A layer is arranged on an implant for bone or tissue structure. The layer constitutes a boundary or barrier between the actual or unoxidized body of the implant and the structure for the purpose of increasing retention and has, in this context, a substantial thickness. The layer is designed with a channel network which gives the layer a substantial porosity. The channel network is designed with mouths which face towards the structure and whose respective cross-sectional diameters, at the surface of the layer facing towards the structure, are substantially less than the respective extents of the channels in and down into the layer as seen from the said surface.
