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.
Clinical Relevance: Understanding the mechanical properties of milled human allograft is important when impaction grafting is used for mechanical support. A simple means of improving the mechanical strength of graft produced by currently available bone mills, including an intraoperative washing technique, is described.
Douglas G. Dunlop, MD, FRCS, FRCSEd (Tr and Orth); Trauma and Orthopaedics, Southampton University Hospitals NHS Trust, Mailpoint 45, Southampton General Hospital, Tremona Road, Southampton SO16 6YD, England. E-mail address: email@example.com
Nigel T. Brewster, FRCSEd (Orth); Freeman Hospital, High Heaton, Newcastle Upon Tyne NE 77 DN, England. E-mail address: firstname.lastname@example.org
S.P. Gopal Madabhushi, PhD; Schofield Centrifuge Centre, Trumpington Street, Cambridge CB2 1PZ, England. E-mail address: email@example.com
Asif S. Usmani, PhD; P. Pankaj, PhD; School of Civil and Environmental Engineering, University of Edinburgh Crew Building, The King's Buildings, Edinburgh EH9 3JN, Scotland. E-mail address for A. Usmani: firstname.lastname@example.org. E-mail address for P. Pankaj: email@example.com
Colin R. Howie, FRCSEd; New Royal Infirmary of Edinburgh, Old Dalkeith Road, Little France, Edinburgh EH16 4SU, Scotland. E-mail address: firstname.lastname@example.org