Results obtained from single centers worldwide indicate that cemented total hip replacement is the treatment of choice for rheumatoid arthritis1-4; however, in a population-based study of patients fifty-five years of age or younger with rheumatoid arthritis, the survival of cementless proximal porous-coated stems has been better than the survival of cemented stems5. To our knowledge, no nationwide results of total hip arthroplasty in patients over fifty-five years of age with rheumatoid arthritis have been published.
With use of population-based data obtained from the Finnish Arthroplasty Registry, we evaluated the long-term survival of primary total hip replacements that had been performed for the management of rheumatoid arthritis in patients who were fifty-five years of age and older.
The Finnish Arthroplasty Registry
Since 1980, data on total hip replacements have been collected and archived in the Finnish Arthroplasty Registry6,7. Health-care authorities, institutions, and orthopaedic units in Finland are obliged to provide the National Agency for Medicines with information that is essential for monitoring past and current trends for the efficacious use of materials, approaches, and designs used in orthopaedic surgery. In an analysis of the coverage of the Finnish Arthroplasty Registry from 1994 to 1995, the data from the Registry were compared with the data from discharge registries of participating hospitals and it was found that the Registry covered 90% of all arthroplasties actually performed6. Since 1995, the data of the registry have been compared with those of hospital discharge registries every few years. Currently, 98% of all arthroplasties are recorded in the Finnish Arthroplasty Registry8.
Inclusion and Exclusion Criteria
Only patients who had rheumatoid arthritis as a recorded indication and were fifty-five years or older at the time of the primary operation were included. Total hip replacements (cup and stem combinations) that had been used in less than thirty operations during the study period were excluded9,10. Prosthetic component types associated with well-documented poor results11-14 (for example, the 10% aseptic loosening rate of cementless threaded cups after a follow-up of one to ten years14) and implant types that did not fit in any of the groups of interest described below were also excluded.
Analysis of the Implant Groups
The success rates of different implant groups were analyzed5,11,15,16. All implants included in the study were classified as belonging to one of the following total hip replacement groups: (1) a cementless group (a cementless, proximally porous-coated stem with or without a hydroxyapatite coating and a cementless, porous-coated press-fit cup with or without a hydroxyapatite coating); (2) a cemented group 1 (a cemented, loaded-taper stem combined with a cemented, all-polyethylene cup); and (3) a cemented group 2 (a cemented, composite-beam stem combined with a cemented, all-polyethylene cup). By a loaded-taper stem it is meant that the stem becomes lodged as a wedge in the cement mantle during axial loading, reducing peak stresses in the cement mantle. The stem is allowed to subside initially until a stable position is found17,18. By a composite-beam stem it is meant that the proximally textured or roughened stem needs to be rigidly bound to cement. Composite-beam stems are not intended to subside17,19.
Femoral components of the included total hip replacements were separately classified into the following three stem groups: (1) cementless, proximally porous-coated stems with or without a hydroxyapatite coating; (2) cemented, loaded-taper stems; and (3) cemented composite-beam stems.
Acetabular components of the included total hip replacements were separately classified into the following two cup groups: (1) cementless, press-fit porous-coated cups with or without hydroxyapatite; and (2) cemented, all-polyethylene cups.
Study Population
During the whole study period (1980 to 2006), 132,540 primary total hip replacements were performed in Finland. Of these operations, 114,620 (86%) were performed on patients who were fifty-five years of age or older. Rheumatoid arthritis was an indication in 6000 (5%) of the 114,620 operations. After exclusion of implants according to our study criteria, 4019 total hip replacements were included in the final analysis (Table I).
