The survival of revisions done in patients with patellar subluxation was worse (p < 0.005) than the overall survival of revisions performed for other reasons (Fig. 4). Pairwise comparisons with use of the log-rank test indicated that the survival of revisions done because of subluxation differed significantly from that of revisions performed because of a fracture of the prosthesis (p < 0.005) but did not differ significantly from the survival of revisions due to loosening, malposition, infection, or other patellar complications.
The five-year survival rate, with repeat revision as the end point, was 93% (95% confidence interval, 88% to 95%) for the AGC Dual Articular prosthesis (Biomet, Warsaw, Indiana), 90% (95% confidence interval, 84% to 93%) for the AGC V2 prosthesis (Biomet), 87% (95% confidence interval, 83% to 90%) for the Duracon prosthesis (Stryker Howmedica Osteonics, Allendale, New Jersey), 98% (95% confidence interval, 96% to 99%) for the Duracon Modular prosthesis (Stryker Howmedica Osteonics), 89% (95% confidence interval, 85% to 93%) for the LINK Endo-Modell prosthesis (Waldemar Link, Hamburg, Germany), and 98% (95% confidence interval, 94% to 100%) for the NexGen prosthesis (Zimmer, Warsaw, Indiana). No patient in the registry had had a P.F.C. Sigma prosthesis (DePuy Orthopaedics, Warsaw, Indiana) for five years, but the survival rate of that prosthesis at 4.5 years was 98% (95% confidence interval, 93% to 99%). The NexGen and Duracon Modular prostheses had better survival rates (p < 0.05) than the Duracon implant, which had the shortest time-to-event survival.
Cement fixation (p < 0.005) and bone-grafting (p = 0.05) improved prosthetic survival, whereas hybrid fixation did not differ significantly from cementless fixation with regard to prosthetic survival (Fig. 5).
The diagnosis, type of prosthesis, primary complications, and type of hospital did not significantly affect the prosthetic survival.
Multivariate Cox Regression Analysis
Significant variables were included in the multivariate Cox analysis. Only age group was significant (p < 0.005) in the initial multivariate analysis, which included all of the variables that were found to be significant in the univariate analyses—that is, age group, year of the first revision operation, time between the primary and revision operations, reason for revision, brand of prosthesis, fixation, use of bone grafts, and gender. However, 57% (1514) of the 2637 knees were missing from the analysis, mainly because of missing information regarding the brand of prosthesis and the fixation method. While our statistical software package can include cases with missing values, this was not considered appropriate6, and a subanalysis was done without those variables (that is, after omission of the brand of prosthesis and fixation method). In this analysis, an age of more than seventy years, having the first operation after 1996, five years or more of service of the primary prosthesis prior to the revision, and absence of patellar subluxation were all found to be significantly associated with improved survival of the revision.
We found no extreme values or outlier observations that would have dominated the model individually and resulted in a misrepresentative model.
One of the assumptions in the Cox regression analysis is that the observations are independent of each other. In reality, the two knees of one patient are not independent of each other, and this must be considered when both knees are operated on. It has, however, been reported that the effect of not accounting for bilateral prostheses is minute, and that the risk of revision of knee prostheses can be analyzed without consideration of the dependency3. In the present study, a relatively small proportion (11.3%, 297) of the 2637 knees were in a patient with a bilateral revision, and the survival analyses were carried out without taking bilaterality into account. A subanalysis indicated that bilateral knee revision was associated with better survival than was unilateral knee revision. This finding is in accordance with the finding of better survival of bilateral primary total knee replacements compared with unilateral primary total knee replacements2.
Age group was a significant predictor of prosthetic survival, with an age of greater than seventy years being associated with better survival (p < 0.005). The quality of bone declines with age as a result of senile osteoporosis. However, the reduced physical activity of elderly people diminishes cyclic loading and micromotion. A reduced activity level and low body weight may explain why a long interval between the primary and revision operations predicts a long survival of the revision replacement, but this cannot be proven because physical activity and weight are not recorded in the Finnish Arthroplasty Registry. To some extent, the effect of the patient's age may reflect surgeons' reluctance to perform repeat revisions in elderly and frail patients. Age in itself cannot be considered as a contraindication for revision knee arthroplasty. On the contrary, it seems that revision knee prostheses can be expected to have a long service life in elderly patients.
