In the last few years, the advances in three-dimensional medical image processing have made possible operations like planning or simulation over real data. Different representations of structures or models have been proposed, being the implicit surfaces one of the most flexible models for processing. This paper introduces a new method for computing the implicit surfaces from the explicit representations of the objects segmented in three-dimensional images. This proposal is based on the approximation of the surfaces using distance functions and natural neighbor interpolation. The system has been tested over CT images of tibia and femur where the explicit representation has been extracted through a TAV model [1]. The results obtained show the suitability of the method for a correct representation of the target objects.