Freeze-drying

Freeze-dried cancellous allograft is known to have inferior mechanical characteristics compared with fresh-frozen bone and is rarely used in bone impaction grafting. In vitro impaction studies comparing freeze-dried irradiated allograft with fresh-frozen, have shown that both types of graft reach the same maximal stiffness of 55 MPa, but the freeze-dried bone required three to four times fewer impactions to reach this level.⁵¹ In a separate study, the same authors showed less subsidence in a full hip simulator model with freeze-dried graft.⁶⁴ Benefits may include a reduced risk of femoral fracture because of the lower impaction energy required and a reduced risk of disease transmission. However, caution is required when interpreting these studies, as the fresh frozen graft used as a control was not rinsed, a factor that can greatly influence the compactability and stability of the graft. Only few clinical studies have reported the results of this type of graft. A very short-term study, documented no failures at a mean follow-up of 14 months in 40 femoral revisions.⁶⁵ The authors used bone scintigraphy in nine patients and found high levels of new bone formation around the prosthesis. In addition, a medium term outcome study of both femoral and acetabular revisions in 32 patient showed a 91% survivorship with all failures on the acetabular side.⁶⁶ Thien et al.⁶⁷ reported an overall survival rate of 86% at an average follow-up of 7 years for 7 acetabular revisions using impacted freeze-dried cancellous bone chips.