An arrangement and an implant are provided for increasing the stress resistance of the implant arranged in an upper jaw bone (1). The implant has access via parts (9, 4') to the sinus cavity (5). At said parts, the implant is arranged with a convex or rounded front surface which, upon access, lifts the sinus mucous membrane, without piercing the latter, and thus forms a closed space (7) between the parts and the underside of the mucous membrane. The implant is provided, at least at said parts, with growth-stimulating substance or substances (15) which interact with cell-containing body fluid (8) which penetrates or has penetrated into the space, so that new bone (11) is formed around said parts of the implant.