Characterization of Outer Insulation in Long-Term-Implanted Leads.

Success of pacemakers and implantable cardioverter defibrillators may be limited by premature lead failure. Lead insulation polymers, such as polyurethane (PU) and polydimethylsiloxane (PDMS), are reported to degrade over time in vivo. PU is known to undergo oxidation, whereas PDMS undergoes surface hydrolysis. Previous studies have characterized polymer degradation in vitro, in animals or in short-term human study; however, complex effects of the biochemical and mechanical environment on the lead insulation can only be fully understood by evaluating long-term-implanted leads. Therefore, we established a retrieval program to systematically characterize the chemical and surface changes in 37 of 104 retrieved pacing and defibrillator leads, implanted for ≥5 yr. Fourier transform infrared (FTIR) spectroscopy was used for chemical analysis, and a scanning electron microscope was used for surface degradation evaluation. PDMS leads were investigated for changes in the ratio of Si-O-Si to Si-C peaks, whereas PU degradation was evaluated by changes in ether (C-O-C), carbonyl (C=O), methylene (C-H), and amino (C-N/N-H) peaks. Under SEM, PDMS showed enhanced roughness but no statistical increase in Si-O-Si bonds. PU showed uniform cracking throughout the lead body and statistical changes in each of the oxidation indicative peaks. Overall, both polymers showed surface changes in the physiological environment, but PU was the only material to show chemical changes. This work is a large-scale characterization study on long-term-implanted leads that confirmed PU oxidation but not hydrolysis of PDMS in vivo. It provides important insight for manufacturers when making design improvements and for surgeons when making decisions about lead implantation.

Effects of gamma and e-beam sterilization on the chemical, mechanical and tribological properties of a novel hydrogel.

Hydrogels are known to possess cartilage-like mechanical and lubrication properties; however, hydrogel sterilization is challenging. Cyborgel(™), a proprietary hydrogel, is intended for use as a cartilage replacement implant. This study evaluated the effect of 30-35 kGy e-beam and gamma radiation on the polymer swell ratio, and the mechanical, chemical and tribological behavior of this hydrogel. Three different formulations were mechanically tested, and material parameters were identified using finite element analysis. FTIR spectroscopy was used to investigate chemical changes. Wear test was carried out for 2 million cycles in bovine serum, followed by 2 million cycles in distilled water. No significant difference was found in the swell ratio, mechanical and tribological properties of control hydrogel samples and those exposed to e-beam or gamma radiation; however, chemical spectra of e-beam sterilized samples revealed minor changes, which were absent in unsterilized and gamma sterilized samples.

Can pin-on-disk testing be used to assess the wear performance of retrieved UHMWPE components for total joint arthroplasty?

The objective of this study was to assess the suitability of using multidirectional pin-on-disk (POD) testing to characterize wear behavior of retrieved ultrahigh molecular weight polyethylene (UHMWPE). The POD wear behavior of 25 UHMWPE components, retrieved after 10 years in vivo, was compared with 25 that were shelf aged for 10-15 years in their original packaging. Components were gamma sterilized (25-40 kGy) in an air or reduced oxygen (inert) package. 9 mm diameter pins were fabricated from each component and evaluated against CoCr disks using a super-CTPOD with 100 stations under physiologically relevant, multidirectional loading conditions. Bovine serum (20 g/L protein concentration) was used as lubricant. Volumetric wear rates were found to vary based on the aging environment, as well as sterilization environment. Volumetric wear rates were the lowest for the pins in the gamma inert, shelf aged cohort. These results support the utility of using modern, multidirectional POD testing with a physiologic lubricant as a novel method for evaluating wear properties of retrieved UHMWPE components. The data also supported the hypothesis that wear rates of gamma-inert liners were lower than gamma-air liners for both retrieved and shelf aging conditions. However, this difference was not statistically significant for the retrieved condition.

Mechanically assisted taper corrosion in modular TKA.

The purpose of this study was to characterize the prevalence of taper damage in modular TKA components. One hundred ninety-eight modular components were revised after 3.9±4.2 years of implantation. Modular components were evaluated for fretting corrosion using a semi-quantitative 4-point scoring system. Design features and patient information were assessed as predictors of fretting corrosion damage. Mild-to-severe fretting corrosion (score ≥2) was observed in 94/101 tapers on the modular femoral components and 90/97 tapers on the modular tibial components. Mixed alloy pairs (p=0.03), taper design (p<0.001), and component type (p=0.02) were associated with taper corrosion. The results from this study supported the hypothesis that there is taper corrosion in TKA. However the clinical implications remain unclear.