The lower wear rate for the fixed bearing inserts in polished CoCr trays compared with inserts in rough Ti trays confirms findings from in vitro wear studies [4, 19] and retrieval analysis . A further indication of the difference in wear performance is that inserts from Ti trays showed wear rate increasing with in vivo duration, while the inserts from polished CoCr trays did not. Increasing backside wear rate in fixed bearings would be expected due to wear of the insert locking mechanism, a process that has been reported in previous studies of modular fixed bearing knees [11, 31].
Our observations suggest decreasing the roughness of the modular tray has a notable impact on clinical wear. This is demonstrated by the RP inserts, which are free to rotate on a polished tray yet showed lower wear rate than the fixed bearings taken as a whole. It is demonstrated further within the fixed bearing series, which showed a wear rate for inserts from polished CoCr trays lower than both the fixed bearing inserts from rough Ti trays and the RP inserts. The findings suggest limiting insert-to-tray motion alone is not fully effective in reducing wear; optimizing the metal counterface and the relative motion is important. Although there is widely reported evidence of substantial motion of RPs relative to the trays [20, 22, 23, 28], the central post constrains the relative motion to be unidirectional. The backside surface of fixed bearing knees experiences much less extensive motion, but it is multidirectional. Increased wear of polyethylene under conditions of multidirectional motion has been documented by in vitro wear studies [4, 5, 7, 29, 37].
We found no difference in measured wear rate between CR and PS knees. An important design premise of PS knees is that kinematics can be maintained while tibiofemoral surface area is increased and contact stress is accordingly decreased [2, 6, 8, 34]. One hypothesis for the lack of differential performance is that the increased conformity also increases tibiofemoral transmission of torque, which drives rotational motion at the insert-tray interface. Progressive wear of a fixed bearing locking mechanism and a rough tray counterface would be expected to result in increasing backside wear rate, which is what we saw in the fixed bearings from Ti trays. Although the in vivo duration of the CoCr fixed bearings is shorter, that design appears to accommodate backside motion with less insert wear than with the rough Ti counterface.
In summary, we found the wear rate of RP inserts was less than that of fixed bearing inserts, which is in contrast to the indications from retrieval studies based on surface damage assessment. The wear rate of fixed bearing inserts in polished CoCr trays was less than their counterparts in rough Ti trays and was less than the RP inserts. The wear rate of PS and CR inserts was not different. Our findings indicate a polished tray counterface reduces insert wear and the wear rate of inserts against a polished CoCr tray does not appear to increase over time.
The authors thank the surgeons who have collaborated with our institutions by sending retrieved devices for analysis. We also thank DePuy Orthopaedics, Inc, for providing the design and manufacturing data for the implants studied in this investigation, without which this type of clinical retrieval analysis is not possible.
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