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Mechanical characteristics in impaction allografting - the role of graft density and cement penetration profile Albert, Carolyne Izabel

Abstract

Revision total hip arthroplasty with femoral impaction allografting has an attractive potential for restoring bone stock in femurs with bone loss caused by the failure of hip implants. However, problematic implant subsidence is often reported after this procedure. A lack of understanding remains over the mechanisms that cause subsidence. The objectives of this study were to: a) explore the relationships between subsidence and morphometric features of the graft and bone cement regions after femoral impaction allografting in a cadaveric femur model; b) characterize mechanical properties of the graft bed as a function of impaction force, and explore new alternative graft compaction methods; and c) develop a finite element model to investigate the key mechanisms that contribute to initial implant subsidence. High levels of cement penetration into the graft bed were observed, resulting in extensive regions of cement contact with the host bone in a cadaveric femur model. The implant subsidence correlated negatively with the amount of cement-endosteum contact. The density, compression stiffness and shear strength of the graft were proportional to the impaction force. A slower alternative graft compaction method resulted in higher graft stiffness and shear strength than traditional graft impaction, but the benefit of this new compaction method was small compared to the effect of increasing the impaction force. In a finite element model, the relationship between graft density and subsidence was dependant on cement penetration profile. Without cement-endosteum contact, subsidence decreased with increasing graft density; however, graft density did not affect subsidence in constructs with cement-endosteum contact. Initial subsidence was primarily attributed to slippage at the stem-cement and endosteum interfaces, and the latter mechanism was greatly affected by changes in graft density and cement penetration profile. This study demonstrated that extensive cement penetration can occur in femoral impaction allografting, which may compromise the potential for new bone formation but may be important in preventing excessive subsidence. The endosteum interface was identified as a key factor in the development of subsidence. Finally, our results indicate that the potential benefit of achieving a denser graft bed depends upon the cement penetration profile.

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