1. A method of segmenting an implant in a radiographic image of a portion of a patient's body that contains said implant, to enable the segmented portion of the image corresponding to the implant to be processed subsequently, the method comprising the following steps performed by a computer: smoothing (S 5 ) the image by a lowpass first filter for reducing noise, in order to obtain a smoothed image; applying (S 6 ) a non-directional spatial derivative second filter to the smoothed image, in order to obtain a filtered image; thresholding (S 7 ) pixels of the filtered image in order to obtain a thresholded image, the thresholding (S 7 ) of the filtered image is hysteresis thresholding, comprising a high first threshold and a low second threshold, the thresholding (S 7 ) of the filtered image comprising retaining pixels having a brightness value greater than or equal to the high first threshold, discarding pixels having a brightness value less than the low second threshold, and retaining pixels having a brightness value lying between the high first threshold and the low second threshold providing they are adjacent to at least one pixel having a brightness value greater than or equal to the high first threshold; selecting (S 8 ) at least one outline obtained in the thresholded image, in order to define the segmented portion of the image corresponding to the implant; modifying (S 12 ) the threshold(s) used during the thresholding step (S 7 ) if the segmented portion that is obtained does not satisfy at least one calculated criterion, and reapplying the thresholding and selection steps (S 7 , S 8 ) in order to obtain a new segmented portion, said threshold modification (S 12 ) includes at least modifying the low second threshold; and if said criterion is not satisfied after a predetermined number of modifications or after lowering the low second threshold down to a minimum, then dilatations are performed followed by erosions of increasing size on the thresholded image until a closed outline is obtained for defining the segmented portion.