Background: Clinical evidence has suggested that the rate of
fracture in allografts sterilized with gamma radiation may be higher than that
in controls. Gamma radiation sterilization has been shown to affect the
post-yield properties of bone but not the elastic modulus. Since most
allograft fractures occur with subcritical loads during activities of daily
living, it may be that the fatigue properties of irradiated allografts are
diminished. In this study, the fatigue crack propagation behavior of cortical
bone sterilized with gamma radiation was compared with that of gender and
age-matched controls. We hypothesized that gamma radiation significantly
reduces the resistance of cortical bone to fatigue crack growth.
Methods: Specimens for fatigue crack propagation testing were
machined from four pairs of fresh-frozen human femora obtained from four
individuals (a younger male, younger female, older male, and older female
donor). Half of the specimens were sterilized with 31.7 kGy of gamma
radiation. The specimens were cyclically loaded to failure in a servohydraulic
testing system, and crack growth was monitored. The cyclic stress intensity
factor and the fatigue crack growth rate were calculated to examine the
kinetics of fatigue crack growth. Following testing, the damage zone around
the fracture plane was analyzed histologically.
Results: The morphology and kinetics of crack growth in irradiated
specimens differed from the control data. Overall, the irradiated bone was
significantly less resistant to fatigue crack growth than was control tissue
(p < 0.05). There was less microdamage associated with fracture in the
irradiated specimens than in the control specimens, with the exception of the
bone from the older female donor.
Conclusions: Gamma radiation sterilization significantly reduces the
fatigue crack propagation resistance of cortical bone. Irradiated specimens
also demonstrate a smaller amount of microdamage along the fracture plane.
These findings may be due to ultrastructural alterations in the collagen
matrix caused by radiation.
Clinical Relevance: This study suggests that, despite having
pre-yield mechanical properties that are similar to those of nonirradiated
bone, gamma-radiation-sterilized allograft may be more predisposed to fracture
even under the subcritical loads that occur during the activities of daily