The present invention provides a system and a method for forming in vivo a junction between a first and a second element of a modular endovascular prosthesis used for repairing defects in vessels and other lumens within the body of a patient. The first element is adapted to have a receiving element, and the second element is adapted to have a protruding element. In the final deployed configuration of the endovascular prosthesis, the protruding element of the second element is adapted to be engaged with the receiving element of the first element, substantially preventing axial separation of the elements relative to each other. The receiving element of the invention may have various forms, and may include a fold in the wall of the first element, an expandable framework having protruding struts, or a flexible thread. The protruding elements of the invention may be formed from an expandable framework having protruding struts, or may be formed from a fold in the wall of the second element, or may comprise a flexible thread.