The aim of the present study was to assess mid-term radiological outcomes in THA and investigate the migration and wear rate of the third generation of a cementless isoelastic monoblock cup with vitamin E-blended HXLPE. At mid-term follow-up, almost no cup-related complications were observed, and none of the investigated implants required revision surgery. Radiologically, no direct signs of cup loosening were obvious, and no cases osteolysis were observed. Clinical outcomes were good, with very high patient satisfaction scores.
Early migration is considered a predictor of aseptic loosening [5, 34], and mean cup migration >2 mm in the first 2 years was shown to correlate significantly with long-term aseptic loosening [21–25, 31]. The present investigation found a mean total migration of 1.34 mm at 5 years. Mean migration of 0.86 mm was seen at 2 years, which was far below the above-mentioned 2 mm limit. Additionally, loosening was defined as an overall migration increase of 0.5 mm per year [30, 31]. We measured a migration rate of 0.22 mm per year at 5 years, which was also far below the threshold for aseptic loosening.
Few previous publications have studied the newest cup generation. Wyatt et al. [5] also analyzed cup migration, however, but only a few EBRA measurements (n = 13) were included. They found a mean migration of 1.5 mm at 5 years, which was confirmed by the present study. Since most migration occurred within the first 12 weeks after surgery, the authors concluded it was due to the initial cup seating and all components can be considered stable thereafter [5]. The present study also found that migration stagnated at 1-2 years postoperatively, and secondary stabilization subsequently occurred in most cases. After the first 2 years and the onset of secondary stability, only slight further migration was observed.
The cause of migrations can be both inadequate initial fixation with insufficient primary stability or loss of fixation during follow-up [31, 34]. Both scenarios might indicate an increased risk of failure. However, minor migration over years is often asymptomatic [6, 34]. In the present study, the clinical results were excellent with no signs of failure despite minor continuous migration. Longer-term follow-up will be needed to confirm these findings.
Wyss et al. [6] investigated the second generation of the isoelastic monoblock cup (RM Pressfit, Mathys Ltd.) in a mid-term follow-up. Similar results to the present study were found, even though all surgeries were performed using a transgluteal approach, patients were only alowed partial weight bearing, and flexion was initially limited to 70° [6]. It is obvious that migration and subsequent loosening might reflect the quality of operative technique - particularly the reaming process - and implant selection [31]. One study reported a correlation between cup inclination and cup diameter with early migration [22]. However, the postoperative treatment protocol is likely to affect early migration. In the present study, all surgeries were performed using the minimallyinvasive anterolateral approach, theoretically making cup positioning more challenging and potentially affecting migration. It is remarkable that similar results were achieved in the present study compared to the investigation by Wyss et al. [6], although our postoperative treatment protocol was far more aggressive.
Polyeythelene wear is another indicator of aseptic loosening of endoprosthetic components and causes osteolysis of the acetabular and femoral bone stock. This aspect was considered in the present investigation. It is critical to further improve UHMWPE to decrease wear and potentially increase the lifetime of acetabular components.
Previous in vitro studies demonstrated that vitamin E-blended HXLPE improves fatigue strength and protects against oxidative damage [11, 15, 17, 18]. The protection of HXLPE with vitamin E could imbue excellent oxidation resistance [11, 13, 15, 35] and potentially reduce wear. Beck et al. performed mechanical in vitro testing and found a significantly lower wear rate of vitamin E-blended HXLPE compared to standard gamma-sterilized UHMWPE [11]. Decreasing wear rate and oxidative degeneration may reduce osteolysis, which in turn could decrease rates of aseptic loosening and failure.
An early study found that osteolysis was rarely observed in THA patients with wear below the threshold of 0.1 mm per year [19]. In the present investigation, annual wear rates were far below this benchmark at 0.06 mm per year. Since most femoral head displacement in the first postoperative year may be associated with creep, annual rates of actual wear are to be expected even lower. Dumbleton et al. found that below a rate of 0.05 mm per year, osteolysis will practically not occur [19]. Earlier studies using the second generation of the isoelastic monoblock cup reported slightly higher annual wear rates compared to the present results. Wyss et al. [6] and Lafon et al. [7] found 0.09 mm and 0.07 mm per year, respectively. Rochcongar et al. recently performed a prospective randomized controlled study comparing the RM Pressfit cup (UHMWPE) to the RM Pressfit vitamys cup (HXLPE/VitE) and reported that wear rates over the first 3 years following surgery were lower for HXLPE/VitE group [2]. This could directly affect the need for late revision surgery. These results were again confirmed in a recent randomized-controlled trial comparing annual wear rates of the RM Pressfit cup (UHMWPE) to those of the RM Pressfit vitamys cup (HXLPE/VitE). At 2 years [36] and 6 years [37], the wear rates were significantly lower for the HXLPE/VitE cup.
At mid-term follow-up, no adverse events occurred and none of the investigated cups showed signs of failure, and not a single revision surgery was necessary in our study. A sclerotic line in zone 2 was observed in some of the patients at the cup dome, without increasing the risk of subsequent loosening. This is likely because the aspheric design of the cup has a flattened dome. Osteointegration seemed to be complete and stable in zones 1 and 3 in those cases.
Moreover, our encouraging clinical results with marked improvements in functionality and activity level confirm earlier studies [2, 4–7, 9, 10, 14, 18] and strongly support the concept of a cementless isoelastic monoblock cup with vitamin-E blended HXLPE.
Some limitations of the present study have to be acknowledged. The first is the mid-term follow-up period of 5 years. Although only long-term results should be considered regarding implant survival, early evaluation of radiological alterations, migration, and wear is helpful to identify future undisirable results. Early migration analysis using EBRA has been established as a reference to long-term survival [5, 6, 34]. Second, the method used to measure migration and wear has lower accuracy than radiostereometric analysis (RSA) [32]. As the accuracy has been validated within 1 mm, our results should be interpreted with caution. Nevertheless, EBRA has become a widely used scientific method without the need to implant markers intraoperatively and thus reduces cost and effort. Measured wear might be greater with EBRA than when using RSA, for example, due to probable plastic cup deformation affecting contrast wire shape [21, 32]. Another important limitation is that the EBRA software failed to evaluate all available radiographs. The image requirements for EBRA measurement are considerable, leading to a high rate of radiographs being rejected by the EBRA software. In the present study, reliable EBRA measurements were only obtained in 42 out of 81 hips at mid-term. Since several radiographs were taken at different time points during the follow-up period, the mean migration and wear results are reliable.