The results of this systematic review show a clinically relevant association between early migration, as measured with RSA, and long-term clinical failure resulting in revision for aseptic loosening. Each mm of migration was associated with an increase in 5-year revision rate of 8%, which remained after correction for age, sex, diagnosis, hospital type, continent, and study quality. This is more than twice the standard revision rate of several national joint registries (DKAR 2009
, NJR 2009
, AJR 2010
, SKAR 2010
). The results of this systematic review show that RSA studies can identify unsafe TKPs (in terms of aseptic loosening) as early as 1 year postoperatively. Early identification of unsafe TKPs with RSA should prevent their widespread use and save numerous patients from extensive revision surgery, possibly with postoperative complications.
Some strengths of this systematic review are the large numbers of studies included (> 100) and of patients included (> 27,000), which resulted in 28 different PFI combinations. This large variation, which reflects the diversity of TKP designs and fixation methods, ensures wide generalizability of the results. Since the migration and revision rates were from different studies, there were no migration data available in the survival studies to be incorporated into the decision to perform a revision. Thus, there is no incorporation bias in our results. We consider the risk of publication bias in this systematic review to be small, since the results from the meta-analysis are similar to those from the national joint registries, which do not suffer from publication bias. Confounders had only a small influence on the association between early migration and long-term aseptic revision.
We should also consider some limitations. The quality of the survival and RSA studies showed large variation. High methodological quality of all the studies included would have been desirable. Nevertheless, the quality of the survival studies and the RSA studies showed only very small effects on the association between migration and revision rates.
We focused on MTPM at 1 year postoperatively, but other migration parameters and follow-up beyond 1 year would also be of interest (Ryd et al. 1995
). Unfortunately, these parameters were reported too infrequently and inconsistently to permit a meaningful analysis. Future RSA studies could therefore benefit from further standardization, particularly regarding the reporting of the results (Valstar et al. 2005
We also recognize that RSA only evaluates aseptic loosening while other failure mechanisms (e.g. infection, pain, and instability or pseudotumors in metal-on-metal total arthroplasty) cannot be evaluated by RSA. As a consequence, RSA studies are only the first step in the phased, evidence-based introduction of TKPs—as proposed by Malchau (2000)
; see . During phase A, several single-center RSA studies should be performed to determine the safety of the TKP with regard to the risk of revision for aseptic loosening. If the TKP is considered safe, phase B studies should be conducted to evaluate the clinical performance of the TKP regarding pain relief and functioning (clinical scores and patient-reported outcome measures (PROMS)) and to determine the rate of expected or unexpected complications. Since RSA studies have already evaluated the risk of aseptic loosening, follow-up of 2 years instead of 10 years would be sufficient. This reduces the follow-up needed for a successful phased introduction by almost a decade compared to traditional cohort studies. After release on the market (phase C), the performance of the TKP must be monitored by post-marketing surveillance in national joint replacement registries (Schemitsch et al. 2010
). This includes both the revision rate and patient evaluations using PROMS.
Figure 6. Flow chart showing the role of RSA studies in the phased evidence-based introduction of new TKPs, modified according the Malchau proposal. Stabilization is defined as migration of less than 0.2 mm in the second postoperative year (MTPM from year 1 to (more ...)
In this systematic review, RSA studies of 20–60 patients followed for 1 year led to the same conclusion as national joint registries with thousands of patients followed for 5–10 years. A recent publication has shown a 22–35% reduction in the number of revisions of RSA-tested total knee replacements as compared to non-RSA-tested total knee replacements in the national joint registries (Nelissen et al. 2011
). Because inferior designs can already be detected early postoperatively, exposing only a small group of patients to potentially unsafe TKPs, RSA provides the necessary efficiency to make possible phased, evidence-based introduction. Now the observed association between early migration and long-term revision translates into practical thresholds that can lead to clinical guidelines for phased, evidence-based introduction of new TKPs.
Various authors and regulatory agencies recognize the potential of RSA (Grewal et al. 1992
, Bulstrode et al. 1993
, Karrholm et al. 1994
, Ryd et al. 1995
, Malchau 2000
, Hauptfleisch et al. 2006
). The NICE guidelines of 2003 (UK) require adequate long-term clinical data for hip prostheses and indicate that RSA is an alternative to long-term follow-up studies. The Dutch Orthopaedic Society now requires a phased introduction with mandatory RSA studies before any new hip prosthesis is considered for introduction to the Dutch market (2011). Official guidelines for knee prostheses are expected to follow.
In light of the recent disasters with introduction of new orthopedic implants to the market, a phased, clinical introduction for new TKPs is mandatory to prevent patients from receiving potentially unsafe TKPs when standard TKPs with excellent long-term track records are available.
In conclusion, we found a clinically relevant association between early migration of TKPs and late revision for loosening. The proposed migration thresholds can be implemented in a phased, evidence-based introduction, since they allow early detection of TKPs with a high risk of aseptic loosening while exposing only a small number of patients.