Take what you read with a grain of salt: commentary on an article by Richard N. de Steiger, MBBS, FRACS, FAOrthA, et al.: "computer navigation for total knee arthroplasty reduces revision rate for patients less than sixty-five years of age".

Total knee arthroplasty provides durable results and is reliable at relieving pain and improving function in patients with arthritis of the knee. The goal during surgery is to achieve a well-aligned total knee prosthesis with ligament-balancing to minimize wear and implant stress. Restoration of alignment close to the mechanical axis has been one of the main tenets of knee replacement surgery over the past four decades. In their study, de Steiger et al. present interesting and novel data from the Australian Orthopaedic Association National Joint Replacement Registry (AOANJRR). They analyzed data that had been prospectively collected from 2003 through 2012 and compared the revision rates and reasons for revision surgery in a cohort of patients who had undergone primary total knee arthroplasty with or without computer navigation. The principal finding of this study was that while computer-assisted surgery did not significantly reduce the overall revision rate in the entire study population, it did reduce the overall revision rate and the risk of revision for aseptic loosening in patients younger than sixty-five years of age. The authors postulated that the lower rate of failures in these younger patients over this ten-year observational period may have been due to more accurate implant position resulting from the use of computer navigation. This study has several strengths and weaknesses and, therefore, one must critically evaluate the data and methodology and determine which conclusions are supported by the study results. First, the data represent the vast, aggregated contributions by all of the practicing orthopaedic surgeons in Australia. Because individual surgeons vary with regard to their practice pattern, clinical experience, and surgical volume, the aggregated data represent and are applicable to all surgeons. Second, the AOANJRR is well designed and managed with various checks and balances ensuring high surgeon compliance and the capture of reoperations and revisions. Thus, the reported revision and reoperation rates are accurate, and the risk of sampling bias is minimized. Finally, the registry includes detailed data such as implant design, mode of fixation, and surgeon volume. Consequently, these data can be effectively used for postmarket surveillance of hip and knee implants and help identify trends and early failures as in the case of metal-on-metal hip replacements. However, as detailed as the AOANJRR is, the main problem with registry data is that they provide only a “bird’s-eye view” of a situation and lack specific clinical details necessary to unravel the contributions of important factors such case complexity, surgical technique, and patient factors. Additionally, factors that may seem to be important at the registry level may in fact be affected by many other confounding variables at the patient level. For example, this same registry showed that metal-on-metal hip resurfacing was associated with an increased risk of revision compared with total hip replacement. Although the initial reaction would be to condemn this entire technology, further analysis showed that men under the age of sixty-five actually had lower five-year revision rates than those who had undergone total hip arthroplasty. Furthermore, this registry does not collect radiographic data and, therefore, no data on alignment, radiolucent lines, or osteolysis were available for analysis in this study. Thus, one must question whether the relatively lower risk of revision when the total knee arthroplasty had been performed with computer navigation in patients under the age of sixty-five is solely attributable to the technology and the benefits of improved component positioning including rotational alignment. The causes leading to revision total knee arthroplasty can be complex and multifactorial. Often, multiple factors contribute to pain and dysfunction. While the AOANJRR contains data on the etiology of revision (loosening, infection, instability, etc.), the process of assigning the principal cause for a reoperationmay not be always clear-cut. How should a failed total knee arthroplasty be classified if component malrotation led to instability? If the radiographs show some radiolucent lines, is instability the cause of the loosening or the other way around? Also, the fidelity with which surgeons classify the cause of failure can be a source of error and bias. Without detailed clinical data, it is not possible to carry out a detailed analysis and determine the drivers of revision surgery. This is the underlying problem with registry data. De Steiger et al. did not find any relationship between surgical volumes and implant survivorship in their study. Computer navigation has been shown to decrease the number of outliers and improve overall component position in total knee arthroplasty regardless of surgical volume. They therefore hypothesized that, because computer navigation allows the surgeon to achieve a specific component position and alignment more accurately, the observed difference in revision rate had to be the result of better implant position e40(1)