In an element for implantation in body tissue, particularly bone tissue, consisting of a biologically flawless material with a micro-pitted surface, the pores in the surface have a diameter many times smaller than has been previously known in order to permit the occurrence of such a tight and extensive boundary zone around the implanted element that this achieves reinforced and inextricable anchoring in the tissue. The pore diameter may be as little as about 10 nm and as large as a few multiples of the normal diameter of the cells in the tissue, preferably no larger than the cell diameter, i.e. about 1000 nm. Optimal results are obtained with pore diameters equal to or smaller than about 300 nm and a finely pored rutile layer has been found to give a particularly strong and durable joint with the growing tissue. Preferably at least one deposit of an agent facilitating and/or accelerating the growing-together process is arranged on or in the element. The element may be shaped with grooves, corrugations, channels etc. and be provided with an opening for tissue to grow through. The element is extremely suitable as anchoring device for a prosthesis or partial prosthesis and may be made integral therewith.