[Wear and Roughness of Bearing Surface in Retrieved Polyethylene Bicon-Plus Cups]

PURPOSE OF THE STUDY By 7th December 2016, 4,755 Bicon-Plus cups in total were implanted in the Czech Republic. Some of them have been continuously re-operated, while the most frequent reason of failure is polyethylene wear and aseptic loosening. The present study is focused on surface analysis of retrieved polyethylene Bicon-Plus cups and the determination of the roughness of their bearing surfaces. MATERIAL AND METHODS In this study, we had 13 high molecular weight polyethylene cups with the average time in situ of 8.11 years (3.6-13.7, SD 3.2) before the retrieval. The study population was composed of 3 men, 10 women, with the mean age of 53.31 years. An optical scanning method, based on the principle of active triangulation, was used to determine wear rate. The rate of wear was identified by means of an obtained scan subsequently processed with the use of the GOM Inspect software. The roughness of surfaces was analysed with the application of Contour GT-X8 profiler using the principle of phase shifting interferometry. Measurements of surface topography of the retrieved cups were performed on the entire bearing surfaces. For the individual surface changes, a typical range of surface roughness, describing the particular wear character, was determined. By means of morphology analysis of the tested implants, three areas were identified: unworn area; area representing the worn part of the cup; and the area roughened by parallel grooving. The total surface roughness was evaluated as an arithmetic mean of the measured values. Subsequently, the values were sorted based on frequency and were classified into categories defining the particular wear mechanisms. RESULTS Wear rate of the retrieved acetabular cups was evaluated based on the wear direction vector and the size of linear wear. The average linear wear was equal to 0.13 mm/year (ranging from 0.26 to 2.29 mm/year), and the mean value of total volumetric material loss was 44.37 mm3/year (the range being from 51.80 to 1,119.7 mm3/year). Using the optical profilometer, a map of roughness distribution of the individual cups was obtained. For each implant, 76 values of roughness were evaluated. With the respect to average roughness, the samples were sorted to various categories describing: surface polishing; abrasiveadhesive wear; surfaces with preserved grooving; substantial plastic deformation. DISCUSSION The results clearly showed an increase of wear depending on implant survival; however, the tendency is not linear. This fact can be attributed to a larger amount of abrasive particles, causing an increase of wear or occurrence of surface wear in terms of micro cracks and oxidation degradation of polyethylene. This study indicates that geometry, positioning, and cup alignment during the implantation have a fundamental impact on the cup durability. Further correlation, which was observed in the case of the cup with prevailing roughness in the range from 0.1 to 0.3 μm, is a relatively wide wear vector angle determined with the use of the optical method. Considering the implants with the longest survival time with no loosening of the acetabular cup, the mean angle of direction vector was 56.8° (SD 2.1°). CONCLUSIONS The present study provides the results of morphology analysis of the retrieved Bicon-Plus cups. In general, relatively high wear rate, mainly of abrasive-adhesive character was identified. The dependence between wear and implant in situ longevity was not clearly linear, which suggests the influence of other parameters on the polyethylene wear rate. An important role of implant positioning on survival was also revealed. Moreover, it seems that it can be a more important parameter than the characteristics of the patient. Key words: total hip arthroplasty, Bicon-Plus cup, retrieval analysis, surface analysis, wear measurement, roughness, deformation, survivorship.