Diffusion of vitamin E in ultra-high molecular weight polyethylene
Introduction
Ultra-high molecular weight polyethylene (UHMWPE) has been the material of choice for the load bearing and articulating components in total joint arthroplasty [1]. Adhesive/abrasive wear of UHMWPE has been the leading cause of peri-prosthetic osteolysis [2], [3], compromising long-term performance of total joints. One method of improving the wear resistance of UHMWPE is radiation crosslinking [4], [5], [6]. Radiation crosslinking to a dose of 100 kGy has been shown to decrease adhesive/abrasive wear substantially in both in vitro experiments and in vivo clinical studies [7], [8].
Highly crosslinked, wear resistant UHMWPEs currently in clinical use are prepared by irradiation and subsequent thermal treatment. The latter is used to decrease or eliminate the residual free radicals caused by irradiation. The residual free radicals are typically trapped in the crystalline regions and cause oxidative embrittlement in the long term [9]. Melting of irradiated UHMWPE decreases the concentration of residual free radicals to undetectable levels and such components have been shown to be oxidation resistant both in vivo and in vitro [10]. One disadvantage of melting is the decrease in the crystallinity of irradiated polymer, which also decreases some of its mechanical properties [11].
We recently advanced an alternate method of preventing oxidation in irradiated UHMWPE through the stabilization of the residual free radicals with the antioxidant vitamin E (α-tocopherol) [12]. Vitamin E stabilization replaces post-irradiation melting and hence prevents the loss of crystallinity without sacrificing wear or oxidation resistance [13]. Vitamin E is a natural lipid whose major role in vivo is to donate a hydrogen atom to free radicals formed on lipids to hinder lipid peroxidation in cell membranes [14], [15]. Its lipophilicity, due to its phytyl tail (Fig. 1), allows it to penetrate through cell membranes, and also provides miscibility with polyethylene.
There are two methods of incorporating vitamin E into UHMWPE. One is to blend vitamin E with UHMWPE powder prior to consolidation. Once consolidated, the blend can be crosslinked with the use of ionizing radiation. However, the presence of vitamin E in UHMWPE during irradiation reduces the efficiency of crosslinking [16], [17]. An alternative method is the diffusion of vitamin E into UHMWPE following radiation crosslinking [12]. The crosslinking efficiency of UHMWPE is not adversely affected in this method since vitamin E is not present during irradiation.
Therefore, it is desirable to study the diffusion of vitamin E in crosslinked UHMWPE. The molecular weight of this linear polyolefin is typically on the order of 1–6×106 g/mol. The long chains of UHMWPE allow a 50–60% semi-crystalline structure with crystallites exhibiting a peak melting transition of approximately 135 °C when crystallized from the melt at low pressures. These crystallites are impermeable to even small molecules such as oxygen and hence, would be impermeable to a relatively large molecule like α-tocopherol (430.7 g/mol). Therefore, the diffusion of vitamin E in polyethylene is primarily through the amorphous phase.
Diffusion through a semi-crystalline polymer cannot be characterized and predicted solely as diffusion through an amorphous volume because crystalline regions are expected to hinder the diffusion of molecules also by restricting diffusion pathways. Therefore, the diffusion coefficient of the amorphous region would be decreased due to impedance created by the crystallites. Since crystals start melting at about 100 °C in UHMWPE (Fig. 2), the impedance of crystallites are expected to decrease with increasing temperature above 100 °C until the peak melting point at about 135 °C. Likewise, chemical crosslinks introduced into the amorphous phase are expected to decrease diffusion provided that chain scission as a result of irradiation has not decreased the molecular weight.
In order to interact with the residual free radicals and prevent oxidation, vitamin E has to be present throughout an irradiated UHMWPE joint implant [18] without detrimentally affecting morphological and mechanical properties. The diffusion of vitamin E in UHMWPE has not been extensively studied. Wolf et al. [19] have shown that vitamin E penetration through crosslinked UHMWPE can be achieved by using doping in pure vitamin E and further homogenization in inert atmosphere and supercritical carbon dioxide at temperatures above the melting point of crosslinked UHMWPE. This does not take into account the effect of the crystalline regions of UHMWPE, which need to be preserved during vitamin E stabilization to maintain the mechanical properties of crosslinked UHMWPE [11], [12].
We studied the diffusion behavior of virgin and radiation-crosslinked UHMWPE as a function of temperature and time. We first tested the hypothesis that the diffusion through irradiated UHMWPEs would be hindered due to increased crosslink density. Furthermore, we studied the effects of a subsequent homogenization step in inert atmosphere below the melting point in diffusing a high surface concentration of vitamin E throughout a desired thickness of UHMWPE. We developed an analytical model for the diffusion of vitamin E in UHMWPE to predict diffusion times to penetrate desired diffusion path lengths.
Section snippets
Radiation crosslinking
Slab compression molded GUR1050 UHMWPE (Orthoplastics, Lancashire, UK) was packaged in aluminum foil packaging (5610, Technipaq, Inc., Crystal Lake, IL) in vacuum and irradiated to 65 and 100 kGy using 60Co gamma irradiation (Steris Isomedix, Northborough, MA). Unirradiated virgin UHMWPE was used as a control.
Determination of crosslink density
Crosslink density measurements were performed on the 65- and 100-kGy irradiated UHMWPE (n=3 each) using a thermal mechanical analyzer (DMA 7e, Perkin Elmer, Wellesley, MA). Thin sections
Results and discussion
Our aim was to study and model the diffusion of vitamin E in virgin UHMWPE and radiation-crosslinked UHMWPE in order to predict diffusion profiles following doping of UHMWPE by soaking in vitamin E with subsequent homogenization at an elevated temperature. A Fickian model provided good fits with experimental data after soaking and after post-soak homogenization.
In addition to the chemical interactions between UHMWPE and vitamin E, there are two structural features of semi-crystalline UHMWPE
Conclusion
The diffusion behavior of irradiated UHMWPE was different from unirradiated UHMWPE due to lower surface concentration, higher diffusion constant, and higher activation energy although radiation dose did not have a large effect on these factors for 65 and 100 kGy. By using a Fickian model, we were able to describe the diffusion behavior and predict vitamin E concentration profiles for vitamin E-doped samples. Further, we developed a method to penetrate vitamin E in UHMWPE joint components by
Acknowledgment
This study was funded by NIH R01 AR051142.
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