was. This feature was consistent for different bone mills that produced graft of different particle-size distributions and shear strengths.
  
  
  CONCLUSIONS
  Removal of fat and marrow fluid from milled human allograft by washing the graft allows the production of stronger compacted graft that is more resistant to shear, which is the usual mode of failure. Further research into the optimum grading of particle sizes from bone mills is required.
authors.affiliation: Edinburgh Orthopaedic Research Department, Princess Margaret Rose Orthopaedic Hospital, Edinburgh, Scotland. douglas.dunlop@virgin.net
authors.forename: Douglas G
authors.initials: DG
authors.lastname: Dunlop
authors.forename: Nigel T
authors.initials: NT
authors.lastname: Brewster
authors.forename: S P Gopal
authors.initials: SP
authors.lastname: Madabhushi
authors.forename: Asif S
authors.initials: AS
authors.lastname: Usmani
authors.forename: P
authors.initials: P
authors.lastname: Pankaj
authors.forename: Colin R
authors.initials: CR
authors.lastname: Howie
mesh_terms: D016025:Bone Transplantation / Q000379:methods*; D001842:Bone and Bones / Q000502:physiology*; D006801:Humans; D010316:Particle Size; D033081:Shear Strength; D013048:Specimen Handling / Q000379:methods*; D019737:Transplants