The results of new technologies such as the current-generation SRA devices reported from highly specialized centers are often the best possible results and do not necessarily reflect what is achieved with general use. Registries not only capture the survival rates of SRA performed nationwide, but also capture those of conventional THA. It was our aim to evaluate and compare the midterm survivorship and the prognosticators for failure of SRA with those of THA. Specifically, we reviewed reports from these NJRRs to (1) determine whether the results of SRA are indeed less reproducible than those of THA; (2) identify the failure modes; (3) determine whether we could identify prognosticators for early failure that might enhance survival of SRA by better patient selection; (4) determine whether the prognosticators were similar for SRA and THA; and (5) determine the survival of revisions of SRA.
Registries have a number of limitations. First, the weakest potential link remains the quality of data collection. This possible hazard is efficiently bypassed in the registries with the longest track records [5
] with capture rates exceeding 95% of cases performed. Unfortunately, the data from only a few registries are directly accessible and the data of some registries are only early followup data [5
]. Second, failure of hip arthroplasties is often multifactorial [33
]. Registry data lack specific parameters important for the survival of the implant such as patient-related data (eg, body weight, femoral head cysts [1
]), surgical experience [4
], and surgical technique (eg, computer-assisted navigation [10
]). Third, most often only revision is considered a failure. Everything comes down to the definition of “failure,” which in survival analysis is interpreted as an event instead of a process such as loosening of a component [11
]. Radiographic features such as stem shaft angle and abduction of the socket are important for failures in SRA [1
], but their influence as a predictor for failure will not be noticed if they are not captured in the database. Postoperative complications (eg, nerve palsy [6
]) can be considered as important modes of failure. The overall complication rate varied from 0% to 18% in a meta-analysis of 13 trials with 3594 SRAs. The mean revision rate, however, was 3.7% (Table ). The overall success rate would thus be an overestimation if only revision is considered as the end point. Functional parameters are especially important with regard to “failure” of SRA in the young, active patient. Some questionnaires (EQ-5D index) are now implemented in the Swedish Hip Registry, but no data on SRA are currently available. It will take approximately another 3 to 5 years before sufficient prospective data will be available [22
]. Therefore, taking all of these considerations together, the current “failure” rates of SRA and conventional THA, solely based on registry data with revision as the end point, may actually be an underestimation. Fourth, poorly performing implant designs become obvious from the registry data after 5 to 7 years [13
], if not sooner. This was reproduced with specific SRA implants [5
]. The differences between well-performing implants are much harder to distinguish. Nevertheless, this would be desirable because the performance of most implants is continuously improving and we strive to identify the best implants. Confounding factors other than age, gender, and diagnosis are often not considered and therefore small differences between well-performing implants must be interpreted with caution. Additionally, lower rates of revision surgery have been observed in countries with longstanding registries [9
]. Registry results provide a feedback loop to the surgeons and have an impact on indications, procedures, and implants used [5
]. For example, since 2003, there has been little change in the proportion of females having primary THA in the Australian NJRR. However, there has been a 5% decline in females undergoing SRA over a time period of 4 years, probably indicating the early higher revision rates in females with SRA had an impact on the indications for the procedure [5
]. In other words, there is some form of positive bias in general practice introduced by the use of NJRR. Finally, it is important to understand that registries may be capturing implants in different phases of their implementation. In other words, an implant that has been on the market for 10 years when a registry begins will not have any learning curve captured in the registry data compared with an implant that is released after the registry has begun. In the early years of a registry, therefore, there is a bias introduced against new implants and technologies associated with a learning curve. Some may argue that is valid and all new implants should be assessed and compared with current well-performing implants. Others may argue that this is an unfair bias potentially restricting the ability to evaluate any proposed long-term benefits of an implant by only focusing on the very short-term results. Obviously, once a registry is well-established and has been in place for many years, this becomes less of a concern as each implant’s early-year results can be evaluated and compared.
Despite these limitations, registries provide an enormous amount of data with relevant information regarding survival rates that can be correlated to prognosticators such as primary diagnosis, age, and gender. The early failure rates of SRA as captured in the NJRR were somewhat higher but comparable to those reported in the literature (Table ). However, they were substantially higher than those of THA after adjustment for age and gender.
Important information can be derived from carefully evaluating the revision data in terms of which component failed and the mode of failure. Aseptic loosening was a problem equivalent to periprosthetic fracture as an indication for revision of SRA in 31% of the cases [39
], which was similar to the incidence reported in the literature [1
] and 15% less than the incidence in revision of THA [38
]. In SRA, 55% of the time a stem-only revision was performed. In THA, 55% of the time a cup only revision was performed. In other words, the resurfacing shells and the THA stems underwent the fewest revisions. Would the combination of both lead to fewer revisions in the younger patient population? Unfortunately, no data on large-diameter metal-on-metal THA are available yet.
The effects of gender and age on survivorship are interrelated. There was an opposite effect of gender as a prognosticator for failure of SRA in comparison to THA, which could not be detected from the published studies on SRA survival (Table ). Females with a SRA have a higher risk for failure than age-matched males and females with a THA. In contrast, males younger than 65 years of age at the time of surgery had comparable, and even slightly improved, 5-year survival rates with SRA than with THA. In addition, it would appear that one of the most important predictors for 5-year survival of SRA was the use of component sizes 50 mm or greater in diameter. The survival rates of both females and males with these component sizes were equal to each other.
Patients with OA as the primary diagnosis for hip arthroplasty had the best survival rates and the use of SRA for DDH and AVN should be cautioned.
The premise that a revision of SRA leads to equal results of a primary THA should be cautiously reevaluated because the 5-year rerevision rates of SRA (11%) were higher than those of primary THA at 5 (2.7%) and even 7 (3.4%) years. These rerevision rates were actually equal to the rerevision rates of an all-component THA revision.
In summary, the knowledge of SRA is evolving and the techniques are changing. Despite their limitations, registries should be considered as a sensitive detection tool that can provide early data on component outcomes, often sooner than randomized, controlled trials, depending on the outcome measure being evaluated. Current registry data on SRA demonstrate an overall increased failure rate compared with THA in all patients with primary OA, with the exception of the younger male patient and patients who can accommodate components with head sizes over 50 mm in diameter. However, males younger than 65 years of age and women with larger component sizes receiving SRA have equivalent short-term survivorship as patients receiving THA when stringent SRA criteria are adhered to. It must also be emphasized that the survivorship of a revision of a failed SRA should not be considered as equivalent to a primary THA because the rerevision rate is much higher and instead is equivalent to the rerevision rate of a failed THA.