Giovanni Dosi's technological paradigm theory was developed in part to correct the then dominant practice to focus on either "demand-pull" or "technology push" when explaining technical change. In Dosi's view, demand can act as a "focusing device" for the supply side to select among the available technological paradigms for development. In this article, we study a specific case, the technological evolution of a surgical implant (an artificial disc in the spinal column), to analyse this selection dynamics.