Unicompartmental knee arthroplasties (UKA) are widely used to treat isolated unicompartmental knee osteoarthritis (OA). Advantages of a UKA include lower postoperative morbidity, quicker return to activities, and more normal feeling of the knee in comparison to the advantages after a TKA [1, 8, 9], and some single-center studies report UKA results that are comparable to those of TKAs [12, 13, 15, 18, 22]. However, these studies are not supported by available implant register data from Finland, Norway, Sweden, Australia, New Zealand, and the United Kingdom [10, 23-27], which repeatedly report inferior midterm survivorship of UKAs compared with TKAs.
However, less is known regarding comparative survivorship between UKA and TKA for the longer term. In addition, comparing survival of UKAs and TKAs directly by using arthroplasty register survival reports may be inadequate because of differences in indications, implant designs, and patient demographics between patients having UKAs and TKAs which are not consistently considered in registry reports.
The purpose of this population-based study was to assess survivorship of UKAs performed for patients with primary knee OA in Finland during a 27-year period and to compare the survivorship rate of patients who underwent UKAs with that of a cohort of patients who received cemented TKAs during the same period.
Patients and Methods
Patients Undergoing UKA
Records for patients who had UKAs from 1985 to 2011 for treatment of primary knee OA were extracted from the data in the Finnish Arthroplasty Register. We obtained information regarding 5211 UKAs with one of 11 different implant designs. After identifying all the UKAs, we excluded the designs that had been implanted in less than 100 knees during the study period (n = 208) or for which fewer than 20 knees were at risk after 10 years (n = 112). Similar exclusion criteria were used in previous register-based studies [2, 4]. In addition, we excluded UKAs which had been performed for reasons other than primary knee OA (n = 178). This resulted in retention of data for 4713 UKAs, which represented 90% of all UKAs (Table 1). The number of UKAs performed was relatively small until 1998 when the Oxford® Phase 3 (Biomet Inc, Warsaw, IN, USA) was introduced (Fig. 1).
Patients Undergoing TKA
The TKA group included all patients who had undergone cemented TKAs for primary knee OA during the study period. Constrained models other than cruciate-retaining or posterior-stabilized prostheses were excluded. In total, we included 83,511 TKAs (Table 1). The number of TKAs performed has increased steadily during the study period (Fig. 2).
To obtain TKA designs corresponding to four UKA designs included in the study, we extracted data for four TKAs during the study period that fulfilled the inclusion criteria; ie, more than 100 procedures performed with 20 or more at risk after 15 years and limited to those performed for primary knee OA. The data for these TKAs were extracted starting with the most commonly used implants then proceeding to the less frequently used designs.
Demographic Differences and Changing Demographics Among Patients
Patients undergoing UKAs were younger than patients undergoing TKAs, with mean ages of 63 years (range, 33-91 years) and 69 years (range, 23-96 years), respectively (p < 0.001). The mean age of the patients undergoing UKAs also decreased during the study period, being 69.1 years between 1985 and 1993, 66.4 years between 1994 and 2002, and 62.1 years between 2003 and 2011. The corresponding mean ages of patients undergoing TKAs were 69.5, 70.4, and 69.1 years, respectively (Fig. 3). The decline in mean age during the study period also was steeper in patients undergoing UKAs compared with patients undergoing TKAs (p < 0.001).
In the Kaplan-Meier survivorship analysis, the survival end point was defined as revision of the knee for any reason. Revisions included changing, removing, or adding any component to the prosthesis. Information regarding patient deaths or emigrations was obtained from the Finnish Population Centre. In the Kaplan-Meier analysis, patients were censored at the date of death or emigration, but they were not excluded from the study. Univariate analysis for comparison of the two groups was performed using the log-rank test.
The Cox proportional hazard model was used to obtain the hazard ratio (HR) and the 95% CIs. Three different Cox proportional hazards models were created to compare: (1) UKAs and TKAs, (2) different UKA designs, and (3) different TKA designs. Age and sex were used as adjusting factors. Age was categorized (≤ 65 years and > 65 years), because the linearity assumption did not hold. Differences between groups were considered statistically significant if the p value was less than 0.05 in a two-tailed test. SPSS software (Version 19.0, IBM, Armonk, NY, USA) was used for the statistical analyses.
Overall, Kaplan-Meier survivorship of UKAs was 89.4% (95% CI, 88.4-90.2) at 5 years, 80.6% (95% CI, 79.4-81.7) at 10 years, and 69.6% (95% CI, 68.2-70.9) at 15 years. Survivorship of TKAs was 96.3% at 5 years (95% CI, 96.2-96.4), 93.3% at 10 years (95% CI, 93.1-93.4), and 88.7% at 15 years (95% CI, 88.5-88.9) (Fig. 4).
