Ten-Fold Increase in MOM Wear Rates Resulting From 3rd Body Metal Particulates: A Simulator Model

Abstract

It has come to light that one significant mechanism for MOM failure may be repeated subluxation or impingement episodes leading to edge wear and release of 3^rd body particles. This MOM debris-challenge model simulates a patient who experienced one subluxation or impingement event and then continues to walk normally until the next event occurs one week later. Our model assumes that 100–200 particles (debris size 100–200 μm) would be released into the joint space at each subluxation or impingement event. The question then becomes: what is the effect of the patient walking on that single dose of particulates over the next week (or 500,000 cycles in simulator test mode).

Nine 38 mm CoCrMo bearings (DJO Inc., Texas) were run inverted in a12-station hip simulator (SWM, Monrovia, CA). The test was 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 for the first 3 Mc, followed by 10 mg of debris particles from 3–5 Mc. 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. Serum was changed out every 500,000 cycles and a fresh dose of debris added. All bearings were ultrasonically cleaned and examined using white light interferometry (WLI, Zygo Corp) and SEM (EVO MA15, Zeiss). Wear rates were determined gravimetrically and serum discoloration was noted at each test interval.

Titanium alloy and CoCr debris produced darkened serum within the first hour of the test and remained so for the duration (500,000 cycles). Serum color with cement debris remained an opaque golden color throughout the test run. The debris challenge provoked the largest MOM wear response using Ti6Al4V particulates (6.7 mm³/Mc), slightly milder with CoCr particulates (4.5 mm³/Mc) and minimal with PMMA particulates (0.5 mm³/Mc). Compared to bone cement debris chambers (which had wear rates comparable to non abrasive MOM bearing tests), CoCr debris created a 9-fold higher MOM wear and titanium alloy debris created a 14-fold higher MOM wear. These observations indicated that only the metal debris elicited an ‘Adverse’ wear response with MOM bearings.