The aim of the present study was to assess the mid-term radiological outcomes and in particular to 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 could be observed and none of the investigated implants required revision surgery. Radiologically, no direct signs of cup loosening were obvious and in none of the cases osteolysis could be observed. Clinical outcome and patient satisfaction resulted in very high scores.
Early migration is considered a predictor of aseptic loosening [5, 34], as a mean cup migration of more than 2 mm in the first two years has previously been shown to correlate significantly with aseptic loosening in the long-term [21–25, 31]. The present investigation resulted in a mean total migration of 1.34 mm at five years. Furthermore, at two years a mean migration of 0.86 mm was seen, which is 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]. Our results demonstrated a migration rate of 0.22 mm per year at five years, which, again, is below the defined threshold for aseptic loosening.
To date, only few previous publications can be found regarding the newest generation of the investigated cup. Wyatt et al. [5] also analyzed cup migration, however, only few EBRA measurements (n = 13) could be included. They found a mean migration of 1.5 mm at five years, which can be confirmed by the present study. The majority of migration occurred within the first 12 weeks after surgery, thus, the authors concluded it can be explained just by the initial cup seating and all components can be considered stable thereafter [5]. The present study also shows that migration stagnates at 1-2 years postoperatively and subsequently in most cases secondary stabilization occurs. After the first two years and the onset of secondary stability, only slight further migration could be observed.
The cause of migration can be both, inadequate initial fixation with insufficient primary stability or the subsequent loss of fixation during follow-up [31, 34]. Both scenarios might indicate an increased risk of failure. In contrast, however, it has been shown that minor migration over years often remains asymptomatic [6, 34]. In the present study clinical results are excellent and no signs of failure were detected, despite minor continuous migration. Longer-term follow up will have to confirm these findings in the future.
Wyss et al. [6] investigated the second generation of the isoelastic monoblock cup (RM Pressfit, Mathys Ltd., Bettlach, Switzerland) in a mid-term follow-up. Almost the same results were found, as compared to the present study. However, in contrast to the present investigation, 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 the occurance of migration and subsequent loosening might reflect the quality of operative technique – particularly the reaming process- and implant selection [31]. Studies have also shown a correlation between cup inclination and cup diameter with early migration [22]. However, also the postoperative treatment protocol is likely to have an impact on early migration. In the present study, all surgeries were performed using the minimally-invasive anterolateral approach, theoretically making the cup positioning more challenging, potentially affecting migration. It is remarkable, that similar results are achieved in the present study compared to the investigation by Wyss et al.[6], although the postoperative treatment protocol is far more aggressive.
Besides migration, polyeythelene wear is also an indicator of aseptic loosening of endoprosthetic components, by causing osteolysis of the acetabular and femoral bone stock. Thus, this aspect has additionally been taken into account in the present investigation. It is of major interest to further improve UHMWPE in order to decrease wear and potentially increase the lifetime of acetabular components.
Previous in-vitro studies have already 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 may lead to excellent oxidation resistance [11, 13, 15, 35], which potentially leads to a reduction of wear. Beck et al., performing a mechanical in vitro testing, found a significantly lower wear rate of vitamin E-blended HXLPE compared to standard gamma-sterilized UHMWPE [11]. This, by decreasing wear rate and oxidative degeneration may potentially result in less osteolysis, which in turn might lead to decreased rates of aseptic loosening and failure.
Early studies found that osteolysis is 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 at 0.06 mm per year and thus far below this benchmark. When considering, that during the first year following surgery most of the displacement of the femoral head may be associated with creep, annual rates of actual wear are to be expected even lower. Moreover, 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, showed slightly higher annual wear rates, compared to the present results. Wyss et al.[6] found 0.09 mm per year and Lafon et al.[7] found 0.07 mm per year. Rochcongar et al. recently performed a prospective randomized controlled study comparing RM Pressfit cup (UHMWPE) to RM Pressfit vitamys cup (HXLPE/VitE) and found that wear rates over the first three years following surgery were lower for the HXLPE blended with vitamin-E [2]. This might directly affect the need of late revision surgery. These results have been confirmed by a very recent randomized-controlled trial, again, comparing annual wear rates of the RM Pressfit cup (UHMWPE) to those of the RM Pressfit vitamys cup (HXLPE/VitE). At two years [36], as well as at 6 years [37] wear rates were significantly lower for the HXLPE/VitE cup than those for the UHMWPE cup.
At mid-term follow-up, in the present study, no adverse events occurred and none of the investigated cups showed signs of failure, making not a single revision surgery necessary. A sclerotic line in zone 2 can be found in some of the patients at the dome of the cup, without containing the risk of subsequent loosening. This is most probably because of the aspheric design of the cup with flattening of the dome. Osteointegration seems to be complete and stable in zone 1 and zone 3 in those cases.
Moreover, 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 to the present study have to be acknowledged. First is the mid-term follow-up of 5 years. Although only long-term results should be considered valid regarding the investigation of implant survival, early evaluation of radiological alterations, migration and wear, however, is helpful to identify future undisirable results. Early migration analysis using EBRA has been established in several studies providing a reference to long-term survival [5, 6, 34]. Second the method used to measure migration and wear lacks some accuracy in comparison to radiostereometric analysis (RSA) [32]. As the accuracy has been validated to be within 1 mm, results should be interpreted with caution. Nevertheless, EBRA has become a widespread scientific method without the need to implant markers intraoperatively and thus without causing intense cost and effort.
Measured wear might be greater than when using RSA, for example, due to a probable plastic cup deformation affecting the shape of the contrast wire [21, 32].
Another important limitation of this study is the EBRA software failing to evaluate all radiographs. The image requirements for EBRA measurement are quite challenging, leading to a high rate of radiographs which were not accepted by the EBRA software. Thus, in the present study, reliable results of the EBRA measurements could only be obtained in 42 out of 81 hips at mid-term. As several radiographs were taken at different time points within the follow-up period, the mean migration and wear results are reliable.