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Most research on procurement of medical devices focuses on buyer behaviour eg in the UK, purchasing strategies by the NHS. This paper is a rare case study of the other side of the market. We investigate competition in the artificial hip implant market in England and Wales, using National Joint Registry data for 2005-18. The analysis is in three parts. We first proceed as would a competition agency, by assessing the nature and structure of the market. We find a highly concentrated duopolistic market structure, in which there has been no significant entry or exit, except a merger between the much smaller 3rd and 4th largest suppliers. Viewed through the eyes of a competition economist, such a structure might be indicative of weak competition. However, concentration is not necessarily synonymous with low competition: it might be that technology dictates that only a small number of firms can survive profitably in the market, but that there is still the cut and thrust of competition leading to forever changing identities of the leading firms. In the second part of the paper, we address these possibilities in two ways, using a novel exploration of market share dynamics and an international comparison. On dynamics, we find an important difference between the two sectors of the market: in traditional cemented prostheses, there is little evidence of any share mobility amongst firms. Indeed, the only discernible change is that Stryker, already the dominant firm, has steadily further increased its market share over time. On the other hand, the uncemented sector has exhibited much more share mobility. Interpreting the uncemented sector as a comparator, this suggests …