VITAL-E & VITAL-XE®
Cross-linked polyethylene with added Vitamin E
VITAL-XE® is a Polyethylene (UHMWPE) with the addition of 0,1% (weight) of Vitamin E (alpha-Tocopherol) that Permedica utilizes for the manufacturing of joint inserts. The Vitamin E is mixed into the polyethylene powder immediately before the formation of the raw material bar.
Vital-XE® is a cross-linked polyethylene, beta-ray irradiated with a 60 kGy dose and then ETO sterilized.
VITAL-XE® is used for the realization of joint inserts in total hip arthroplasties.
VITAL-E® is the non-irradiated version, therefore without crosslinking of the Vitamin E polyethylene, and sterilized ETO. Vital-E® is used for non-cross-linked knee prosthetic inserts, in addition to humeral heads and glenospheres for shoulder prostheses.
OXIDATION
The degradation of UHMWPE and polymers in general is a process that alters the chemical structure of the material that diminishes the molecular weight of the material. Degradation diminishes the polyethylene mechanical characteristics and therefore, causes precocious wear.
Degradation of a polymer comes with the breakage of the chemical bonds and the creation of new free radicals, when for example the material is exposed to a quantity of energy superior to that of the chemical links themselves that make up the molecular chain of the polymer. In the presence of oxygen this process is called oxidative degeneration or oxidation.
Once the oxidation process begins, it is irreversible. The formation of new free radicals that react with oxygen then trigger the breakage of other chemical ties which then generate other free radicals and so on, creating a chain reaction that quickly self generates. The oxidation velocity depends on the number of free radicals present and from the amount of oxygen that could be present in the atmosphere during the manufacturing and sterilization processes of the polyethylene, which can also penetrate during the storage of the product or once implanted in the patient.
Polyethylene used in joint replacement systems typically undergoes elevated doses of energy in the form of gamma or beta ray irradiation to sterilize or increase the polyethylene level of crosslinking. Such irradiation produces the formation of active free radicals inside the material. The major part of the free radicals recombine forming new traversal links during the amorphous stage of the polyethylene – the crosslinking stage. The free radicals instead, that are formed during the polyethylene crystalline stage long remain trapped without recombining into new links. In the presence of oxygen these radicals are responsible for the progressive breakage of the polymeric links through oxidative chain reactions and consequently, are responsible for the reduction of the polyethylene mechanical properties.
Oxidation determines the degradation of the polyethylene molecular structure and therefore, in time causes a progressive decline of the mechanical and tribological properties.
VITAMIN E AS A STABILIZING ADDITIVE FOR POLYETHYLENE
Stabilization of the polymers is principally based on the reduction of the oxygen concentration or on the reduction of the number of free radicals or their reactivity, or to reduce or theoretically eliminate the problem of polyethylene oxidation. In the case of (UHMWPE) polyethylene, in vacuum sterilization or in modified atmosphere brings advantages, but oxygen is anyway present within the material, penetrating in a diffused way during the production process; and therefore it is materially impossible to completely eliminate the oxygen from the polyethylene.
Many additives exist in the market able to limit the reactivity of free radicals, unfortunately most of these have issues of biocompatibility, something absolutely necessary for products to be implanted in the human body.
A biocompatible drug which has been studied in literature for some time for the stabilization of polyethylene in medical use is Vitamin E (alpha-Tocopherol). Vitamin E is present in many vegetables and represents a nutrient that is essential for man.
The stabilization of UHMWPE through Vitamin E was introduced as an alternative method to guarantee resistance to oxidation without sacrificing the UHMWPE mechanical properties. There are two methods for introducing low concentrations
( 0.05%-1%) of Vitamin E into UHMWPE. The first consists of mixing a liquid form of Vitamin E into the UHMWPE powder before the extrusion process and then crosslinking the bar. The second method consists instead of blending the Vitamin E into the already consolidated and cross-linked UHMWPE.
The addition of minimum concentrations of Vitamin E into the UHMWPE eliminates the problem of oxidation and consequently increases the longevity.
BIBLIOGRAPHY
Torre M, Brach Del Prever E, Costa L, Romanini E, Masciocchi M, Manno V. Materiali innovativi in ortopedia: il polietilene reticolato.
Ann Ig 2011;23:81-90.
