In this study of a well defined RA population, we demonstrated a reduced incidence of primary THA in 2002 to 2006 compared to 1998 to 2001. By contrast, there was no significant change in the incidence of TKA. Our results are compatible with previous studies indicating a reduced rate of joint surgery overall in patients with RA [5
], and a decreased rate of arthroplasty relative to the non-RA background population [4
]. In contrast with our findings, the California State Database reported a decrease over time in 1983 to 2001 in the rate of primary TKA in patients with RA [20
]. Changes in coding practice and reimbursement for joint replacement surgery may explain the results from the latter study, although we cannot exclude that a global decline in the frequency of knee arthroplasties has been followed by stabilization or increase after the turn of the century.
Our results also differ from recent data from a Scandinavian survey of the national knee arthroplasty registers, in which a reduced incidence of TKA in Sweden over time was found [8
]. Several methodological issues may explain these discrepancies. First, the present study used a defined set of RA patients with a validated diagnosis, whereas in the national TKA study the diagnosis of RA was based on the report from the orthopedic surgeon. Increasing misclassification of overall milder cases of RA over time would affect analyses of longitudinal trends. Second, we studied the incidence of the first TKA in the local RA population; whereas the national study investigated the incidence of all RA related TKA, with the entire Swedish population as the denominator. In the national study, a systematic change in the reporting of RA patients to the register may bias the findings. On the other hand, the precision of the estimates in the present study are limited due to the small sample size. Finally, there was no decrease in the incidence of RA related TKA in Denmark in the Scandinavian survey, indicating that geographical differences may play a role.
Potential explanations for the reduced rate of THA include reduced RA related joint damage due to better management of RA. Treatment strategies for RA have changed markedly over the past three decades, with the introduction of early and aggressive treatment. In the present study population an increasing use of DMARDs (52% to 87%) and TNF-inhibitors (0% to 20%) from 1997 to 2005 together with substantial improvements in median health assessment questionnaire disability index and Short Form (36) health survey score levels have previously been reported [21
]. The timing of our study thus coincides with the establishment of tumor necrosis factor (TNF) inhibitors in the standard of care of patients with severe RA. These agents were introduced in the late 1990's, and used more extensively after 2002. There is extensive evidence for a reduced peripheral joint damage in patients with RA treated with TNF inhibitors [22
], and such treatment could also prevent hip destruction. Other changes in RA management may also have contributed to the decline in THA surgery.
The contrasting pattern for knee arthroplasties may indicate that mechanisms of joint destruction may be partly different in knees and hips. In a systematic study of multiple sections from the cartilage-pannus junction of RA joints, invasive pannus formation with major cartilage degradation was more frequent in hip joints, and osteophyte formation was more frequent in knee joints . TNF inhibitors and other drugs could, therefore, be less successful in preventing knee damage. A second possible explanation could be that criteria for knee arthroplasty in RA have changed over time or TKA has become more available compared to THA. Such hypothetical changes could possibly result in TKA being performed in patients with less severe disease, relative to THA. However, there are presently no data supporting such changes in Sweden.
Limitations of the present study are due to the sample size, which results in a limited number of primary joint arthroplasties. In addition, the patients were mostly Caucasians from a single urban area in southern Sweden, and the findings may not apply to other settings. In theory, the results could partly reflect local changes in this district, but we consider it unlikely that changes in indications for surgery would be substantially different from other areas in Scandinavia. Our study is based on only eight years of observation, but the need for total joint arthroplasty is an important severe long-term outcome of RA and should be investigated over longer periods. On the other hand, some long term studies have suffered from problems related to changes in coding practice (such as the transition from the International Classification of Diseases (ICD) -9 to the ICD-10 system in 1998 for a previous national Swedish study) [6
The mean age of the cohort increased slightly over time. Based on this, a minor increase in the rate of incident arthroplasties would be expected, but this is unlikely to have had any major impact on our results. Changes in body mass index and physical activity may also have influenced our results, but no such data are available.
Major strengths of our study include the community based approach, which limits selection bias, and the validated RA diagnosis based on the 1987 ACR criteria, which contrasts with studies based on patient administrative databases alone. Furthermore, the use of national registers for joint arthroplasties mean that we were likely to identify virtually all THA and TKA in the cohort, including those performed in hospitals in other parts of Sweden.