Primary Operations
The mean age and sex distributions of the patients were recorded (Table I). In patients who were fifty-five years of age or older with rheumatoid arthritis, 106 different stem designs were used in Finland during the study period. Of these stem designs, eighty-two designs (only 8% of the total number of all stems) were used in fewer than thirty operations. Cementless stems were used in 23% of the primary operations. In patients who were fifty-five years of age or older with rheumatoid arthritis, 110 different cup designs were used during the study period. Of these cup designs, seventy-four designs (only 8% of the total number of all cups) were used in fewer than thirty operations. Cementless cups were used in 29% of the primary operations.
Revision Operations
Revisions were linked to the primary operation with use of the unique personal identification number assigned to each resident of Finland. Revision was considered as either an exchange or removal of the cup and/or stem or an exchange of the liner. Infections are mainly treated with a two-stage revision in Finland. Only the first operation, i.e., removal of the prosthesis, is recorded as the first revision in the Finnish Arthroplasty Registry. The second operation, i.e., a repeat replacement, is recorded as a rerevision and is not included in the study. During the study period, 371 revision operations were performed for patients in the study group (Table II).
Statistical Analysis
The end point for survival was defined as revision when either one component (including the liner and the femoral head) or the whole implant was removed or exchanged. Both revision for any reason and revision for aseptic loosening served separately as end points. Revisions for any reason included (in addition to revisions for aseptic loosening of the cup and/or the stem) revisions for infection, dislocation, malposition, periprosthetic fracture, fracture of the prosthesis, and other reasons (including liner exchange). Kaplan-Meier20 survival data were used to predict the survival of the implants at five, ten, and fifteen years. At each follow-up time point, survival rates were analyzed only for implants with more than twenty patients at risk21. Survival data obtained by Kaplan-Meier analysis were compared with use of the log-rank test. Patients who had died or had emigrated from Finland during the follow-up period were excluded at that point. The Cox multiple regression model22 was used to study differences among groups and to adjust for potential confounding factors. The factors studied with the Cox model were implant groups, age, and sex. When stem groups were analyzed with the Cox model, loaded-taper cemented stems with well-documented, good long-term results23-27 served as the reference group. Similarly, all-polyethylene, cemented cups23-28 served as the reference group on the acetabular side, and cemented total hip replacements with loaded-taper stems23-31 served as the reference group in analyses of total hip replacements. When the effects of age and sex on implant survival were analyzed with the Cox model, adjustment was also made for implant15. Cox regression analyses provided estimates of survival probabilities and adjusted risk ratios (RR) for revision. Estimates from the Cox regression analyses were used to construct adjusted survival curves at mean values of the risk factors. The Wald test was applied to calculate p values for data obtained from the Cox multiple regression analysis. Differences among groups were considered to be significant if the p value was <0.05 in a two-tailed test.
We had permission from the National Agency for Medicines to perform the current study. Because of the registry-based character of our study, no other approvals were needed.
Source of Funding
The funding source (Sigrid Jusélius Foundation) did not play a role in the investigation.
Survival of Stems with Revision for Aseptic Loosening as the End Point
The fifteen-year survival rates of cementless stems and loaded-taper cemented stems (cemented group 1) were higher than that of cemented composite-beam stems (cemented group 2) (Table III). In the Cox regression analysis, cementless stems had a significantly lower risk of revision (p = 0.006) than did the cemented reference group (Table III, Fig. 1-A).
Cohort Effect Among Stem Groups
In the Cox regression model, cemented loaded-taper stems that were implanted between 1980 and 1993 had a higher risk of revision than did stems of the same category implanted between 1994 and 2006 (RR, 3.54; 95% confidence interval [CI], 1.35 to 9.30; p = 0.01). Cementless stems (RR, 2.00; 95% CI, 0.37 to 10.74; p = 0.4) and cemented composite-beam stems (RR, 0.98; 95% CI, 0.64 to 1.51; p = 0.9) did not show any significant differences in survival between the two cohorts (1980 to 1993 and 1994 to 2006).
