Allograft bone decreases in strength in vivo over time

Allograft bone is the primary source of graft material for structural oncological limb salvage procedures. Failure rates after massive allograft reconstructions have been reported as high as 60% at 10 years, which are associated with a multitude of biologic processes influencing the graft incorporation and functional capacity. It is unknown if mechanical failure is associated with a gradual loss of bulk material properties of the bone (strength and modulus), loss of bone mineral density, osteoclastic resorption of the allograft, unrepaired allograft microfractures or microcracks, and/or local stress concentration within the tissue. Allograft material properties, bone mineral density, microcrack prevalence, and cortical porosity were quantified in 13 failed human allograft retrievals ranging in longevity from 1 to 13 years in vivo. Nonimplanted allograft tissue (n = 27) served as the baseline for comparison. A 50% loss in strength of allograft tissue was noted after 10 years in vivo. Loss of strength was correlated with an increase in microfracture prevalence and decrease in bone mineral density within the retrieved allograft cortex. This study suggests functional failure of allograft limb salvage procedures may, in part, be attributed to degradation of the tissue's material properties, bone mineral density and prevalence of microcracks.