Abstract
The MOM controversy continues with many prevailing opinions as to the causes of failure in contemporary designs. There has been a great deal of focus on breakdown in fluid-film lubrication with respect to cup positioning and edge wear at its rim. However there has been very little discussion on the problems of 3rd body abrasion. In only one study was there a description of unusually large abrasive marks on retrieved femoral heads (McKee Farrar MOM), revealing 100 μm wide scratches, attributed to circulating particles fractured during impingement episodes. With contemporary MOM devices, there is the potential for abrasion by particulates of CoCr, PMMA and Ti6Al4V. However it has been difficult to formulate a coherent simulator model for 3rd-body abrasive wear, given the unpredictable nature of impingement damage releasing abrasive particles into the patient's hip joint. Thus this study sought to identify if metal or cement particulates were capable of creating 100 μm wide scratches as seen on retrieved MOM and develop a simulator model for 3rd body abrasive testing on MOM bearings.
Six 38 mm CoCrMo bearings (DJO Inc., Texas) were run in a12-station hip simulator (SWM, Monrovia, CA) with cups mounted both anatomically and inverted (3 MOM each). The tests were run in standard simulator mode (Paul gait load cycle: 0.2–2 kN, frequency 1 Hz) with the addition of 5 mg of debris particles. Commercially available CoCr (ASTM F75) and titanium alloy (ASTM F136) particles and broken polymerized bone cement particles were used in the size range 50–200 μm. The simulator was run for only 10 cycles and the MOM parts removed for study. All bearings were ultrasonically cleaned and heads were examined using white light interferometry (WLI, Zygo Corp). Grooves were characterized using surface profiles to measure width, depth, and rim height. SEM imaging (EVO MA15, Zeiss) and EDS imaging (X flash detector 4010, Bruker AXS) was performed in areas of grooving and suspected transfer layers.
CoCr debris produced broad, curvilinear grooves with widths ranging from 20–170 μm, depths from 0.3–1.5 μm, raised rims, longitudinal striations and chatter marks. Titanium alloy debris produced arrays of very shallow scratches accompanying larger grooves. These larger grooves measured 20–110 μm wide and 0.4–1.9 μm deep.
EDS imaging showed the smears and islands contained the elements Ti, Al and V representative of the Ti6Al4V alloy. WLI imaging showed these metal deposits (250–900 um wide) were raised >10 um above the surface. Particularly conspicuous was evidence of considerable smearing on CoCr surfaces, with linear streaks ranging 150–300 μm wide. Bone cement debris proved incapable of grooving the CoCr surface, the only scratches observed were those comparable to normal carbide scratches.
