A 100% 10-year survivorship was reported for a cohort of patients with PFC cruciate-retaining prostheses followed at 5-year intervals [20
]. The current study presents survivorship at a minimum of 15 years along with the previously reported clinical and radiographic variables. There are two aspects of our analysis, evaluation of the surviving knees from the initial study group and analysis of the six failures. The polyethylene was thin (8 mm in 77% of the knees) and we were concerned this predisposed the implant to failure. Because the failure mode appeared limited to, or initiated by, polyethylene failure, we were concerned about the short-term survivorship for polyethylene exchange. With the current minimum 15-year evaluation, we attempted to address: (1) functional scores, (2) rates of radiographic failure, (3) overall revision rates, (4) mode of failure after 10 years, and the fate of implants that had been revised.
We acknowledge several limitations of our study. First, is the small number of failures available to study. Because the modes of failure varied, we were unable to perform subcohort analyses. However, the data suggest high short-term survival for patients with polyethylene exchange. Second, this was an older population. The average age of the overall cohort at the time of the index procedure was 71.2 years, similar to other contemporary TKA cohorts (Table ). Thus, we had relatively few patients to follow, although for most patients the implant provided function for the duration of their lives.
Minimum 15 years followup studies of other TKA cohorts
Strengths of the study include the fact that the operating surgeon (SSK) was not involved in the followup or evaluation of the long-term results. One of the other authors (DDG) performed all the clinical followups during the 15-year interval and two other authors (JJC and ASM) performed all of the radiographic analyses and data collection. All patients were called and requested to return for followup at 5, 10, and 15 years after their surgery.
Numerous TKA cohorts have been followed for 15 years or longer with survivorship rates consistently exceeding 90% [7
] (Table ). Although 15-year studies on modular TKA were reported [13
], there are relatively few studies, especially when one considers the frequency with which modular TKA components are used. In addition, as compared with our study, the cohort of Rodricks et al. had more limited radiographic followup [27
], and that of Dixon et al. included hybrid replacements [13
]. We obtained clinical followup information for 91% of our living patients (32 patients, 43 knees). Three patients (four knees) had dementia. The Knee Society scores obtained in this study were comparable to those of other studies with similar followup (Table ). We also report WOMAC scores, which were not reported in similar studies.
We obtained radiographic followup for 80% of our living patients (28 patients, 38 knees). Radiolucent lines were present in 34% and were incomplete and nonprogressive. The range of radiolucent lines has varied in reports of the same implant (13–62%); however, in each of those studies, the lines were incomplete and nonprogressive [13
]. All studies reported concerns regarding osteolysis [13
]. Early changes consistent with osteolysis were evident in four patients in our study.
In the current study, the PFC prosthesis performed well with respect to survivorship (Fig. ). Fetzer et al. previously reported a survivorship of 100% at 10 years [20
]. All revisions occurred greater than 11 years after the index procedure. All failures were related to polyethylene wear leading to osteolysis and/or loosening. Our survivorship, radiographic, and clinical outcomes are comparable to those of other studies with similar 15-year followups, including studies of similar prostheses designs (Table ). Unlike other studies with a mixture of failure modes (infection, patellar failure), our failures can be attributed to the durability of the polyethylene. In two of the six failures, intervention occurred before major osteolysis. When osteolysis compromised component fixation, the femoral component was involved in all four cases, the tibial component in two of those four. Although we had small numbers, our finding that osteolysis was more likely to occur on the femoral side also was reported by Griffin et al. [21
In our cohort, relatively thin polyethylene was implanted. In 78 of 101 knees, an 8-mm insert was implanted. However, taking into account the tibial base plate, the actual minimum thickness of an 8-mm insert implanted was 5.3 mm. All failures occurred in patients with 8-mm inserts although given the small numbers of thicker (10, 12.5, 15 mm) inserts implanted we observed only a p = 0.051 level of significance. The PFC implant studied here also uses a relatively flat, nonconforming surface, particularly the posterior-lipped insert. Despite the potentially deleterious effects of thin [2
], flat [4
] inserts on polyethylene wear, the revision rates in our patients were low and occurred late.
Our study cohort consisted entirely of polyethylene inserts gamma-irradiated in air, a process that has been associated with increased rates of polyethylene wear [9
], particularly in combination with increased shelf life of polyethylene before implantation [8
]. The catastrophic early failure observed in PFC knee prostheses of the same design implanted after 1991 [18
] was not evident in our cohort, which included patients who had TKAs performed just before 1991. Polyethylene shelf life data were not available for analysis in the current study.
At 6.7 years followup (range, 4.7–9.1 years), we found no evidence of clinical or radiographic concerns in the six patients who had undergone revision. Although two of the six patients had complete revisions, four were able to benefit from the modularity of the tibial tray. The potential economic impact of modular exchange versus complete revision is presented in Appendix 1. The limitations to the cost analysis are that it is based on the assumption that the initial decision at the time of primary total knee replacement was to use a more expensive modular device versus a nonmodular device, and that the analysis was based on our small numbers of overall revisions (six).
Although two studies reported high rates of early failure after polyethylene exchange revision TKA [1
], our four patients undergoing polyethylene exchange (two isolated polyethylene exchange and two polyethylene exchange associated with femur/patellar revision) reported no knee pain and had no evidence of progressive radiographic abnormality at an average of 7.2 years (range, 4.7–9.1 years) from the revision procedure. This was possible because of the regular followup (every 5 years) of our patients. There is one other study advocating consideration of modular exchange when the component fixation is intact. In a recent retrospective review, an 84% success rate of polyethylene exchange at a minimum of 24 months postrevision in well-aligned knees was reported [21
The PFC modular, cruciate-retaining total knee prostheses performed well clinically and radiographically at a minimum of 15 years postoperatively. Osteolysis is recognized as the dominant mode of aseptic failure in TKAs, particularly those in which modular-design prostheses are implanted [9
]. Because osteolysis can be asymptomatic for a relatively long time, close followup, especially after 10 years, is warranted. Modularity does offer the advantage of a low-morbidity, cost-saving option for revision TKA (Appendix 1.) Use of this potential benefit requires close clinical followup of patients undergoing TKA.