It has been stated that a criterion of a threshold concept is that it is troublesome or difficult to understand. However, not all difficult concepts are also threshold concepts. This article explores this difference through the context of students' difficulties in learning prosthetics and two models of describing threshold concepts. Students and staff at two UK universities teaching prosthetics and orthotics participated in interviews and questionnaires. Data were analysed using an interpretative phenomenological analysis approach. Five themes of difficulty emerged from the data, two of which were difficult concepts and three of which were difficult and threshold concepts. These themes involved issues with numbers, the human body, disciplinary language, gait and a holistic perspective. The differentiation was that threshold concepts involved both conceptual and ontological transformation and the use of procedural concepts with associated memories in order to develop disciplinary ways of viewing a concept.