The preform has a grid (1) formed by a network of weft threads (2) and wrap threads (3) coated in an impregnation methacrylate resin (6). A central space (8) between meshes is free from fibers and resin, higher than 25 percent of a total surface of the grid and determines a surface having free spaces. The weft threads are fixed on the wrap threads at nodes of the network to authorize handling of the preform and formation of the preform on a laboratory model without creeping impregnation resin in the space. A thermoplastic membrane is made of ethyl-vinyl-acetate, and plated on the grid. The preform is made of composite material constituted by resin, particles and fibers in a state before polymerization. An independent claim is also included for a method for forming a reinforcement grid of a dental prosthesis.
