Abstract
Summary
Understanding of the role of the radical-generating ability of wear particles of the existing and new implant materials as well as application of efficient antioxidants is one of the necessary conditions for improvement of the results of joint replacements.
Introduction
Functioning of joint prostheses is accompanied by a continuous formation of wear particles and their accumulation in surrounding tissues. The impact of microroughnesses of joint prosthesis friction units may bring about chemical bond breakage and free-radical generation on a newly-formed wear surface. Wear particles of orthopedic alloys are capable to produce free radicals, and Co-Cr-Mo alloy particles are especially active. Free radicals generated by wear particles can cause oxidation and reduced wear resistance of polyethylene. Oxidised polyethylene particles stimulate the activity and release of bone-resorbing cytokines by human monocytes/macrophages. The ability of free radicals to cause damage to surrounding tissues and implant components makes it necessary to estimate comprehensively the radical-generating activity of wear particles of different orthopedic materials and develop the ways of its inhibition.
Methods
Artificial Co-Cr-Mo alloy wear particles were obtained using dry friction of a ball against a disk. The radical-generating ability of orthopedic alloy wear particles was estimated by oxygen consumption using the model reaction of cumene oxidation. The radical-generating ability of wear particles was determined at different moments after their formation and storage at room temperature and humidity. In the experiments, a pro-inflammatory action of wear particles during their continuous formation was also simulated. Fresh cobalt alloy wear particles were used for a consecutive triple oxidation of 2 ml of cumene at a particle concentration of 0.3 mg/ml. After the first 40 min oxidation, a suspension of particles in cumene was centrifuged, and the used particles were removed. Fresh particles were added to oxidised cumene, and the second and third oxidations were carried out in a similar way. The ability of some antioxidants to inhibit the radical-generating ability of cobalt alloy wear particles was also determined.
Results
Fresh cobalt alloy wear particles demonstrated an expressed radical-generating ability which remained practically at the initial level after a one-week storage. The ability gradually reduced in the process of storage. After a one-month storage the particles’ radical-generating ability decreased 2.6 times. A six-month storage of cobalt alloy particles resulted in a tenfold reduction of the radical-generating ability as compared to that of fresh particles. The intensification of radical formation was studied during three consecutive oxidations of cumene by wear particles. It was established that each consecutive oxidation of cumene by fresh wear particles occurred with a growing radical-generation ability. That parameter of the newly-formed particles increased more than two- and threefold during a consecutive double and triple cumene oxidation, respectively. Synthetic antioxidant BHT and natural antioxidant alpha-tocopherol were used for inhibition of wear particles-initiated free-radical reactions. Introduction of the antioxidants inhibited cumene oxidation with an antioxidant dose-dependent duration of this effect. In a mixture of alloy and orthopedic polyethylene particles, alpha-tocopherol completely inhibited the radical-generating activity of alloy particles thus preventing the polymer's oxidative destruction.
Conclusion
The use of commercially available particles of orthopedic alloys with an uncontrolled duration storage in experiments considerably reduce or do not reveal the negative effects conditioned by their radical-generating ability. A proper study of the effect of the radical- generating ability of wear particles on the properties of implant components and surrounding tissues is possible only with the use of fresh particles. Permanent generation of free radicals in the process of wear of joint prosthesis metal components creates conditions for self-potentiation of negative free radical reactions during joint replacement. This requires the necessity of a preclinical estimation of the radical-generating ability of orthopedic materials and application of efficient antioxidants during the post-implantation period.
