Corrosion and Oxidation Analyses on metal-on- polyethylene total hip implants retrieved due to pseudotumor formation

Wang, Qiong (Department of Materials Engineering, University of British Columbia)
Eltit, Felipe (Department of Materials Engineering, University of British Columbia)
Duncan, Clive P. (Department of Orthopedics, University of British Columbia)
Garbuz, Donald S. (Department of Orthopedics, University of British Columbia)
Greidanus, Nelson (Department of Orthopedics, University of British Columbia)
Masri, Bassam A. (Department of Orthopedics, University of British Columbia)
Wang, Rizhi (Department of Materials Engineering, University of British Columbia)

Introduction

There are two typical designs of total hip arthroplasty (THA) in terms of bearing couple materials. One is a metallic femoral head articulating against polyethylene acetabular liner (MOP, or metal-on-polyethylene) and the other is metallic head on metallic cup bearing (MOM, or metal-on-metal). The latter MOM bearing hip implants with modular design, were frequently associated with the development of benign fibroma described as pseudotumor. The highly cross-linked polyethylene (PE) with improved wear and fatigue resistance makes the MOP bearing a promising design in total hip arthroplasty. Unfortunately in 2012, a typical inflammatory pesudotumor was reported from a MOP hip implant failure[1]. We have also found similar adverse reactions in this group. These lasted observations have brought serious concerns to the biomedical community and the patients undergoing hip replacement. The mechanism for formation of pseudotumors in this population is largely unknown. As a first step, we evaluated a group of MOP implants retrieved from patients with pseudotumor in an effort to determine the possible causes of adverse response to this bearing couple. We studied the oxidization and wear of the PE liners and analyzed the corrosion product located in the metallic head socket. These findings will help us to link the pseudotumor formation with the specific material or mechanical designs.

Materials and Methods

Patient information: 7 MOP hip implants with the periprosthetic tissues were retrieved during revision surgery from patients with pseudotumor following the ethics approval. All of these MOPs were the same model of Ti-6Al-4V stem coupled with Co28Cr6Mo head wearing against highly cross-linked PE liner. And the duration of implants in vivo varied from 23 to 80 months. Analysis of High Cross-linked PE liner: The oxidation index of PE was evaluated following ASTM F2102-06 standard by a Fourier transform infrared spectrometer (FTIR, Bruker EQUINOX 55) with microscope attachment. The oxidization value of each test was calculated as the ratio of the carbonyl absorption area near 1720cm-1 to the C-H absorption 1370 cm-1 peak. A portion of wearing PE liner was sectioned and sputtered with gold coating. Optical microscopy (Nikon SMZ 1000) and Scanning electron microscopy (SIGMA for materials, Zeiss) were used to study the surface wear morphology. Corrosion Analysis: Cellulose acetate tape was used to make extraction replicas of the corrosion products on the head sockets or the modular adaptors. The replicas were then analyzed with an SEM (SIGMA for materials, Zeiss) equipped with an energy dispersion X-ray spectrometer (EDS).

Results

No obvious oxidizations were found on the PE samples. The highest oxidization value 0.39 obtained in this study is at very low level (See Fig. 2). All the PE liners were rated for surface damage on a scale of 0(none) to 3(severe) following the protocol developed by Hood[2]. The samples presented variable levels of wear damage, which were not proportional to the time in vivo. High Resolution SEM showed two typical morphologies with different compositions in these corrosion products (Fig. 3).Aggregation of nano-size particles contained Cr, Mo and Ti elements while bulk debris were rich in Cr,P and Mo without Ti.

Discussion and Conclusion

There is an assumption that the free radicals associated with oxidization in PE may cause adverse tissue reaction[3]. Here in our study, all of these PE liners retrieved from patients with pesudotumor presented negligible oxidization. Besides, the wear situations were varied. Therefore, no significant association between the PE oxidation and PE wear and the pesudotumor formation could be concluded based on our current study. The formation of corrosion debris was believed a corrosion process with mechanical assistance[4]. The consistent fretting between the CoCrMo and Ti alloy at the junction may destroy the passivation layer, accelerating the oxidization of alloys and release of metallic ions. Since both cobalt and chromium are known to be cytotoxic at certain concentration level, the release of the metallic particles and ions at the modular junction could act as positive stimuli to pseudotumor formation. Future research will be conducted on the metallic debris in the periprosthetic tissues. Comparison of the metallic debris between tissues and implants will help to confirm our primary conclusion.

Table 1. Oxidation index and wear damage of the retrieved implants

Fig 2. The FTIR spectrum obtained on sample MOP6 at around 500µm depth, which showed the highest oxidization value of all samples

Fig 3. Scanning electron micrograph showing aggregates of nano-particles with significant Ti element(Left) and blocks of debris rich with Cr but Ti in the corrosion product on MOP2 case(Right). Inset is energy dispersive X-ray spectra

References

1. Walsh, A. J.; Nikolaou, V. S.; Antoniou, J. Inflammatory pseudotumor complicating metal-on-highly cross-linked polyethylene total hip arthroplasty. J. Arthroplasty 2012, 27, 324.e5–8. 2. Hood, R. W.; Wright, T. M.; Burstein, A. H. Retrieval analysis of total knee prostheses: a method and its application to 48 total condylar prostheses. J. Biomed. Mater. Res. 1983, 17, 829–42. 3. Kehrer, J. P. Free radicals as mediators of tissue injury and disease. Crit. Rev. Toxicol. 1993, 23, 21–48. 4. Swaminathan, V.; Gilbert, J. L. Fretting corrosion of CoCrMo and Ti6Al4V interfaces. Biomaterials 2012, 33, 5487–5503.

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