A medical implant device comprises a substrate (10) having an undulating surface provided by peaks (12) which are separated by recesses (14). The device includes a porous coating layer provided on the undulating surface of the substrate which comprises a plurality of particles (16). The spacing between adjacent peaks on the surface of the substrate is less than the particle size of the particles. The particles are bonded to the peaks on the surface of the substrate and adjacent particles are bonded to one another.