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. We conclude Dosi's theory can be extended by adding some technological signals interacting with demand forces in this pre-market selection process, which help to explain the historical supply side choices in the evolution of the artificial disc.