As a consequence of its recent introduction to clinical use, there currently are no publications describing the clinical wear properties of the X3™ acetabular cup insert. One hip simulator study reports the in vitro wear of this insert [16
], and therefore our purpose was to compare these in vitro wear results with those after clinical use during a 2-year period. Furthermore, as this is the first report of the clinical wear characteristics of a second-generation HXLPE acetabular liner, we were interested in comparing the results from this study with literature reports of first-generation annealed highly cross-linked UHMWPE liners. We hypothesized the findings of this study would be consistent with the in vitro wear results and show clinical wear comparable or less than wear for first-generation HXLPE liners.
The high sensitivity of the RSA measurement technique, in conjunction with the ability to extrapolate results of short-term average linear wear rate to the average long-term linear wear rate for a population, makes this a useful tool for screening newly introduced prostheses [13
]. Despite this, the RSA technique has limitations in accuracy and precision [4
] that make it challenging to measure very small amounts of wear that are less than the detection threshold. In an optimal experimental setup, the accuracy of RSA is reported to range from 0.022 mm to 0.086 mm depending on the vector direction [7
]. Our study is further limited in that no measurement of precision was made for this data set. The precision of our results should be similar to those validated previously for the combined liner and marker technique [4
Dowd et al. [15
] reported a linear increase in true wear with time is characteristic of polyethylene acetabular liners. However, we measured an uncharacteristic pattern of proximal femoral head penetration in that numerous patients had negative wear, particularly within the first year of the study. This finding has been reported previously [6
] and is a result of femoral head penetration measurements lying within the accuracy limit of the technique and therefore being outside the limit of detection. The migration calculated between 1 and 2 years represents the actual rate of wear, but the numerical value of this should be interpreted with caution as it also lies within the detection threshold of the RSA technique.
The calculation of annual wear in this study was based only on wear that occurred between 1 and 2 years. Although the amount of head penetration was recorded at three times, the measurements were relative to the immediate postoperative radiographs and consequently included the initial creep and bedding-in of the liner. Studies have shown the majority of bedding-in occurs within 2.5 million cycles [21
], which usually is complete after approximately 1 year [47
]. This being the case, only wear measured between the first and second years was considered true wear. This is supported by the findings of Glyn-Jones et al. [24
] in an RSA study of the creep and wear characteristics of HXLPE. They concluded femoral head penetration within the first 6 months was dominated by creep whereas penetration after 1 year was virtually all attributable to wear.
In the only published hip simulator study comparing the X3™ liner with conventional UHMWPE and a first-generation annealed HXLPE liner (Crossfire®
; Stryker Orthopaedics), the X3™ liners had a markedly lower wear rate than the conventional and first-generation HXLPE liners [16
]. Based on their findings, Dumbleton et al. [16
] predicted the clinical wear rate of the X3™ liners should be 14 μm/year. The wear rate of 15 μm/year of median proximal wear measured in our study between 1 and 2 years is consistent with the predicted wear rate of 14 μm/year but should be considered a serendipitous result as this amount of wear is within the limits of accuracy for RSA and is not valid at this time. If we assume a conservative detectable limit for measuring wear of 80 μm, which is consistent with the precision measurements reported by Borlin et al. [4
] for this technique (68 μm, 98 μm, 138 μm in the x, y, z axes, respectively), it would take more than 5 years before there is evidence of measurable wear. Similar findings have been reported for first-generation HXLPE liners, which highlights the need to evaluate HXLPE over a period of at least 5 years [9
]. The annual 2-D wear rate calculated in our study was considerably less than for proximal wear; however, linear measurements of 2-D wear are thought to underestimate the true wear rate [17
] and therefore may not truly represent the wear rate for this type of polyethylene.
First-generation annealed Crossfire®
liners are reported to have an annual wear rate of 36 μm/year based on a 5-year evaluation of plain radiographs [12
]. The annual wear rate for the X3™ liner found in our study (15 μm/year) is 58% less than this, which is consistent with the hip simulator results of Dumbleton et al. [16
], who found the X3™ material had 62% less wear than Crossfire®
liners. Rohrl et al. [44
] reported a mean wear of 23 μm between 2 and 24 months for Crossfire®
inserts. This is similar to the mean proximal head penetration we found (28 μm) for the X3™
liner; however, an accurate comparison requires a longer study to quantify the potential differences in wear between these materials.
A low rate of polyethylene wear is advantageous as it reduces the likelihood of wear particle-induced osteolysis and the subsequent need for revision arthroplasty owing to aseptic loosening. Dumbleton et al. [17
] have assigned an osteolysis threshold for wear of 0.1 mm/year, below which osteolysis occurs infrequently, and a rate of 0.05 mm/year, which is considered safe, as the occurrence of osteolysis is almost eliminated. The annual wear rate calculated in our study was well below this threshold. We can expect the need for revision arthroplasty attributable to wear particle-induced osteolysis to be unlikely at least in the short term. The functional biologic activity of this material is likely to be lower than conventional polyethylene owing to a combination of similar specific biologic activity and lower wear rate [16
Our study showed that wear of X3™ acetabular liners after 2 years is less than a clinically quantifiable level, making accurate comparison with first-generation Crossfire® liners challenging. A longer period of evaluation is required until wear reaches a level that is clinically detectable. However, it is clear X3™ liners have wear properties superior to those of conventional polyethylene. Our measurements between 1 and 2 years followup suggest wear is nearly undetectable, which is encouraging for the future clinical performance of this material.