SELECTED PROCEEDINGS FROM THE 2017-18 EUROPEAN KNEE SOCIETY MEETINGS
Where Are We Now?
In the current study, Klasan and colleagues  found that 16 of 23 patients who received a metal-on-metal (MoM) hinge TKA for a complex primary or revision operation demonstrated elevated serum metal ion levels at a median followup of 28 months. This observation confirms those of two earlier studies [4, 6], both of which also reported elevated blood/serum metal ion (both cobalt and chromium) levels most patients after complex revision knee arthroplasty using a MoM hinge. Taken together, it seems reasonable to conclude that elevated serum metal ion levels are a concern in these patients.
The current study  also extends the findings on those earlier studies [4, 6], since it is, to my knowledge, the first study to assess blood metal ion levels in patients with a conventional hinge revision knee arthroplasty, whereas those earlier reports evaluated ion levels in patients who had received a megaprosthesis-type hinge TKA. This is important because conventional hinge TKAs are much more often used than megaprostheses, and it also illustrates that the rather small hinge mechanism is an inevitable source of metal wear debris in MoM hinge TKAs.
Where Do We Need To Go?
After reading the current study , several clinically important questions still remain, including: (1) Will the metal ion levels increase or decrease over longer followup? (2) What will be the consequences of increased metal ion burden in these knees? (3) What is the best followup protocol for these patients? (4) What are the indications for cross-sectional imaging in these patients? (5) What will be the incidence of adverse reaction to metal debris (ARMD) in these patients? (6) What will be the failure mechanisms of these knees (loosening, ARMD, or something else) and how long will these knees endure? And (7) what is the ideal threshold for rerevision surgery?
How Do We Get There?
It will be difficult to further study the consequences of elevated blood metal ion levels in these patients because MoM hinge revision knees have been used in a rather limited patient population, and many countries where these procedures have been performed do not have national arthroplasty registers. As a result, turning to the modern national arthroplasty register system may not be fully informative.
There seem to be three realistic ways to tackle this issue. First, we need to rely on the experience and the lessons that we’ve learned when dealing with the MoM hip arthroplasties. This applies both to followup, indications for cross-sectional imaging , and indications for revision surgery . Second, we should look for multicenter collaboration to perform adequately powered studies. Although multiregistry collaboration could be a consideration here, it is my belief that registries do not help in solving these research questions, as they do not contain information on blood metal ions, cross-sectional imaging, revision surgery reports, nor histopathological samples. That’s where multicenter collaboration could play a role, as the centers may collect all this information. From arthroplasty registers, one can only assess implant survivorship, and in this case it’s not enough. Third, dedicated implant-retrieval centers will be a vital part of this research . Explanted MoM hinge knees should undergo strict retrieval analysis to assess the magnitude of wear that takes place in the hinge mechanism. The explants should also be assessed for corrosion both in the hinge mechanism and also in the other taper junctions (between the stem extension and the femoral/tibial component). Additionally, when these knees are rerevised, histopathological assessment of the synovium is important to clarify whether the tissues evince ARMD-type features. Currently, little is known on how the synovium of the knee joint reacts to the particulate debris produced by all knee arthroplasties. Unfortunately, there are no shortcuts in this field of research, and answering these important questions will take time. Until we get those answers, though, I strongly discourage the use of MoM hinge knee designs, as there are safer options available. The recently published long-term outcomes of conventional hinge TKAs have been acceptable both in primary and in revision situations: In the Australian arthroplasty register, the 10-year revision rate of all hinge knee implants was 14.4% when used in primary TKA . Further, Cottino and colleagues  recently reported a 22.5% revision rate at 10 years for a cohort of 408 consecutive rotating-hinge TKAs, majority of which (82%) were implanted in complex knee revisions. Knee revision surgery is difficult and complicated enough; we certainly don’t need additional metal-debris-related complications to further compromise the results of these procedures.
1. Australian Orthopaedic Association National Joint Replacement Registry (AOANJRR). Hip, Knee & Shoulder Arthroplasty: 2018 Annual Report. Adelaide: AOA, 2018. Available at: https://aoanjrr.sahmri.com/en/annual-reports-2018
. Accessed February 7, 2019.
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5. Klasan A, Meine E, Fuchs-Winkelmann S, Efe T, Boettner F, Heyse TJ. Are serum metal ion levels a concern at mid-term followup of revision knee arthroplasty with a metal-on-metal hinge design? Clin Orthop Relat Res. [Published online ahead of print]. DOI: 10.1097/CORR.0000000000000638.
6. Laitinen M, Nieminen J, Reito A, Pakarinen TK, Suomalainen P, Pamilo K, Parkkinen J, Lont T, Eskelinen A. High blood metal ion levels in 19 of 22 patients with metal-on-metal hinge knee replacements. Acta Orthop. 2017;88:269–274.
7. Matharu GS, Eskelinen A, Judge A, Pandit HG, Murray DW. Revision surgery of metal-on-metal hip arthroplasties for adverse reactions to metal debris. Acta Orthop. 2018;89:278–288.
8. Matharu GS, Judge A, Eskelinen A, Murray DW, Pandit HG. What is appropriate surveillance for metal-on-metal hip arthroplasty patients? Acta Orthop. 2018;89:29–39.