Survival of Cups with Revision for Aseptic Loosening as the End Point
The fifteen-year survival rate of cementless and cemented cups was similar (Table III). In the Cox regression analysis, cementless cups had a significantly lower risk of revision (p = 0.02) than did cemented all-polyethylene cups (Table III, Fig. 1-B).
Cohort Effect Among Cup Groups
In the Cox regression model, there was a trend for cementless porous-coated cups implanted between 1980 and 1993 to have a higher risk of revision than did cups of the same category implanted between 1994 and 2006 (RR, 2.75; 95% CI, 0.87 to 8.64; p = 0.08). Cemented cups (RR, 1.28; 95% CI, 0.84 to 1.93; p = 0.2) did not show a difference in survival between the two cohorts (1980 to 1993 and 1994 to 2006).
Survival of Total Hip Replacements with Aseptic Loosening as the End Point
The fifteen-year survival rates of cementless total hip replacements and cemented total hip replacements with loaded-taper stems (cemented group 1) were higher than that of cemented total hip replacements with composite-beam stems (cemented group 2) (Table III). In the Cox regression analysis, cemented total hip replacements with composite-beam stems (cemented group 2) had a significantly higher risk of revision (p < 0.001) than the reference group (Table III, Fig. 2-A).
Survival of Total Hip Replacements with Revision for Any Reason as the End Point
The fifteen-year survivorship of cemented total hip replacements with loaded-taper stems (cemented group 1) was higher than that of cemented total hip replacements with composite-beam stems (cemented group 2) (Table III). In the Cox regression analysis, cemented total hip replacements with composite-beam stems had a significantly higher risk of revision (p = 0.001) than did cemented total hip replacements with loaded-taper stems (Table III, Fig. 2-B).
We found that both cementless stems and cups, analyzed separately, had a significantly lower risk of revision for aseptic loosening than did cemented implants in patients who were fifty-five years of age or older with rheumatoid arthritis. Also, the long-term survival rate of cementless total hip replacements was comparable with that of cemented total hip replacements when revisions for any reason were used as the end point.
This registry-based study has certain limitations. Prior to 1994, 10% of the total hip replacements were missing from the Finnish Arthroplasty Registry6. These total hip replacements that were lost to follow-up could have been failures and caused bias in our study. It is also possible that a few centers performed the majority of the replacements and certain complications could have occurred more often at certain centers. The number of total hip replacements performed for the management of rheumatoid arthritis is lower than the number of total hip replacements performed for the management of osteoarthritis16. However, in the current study, the number of hospitals that implanted cementless total hip replacements was similar to the number of hospitals that implanted cemented total hip replacements (Table I), and therefore we do not think that there was a bias.
A limitation of registry-based studies is that only a revision operation is used to define failure. There might be patients with polyethylene failure, osteolysis, or loosened implants who are too ill to undergo revision surgery, who simply prefer not to do so, or who are not even aware of the problem. Furthermore, differences in regard to bearing surfaces and material properties of liners could not be analyzed due to the data limits of the Finnish Registry. Selection bias as a limitation of a registry-based study may develop when some surgeons, for example, do not use cementless implants under certain circumstances, such as when patients have rheumatoid arthritis. However, this kind of selection bias can in theory only be avoided by conducting a randomized controlled trial.
Cemented total hip arthroplasty has traditionally been considered the gold standard for the treatment of end-stage joint disease in patients with rheumatoid arthritis1-4. However, the rate of aseptic loosening of acetabular components has been high and a greater incidence of both acetabular and femoral loosening has been reported in patients with rheumatoid arthritis than in those with osteoarthritis2,32,33. This finding has been attributed to periarticular osteopenia associated with inactivity, the use of medications such as corticosteroids and antimetabolites, regional hyperemia, and increased bone turnover34. However, in a population-based registry study from Norway, no difference in survival of total hip replacements between patients with rheumatoid arthritis and patients with osteoarthritis was found15. Furthermore, it has been stated that cementless total hip replacements and cemented total hip replacements have comparable long-term survival rates in patients who are fifty-five years of age or older with osteoarthritis16.
