Particle size analysis of implant debris (ASTM 1877-05) has typically been conducted by particle counting using scanning electron microscopy (SEM).  Competing technologies such as Laser Diffraction (LD), which provide both volume- and number-based distributions, call into question when is each technique appropriate and/or inappropriate. We hypothesized that there are specific conditions when routine analysis of implant debris is mis-characterized by one or the other technique.  We tested this hypothesis by compiling the analyzed results of simulator fluids of implant debris from 5 types of implant simulator studies (metal-on-metal, metal-on-polymer, polymer-on-polymer, and ceramic-on-ceramic bearing surfaces). Debris included Co-alloy, Cr-O-P, Ti-6AL-4V, PEEK, UHMWPE, PTFE and Alumina particles. Inappropriate use of both SEM and LD analysis was found in several critical instances: 1) <0.001mg/mL of debris, LD reached method detection limits and lost “sight” of particles <1um, 2) the wider the distribution (0.1 to 1000um) the greater the misrepresentation by the a number-based SEM analysis. 3) LD analysis is unable to “see” certain types of material (e.g. PTFE) when the refractive index of the material is similar to organic and aqueous mediums (Index=1.3). 4) Generally, SEM analysis over represents smaller particles (<5um) identified in high magnification images because their average is weighted at 10,000 times that of the low magnification SEM images for average calculation.  5)  Laser diffraction analysis measures millions of particles; however, it lacks the capability to yield morphologic data.  These differences are critical to implant debris analysis where if the inappropriate method is chosen, inappropriate conclusions will be drawn.