The aim of the present study was to assess the mid-term clinical and radiological outcome 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, 35], as a mean cup migration of more than 2 mm in the first 2 years has previously been shown to correlate significantly with aseptic loosening in the long-term [21–25, 33]. The present investigation resulted in a mean total migration of 1.34 mm at 5 years. Furthermore, at 2 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 [32, 33]. Our results demonstrated a migration rate of 0.22 mm per year at 5 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 5 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 2 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 [33, 35]. 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, 35]. 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 [33]. 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 has been reported that oxidation of polyethylene particles is a factor inducing osteolysis [11, 29, 36, 37]. Therefore, 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 leads to excellent oxidation resistance [11, 13, 15, 38], which potentially leads to a reduction of wear. Beck et al. reported, that the wear rate of vitamin E-blended HXLPE is at least seven times lower than the wear rate of the standard gamma-sterilized UHMWPE [11]. This, by decreasing wear rate and oxidative degeneration potentially results 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 are at 0.06 mm per year and thus far below this benchmark. 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 y and Lafon et al.[7] found 0.07 mm per y. Rochcongar et al. recently performed a prospective randomized controlled study comparing RM Pressfit cup (UHMWPE) to RM Pressfit vitamys cup (HXLPE/VitE) and confirmed 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. Engh et al. compared cross-linked (HXLPE) and non-cross-linked (UHMWPE) polyethylene and showed that lower wear occurred in the cross-linked group and fewer revision surgeries were necessary [39].
At mid-term follow-up, 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, 35]. Second the method used to measure migration and wear lacks some accuracy in comparison to radiostereometric analysis (RSA) [34]. 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, 34]. Nevertheless, the need to implant markers intraoperatively restricts the usage of RSA significantly and would have caused intense cost and effort.
Another 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 are not accepted by the EBRA software. However, with included data of 42 patients at mid-term follow-up, to our knowledge, no equivalent high number has been analyzed so far in previous studies. As several radiographs were taken at different time points within the follow-up period, the mean migration and wear results are highly reliable.