Aseptic loosening was the most common reason for revisions in patients undergoing UKAs and TKAs but was more common among patients undergoing UKAs (HR, 4.42; 95% CI, 3.7-4.8; p < 0.001 adjusted by sex and age) (Table 2). In patients undergoing UKAs, neither age (≤ 65 years or > 65 years) nor sex affected the revision rate. In patients undergoing TKAs, patients 65 years or younger had an increased revision rate compared with patients older than 65 years (HR, 2.1; 95% CI, 1.9-2.2; p < 0.001). UKAs had inferior survivorship compared with TKAs, even after adjusting for patient age and sex (HR, 2.2; 95% CI, 2.0-2.4; p < 0.001). Survivorship varied by UKA prosthesis design (Fig. 5), and the PCA® (Howmedica, Rutherford, NJ, USA) had shorter survivorship when compared with other designs (Table 3); Kaplan-Meier analysis performed on UKAs with PCA® prostheses removed (data not shown) did not change the overall conclusion of our study, because the PCA® accounted for a relatively small proportion of implants in the UKAs reported in the register. For comparison, we also obtained survival data for four different TKA prosthesis designs that corresponded to the UKA prosthesis designs (Table 4).
In the arthroplasty register reports, overall survivorship of UKA is poorer compared with TKA [23-27]. However, direct comparison of UKA and TKA survival may be inadequate because of different implant designs, indications for surgery, durations of followup, and differences in patient demographics. We compared age- and sex-adjusted survival for patients who had UKAs and cemented TKAs performed for primary knee OA during a 27-year period. In our study, the overall long-term survivorship of UKAs performed for primary knee OA, even the best-performing UKA prosthesis design, was inferior to that for cemented TKAs. In addition we found that the age of patients having TKAs and UKAs decreased during the study period, but the decline in age of patients having UKAs was significantly steeper. Finally, the overall number of UKAs performed decreased during the last 5 years of the study.
The weaknesses of the current study are those common to all register-based studies. Detailed data regarding the patients’ medical histories or knee radiographs were not available. Additionally, we do not have any information regarding the preoperative and postoperative functional outcomes which would better reflect the results of the operations than implant survival alone [5, 28]. However, to overcome the limitations of single-surgeon or hospital studies, there is a need for register-based studies that reflect the results of the operations in widespread use outside the centers of excellence. Second, comparing UKA results with TKA results is complicated. Even if one could match patients by age and sex, which we could not do in this large registry setting, the characteristics of patients undergoing UKAs and those undergoing TKAs may be quite different. In general, it can be assumed that patients undergoing UKAs are more active and their expectations after surgery are higher. Such patients may be willing to accept a higher risk of failure to enjoy some of the perceived benefits of UKA; however, they need to be informed of this risk, and to not overstate the potential influence of age and sex (potential surrogates for activity level), we adjusted Cox models for age and sex. In addition, the Finnish register does not provide surgeon-specific data by volume; a post hoc analysis by hospital volume (greater than versus less than 20 UKAs per year) did not show a difference in survivorship by hospital volume (HR, 1.1, p = 0.26). Finally, the UKA group included an outlier design (PCA®) for which the survival was poor. The inclusion of this implant may bias the results; however, we repeated the Kaplan-Meier analysis with this implant excluded (data not shown), and because the numbers of PCA® prostheses in the registry was relatively small, the overall conclusions of our study remained substantially unchanged.
The reasons for the higher revision rate and decreasing number of UKAs are likely multifactorial. First, there are unique reasons for UKA revisions; in particular, the progression of arthritis to a contralateral compartment, which does not exist for TKAs and may partly explain the higher revision rates. Second, aseptic loosening is more frequent with patients undergoing UKAs compared with patients undergoing TKAs [3, 23-27]. The higher rate of loosening may be explained by the smaller contact area between the implant and bone compared with TKAs. Additionally, the UKA cementing technique is technically demanding, particularly if limited incisions are used and if the bone is sclerotic [7, 14]. We believe that aseptic loosening is a substantial reason for revision, but, especially for the Oxford® UKA prosthesis (Biomet Inc, Warsaw, IN, USA), radiolucent lines under the tibial tray may be misleading , but only in a relatively small number of patients. Even so, it appears that reliable fixation for UKAs remains an unsolved problem . Third, patients undergoing UKAs are younger compared with patients undergoing TKAs, therefore, their expectations are high, and the results of UKAs in these patients may be disappointing . Finally, there is increasing evidence that patients with mild or moderate OA who undergo knee arthroplasty, even those with severe symptoms, have a higher revision rate than patients with severe arthritis [16, 17, 19]. These patients may be overrepresented in the UKA group because the less invasive operation may be performed for patients with less severe arthritis.
Comparing the reasons for failure from the different arthroplasty register reports is difficult. Every register has individual classifications for revisions. For example, the most common reason for UKA revision in the Norwegian arthroplasty register was “pain,” which does not exist in the Finnish or United Kingdom registers [24, 26]. However, the register trends suggest that UKAs are revised more often because of aseptic loosening, pain, and progression of disease compared with TKAs. TKAs are revised more often because of infection [23-27].
A UKA offers tempting advantages compared with a TKA, including lower infection rates, preserving bone stock, minimizing invasiveness, and restoring knee kinematics without sacrificing ligaments . However, in our registry and others [23-27], the survivorship of UKAs is poorer than that of TKAs. Aseptic loosening is a particular problem and tibial component fixation of UKAs appears to remain an unsolved problem. Additional research is needed to develop more reliable fixation of UKA implants. When choosing between a UKA and a TKA, patients should be informed of the advantages of both procedures, but they also should be advised of the generally higher revision risk after UKA.
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