One projection has suggested that by 2030, the number of primary TKAs performed in the United States annually will grow to 3.48 million (a 673% increase) . Along with this increase in primary TKA, the projected demand in revisions is expected to grow by 601%  over the same time period. This increase in demand will represent a major economic burden . Medicare data estimate a USD 4.1 billion (450%) increase in hospital charges for revision TKA and a USD 340 million (160%) increase in surgical charges .
Historically, common causes of failure in TKA have included infection, instability, stiffness, and osteolysis secondary to polyethylene wear [2, 3, 5, 6, 8, 9, 13, 15, 16]. In 2002, Sharkey et al.  reported that polyethylene wear was the most common cause of failure in TKA, accounting for almost 25% of all revisions. Revisions for polyethylene wear accounted for 44% of all late failures, which was defined as revision at least 2 years after the index operation. In the mid-1990s, new manufacturing techniques for polyethylene appeared to have a major impact on implant survivorship. Polyethylene that was gamma-sterilized in air was prone to oxidation, which reduced the resistance of polyethylene to fatigue and thus predisposed it to early mechanical failure . By the late 1990s, major orthopaedic implant manufacturers began polyethylene sterilization in inert environments. Tibial inserts that were gamma-sterilized in inert environments exhibited improved mechanical properties, including a lower incidence of delamination in the first decade of implantation . Whether these changes in manufacturing processes have resulted in a shift in the dominant failure modes after contemporary primary TKA (which may well be different now than they were at the time of Sharkey et al's work in 2002 ) is not known. However, to improve patient health and minimize economic burden, future research must identify the leading causes of TKA failures to help guide our efforts to improve survivorship.
We therefore sought to determine the current reasons for (1) early and (2) late failures after TKA at one high-volume arthroplasty center.
Patients and Methods
We reviewed all first-time revision TKAs at one institution from January 1, 2001, to December 31, 2011, by three surgeons (SBG, WJM, JIH). To filter out the older generation of polyethylene inserts, patients were excluded from the study if their primary arthroplasty dated before 2000. None of the first-time TKA revisions performed during this time period by the authors were excluded for incomplete data.
This resulted in a cohort of 253 revision TKAs in 251 patients (SBG: 90 patients, WJM: 53 patients, JIH: 108 patients) (Tables 1, 2). One hundred forty-four revisions were in females. The mean age of the patients was 64 years (SD 10 years).
We defined time to failure as the interval between the index arthroplasty and revision procedure with early failures involving surgery within 24 months of the index arthroplasty and late failures undergoing surgery later than that. Early failure accounted for 46% of all revisions, whereas late failure accounted for 54%. The mean times to revision were 35 months (SD 26 months) for the entire cohort with 15 months in the early failure group and 52 months in the late failure group.
Clinic and hospital notes, laboratory studies, and radiographs were reviewed. Patient age and sex were recorded as well as time to failure and primary reasons for failure. In cases with multiple failure modes, the primary mode was determined subjectively by the operating surgeon. Reasons included the following categories: infection, instability, stiffness, aseptic loosening, polyethylene wear, and other causes. Other causes of failure included implant malposition, extensor mechanism failure, failed unicompartmental knee arthroplasty, pain, periprosthetic fracture, failed UniSpacer® (Zimmer, Inc, Warsaw, IN, USA), and tibial insert dislodgement without significant wear. All data were recorded and stored in a secured Excel® file (Microsoft Corp, Redmond, WA, USA).
The early failure group consisted of 116 of the 253 revisions (46%). The most common causes of early failure included infection (28 of 116 [24%]), instability (30 of 116 [26%]), and stiffness (21 of 116 [18%]). Failure of osseointegration (16 of 253 [14%]) and other causes (19 of 116 [16%]) were the next most common causes for revision in this early group. Wear (two of 116 [2%]) was the least common cause for early revision.
The late failure group consisted of 137 of the 253 revisions (54%). The most common causes of late failure included infection (34 of 137 [25%]), instability (24 of 137 [18%]), and stiffness (19 of 137 [14%]). Failure of osseointegration (18 of 137 [13%]) and other causes (28 of 137 [20%]) were the next most common reasons for revision in the late group. Wear (13 of 137 [9%]) was the least common cause for late revision.
Other less common causes for revision included implant malposition (18 of 253 revisions [7%]), extensor mechanism failure (nine of 253 [4%]), failed unicompartmental knee arthroplasty (eight of 253 [3%]), pain (six of 253 [2%]), periprosthetic fracture (four of 253 [2%]), failed metallic unicompartmental spacer (two of 253 [1%]), and polyethylene insert dislodgement without significant wear (one of 253 [0.5%]). This group accounted for 19% (48 of 253) of all revisions with 8% (19 of 253) early and 11% (29 of 253) late.
