The invention concerns a method for manufacturing a prosthesis (11) for a fractured long bone of a patient, the method comprising the steps of: A) providing data representative of the fractured long bone, the fractured long bone comprising a diaphyseal fragment (2) comprising a medullary cavity (8); B) based on said data, designing the prosthesis specifically to the patient, the prosthesis comprising a stem part (12) configured to be inserted into the medullary cavity, step B) comprising: a sub-step of choosing, specifically to the patient, a contact zone (40) of the medullary cavity onto which a respective chosen mechanical stress is planned to be applied by the stem part, and a sub-step of designing the stem part so that the stem part may be inserted into the medullary cavity and thus apply the chosen mechanical stress to said contact zone; and C) manufacturing the prosthesis designed at step B).