Atomic force microscopy (AFM) is used to characterise UHMWPE wear debris from a knee prosthesis actuated under constant load. The size and shape of debris particles is quantified in all three spatial dimensions. Artificially limiting the analysis to the two-dimensional projections of the particles onto the substrate plane, it has been found that equivalent shape ratio (ESR) plotted as a function of equivalent circle diameter (ECD) follows a trend observed before. Inclusion of the third, vertical spatial dimension of particle height shows that such two-dimensional analysis, as it is often based on SEM images, can greatly misrepresent the actual particle shape. The three-dimensional AFM information indicates that for the prosthesis and the conditions studied here debris particles tend to be deformed independent of their volumetric size. Fractionation of debris particles according to size was achieved with a new filtration protocol. It is demonstrated that with this protocol debris particles settle uniformly across the filter and that particle abundance as function of size can thus reliable be established.