Historically, polyethylene wear and its sequelae (osteolysis, late instability, aseptic loosening) were common causes for revision TKA. Recently, polyethylene manufacturing has become more consistent; furthermore, a clearer understanding of the importance of oxidation on polyethylene performance led to packaging in an inert environment. These changes may result in fewer polyethylene-related failures, but because, to our knowledge, no analysis has been done to see whether the dominant causes of TKA failure have changed in the wake of these manufacturing process improvements, we sought to examine the most common causes of failure after contemporary primary TKA. Both early and late failures in our cohort were most frequently caused by infection, instability, and stiffness in that order; polyethylene-related revisions were very uncommon. However, the mean time to revision in this series was only approximately 3 years, and the latest revision in this series was performed just over 10 years after the index TKA, so studies of this design with longer followup clearly are needed to draw firmer conclusions about the relative contribution of polyethylene wear as a cause of revisions.
The primary limitation of this study is the lack of longer followup that will be necessary to determine if modern changes to the manufacturing and packaging of polyethylene inserts result in fewer late wear-related failures. Other limitations of our study include its retrospective nature (selection bias) and its reliance on the medical record, which can have errors in documentation. Because this series is from a tertiary referral center, it may not reflect general trends seen in the community setting. Lastly, our study did not account for the differences in primary implant types used and we cannot confirm all inserts included in our study were sterilized in an inert environment.
Infection continues to remain one of the most common causes of failure in TKA. In our study of 253 revisions, 25% of revisions were caused by infection. Similarly, in 2010, Bozic et al.  reported that infection accounted for 25.2% of revisions in a Medicare population. The incidences of revision resulting from infection in other recent series include 32.7% , 38% , and 24% . Based on these findings, infection prophylaxis and treatment should continue to be a focus of future research efforts.
Unlike previous studies, we found fewer revisions were caused by wear/osteolysis/aseptic loosening (6%). In 2002, Sharkey et al.  reported that 25% of all revisions were the result of wear-related causes with 44% of late failures attributable to wear. Vessely et al.  reported 16% wear-related failures in 2006. Gioe et al.  in a community-based registry reported 15% revisions resulting from osteolysis in 2004. Hossain et al.  reported polyethylene wear in 12.3% of revisions in 2010. Given that these studies had longer mean times to revision than our study, it is possible that we would have seen an increase in wear-related failures had our patients presented with longer followup. Other factors besides polyethylene quality that may have contributed to the differing rates of wear-related failures include the subjective determination of the primary failure mode by the research team, the use of highly crosslinked polyethylene inserts, and the inclusion of multiple prosthetic designs in the analyses. Furthermore, a lack of standardized definitions renders a direct comparison between these studies of limited use.
The exclusion of index arthroplasties before 2001, which effectively eliminates a previous generation of inserts that were gamma-sterilized in air, may have contributed to our lower revision rate resulting from osteolysis. Changes in the manufacturing process of polyethylene inserts include gamma sterilization in inert gas as opposed to air, omission of calcium stearate additives in production of polyethylene resins, and methods of final geometry shaping of inserts. This is supported by a study from 2005 in which Collier et al.  examined 300 patients and found a 34% rate of osteolysis in tibial inserts gamma-sterilized in air as compared with 9% in inserts gamma-sterilized in inert gas. Griffin et al.  in 2007 examined wear-related failures in two large cohorts of patients with identical tibial trays with inserts gamma-sterilized in either air or inert gas. At 5-year followup, wear-related failures were 1.1% in inserts sterilized in inert gas versus 8.3% in knees with inserts sterilized in air. Berzins et al.  reported less polyethylene surface damage in tibial inserts manufactured by net-shaped molding in the absence of calcium stearate compared with those inserts manufactured by ram extrusion with calcium stearate additive.
The Swedish Knee Arthroplasty Register has shown a consistent reduction in risk for revision TKA over time with the exception of a period between 2006 and 2010, which was attributed to a spike in infections . This reduction over time may also reflect adoption of a newer generation of polyethylene inserts .
In conclusion, our study demonstrated that infection, stiffness, and instability were the most common causes of failures in TKA. Wear-related failures were decreased compared with previous reports [5, 13], and this may have been influenced by improvements in polyethylene manufacturing and sterilization, although longer mean times to revision in those reports [5, 13] might also have contributed to our findings; longer followup on the newer polyethylene bearings is needed. Strategies to improve outcomes in TKA should be aimed at infection prophylaxis and treatment, surgical technique, and patient selection.
We thank Angela Bye MA, PT, for her assistance with data collection and analysis.
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