Many published studies on clinical performance of knees are based on visual assessment of the surfaces of explanted devices [14
] and provide important information about in vivo kinematics, impingement, debris, modes of material wear, and other aspects of bearing performance. Measuring actual wear (material loss) from clinical knee retrievals is challenging, in large part because an accurate unworn reference (either gravimetric or dimensional) is usually not known. Our objectives were to contribute quantitative wear measurements from retrieved knee bearings to answer whether RP mobile bearings wear more than fixed bearings in vivo, tibial tray roughness has a measurable impact on backside wear, and there is a difference in wear between CR and PS inserts.
Readers should be aware of the limitations of the study. First, we considered a limited series of retrieved bearings of two designs. Our findings might not apply to other designs. Second, the in vivo duration is longer for the fixed bearing series than for the RP series (179 versus 124 months). In consideration of this difference, wear measurements were presented in terms of both depth of wear and wear rate (normalized for in vivo duration) and conclusions are based only on wear rate. Third, the RP wear measurements and the corresponding through-thickness measurements of fixed bearing inserts represent total wear on both the top and bottom articulating surfaces plus other deformation processes such as creep and pitting. Therefore, the reported wear measurements should be considered a conservatively high value of insert thinning due to abrasive/adhesive wear. We report an estimate of wear volume only for backside wear, for which the area of the worn surface could be accurately measured, and therefore direct comparisons with other studies should be made on the basis of backside-only wear volume.
Our findings indicate the additional articulating surface of the RPs did not increase total wear penetration rate on the inserts compared to fixed bearing inserts. This finding does not support the concern raised by the damaged appearance of the rotation surface of retrieved mobile bearing knees [20
] (Table ). The lack of wear bias to the medial side is consistent with other reports of measured wear of mobile bearings [1
] (Table ) but is distinct from our fixed bearing results and those of other reports [12
] (Table ). The RP wear rate of 0.04 mm/year in our study is lower than the 0.09 mm/year reported by Kop and Swarts [24
] for relatively small series of both LCS-RP®
and AP Glide®
inserts and by Atwood et al. [1
] for a series of 100 LCS-RP®
inserts (Table ). It is notable, in the study of Atwood et al. [1
], the wear rate for in vivo duration of more than 2 years was 1/3 of the series average. In our study, the wear rate of the RPs was not dependent on vivo duration, in clear contrast to the wear rate of the fixed bearing series, which increased with in vivo duration. The wear rate of the fixed bearing inserts was within the range of other published results, which range from 0.0041 mm/year for a study based on erasure of engraved backside lettering [13
] to 0.35 mm/year based on through-thickness measurement on a variety of designs [3
] (Table ).
Comparison of mobile bearing knee wear assessments from other studies and our study
Comparison of fixed bearing knee wear assessments from other studies and our study
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
] and retrieval analysis [10
]. 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
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
], 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
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
]. 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.