SELECTED PROCEEDINGS FROM THE 2017-18 EUROPEAN KNEE SOCIETY MEETINGS
Where Are We Now?
Incorrect mechanical alignment has been considered a main cause of polyethylene wear and implant loosening after TKA. Although the risk of aseptic loosening and polyethylene wear generally is low, it is not zero, and other problems (like instability and stiffness ) remain largely unsolved. I believe that component positioning influences all of these important endpoints, and is a primary factor influencing the likelihood that a TKA will provide the patient with a mobile, stable, and well-aligned knee. Because of the importance of alignment, a number of technologies have been developed in the hopes of helping surgeons achieve correct alignment more frequently. But these approaches—including computer assisted surgery (CAS), patient specific instrumentation, and robotics—must not only help us to improve alignment; they also need to be easy and efficient enough to use in routine practice, and they must be cost effective.
The paper by Budhiparama and colleagues  concentrated on accelerometer-based navigation (ABN) systems; these are handheld devices that can guide distal femoral and proximal tibial resections. They may be simpler and less expensive than large computer consoles like those used in traditional CAS systems. In their systematic review that included 10 studies on the two available systems, Budhiparama and colleagues  found inconsistent evidence on mechanical-axis alignment or component positioning improvements. They also found no conclusive benefit in terms of patient-reported outcomes, and with more than 600 patients in each group, it was large enough to detect meaningful differences had they been present. While this review  did not provide long-term assessment of revision risk as one study provided for traditional CAS in TKA , there is no particular reason to think the results will improve over time (indeed, they did not in our long-term study ), and it did not demonstrate any important benefit to ABN systems.
Where Do We Need To Go?
Because of the absence of evidence in support of ABN, it seems to me that future work should be borne under research protocols, until or unless the technology is shown to have a benefit that patients can perceive. Should this occur, we could then have conversations about target alignment values, ranges, and number of outliers. But insofar as this study  and another by Matsumoto and colleagues  found no compelling advantages to ABN, I believe that patients should not bear additional costs nor surgical time associated with its use. The absence of increased surgical time, as recorded in this review and another study , is one reason to consider ABN technology since additional surgical time is considered as one of the limiting factors by orthopaedic surgeons for a wider use of conventional CAS systems in TKA.
If ABN is to see wider use, it may need improvements in terms of consistency of registration of the femoral head , which has implications when assessing the target alignment ranges. More-recent studies have called into question whether such precision of alignment is even necessary , particularly with the improved polyethylene bearings that now are available, and whether alternative alignment targets (such as constitutional varus  or kinematic alignment ) may be as good or better for some individuals.
How Do We Get There?
Since the global usage of TKA is increasing, it is important to explore any technological avenues that can help us to improve long-term results. Although the current review suggested that ABN is unlikely to do that , I point out that it included only four randomized trials. There is room for more work on this subject, but any such work should be considered (and funded as) clinical research with the need for several concomitant studies demonstrating the system to be beneficial for the patient.
If such research is to be conducted, it will be essential to further define what the most important anatomic endpoint ABN could improve upon. If most instances of loosening are tibial, perhaps that is where the emphasis should be; by contrast, it might be the overall limb axis. Future studies should help determine this. We must remember that these systems do not guide us in terms of tibial or femoral rotation, which also is important. We also need to think about the clinical endpoints that matter and the kinds of studies that might best guide us. For example, one randomized trial found computer navigation in bilateral TKA to provide no benefit in terms of pain, function, or survivorship , while the Australian Joint Replacement Registry demonstrated a reduction in aseptic loosening when using CAS TKA in patients younger than 65 years of age .
I believe that precision and reproducibility are essential to a high-quality, well-performing TKA, and although past results with computer navigation and ABN have been mixed, my hope is that refinements to those approaches will deliver better results in the future.
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