The outcomes of primary arthroplasties have been relatively good in patients with rheumatoid arthritis2, although this disease is characterized by destruction of cartilage and bone, ligamentous laxity, and juxta-articular and generalized osteoporosis. The differences in prosthetic survival between knees with a diagnosis of osteoarthritis and those with rheumatoid arthritis, and between men and women, were significant in the study of primary knee replacements (p < 0.001 and <0.0001, respectively)2. This was not the case in our study of revision knee replacements. This difference may be due to the fact that patients who had already been treated with primary knee arthroplasty had adapted to a reduced activity level so that any pre-existing differences were diminished.
The type of hospital where the patient had undergone the revision arthroplasty (that is, at a university, central, regional, or other type of hospital) was not a significant predictor of prosthetic survival in our analyses. However, it should be noted that, during the follow-up period, specialization in the different fields of orthopaedics very often had not yet developed, even in large units, in Finland7. Therefore, some orthopaedic surgeons performed only a few arthroplasties in a year. On the other hand, large hospitals that had achieved a good reputation in a certain field may have attracted more patients with more difficult cases, and this may have worsened the results in those hospitals. Concurrently, the high number of revision operations needed and the long waiting times sometimes led to a revision being performed only when it could no longer be avoided7. This may have improved the prosthetic survival figures for individual units. However, the registry provides the nationwide mean for the outcome of revision total knee arthroplasty in Finland, which can be considered to be one of its major advantages.
Patellar subluxation seems to adversely affect the outcome of revision knee replacement, indicating that surgeon-related factors are in part responsible for the failure of some of these procedures. This finding is consistent with those of our previous studies, which demonstrated an excellent or good result in 82% (fifty-eight) of seventy-one patients, a high (94%) eight-year survival rate combined with a low (8.5%) complication rate (six of seventy-one), and very few and asymptomatic radiolucent lines around Total Condylar III revision total knee prostheses (Johnson and Johnson, Braintree, Massachusetts) when these operations were performed by a few experienced revision surgeons8,9. Furthermore, only 5% (115) of the 2443 revisions in the present study for which the reason for revision was known resulted from infection. This might to some extent represent underreporting, but that seems unlikely considering the wide coverage of the registry, which was estimated to be 90% to 95% even in the early 1990s, when registration was still voluntary10.
To improve feedback and quality control, the Finnish Arthroplasty Registry is being further developed so that it will, perhaps in the near future, be possible for individual surgeons to confidentially check online their personal performance in relation to the nationwide norm. The surgeon is responsible for the selection of the brand of prosthesis, the fixation method, and the use of bone grafts, which may affect the survival of revision total knee replacements. Many different brands have been used in Finland, and only the most commonly implanted ones were included in our analyses. Many of the new brands had been used in very small numbers of knees, so more observations are needed before reliable survivorship analyses of those brands in a clinical setting can be performed.
Rand et al. found that primary total knee arthroplasty can be expected to have the most durable results in women who are more than seventy years of age, have inflammatory arthritis, and are treated with certain types of prostheses2. They also reported that the cruciate-retaining design was better than the cruciate-sacrificing design in primary total knee arthroplasty. Detailed information about the status of the cruciate ligament was not available in the Finnish Arthroplasty Registry. On the basis of the results of the present study, it can be concluded that revision total knee arthroplasty can be expected to have the most durable results in patients who are older than seventy years of age and in whom the primary implant had been in service for a long period of time. Absence of patellar subluxation is also a positive indicator.
Gender and diagnosis did not predict the survival of the revised total knee replacements, perhaps because the patients had adapted to a reduced activity level after the primary total knee arthroplasty. Recent advances in implant materials, designs, and operative and fixation techniques presumably have improved implant survival. Indeed, the results of revision total knee surgery in Finland have improved since the early 1990s.
Tables showing demographic data, factors included in the analyses, and the estimated hazard ratios for the factors are available with the electronic versions of this article, on our web site at jbjs.org (go to the article citation and click on “Supplementary Material”) and on our quarterly CD-ROM (call our subscription department, at 781-449-9780, to order the CD-ROM). ▪
In support of their research for or preparation of this manuscript, one or more of the authors received the Academy of Finland Center of Excellence Grant. None of the authors received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, educational institution, or other charitable or nonprofit organization with which the authors are affiliated or associated.
Investigation performed at COXA Hospital for Joint Replacement, Tampere; ORTON Orthopaedic Hospital, Invalid Foundation, Helsinki; National Agency for Medicines, Helsinki; and Department of Medicine, Helsinki University Central Hospital, Helsinki, Finland
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