An increase in wear resistance and a decrease of periprosthetic osteolysis were suggested in several previous reports about mid-term and long-term results of THA using 1st-generation HXLPE. Previous studies reported that the steady-state wear rate of annealed HXLPE (Excellink; Kyocera) was 0.017 to 0.02 mm/year at 7 to 13-year.15–17 The steady-state wear rate of remelted HXLPE in previous studies was 0 to 0.042 mm/year at 11 to 15-year (Longevity; Zimmer Biomet).18–20
However, oxidative degradation and reduced mechanical properties of these materials were considered to be another risk factors for material failure of THA. Muratoglu et al. showed that minimal oxidation occurred at the time of explantation in 34 retrieved remelted HXLPE acetabular liners (Longevity); however, oxidation levels increased during shelf storage, with a concomitant decrease in crosslink density and increase in crystallinity, whereas no detectable oxidation was found in the shelf-aged control liner.21 Wannomae et al. demonstrated that embrittlement, oxidation, and an increase in crystallinity were shown in 14 surgical explants of annealed HXLPE acetabular liners (Crossfire; Stryker, Mahwah, NJ).8 Currier et al. showed that the measurable oxidation occurred in 22% of 50 retrieved remelted HXLPE acetabular liners (Longevity, n = 21, Marathon, DePuy, Warsaw, IN, n = 26; XLPE, Smith and Nephew, Memphis, TN, n = 3) and 19 tibial inserts (XLK, DePuy, n = 14; Prolong, Zimmer Biomet, n = 2; Durasul, Zimmer Biomet, n = 3) after an average of 2 years in vivo.22 MacDonald et al. showed that oxidation occurred in retrievals of 80 annealed HXLPE components (Crossfire) preferentially at the rim of the liners after an average of 3.5 years in vivo.23
The stabilization of HXLPE with vitamin E either through diffusion or blending was introduced recently to increase resistance to oxidative stress.24 Rowell et al. showed relative resistance to oxidation in retrieved vitamin E-diffused components (E1) after 3-years implantation in comparison to retrieved remelted liners (Longevity) and sequentially annealed liners (X3, Stryker) after short-term implantation.25 Currier et al. showed that minimal oxidation was found in retrieved vitamin E-diffused HXLPE (9 hips, 7 knees; Zimmer Biomet) after an average of 17 months in vivo and vitamin E-blended HXLPE (2 hips, 1 knee; Zimmer Biomet), and statistically significant difference between the two materials was not found.26
As an alternative path, HXLPE hip liners grafted with MPC on their bearing surface to reproduce the same tribophysics of the human cartilage were commercialized as a new generation of artificial hip joints capable to prevent aseptic loosening. Grafting of the MPC onto the HXLPE surface of hip acetabular liners is performed through a photo-induced polymerization technique, producing a covalent bond between MPC and PE polymers.10
Some clinical studies have demonstrated the safety and efficacy of vitamin E-diffused or vitamin E-blended HXLPE and MPC-grafted HXLPE at short-term and mid-term follow-up. Previous studies reported that the steady-state wear rate of E1 was − 0.01 to 0.02 mm/year at 5 to 7-year.27–30 Busch et al. showed that the penetration rate of vitamin E-blended HXLPE (Vitelene; Aesculap AG, Tuttlingen, Germany) was 0.02 mm/year at 5 years.31 Moro et al. reported that the steady-state wear rate of Aquala Q3 was 0.002 mm/year at 5 years.32 Similarly, present study showed low wear rates of E1 at 9 years and Aquala Q3 at 8 years (Table 3).
Table 3
Wear rate of 1st generation HXLPE and vitamin E-supplemented HXLPE
Study | Year | Follow-up (years) | Type of HXLPE | Head Diameter (mm) | Penetration Rate (mm/year) | Steady-state Wear Rate (mm/year) |
Hasegawa et al.15 | 2013 | 7 | Annealed | 26 | | 0.02 |
Naito et al.16 | 2021 | 13 | Annealed | 26 | 0.04 | 0.02 |
So et al.17 | 2015 | 12 | Annealed | 22 | | 0.017 |
Moon et al.18 | 2021 | 15 | Remelted | 22/28 | | 0.028 |
Lachiewics et al.19 | 2016 | 11 | Remelted | 26 | | 0 |
28 | 0.031 |
32 | 0.021 |
36/40 | 0.042 |
Naito et al.20 | 2021 | 12 | Remelted | 32 | 0.05 | 0.01 |
Nebergall et al.27 | 2016 | 5 | Vitamin E-diffused | 32 | | 0.005 |
Shareghi et al.28 | 2017 | 5 | Vitamin E-diffused | 32 | | 0.02 |
Galea et al.29 | 2019 | 7 | Vitamin E-diffused | 32/36 | | -0.01 |
Kjærgaard et al.30 | 2020 | 5 | Vitamin E-diffused | 32/36 | 0.018 | -0.006 |
Busch et al.31 | 2020 | 5 | Vitamin E-blended | 32/36 | 0.02 | |
Moro et al.32 | 2017 | 5 | MPC-grafted | 26 | | 0.002 |
Present study | | 9 | Vitamin E-diffused | 28/32 | 0.04 | 0.004 |
| | 8 | MPC-grafted | 28/32 | 0.05 | 0.007 |
HXLPE highly cross-linked polyethylene, MPC 2-methacryloyloxyethyl phosphorylcholine |
This is the first report about the comparison of polyethylene wear between vitamin E-diffused HXLPE and MPC-grafted HXLPE. Whereas the results of the present study were comparable to earlier studies, the duration of follow-up was longer than previous studies.
Galea et al. showed that radiolucent line was developed in 18% of total hip arthroplasty patients with Regenerex at an average 7-year follow-up.29 However, the clinical relevance of the radiographic finding is unclear. In the present study, no hip showed a radiolucent line that appeared for the first time at later follow-up.
We have previously reported that MPC polymer quickly peeled off from the bearing surface in the retrieved liners, one of these liners was retrieved due to infection in this study.33 However, the wear rate in this study was very low and comparable with previous studies.
No correlations were found between wear rates of vitamin E-diffused or MPC-grafted HXLPE and age, BMI, cup inclination angle, and polyethylene thickness. These findings suggest the usefulness of these liners for a wide range of cases.
This study has some limitations. First, this study included no comparison with conventional UHMWPE or 1st-generation HXLPE. Previous reports have reported that the wear rates of 1st-generation HXLPE and vitamin E-supplemented HXLPE were very low, and there were no significant differences between the two materials.28–31 Second, retrieval analysis was not conducted in VEPE group because no hip underwent revision. While Spece et al. reported that oxidation occurred in retrieved vitamin E-diffused HXLPE for total knee arthroplasty,34 the safety and efficacy of vitamin E-diffused HXLPE were demonstrated in this study.