"Improvement of the biophysical properties of bone tissue sterilized by gamma radiation using different antioxidants"

Abstract

The critically sized bone defects are gaps which are too wide for the body’s physiology to heal by itself. Human bone transplants (allografts) are used for orthopaedic restoration of structural abnormalities arising from traumatic injuries; disorders like bone cancer and revision arthroplasty. In such cases, a certain kind of reconstruction is required, including grafting material to fill the gap and preserve the structure and function. The several benefits of utilizing an allograft over an autograft are availability, no donor site morbidity, availability of different sizes and shapes, and faster operating time and healing time. Nevertheless, the utilization of a donor allograft raises the risk of transmission of viral and bacterial diseases to the recipient that may contribute to substantial secondary complications. Gamma radiation sterilization is the procedure followed by most tissue banks to sterilize bone allograft to reduce or eliminate pathogens (bacteria, viruses, fungi) and lower the risk of recipient infection. It has become a common technique for sterilization due to of its effectiveness in viral and bacterial bioburden reduction and its high penetration depth. The ‘‘standard dose’’ of gamma irradiation varies from 15 to 35 kGy but this depends on the bioburden and desired sterility assurance level (SAL). SAL is set by US Food and Drug Administration to 10-6 (probability of 1 in 1,000,000 of finding a non sterile unit) for implantable medical devices that come in contact with human tissue. The concept of SAL is to ensure microbial inactivation after sterilization. Although a radiation dose of 25 kGy is the reference dose recommended by the International Atomic Energy Agency (IAEA) for the sterilization of tissue grafts, greater doses are required to sterilize against radiation-resistant viruses However, gamma radiation sterilization impairs the material properties of bone which is a major clinical concern since bone grafts are used in load bearing applications. The majority of gamma radiation damage is induced by damaging species (i.e. free radicals) resulting from the radiolysis of water molecules that might be minimized with the usage of free radical scavengers (e.g. antioxidants).

The aim of this study was to assess the potential of both Hydroxytyrosol (HT) and Alpha Lipoic Acid (ALA) as free radical scavengers to preserve the mechanical properties and chemical composition of gamma-irradiated cortical bone. Fresh (90) bovine femurs (aged between 18 and 24 month) were used in this study. Four groups were included for each antioxidant namely: control, antioxidant treated, irradiated and antioxidant pretreated plus irradiated. The concentration of 9-mM ALA for 3-day and 0.25-mM HT for 7- days bone soaking has been chosen according to a pilot study. There were three irradiated groups; one of them was only irradiated by 35 kGy, while the other two groups were first treated with ALA (9 mM for 3 days) or HT (0.25 mM for 7 days) and then irradiated to the same dose in the presence of the antioxidants.

For the aforementioned target, specimens were subjected to three point bending and microhardness tests to assess the capacity of both free radical scavengers for reducing gamma radiation deterioration of the mechanical properties of bone. In addition, a validation test of radiation sterilization had been carried out to ensure that both antioxidants did not protect the pathogenic organisms existing in bone specimens. Also X-ray Diffraction Analysis )XRD( and Fourier Transform Infrared Spectroscopy-Attenuated Total Reflection )FTIR-ATR) techniques were used to analyze the changes in crystallinity and chemical composition induced by irradiation in the presence of free radical scavengers.