What is a “Normal” wear pattern for metal‐on‐metal hip bearings?

Abstract

In addition to classical run-in and steady-state wear phases, metal-on-metal (MOM) hip bearings have encountered "runaway wear" (RAW) trends in simulator studies. This puzzling behavior has resulted in 2- to 19-fold wear increases compared with other apparently "identical" bearings. MOM bearings have shown five identifiable RAW wear patterns in joint simulators; therefore, additional descriptive terms were used here to indicate various observed patterns, for example, "breakaway wear" (BAW), which was defined as a higher wear trend that recovers to steady-state wear. As these trends commonly occur for MOM, this raises the question of what can be considered "normal" behavior or "abnormal"? In an effort to identify possible causes for this behavior, the current study investigated six Co-Cr bearings, which closely matched with respect to geometrical tolerances. Despite close control of design and test variables, BAW occurred in 30% of the MOM bearings, producing a threefold wear increase above otherwise identical MOM bearings within the same group. The majority of the BAW (85%) occurred on the cup side and was validated by growth of wear scars and concentrations of metal ions. One bearing that showed continuing BAW at 5 Mc revealed a cup that was 50% smoother than other cups whereas its mating head was 50% rougher, thus signifying that highly polished areas were sites of the highest MOM wear. The two BAW bearings with high wear showed the greatest conformity at 5 Mc, in apparent contradistinction to classical lubrication theory.

Risk of Impingement and Third-body Abrasion With 28-mm Metal-on-metal Bearings

“…Thus, our assumption that such wear patterns were indeed ''normal'' may be incorrect. Simulator studies with large-diameter MoM have described wear areas mainly in cups [5,44,46]. In our 60-mm simulator study [5], the resulting Hemi% ratios represented 29% and 8% for heads and cups, respectively.…”

“…1A: Type A) and one with a 205°cup-face angle (Type B). MoM bearings were examined for sites and patterns of habitual wear created by patient flexion-extension and abduction-adduction activities [5,9,33,56]. Bands of protein deposits were noted but not included for study [7].…”

Wear of CoCrMo alloys used in metal-on-metal hip joints: A tribocorrosion appraisal

“…Kretzer [46], Rieker [52] and Büscher [12] maintained a lubricant temperature of 37 1C while Bowsher [48] [52] were reported. Clearly the judgement of wear severity through wear rates must take into account this run-in effect, and in particular the relation between run-in and test duration.…”

“…The number of loading cycles varies typically from 3 up to 10 Mcycles at frequencies of 1 or 1.13 Hz. Table 2 summarizes the key features of published papers on the characterisation of metal on metal CoCrMo artificial hip joints using simulators [12,[46][47][48][49][50][51][52][53][54][55][56][57][58]. Among the authors, only Yan [49] measured the evolution of open circuit potential during a simulator test.…”

“…With approximately 40 components [63] bovine serum is a very complex liquid with a broad reactivity spectrum. It is seldom used without additions of biocides (such as sodium azide NaN 3 ) and complexing agents (such as EDTA) to avoid metal precipitation [46][47][48]51,[53][54][55][56][57]. Moreover, it can be used at different dilutions in water [51] or Ringer's solutions [52] depending on laboratory.…”