The results of the current study are based solely on the orthopaedic registry data. Unfortunately, we are not able to obtain drug protocol data. However, many patients in this study have probably used drugs such as chloroquine, auranofin, or methotrexate during some phase of their disease and newer drugs such as tumor necrosis factor alpha-antagonists more recently. In the cohort-effect analysis of the current study, the survival rates of cementless stems that were implanted between 1980 and 1993 did not differ from the rates of those implanted between 1994 and 2006. Thus, it might be inferred that the development of drug protocols did not have an effect on the fixation of cementless stems. On the acetabular side, newly implanted cementless cups performed better than those implanted earlier, probably due to the development of liners. However, theoretically it is possible that a new drug protocol enhanced fixation of the cup. At least, it can be stated that such methods did not worsen cup fixation.
The cemented implants had longer average follow-up times since surgery because they were used earlier in the study period. The only major change in addition to new drugs during the time period was the development of cementing techniques. Modern cementing techniques include washing the bone with pulsatile lavage and then drying the bone, using cement plugs, pressurizing the cement, and using centralizers. It was surprising to us that the survival of the composite-beam stems was not any better in more recent years with the development of these techniques. The survival of loaded-taper stems, however, was higher in the later cohort. It seems that the survival of composite-beam stems cannot be improved with better cementing techniques. Otherwise, the surgical procedures and rehabilitation protocols did not differ from each other over the study period.
Recently, good results have been reported for cementless total hip replacements in rheumatoid arthritis35-39. In a study based on data from the Finnish Arthroplasty Registry, it was concluded that uncemented proximally porous-coated stems are the implant of choice for younger patients with rheumatoid arthritis5. However, the risk of cup revision was higher for cementless cups than for all-polyethylene cemented cups.
In the present study of patients who were fifty-five years of age or older, cementless stems and cups, analyzed separately, had a significantly lower risk of revision for aseptic loosening than did cemented implants (p = 0.006 for stems and p = 0.02 for cups). When all revisions were taken into account in the survival analyses, the long-term survival rate of cementless total hip replacements was comparable with that of cemented total hip replacements. During the study period, in almost every center there has been a shift toward the use of noncemented implants in patients with osteopenia as well as in patients who have normal bone density, with the same reported results. Cementless implants seem to perform well at a population-based level in patients who are fifty-five years of age or older with rheumatoid arthritis (a group of patients considered to be relatively osteopenic), at least when compared with patients who are less than fifty-five years of age with osteoarthritis. We are not aware of any other population-based report on this subject.
The results of the present study suggest that cementless cups can resist aseptic loosening very effectively in patients who are fifty-five years of age or older with rheumatoid arthritis. However, a large number of wear-related revisions of modular cementless cups are a worrisome finding that clearly emphasizes the need for more wear-resistant articulations for cementless cups. The problem of incongruity between the cup and the liner in the two-piece acetabular designs with incomplete locking mechanisms was emphasized in our study due to the large proportion of cementless cups with poor liners, which is a critical issue that has been reported previously from the Finnish Arthroplasty Registry11,16,40.
Poor long-term survivorship of composite-beam stems in elderly patients with osteoarthritis has been reported previously16. In the current study, composite-beam stems did not show any differences in survival between the cohorts of 1980 to 1993 and 1994 to 2006, whereas the survival of loaded-taper stems was significantly better in the later cohort. Systematic instruction regarding proper cementing techniques and reporting of the results in the 1990s have clearly had an effect on the results seen with loaded-taper stems but not on the results with composite-beam stems. According to the current data, there are more reliable options than cemented composite-beam stems for patients who are fifty-five years of age or older with rheumatoid arthritis.
In conclusion, cementless and cemented total hip replacements produced comparable long-term results in patients who were fifty-five years of age or older with rheumatoid arthritis.