Radiopaque polyethylenes to aid surgical device positioning and patient follow up

Introduction: The X-ray attenuation of polyethylene is comparable to that of tissue, so it is not currently possible to visualise polyethylene medical devices using X-ray or CT imaging. The clinical impact of this is surgeons cannot use techniques such as fluoroscopy to assist with implant positioning, and cannot monitor implant performance and/or condition after the surgery using X-ray based methods. These issues are particularly relevant to devices such as ligament repair sutures [1] or maxillofacial implants [2] where accurate positioning is essential, and joint replacement devices where early diagnosis of complications such as wear can be critical. This work uses oil-based contrast agents to create radiopaque polyethylene devices, and examines the potential of this biomaterial. Experimental methods: Ultra-high molecular weight polyethylene (UHMWPE) in solid form (GUR 1050 moulded sheets, Celanese, Germany), and UHMWPE braided into sutures (ACL Tightrope, Arthrex, Germany), were examined. For the solid UHMWPE, tensile test samples (ISO‑572, type 1AB) were immersed in pure iodised oil (Lipiodol, Guerbert, France) and held at 85oC, 105oC, and 115oC for 12 h. The UHMWPE sutures were cut into 500 mm lengths and immersed in pure iodised oil at 80 oC for 1h. Tensile tests were performed using an electromechanical test machine (Instron) in accordance with the standards (ISO 572 for the solid UHMWPE, ISO 2062 for the sutures). The radiopacity was quantified using μCT imaging (Nikon XT H 225 kV) alongside a sample of distilled water for calibration, the grayscale values were then converted into Hounsfield units (HU). The concentration of iodised oil was determined using Fourier Transform Infrared (FTIR) Spectroscopy (32 scans, 4000 – 600 cm-1, assigned peaks summarised in Table 1), and the FTIR spectra were also used to quantify polyethylene oxidation. The crystallinity of UHMWPE was quantified using Differential scanning calorimetry (DSC). All experiments used a sample size of 5, included untreated controls, and were compared statistically using Kruskal-Wallis or Mann Whitney U tests. Results and discussions: The iodised oil diffused into both the solid and suture UHMWPE samples at the elevated temperatures. In the solid samples held at 115oC the treatment caused a mean weight increase of 33±4%, and a weight increase of 32±3% in the sutures. The radiopacity significantly increased in all samples (p<0.05, see Figure 1: X-ray of a treated partial joint replacement). The average radiopacity of the treated sutures was 744.5±85 HU, and the solid UHMWPE held at 115oC was 1562.0±426 HU. The solid UHWMPE had similar weight increase but a higher HU than the braided UHMWPE; this may be due to the higher porosity of the braided sutures. The tensile tests found no difference in ultimate tensile strength (p=0.22) or elongation at break (p=0.1) of the solid or sutures samples, but the tensile modulus was lower in the treated samples (p=0.013). No change was observed in the crystallinity or the melting point of the polyethylene, and no oxidation was detected from the FTIR spectra. Conclusions: Radiopaque polyethylene has been created successfully through diffusion of iodised oil. No difference was found in the crystallinity, oxidation, or mechanical properties of the solid or suture samples, with the exception of the tensile modulus. Current work is examining the long-term behaviour of this material (fatigue, creep, wear, aging and leaching) so its safety for clinical use can be assessed. References: [1] Jameson, S. S. et al. The Knee 19, p14-19 (2012) [2] Oh, J. Maxillofacial plastic and reconstructive surgery 40, p2 (2018) [3] Zagonel, G.F. et al. Talanta 63, p1021–1025 (2004). [4] Carrión-Prieto, P. Pharmacogn Journal 9, p157-162 (2017).